+ Reply to Thread
Page 1 of 3 123 LastLast
Results 1 to 15 of 40

Thread: DIY Axially symmetric oblate spheroid CD waveguides, in solid Oak

  1. #1
    Member jack_bouska's Avatar
    Join Date
    Aug 2004
    Location
    London U.K.
    Posts
    83

    DIY Axially symmetric oblate spheroid CD waveguides, in solid Oak

    I posted some images of my recent waveguide construction on the "Horn system pictures" thread, <http://www.audioheritage.org/vbulletin/showpost.php?p=122818&postcount=49>, and in post #51 "Titanium Dome" followed my post with a request for the & measurements I made on my waveguides.
    Rather than clutter up the "Horn system pictures" thread, I have chosen to start a new thread under the DIY section, with more details of the design and images.
    The two new waveguides are mounted on a pair of 1" exit TAD 2002 compression driver, and a 2" (49mm actually) exit a pair of JBL 2441 compression driver.
    I purchased the TAD new, however the JBL 2441 originally saw duty as part of the sound reinforcement system of the Calgary Saddle dome, home of the 1988 Olympic games <
    http://www.pengrowthsaddledome.com/foundmain.html>. It may be unusual to use a device which once shared about 1/15 of the coverage duty for a 20,000 seat stadium in my modest sitting lounge, but it should be no surprise to this membership that achieving realistic dynamic range with low distortion does require some extreme measures to achieve our acoustic goals
    The full list transducers in the speaker system consist of (refer to picture 4 in this post)
    2 x TAD 2002 (4khz - 20khz)
    2 x TAD 2441 (1khz-4khz)
    4 x JBL 2123 10" cone midrange (250hz - 1khz)
    4 x JBL 1401nd 14" cone upper bass (60hz - 250hz)
    2 x Altec 3182 18" cone sub bass (7hz-60hz)
    The waveguides were designed using information from published papers by Earl Geddes. (reprints can be found at: <
    http://baseportal.de/cgi-bin/baseportal.pl?htx=/Data/exdreamaudio/download> )

    Images and measurments graphs to follow:

    Jack Bouska

  2. #2
    Member jack_bouska's Avatar
    Join Date
    Aug 2004
    Location
    London U.K.
    Posts
    83

    OVERVIEW:

    The first four pictures, in this post illustrate the Oak waveguides during construction, mounting, and in their final location in my listening room. The first images shows the "green" untreated French Oak on the faceplate lathe partway through turning. Note the blue plastic template used to guide the turning operation. I nicknamed the horn flare "the rams head horn" for obvious reasons.
    The second image shows how the horns are mounted to a retaining flange which forms part of the brackets which retain the compression drivers. The horns are not bolted directly to the compression drivers. BluTak is used as a flexible gasket to ensure a smooth transition from waveguide throat to driver exit.
    The third image shows a front perspective close up of the mid-high section of the loudspeaker.
    The fourth image shows a full view of the components, left to right: TX columns contain an Altec 18" in 4 meter folded transmission line cabinet, centre: "stack of loudspeakers" and on the right, the equipment rack with CD, 2x Behringer DCX2496 and the 5kW Tripath power amplifier "plate"
    The full system can play peaks well in excess of 130dB, over a power range of 10hz to >20khz
    I nicknamed the loudspeaker system "IMPACT STACK"
    Jack Bouska
    Attached Images Attached Images     

  3. #3
    Member jack_bouska's Avatar
    Join Date
    Aug 2004
    Location
    London U.K.
    Posts
    83

    WAVEGUIDE SHAPE AND CONSTANT DIRECTIVITY MEASUREMENTS:

    The first two pictures in this post show the waveguide design profile for the 2" throat, and the 1" throat compression drivers respectively. These images can be downloaded, scaled (to match the dimensions listed on the diagrams), and printed to produce templates should anyone wish to construct a copy of these specific waveguides.
    Image 1: The 1" exit waveguide is stock Oblate spheroid shape, with a 1/2 round radius at the mouth The 2" exit waveguide is a compound shape, starting with oblate spheroid from mouth to 3/4 horn length, merging (seamlessly) into a tractricx taper for the last 1/4 of the waveguide, to the mouth, finishing in a 1/4 round radius curve to the outside edge.
    When I worked up a waveguide design which followed E. Geddes recommendation, to terminate the mouth with a radius edge equal to 1/4 wavelength at lower frequency. limit, this resulted in a radius of over 3" for 1khz, which meant that for a full 1/2 round mouth, I would need an annular ring 6" wide all the way around the mouth.
    Image 2: My oblate spheroid mouth is 5" in diameter, which would have made the whole horn 17" in diameter, much too big. Instead, the Tractrics horn mouth termination (which goes from 40deg curving around to 90deg) is only about 1 1/2 inches wide, and I add a 1/4 round radius just to smooth the edge. This makes the whole diameter much more tractable at less than 10" diameter
    The last three images show the directivity response of a 2" throat tractrix horn, followed by the 2" waveguide, and finally the 1" waveguide (mounted on the JBL 2441, and TAD 2002 respectively). The measurements were made with a Panasonic mic element, Altec microphone preamp, SoundBlaster Audigy ZS 24/96 pcmcia sound card, IBM laptop and ETF5 software. The measurements were made at low SPL (~88dB) at a distance of 50cm from the mouth.
    Image 3: The spl response vs. angle for the Tractrix horn is seen to "fan out" fairly rapidly from about 2.5khz upward, with the curves widening as frequency increases. Only the 90 deg (on axis) curve stays close to flat. (These curves were measured without using any horn compensation) The curves are annotated as angle away from horn axis. This angular coverage is about the same as I would expect from a cone of approximately the same diameter. (not CD)
    Images 4-5: Both the 2441 and TAD curves illustrate good examples of constant directivity using the axially symmetric oblate spheroid waveguides invented by Earl Geddes. The spl response curves cluster closely (within a few dB) over the full +/- 40 degree design coverage. The curves are annotated as angle away from the plane of the waveguide mouth (so 90deg is directly on axis). Both these devices have response equalization applied using a Behringer DCX2496 digital crossover.
    The spl response curves which are outside this angular coverage (yellow, black and red curves) are seen to drop in amplitude very rapidly with rising frequency, which indicates that the CD waveguide "does what it says on the tin"
    Image 4: The 2" throat device on the 2441 is good from about 1khz up to 10khz (where the aluminium diaphragm break-up starts), and the phase plug no longer produces nice planar waves above that frequency.
    Image 5: The 1" throat device on the TAD appears to be good from 1khz up to beyond 20khz. No evidence of diaphragm break-up is visible. (Beryllium, the right material for this job)
    Jack Bouska
    Attached Images Attached Images      

  4. #4
    Member jack_bouska's Avatar
    Join Date
    Aug 2004
    Location
    London U.K.
    Posts
    83

    AVERAGE RESPONSE CURVES AND IMPULSE RESPONSE

    The first four pictures in this post show the uncorrected, and EQ'd-corrected frequency response of the two waveguides, using white noise, averaged over a 4ft x 4ft x 2ft volume at/around the listening position, followed by Fourier transform spectral analysis averaged over about 45 seconds of recorded noise waveform. Use of spatial averaging smoothes through the short period room modes (peaks and nulls), but highlights the response anomalies which are inherent to the device under test. This form of measurement is a balance between the direct arrival energy, and the reverberant sound field in the listening room. (as opposed to the previous ETF5 measurements, which are intentionally restricted to direct arrival only, via close mike placement, and short window on the FFT transform).
    The volume of averaging is around the defined listening position, and stays well within the +/- 40deg coverage angles. Both left and right channels are run simultaneously, with uncorrelated white noise between channels. Amplitude is less than 90dB, distance approximately 7-11ft from each channel.
    Note that the compression driver/waveguide combinations are not useable without response equalization applied using a Behringer DCX2496 digital crossover) shown annotated on the graphs. The waveguide CD devices do not have the characteristic on axis high frequency boost noted with exponential or tractrix horns.
    Image 1: shows the response of the JBL 2441 2" throat drivers on the waveguide, using spatially averaged white noise.
    Image 2: shows the response with final EQ applied. I chose to use a parametric EQ (Band pass) centred on 2.62khz with a Q of 0.4, level -8dB cut.
    Note the very low level of ripple on both these graphs (around +/- 1db worst case). This is a very good indication of the amount of internal (axial) reflections within the waveguide and coupling throat of the JBL. Poor acoustic impedance matching at the mouth can cause reflections which send energy backwards in the waveguide to be reflected at the phase plug, and generate resonances. These axial modes generally exhibit themselves as the classic "horn honk" and sound similar to the distortion you can hear when speaking through the cardboard tube from kitchen food wrap (or toilet paper tubes). The resonance modes (reflections) are generally visible as ripple in the response of most horns, and a smooth curve is evidence of low levels of reflected energy, and high quality tone. My choice of tractrix mouth instead of 1/2 round radius was apparently a wise one, as borne out by these graphs. The sound is indeed very natural and clear. To quote my audiophile friend on his first listen: " I have never heard horns sound like real music before this!"
    Image 3: shows the response of the TAD 2002 1" throat drivers on the waveguide, using spatially averaged white noise.
    Image 4: shows the response with final EQ applied. I chose to use a parametric EQ (Band pass) centred on 4.238khz with a Q of 0.5, level -7dB cut, cascaded with a HighPass shelving filter set at 20khz, +15dB, single pole 6dB/oct.
    Note the modest increase in the level of ripple on the TAD response (now +/- 2dB). I attribute this to two possibilities: 1) the use of a small 1/2 round radius on the horn mouth which is intended only to operate above 4khz (the crossover frequency for this device) and 2) reflections in the phase plug and throat of the device itself. I have noted these same ripples, at the same frequency locations, on a number of horns, of different taper and length, when used with this same compression driver.
    When the crossover is applied, the same graphs show marked reduction of ripple below 4khz (in the crossover band), which indicates that offending reflections (poor mouth termination) can be suppressed by restricting the bandwidth to the range appropriate for the round over at the mouth (1/4 lambda = r)
    Image 5: shows the pair of impulse response graphs, using the 90deg measurement from ETF. In the interest of time (I have to go now) I will describe these two graphs in the next post, later this week.
    For now the above graphs give a fair overview of the performance of these devices, and I promise more information to follow shortly
    Jack Bouska
    Attached Images Attached Images      

  5. #5
    Member jack_bouska's Avatar
    Join Date
    Aug 2004
    Location
    London U.K.
    Posts
    83

    ETF5 GRAPHS

    The last image in the preceding post (#4 above) showed the pair of impulse response graphs for the compression drivers on waveguides (JBL top, TAD bottom). The amplitude scale is linear (+/- 32k, 16 bit wave file), and the time scale is in samples @ 48khz sample rate. The display window is 120 samples long, or 2.5ms. The energy on both devices is seen to decay by 99% in the first 1/2 millisecond, and further attenuate to approximately 99.9% within the first full millisecond. Both wave shapes exhibit a classic high-pass differentiation filter shape (prominent peak-trough), followed by an exponentially decaying tail of band limited lower frequencies. The impulse response of both devices also display a 2nd negative pulse following the first trough. I suspect this is a function of the digital EQ applied via the Behringer DCX2496 crossover, and I intend to investigate by running some "raw" measurements, driving the compression drivers directly from the soundcard/amp with no equalization applied.

    The first image in this post shows the ETF5 cumulative spectral display, in linear scale format. The spectral plot serves two useful diagnostic purposes: first it illustrates the decay in energy with time, (similar to a waterfall), by using four overlapped FFT graphs (each 2.6ms long), with start times of 0,1,2,3ms respectively. Second, the linear scale spectral graph can be used to identify/analyze internal mouth-throat-phase plug resonances (axial modes), which cause response ripples due to comb filtering of the reverberation pattern.

    The 2441 graph (top) shows that between 1.5khz and 10khz, the energy from 1-3ms is on average about 20db lower than the first arrival, which implies very little energy storage within the device or diaphragm. Much of the measured energy beyond 1ms is likely reflected from surfaces near the microphone (floor, walls, furniture), while some of it is related to the unavoidable effect of abrupt horn mouth termination, and the acoustic impedance contrast associated with the rapid change in coverage angle from +/- 40deg to full space (+/- 180deg). At the rim of the waveguide (or any ordinary horn), the acoustic impedance contrast generates diffractions, sending some energy backwards within the waveguide from the mouth towards the throat. This is reflected again from the position of the phase plug (another cause of acoustic impedance contrast within the compression driver itself), and the affair sets up a series of decaying "normal mode" reverberation, with reverberation time (period) roughly equal to the length of the horn (plus compression driver coupling throat), divided by speed of sound. (~ 35cm/ms)

    In their article: "Round The Horn" Philip Newell and Keith Holland, (Speaker Builder, 8/94) suggest that much of the classic "horn honk" is attributable to these mouth-throat resonances. For a practical demonstration of how important good mouth termination is for neutral tonality, simply take your favourite magazine, roll it up into a conical shape "megaphone" hold it up to your mouth and clearly utter the phrase: "this is the sound of horn honk". (and you will be speaking the truth). Horn honk can be easily detected by your ears as acoustic resonance and emphasis of some frequencies, along with recognition of the delayed energy caused by the trapped waves. Bad horns have a great deal, and good horns have almost no detectable evidence. The axially symmetric horn tested by Keith Holland was documented as sounding like a quad electrostatic by the listening panel.

    These resonances take the form of delayed signals, which are added back into the acoustic output (with alternating polarity). The delay+addition of signal creates a comb filtering effect, which on a log scale amplitude plot appears as a sinusoidal ripple in the (otherwise smooth) response. The period of this sinusoidal pattern is related to the time constant of the reverberation according to the equations: time period = 1 / frequency (as measured peak to peak on a linear frequency scale spectral graph)
    Inspecting the first image of this post shows the most pronounced ripple to have a peak to peak span of about 400-500hz, which relates to 1/400 = .0025s = 2.5ms or around 87cm, which corresponds to the distance from the horn to the floor, and back to the microphone. Intra-waveguide reflections would need to be less than the 20cm (x2) length along the waveguide axis. (40cm / 35cm/ms = 1.14ms, and 1/(.00114) = 877Hz) This corresponds to ripples with duration of 900hz or longer. Evidence of these periods are difficult to discern on the blue spectral graph, but are noticeable on the later spectral lines, with some periodic notches noticed at 1-2khz. This would imply some reverberation between the throat and mouth of the wooden waveguide section (10cm) or more likely, between the phase plug and the transition to the wooden waveguide section (~6-8cm). The exit point from the compression drivers to the waveguide acts as a circular diffraction aperture, such that the near planar wave front emerging from the compression driver effectively diffracts into a spherical wave front, to ultimately fill the conical expansion of the oblate spheroid contour.

    This ripple (900-2khz period) is very low amplitude, perhaps 1-2dB out of the total 40dB on the vertical axis. This effect is not audible by me, as I rate the waveguides as having near-zero horn honk characteristic.
    Incidentally, the intra-phase plug resonances would generate ripple with a 10kHz period on these graphs, and indeed there does appear to be some visible on these graphs, however I would need to do more work to determine if this is truly a phase plug problem, or if it has been induced by my heavy handed equalization scheme.

    The second image in this post shows the measured phase response of the waveguides, (blue line) compared with the calculated minimum phase for a filter with the same frequency response (using Hilbert transform). Ignore the vertical lines on the top plot, where the measured phase slightly exceeds the +/- 180deg vertical scale (phase wrapping).

    The third image shows the 1/3 octave smoothed, on-axis, frequency response of the waveguides, with a gating of ~10ms. The close mike position, coupled with the short window, mean that these graphs correspond to the direct arrival "anechoic" frequency response (although some pesky reflections do affect the response accuracy). Full equalization has been applied, with the lower frequency of the crossover set to 350hz

    The fourth image of this post contains the waterfall plots for the waveguides, and are included mainly as "eye candy". Apart from the obvious quality of the rapid energy decay with time, also note that the devices are relatively resonance free (apart from the common bump and ridge at 1khz). The ridges down at the noise floor are largely influenced by early reflections from floor and surrounding objects. However one important feature of note is visible on the JBL 2441 graphs at approximately 10khz. A dip in the frequency response (T=0) rapidly turns into a non-decaying ridge along the time axis. This is a clear indication of a diaphragm break-up mode, and is audible when the 2441+waveguide is run full band (1-20kHz). For frequencies below 10kHz the performance is exemplary, and rivals (or perhaps even betters) the performance of the TAD with it's modern phase plug and exotic Beryllium diaphragm. (Note, the JBL 2441 has 1dB more sensitivity, than the TAD 2002, and can handle 9dB more power input-watts).

    The fifth image in this post (and the last in this series) shows the ETF 1/3 octave smoothed (direct arrival weighted) frequency response, compared to the spatially averaged, white noise excitation, spectral estimate (reverberant field weighted). The frequency response is wide and smooth, but more importantly, the direct arrival response closely matches the reverberant field response, which is a direct consequence of employing the constant directivity oblate spheroid waveguides. This allows me to choose a single set of response EQ curves which are appropriate for both the on-axis direct arrival wave front, and also appropriate for the reverberant portion of the wave field in the room. The CD waveguides permit the generation of a smooth 360deg (solid angle) power response, when the speakers are placed in the corners of my listening room.
    Jack Bouska
    Attached Images Attached Images      

  6. #6
    Senior Member
    Join Date
    Feb 2004
    Location
    SoCal, USA
    Posts
    8,895

    Smile Thanks!

    Jack

    That's a plethora of information. Thank you.

    You've clearly been working and thinking hard on these units.

    I see some room treatments there. Comments?
    In.

  7. #7
    Member jack_bouska's Avatar
    Join Date
    Aug 2004
    Location
    London U.K.
    Posts
    83
    Quote Originally Posted by Titanium Dome
    Jack

    That's a plethora of information. Thank you.

    You've clearly been working and thinking hard on these units.

    I see some room treatments there. Comments?
    Mr Dome (can I call you Titanium?)

    Actually, the room has not had any specialized room treatment at all. It is furnished normally, with both soft chairs, and large leather sofa's. The floor is carpeted, and the walls and ceiling are untreated other than normal hanging pictures. The room contains no curtains. Also rather interestingly, the entire construction is cement block, with poured cement floor and ceiling. The room modes are well distributed, but very pronounced. The typical RT60 in the room measures less than 0.4 seconds for frequencies above 120hz. Below 120hz the RT60 increases to a time of 1.2 seconds, owing to the massive cement container construction (dimensions: 18.5 x 14 x 8.1 ft)

    I use high Q notch filters to attenuate the worst of the axial modes.
    I do need to add damping in the low frequencies, and intend to used large diameter rigid fibreglass pipe insulation as an aperodic absorber, placed upright in the corners of the room, just have not gotten around to this yet. In the mean time, the only low frequency absorption comes in the form of the leather sofa.

    When you mention room treatment, I suspect you are referring to the faux-marble columns visible behind the main speaker stack. (in the corners)

    These are actually 4 meter long, folded transmission line loudspeakers, loaded with one 18" Altec each.
    Pictures of the driver are available in my image gallery on this Forum:
    http://www.audioheritage.org/photopost/showgallery.php?cat=500&ppuser=844
    Details of the construction are shown below in the following three images.
    Jack Bouska
    Attached Images Attached Images    

  8. #8
    Administrator Robh3606's Avatar
    Join Date
    Apr 2003
    Location
    Destiny
    Posts
    7,510
    Nice job Jack!!

    So does it sound as nice as it looks??

    Rob

  9. #9
    Super Moderator yggdrasil's Avatar
    Join Date
    Dec 2004
    Location
    Bodø, Norway
    Posts
    1,126
    Fantastic. You must certainly have spent a lot of time/fun planning and finishing off with very skilled craftsmanship.

    Do you have original DCX2496? There was an article in audioXpress this summer about replacing the analog section completely.
    Johnny Haugen Sørgård

  10. #10
    Senior Member morbo!'s Avatar
    Join Date
    Oct 2005
    Location
    australia
    Posts
    399
    Thats totally friggin amazing

    I always wanted cyndricral spreaker`s

    now i got a new project
    gonna take me awhile to save up for 18" drivers
    but i really wanna feel that bass


    thanks again to this forum
    and it`s crazy but well balanced members

    some people get a rush jumping out planes
    personally i cant im imagine the rush of doing that from scratch


    lets hope the cash gods smile on me soon
    http://www.medpot.net/forums/

    daily volcano demo`s
    find out the truth
    tell`em morbo sent you

    mention lansing heritage for 10% off

  11. #11
    Member jack_bouska's Avatar
    Join Date
    Aug 2004
    Location
    London U.K.
    Posts
    83

    So does it sound as nice as it looks?? (part I)

    Quote Originally Posted by Robh3606
    Nice job Jack!!

    So does it sound as nice as it looks??

    Rob
    Rob: (ok you asked for it, part I)

    My simplest answer could be: "they sound much better than they look!", but that statement does not tell you very much.

    In a previous post, I elaborated a little more by saying: "The sound is very natural, and well recorded live instruments sound convincingly very real."

    But to be honest, these are only subjective comments, which need to be taken with a grain of salt, because my own enthusiasm could easily be explainable as a consequence of Beranek's law:

    "It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion." - L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.

    However in my own defence, at least I have been overly generous with providing a comprehensive set of frequency response graphs & measurements to help verify my claims. No doubt you have reviewed all the images above, but you still need to ask if the system sounds as good as it looks, because frequency graphs don't tell the whole story. On the other hand, I have never heard a good system that I measured to have a bad frequency response, so flat, wide, bandwidth, with good phase and rapid energy decay is the mandatory entry level for any system which claims to be world class.

    It's probably best if I describe the system design, and explain the engineering behind why various aspects sound as they do, while I proceed through the list of sonic qualities. As it turns out, I tend to rebuild my speaker system about once every decade. Two years ago, after obtaining four used JBL 1401nd drivers, I spent just over a year (in my spare time) designing this system, and another year (again, in my spare time) constructing it. The system is mostly complete now, so I will likely spend another year writing about it. (here on this forum, then on my own web page, and eventually a full design/construction article for AudioXpress).

    The original system goal was to achieve a wide, flat, frequency response, with low distortion, and wide dynamic range, including high SPL capability, with low acoustical, mechanical, and electrical noise floor. These design criteria match or exceed those required in a good studio monitor (adapted from my website):

    1) Wide bandwidth (starting below 10hz extending above 20khz)
    2) Realistically wide dynamic range (at least 130db SPL peaks, undistorted)
    3) Flat bandwidth with smooth anechoic on axis response (max rate of change < 2db between 1/2 octave bands)
    4) Flat bandwidth with even distribution of full field power response at listening location (Smooth white / pink noise spectrum, power response uniform in horizontal plane below 800Hz, narrowing to horizontal&vertical +/- 40 deg above 1kHz, all +/- 3dB)
    5) Low distortion (Harmonic and IM < 0.1% midband)
    6) Less than 1 db power compression, all amplitudes up to 125db SPL
    7) Stable stereo imaging (via controlled acoustic dispersion and absence of early reflections)
    8) Natural sounding speech and music (freedom from any identifiable timber or tonality.)
    9) A system which is compact enough to fit in an average living room.

    The system has met most of these design goals, with 11 octave bandwidth, +/- 3dB over more than 9 of those octaves, utilizing drivers known to have low distortion, and a peak output SPL of more than 130dB per channel (courtesy the 5kW amplifier rack driving them).

    Before describing the sound as a whole, I will expand a little by discussing each of the constituent components in turn and describe some of the technology and design choices underlying the transducers and cabinets in each frequency band.

    Starting at the bottom end, I can say sincerely that this system can produce a level of deep bass extension and fidelity that 99.9% audiophiles simply have never experienced in any home setting.

    With proper equalization, the -3dB point of the 18" Altec transmission line starts at 7 Hz, and ignoring room modes, is flat up to over 1khz. In practice, the Altec's are crossed over at 60hz low pass with a, 6dB/octave slope. This is well beyond scope of normal commercial HiFi, and in all my visits to high-end shops, and Hi-Fi shows (in London), I have never heard anything to compare. (Some systems go deep, but not loud enough, some visa versa, and most are hopelessly distorted at anything approaching 110dB). The deep bass from my transmission line devices is prodigious (each Altec woofer can produce continuous spl levels of 113dB over the 10hz to 60hz band), with very low distortion. This combination works to lay the very foundation for rock and pop music.

    Suffice to say that this level of power can easily cope with anyone's desire for chest thumping, furniture shaking and window rattling from any collection of Rock, Pop or techno dance disks. But there is much more to it than that. The system can produce uncanny ambience effects which impart a true sense of the venue-hall size and volume.

    The quality of the deep bass (sub 30Hz) makes it possible to deduce if instruments, such as bass and drums, were recorded in a small basement studio, or in a large 60 foot x 40 foot commercial recording hall. The ambience and reverb from other instruments (organ, tuba, double bass, etc) also can impart a sense of the space that the recording took place in. For most systems (that are limited to upwards of 30-40Hz) the spatial cues come from wall reflections higher in the range, but when the same music is played on a system, such as this, that has true deep bass fidelity, the low frequency information adds an extra dimension to the experience.

    This extra bandwidth can be a double edged sword, as these speakers are readily adept at revealing a host of mastering and recording problems (thumps, HVAC rumbles, traffic noise, and wind pops from close mic'd strings) which made it onto the final CD, without the recording engineer ever being aware of the problem. Of course with the Behringer DCX2496 I can add a low cut filter with the push of a button, however these anomalies are so rare on the music I listen to, I just leave the bandwidth as wide as possible. Besides, I get a kick out of being able to reproduce even more low bass on modern electronically generated dance music than the dj/recording engineers who composed the pieces could hear themselves when they laid down the tracks.

    Everyone who auditions my system reports being duly impressed by the quality, fidelity, and sheer impact of the transmission line bass cabinets. One commercial manufacturer (in the U.K.) has actually started prototyping their own version (using 15" drivers) following a listening session at my home. A fellow club member of the LondonLiveDIY HiFi Circle made the following comments after a home meeting:

    "No trip to Jacks can pass without mention of his system's ability to reproduce low frequencies. This was my first experience of his system and I was of course impressed with the lf capabilities. Where there are low frequencies the artist/musician intended this is a real treat. However, many tracks seem to have a good number of thumps and bumps that were clearly not intended. Jack says this is all detail that made it through mastering without being noted because the studio monitors were not going low enough.

    Whatever the explanation, I found it a distraction on some tracks. However, as one who enjoys the sensation of low bass it is certainly good to experience a system that can provide it and I am very glad I went on Saturday.

    Trouble is it leaves one thinking about building speakers!"

    End of part I
    Jack Bouska


    Jack Bouska



  12. #12
    Member jack_bouska's Avatar
    Join Date
    Aug 2004
    Location
    London U.K.
    Posts
    83

    So does it sound as nice as it looks??, part II

    Quote Originally Posted by Robh3606
    Nice job Jack!!

    So does it sound as nice as it looks??

    Rob
    ( ok, you asked for it, Part II)
    Moving up the frequency scale to the next band, are the four JBL 1401nd 14" transducers each in its own separate 22 litre cylindrical cabinet (see my avatar), crossed over at 60Hz and 243hz, 6dB/oct. It turns out that about 40-50% of the total radiated acoustic power is contained within this two octave band, so I can't possibly over-emphasize the importance that these drivers play in the total system performance. (These drivers do most of the proverbial "heavy lifting")

    The JBL 1401nd driver was first used in JBL K2 high-end system and later in the formidable JBL DMS-1 large format studio monitors. The exceptional qualities of the sound of these two speaker systems has been described numerous times elsewhere on these forums, so I probably don't need to elaborate too much on the superb qualities of this particular driver. It is one of Greg Timbers favourite woofers, and he has several in his own home system. (If they are good enough for Greg T ……)

    Given the exemplary pedigree of this driver, the remaining questions are related to specific implementation in a given system. Both the K2-S9500 (and M9500), as well as the DMS-1 systems employed the 1401's in large ported enclosures. These ported enclosures helped stretch the frequency response to about 30Hz on the low end, however in my experience the sound of bass from a port or vent is never quite as good as that obtainable directly from the cone, when the driver is in a sealed enclosure.

    Part of this is because the area of the port is generally smaller than that of the cone, and part of the problem is because of the inherent energy storage in the Helmholtz resonator arrangement of a ported enclosure.

    Sealed enclosures are also less sensitive to box tuning, so driver to driver variations are minimized. The difficulty in using the 1401's in a sealed box is that the Vas, and Qt are both relatively low, which means that the volume of the enclosure must be made quite small to prevent the system Q from being over-damped. With a volume of 22 litres, I get a -3dB point of around 80Hz, and when stuffed with long hair wool, the cabinets have a system Q of between 0.5 and 0.6 (near Bessel alignment). I use a modest amount of equalization to make the drivers flat down to about 30Hz (in room) prior to application of the 6dB crossover at 60Hz. Although this design means the drivers are slightly less efficient than when used in a ported enclosure, I find that this setup results in the best transient response possible, and the best overall tone and scale.

    The conscious decision to utilize small, sealed enclosures in a five way system is appropriate given the incredulous foundation of bottom end afforded by the use of the 18" Altec transmission lines. This freedom from the requirement to produce the lowest frequencies opens up a host of other design benefits from the use of small volume enclosures, namely:

    A) A smaller overall system footprint which fits better in residential sized listening rooms.

    B) Each individual cabinet (and total system) weight is lower than for an equivalent rectangular box (in MDF or plywood).

    C) The small front baffle size aids close spacing and better integration among various drivers in the system.

    D) The total surface area of each small cylindrical cabinet is much less than a large rectangular box, resulting in a lower ratio of enclosure-to-driver surface area, which reduces the radiation of unwanted cabinet vibration.

    E) Cylindrical cabinet construction is very strong and stiff in the axial direction, so the whole cabinet works as true reaction mass acting against cone acceleration forces, resulting in overall less stored energy in system.

    F) Each cabinet is compliantly mounted (using sorbothane), which reduces the level of mechanical coupling, and vibration transmission to the other drivers in the system, as well as assisting with reaction mass functionality (in E).

    G) Any residual vibration in the cylindrical walls will be in phase for antipodal points (180deg), but will be out of phase at the points +/- 90deg around the circumference (typical of bell modes). For lower frequencies, in the far field the interference among these out of phase waveforms will cancel, reducing the already low level of radiated vibration.

    H) The dimensions of the cabinet are adjusted to be less than 1/2 wavelength for all frequencies in the pass band, so each cabinet generates a true omni directional sound radiation pattern.

    I) The front baffle is adjusted to be only slightly larger than the diameter of the driver, so that the response anomalies due to diffraction from the abrupt cabinet edge are moved up in frequency, and out of the pass band of the device. When the crossover is applied, the individual drivers are both omni directional, and diffraction free.

    J) The small internal volume behind each driver does not have any dimensions larger than 1/2 wavelength for all frequencies in the pass band, so the volume will be entirely free of internal resonant modes (which only form at higher frequencies), eliminating any hint of overtones due to internal cavity resonance impacting the overall temporally delayed frequency response of the driver and cabinet. Transient response and energy decay are kept as short as possible.

    K) The cabinets use a novel double wall construction, with a sealed air gap between inner and outer walls, so that shear wave vibration modes are not transmitted from interior to outer wall. This cavity wall construction greatly reduces transmission of vibration from inner to outer cabinet, and into the room.

    L) The system design employs two drivers per channel, in a modified MTM configuration, for better time alignment at listening position, and better stereo imaging in the horizontal plane. The dual position also helps to excite a broader range of room modes, for better balance of mid-bass .

    Implementation of any one, or two, advantages from the above list would be sufficient to claim an advance over the traditional ported enclosure design, however utilizing all dozen of the listed benefits (A-L) provides a substantial integration of incremental improvements, which collectively result in a true step change in acoustic performance compared to ordinary enclosures. Perhaps the best that can be achieved with these drive units.

    As I mentioned before, the 1401 drivers, in almost any enclosure, have enviable pedigree, and as with most JBL woofers, they are truly adept at recreating the chest compression impact that I experience at live concerts, as well as the bite to cello strings, the pluck to bass guitar, and a real sensation that the drum kit is in the room. On well recorded rock and jazz CD's, these drivers re-create the impression that the band is in my house.

    But the illusion does not stop there. The implementation of all dozen benefits (A-L) listed above are not in common use in the industry, so it is unlikely that the acoustic phenomena that I am about to describe will have been experienced by many readers of this forum.

    The collection of design elements in A-L combine to create small, inert, omnidirectional loudspeakers, which effectively suppress; internal resonances, cabinet wall resonances, diffraction response anomalies, and temporal smear due to stored energy. (And yes, you don't need to ask, I have verified this claim by conducting a series of acoustic and accelerometer based measurements at various times during the construction phase of the project) Technically, this contributes to creating excellent impulse/transient response in the time domain, and acoustically, acts to eliminate most of the normal set of cues which we unconsciously use to localize sound sources. The overtones, resonances, and delayed energy which impact tone, and also usually betrays the position of loudspeakers in our room are mercifully absent from this design.

    The best verbal description would be to say that the speakers "disappear" during reproduction of music (poorly mastered R-L *pan-pot* recordings excluded). With eyes closed, or room lights off, it is nearly impossible to point directly at the loudspeakers which are creating the illusion of a broad and accurate 3D soundstage across the front 1/3 of the room, not just beyond the speakers, but beyond the walls as well.

    This is an effect which needs to be experienced to be understood, and so that limits what I can effectively write about it, except to say that the quality of imaging from this system is at a level that I have heard only very rarely, if at all, in other systems, and never before from any traditional "square corner, flat panel" box loudspeaker.

    End of part II
    Jack Bouska

  13. #13
    Member jack_bouska's Avatar
    Join Date
    Aug 2004
    Location
    London U.K.
    Posts
    83

    So does it sound as nice as it looks?? Part III

    Quote Originally Posted by Robh3606
    Nice job Jack!!

    So does it sound as nice as it looks??

    Rob

    The next band up the frequency scale uses the venerable 10" JBL 2123 midrange driver, also housed in a cylindrical cabinet of 4.2 litres internal volume, crossed over at 243Hz 6dB/oct and 1kHz 12dB/oct LR).

    My choice of this 10" driver was inspired by the Drew Daniels articles:
    THE ANCIENT AUDIOPHILE'S QUEST FOR THE ULTIMATE HOME SYSTEM (D. Daniels)
    http://www.audioheritage.org/html/perspectives/drews-clues/audiophile.htm

    and confirmed by continued use in his second system design:
    CONSTRUCTION AND SETUP DETAIL FOR DREW DANIELS' HIGH-EFFICIENCY LOUDSPEAKER SYSTEM
    http://www.audioheritage.org/html/perspectives/drews-clues/system.htm

    Drew chose the 2123 for both these systems, and he aptly describes the sound of those drivers in the previously referenced articles, which I quote just below:

    "As it turns out a mid cone supplying 300 Hz to 1200 Hz gives the proper effortlessness with very little power, and thus has extremely small cone excursions and low distortion.... The power absorbed by the mid cone driver amounts to milliwatts most of the time, which helps to hold harmonic distortion to very low levels, typically well below 1% THD up to dangerously loud volume. I experimented with a dozen midrange drivers before I was confident that the 2123H with its high efficiency and limited excursion linearity would produce sufficiently low distortion. It is a wonderfully transparent driver and a large part of the reason this speaker system sounds like listening to live music rather than loudspeakers." D. Daniels 1990 (If they are good enough for Drew D……)

    I obviously don't need to "preach to the choir" of JBL fans on this forum about how good the 2123 driver sounds, but I will emphasize again that I believe I have "raised the bar" on the level of performance achievable from these units (or any other cone loudspeaker), by careful design of the enclosure, using small cylindrical cabinets employing all 12 of the design innovations listed in A-L above. The cabinet uses identical construction as the 1401 cabinet described above, and benefits from the same technical advancements in minimizing; diffraction effects, cavity, and panel resonance, and time smearing.

    The only difference is in the conscious choice to use a 10" diameter driver over a frequency range extending high enough so that the diameter of the cone becomes a significant fraction of the wavelength of sound at it's upper crossover point. (as explained in D.Daniels tower system writeup, above). This results in a gradual change from omni directional radiation (below 600Hz) to approximately +/- 45deg at 1.5khz. This directivity narrowing, combined with the MTM configuration is intended to provide a smooth transition from the omni directional response below 600Hz, to the constant directivity response afforded by the waveguides crossed over at 1kHz.

    All of the previously described, "invisible-inaudible enclosure" attributes, derived from the innovations in A-L above, also apply to the smaller enclosures as well. In fact this is mandatory, in order to match the unparalleled 3D imaging accuracy of the devices in the bands above and below this one.

    In summary, the small cylindrical enclosures achieve all the design aims I intended. I suggest that the days of assembling a collection of high quality drivers, then mounting them all on the front panel of a rectangular wooden box, with passive crossovers, are over. Simply put: such an outdated method cannot approach the state of the art performance available today.

    The frequencies above 1khz are covered by the waveguide & compression driver combinations which have been described (at length) in the previous posts of this thread, so I will restrict my comments to my subjective impression of their acoustic performance, as the technical aspects are well described previously.

    In uncharacteristic brevity, let me just say: the waveguides sound better than I expected, and perform exactly to my ultimate stretch target, namely accurate and realistic 3D imaging with neutral tonality.

    So each of the constituent parts fulfils it's duty as a stunning individual performer, yet this pales in comparison to the overall performance of the system as a whole. At the heart of this integration are a pair of Behringer DCX2496 digital crossovers, which provide the needed flexibility in crossover frequency and slope, as well as digital accuracy in time alignment and probably most important, comprehensive equalization capability for each individual band.

    The overall choice of a five way design adds a few extra degrees of complication, but in return for battling the frustration of working through ten sets of wires, connections, amps and EQ settings, the reward comes in the form of an expansion of design choices not available with 2-way or 3-way systems.

    The off-loading of the lowest octaves to the Altec allows for employment of small cabinets, and the ability to eliminate in-band diffraction effects is a direct consequence of restricting the diminutive cabinets to operate only within the two octave band in which they are both efficient, and omni directional.

    Also, as each of the cone drivers is capable of operating over a much wider (5+ octave) bandwidth, this implies the frequency response is well behaved beyond outside the selected crossover points, allowing the use of audibly superior, phase perfect, single pole crossover slopes, for each of the cone transducers. This fully compliments the chosen Bessel system alignment, yielding the best possible phase and transient response attainable. Most readers will agree that 1st order xovers are very difficult to implement in a 3-way system, and probably impossible (at anything above whisper volumes) in a two way system.

    I would remind everyone that this system has exceptional dynamic range capability, which also means that it can play VERY loud. (I use an amplifier rack capable of delivering around 5kW total into this load.) No doubt most people have heard commercial HiFi played loudly, at best it is uneventful, and uninspiring, at worst, just horribly distorted, compressed and clipped.

    Again, attempting to avoid "preaching to the JBL choir" on this forum, I assume most members who use pro-sound drivers are familiar with the particularly desirable "non-HiFi" dynamic characteristic of low distortion at high SPL. When I turn the volume up on my system, the normal commercial-hifi cues of increasing distortion with increasing volume are absent, so the listener is immediately imparted with a sense of anticipation: how loud can this thing go? The effect can be downright frightening to owners of electrostatic systems (which have limited dynamic range, by nature), leading one fellow audiophile, after hearing my system, to describe me with the nick-name: "impact Jack". I resemble that remark enough to have adopted the same phrase as the name for my new system, which I call: The Impact-Stack.

    End of part III

    Jack Bouska




  14. #14
    Member jack_bouska's Avatar
    Join Date
    Aug 2004
    Location
    London U.K.
    Posts
    83

    So does it sound as nice as it looks?? Part IV - end

    Quote Originally Posted by Robh3606
    Nice job Jack!!

    So does it sound as nice as it looks??

    Rob
    If you have jumped to the last post in this thread, you may wish to scroll up and read the previous three posts (parts I, II and III) to get the answer in logical sequence.
    (hey, only four posts required to describe a five way system, would have been less, but who knew that this forum limits each post to be 10,000 words or less!)

    So, in searching for an apt description of the overall performance of this Impact-Stack, I decided to leave you with a single anecdotal description derived from a pair of listeners reactions during audition of the following track:

    Chesky Jazz & Tests Volume 2, Catalog No: JD068 Track 47.General Image and Resolution

    In this track, four musicians are recorded in a large, reverberant church venue, with voice and simple percussive instruments, parading up the isle, towards the crossed-array Blumlien stereo microphones, then three times around the mic-pair, and exit stage right. I personally find the presentation of this recording on my system uncannily realistic, and on two separate occasions, I recently played the same track to my 13 year old son, and later to a fellow audiophile from our London club.

    The effect that all three of us experienced, is that of the musicians advancing toward the plane of the loudspeakers, then moving to the right of the room, beyond, and then out in front of the right loudspeaker, and, uncannily, *behind* the listeners head, then beyond the left speaker and back to the front (three times). The effect of the musicians moving in three dimensional space, in an oval pattern around in front, side, and behind the head is one that is rarely experienced with two channel stereo, and relies on wide band, flat frequency response, with adequate suppression of early reflections (which otherwise destroy imaging and interfere with low level resolution retrieval).

    I should mention that both listeners were asked to read the Chesky liner notes prior to auditioning the track, so there is an obvious element of pre-conditioning, however on both occasions (1 week apart), I intentionally sat well behind the listeners, so they could not get any prompts or cues from me during the playback.

    I was surprised, and gratified on both occasions to hear each listener spontaneously emit an excited chuckle at the precise point when the sound of the lead musician seemed to step out of the plane of the loudspeakers, and walk from right to left just *behind* the listeners head!

    This amazing demonstration of the Blumlien 3D technique is one which clearly illustrates what level of realism is achievable with good stereo microphone techniques, as well as showcasing the (verifiable) quality of reproduction, and three dimensional imaging I have managed to achieve with the present system design. The underlying reason that both listeners emitted audible surprise at the same point in the track owes much to the nature of realism in the recording, and the listeners palatable, physically based, expectation and acoustic experience of four people, with percussive instruments, walking around behind them in the same room. All of this is accomplished without resorting to Q-sound DSP-tricks, and so naturally it is very surprising (and pleasing) to hear such realism coming from a two channel playback system.

    Have I done my level best with the current design and implementation, is it now time to hang up my hat? Not by a long shot, I need to complete the new amplifiers which I am building for the compression drivers, and I have a backlog of ideas to try in terms of modified EQ for each driver, as well as switching the crossover slopes to be fully 1st order, top to bottom, as well as general level adjustment, and system tuning (and my to-do, wish list keeps growing).

    So in closing, all hyperbole aside, although the above graphs can convey a great deal of objective information related to the performance of the system, and I can easily be accused of excessive verbosity in my subjective comments, the original question remains :

    "how does it sound?",

    the ultimate answer is: Great! but not nearly as good as it is going to sound after I tinker with it a little bit more!

    End of part III
    - end -

    Jack Bouska


  15. #15
    Member jack_bouska's Avatar
    Join Date
    Aug 2004
    Location
    London U.K.
    Posts
    83
    Quote Originally Posted by yggdrasil
    Fantastic. You must certainly have spent a lot of time/fun planning and finishing off with very skilled craftsmanship.

    Do you have original DCX2496? There was an article in audioXpress this summer about replacing the analog section completely.
    Johnny - I do subscribe to AudioXpress, and did have a good look at the DCX mod article. I should mention that I'm not all that keen on modifying (Poogeing) commercial equipment - I much prefer to build from scratch where practical, or from kit when convenient - over the last 35 years of my active audio hobby, I have followed many a "cold trail" of replacing this capacitor, or that op-amp or power supply, only to find the resultant change would generally fail to meet my expectations for uplift, or the originators claims of sonic improvement. My experience is that overall system design and implementation (in electronics) is usually the dominant factor compared to individual component influence.

    I don't remember the details of the article you mention, but I do remember being underwhelmed by the authors suggestion, deciding that in my case the result would be a significant degradation of the unit's capability (loss of single ended output, and much lower gain).

    As an aside, considering scratch built crossover systems, I am slowly being convinced by Ed Wildgoose

    <
    http://www.duffroomcorrection.com/wiki/Main_Page >

    that it would be beneficial to replace the DCX's with a dedicated rack mount PC (with dvd transport, high quality sound cards, etc), which would also open the possibility of designing my own -ultra high spec - time domain crossover filters, including full room correction facility *intra-band*!

    Wish us luck
    Jack Bouska

+ Reply to Thread

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts