Page 3 of 3 FirstFirst 123
Results 31 to 40 of 40

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

  1. #31
    Junior Member
    Join Date
    Sep 2006
    Location
    Toronto, ON
    Posts
    14
    Absolutely inspirational! Everything about your research, plan and execution are a marvel. I thought that I have done a load of research on my planned projects (>1.5years and counting) but you have given me a few things to think about still.

    speechless....

  2. #32
    Junior Member
    Join Date
    Oct 2005
    Location
    Mountain View, CA 94043
    Posts
    11
    Jack,

    An amazing accomplishment.

    One thing that puzzles me is how you can get such low extension from a TL of that length.

    I calculate 1/2 wavelength corresponding to 4 m as 86 Hz, which I thought would mean LF starting to roll off at 43 Hz.

    Thanks
    Noah

  3. #33
    Member jack_bouska's Avatar
    Join Date
    Aug 2004
    Location
    Calgary, AB, Canada
    Posts
    83

    The quarter wave pipe: How low can you go ?

    Quote Originally Posted by noah katz
    Jack,

    An amazing accomplishment.

    One thing that puzzles me is how you can get such low extension from a TL of that length.

    I calculate 1/2 wavelength corresponding to 4 m as 86 Hz, which I thought would mean LF starting to roll off at 43 Hz.

    Thanks
    Pipes (or any acoustic cavity with planar ends) that are closed at both ends exhibit their lowest resonance mode at a wavelength equal to ½ of the pipe length. (The same is true for room or box mode resonances). This is because both ends of a closed pipe (or a rectangular enclosure) have a positive reflection coefficient, (they act like an acoustic mirror), so half a wavelength looks exactly like an infinitely long wave, for F=1/2 L (and harmonics)

    Pipes which are closed at one end, and open at the other end work differently. The closed end (with or without a loudspeaker) acts just like an acoustic mirror, as described above, however the reflection from the open end will see a negative reflection coefficient, caused by the abrupt drop in acoustic impedance at the open end. The reflected wave will have a 180° phase shift (polarity flip), the equivalent of an acoustic “anti-mirror”. This means that the lowest resonant frequency will now be F=1/4 Length of pipe (plus a slight end correction). Overtones will be at odd multiples of quarter wavelengths. (3/4, 1-3/4 …) An open ended pipe (transmission line) is tuned to ¼ of the pipe length, and is often described as a quarter wave (transmission) line speaker. In practice, the ¼ L 1st resonant mode, and harmonics will be shifted slightly upwards in frequency, depending on specific box construction.

    In the case of my 18” Altec transmission line the tuning is as follows:
    ¼ * (334mps / 4m) = ~ 21.5 Hz.

    The calculated quarter wave resonance may be suppressed or enhanced by the specific construction details of the transmission line cabinet, and TS parameters of the driver. In my case, I undersized the internal volume, and chose a diameter at the driver end which is very close to that of the speaker cone. Given the low QT and high Vas of the Altec 3182, this resulted in an over damped system (Q~ 0.5), with a LF roll off similar to that of an undersized closed box. In other words, the driver dominates the enclosure. The small internal volume also significantly reduces the port output at ¼ L, and higher (with stuffing). The Martin J. King TX simulation for my arrangement is shown below in the first figure in this post. The lower graph shows the cone output in red, and the port output in dashed-blue. The port contribution is minor compared to the cone output (not typical of transmission line speakers in general), resulting in a strong roll-off of the low frequencies. The device acts as a critically damped closed box of relatively small size, compared to the Vas of the driver.

    The Martin J. King simulation indicates the requirement for approximately 12 dB/ octave low frequency compensation, but in practice, in my room, I have applied slightly less than 6 dB/octave, between 20-200 Hz in my system, so the question is: what mechanism(s) provide the additional required gain? (I use 15dB boost, while the simulation suggests >25dB is required at 20Hz)

    As mentioned above, because of the small volume, my system is driver dominated, however this particular construction also generates a Helmholtz resonance at a frequency much lower than the ¼ L internal mode. The column of air behind the speaker cone acts as ported enclosure, with the air-slug mass working against the compliance of the driver. To my knowledge, only Martin J. King has reported this phenomenon in relation to transmission line loudspeakers. I first encountered the effect when I assembled my un-stuffed 4m line, and then pushed + released the Altec cone. The un-damped driver visibly oscillated with high amplitude for several seconds, at a few Hz. I was quite taken aback, never before having seen a driver do this without being connected to an amplifier!

    Subsequent relative impedance graphs confirmed the lowest resonance frequency to be 4Hz in the 4m transmission line. (I used a CROWN DC300A for accurate response down to DC).

    I also measured the transmission line relative impedance with only the internal tapered tube, equivalent to a 2m transmission line. The relative impedance plots for the un-stuffed and stuffed 2m and 4m lines are included at the bottom of this post. The 2m plots clearly show that the un-stuffed transmission line is behaving more like a ported enclosure, with the entire tapered tube acting as a very large & long port, to generate a very low tuning frequency (~8 Hz in the 2m tube). Note that the overtone distribution only vaguely adheres to simple quarter wave pipe theory due to the influence of the acoustic reactance associated with the ported enclosure (Helmholtz) response

    The relative impedance graph for the 4m transmission line shows that the frequency has moved down to 4 Hz. In both cases, long hair wool damping reduces both the fundamental resonance, and overtones resulting in a smooth acoustic output from the cone. Although well damped, I note that the 4Hz resonance is still buried within this system, and so the port output at these frequencies probably augments the lowest frequencies below 10Hz.

    Another contributor to low frequency reproduction in my system comes from “room gain”. Although many posts cite this effect as important for low frequency extension, in a typical room the effect is mainly one of near field proximity to acoustic boundaries, which limits radiation to ½ space, ¼, or 1/8th space, depending on distance from a corner. True room gain is common in small listening spaces, such as automotive interiors, where the cavity volume is so small that all bass waves are much larger than any of the internal dimensions. Domestic listening rooms are generally large, and commercial speakers have typical bandwidths which restrict LF or subsonic output such that true room gain is relatively rare.

    Rare, yes, but not impossible, as explained in the following text, extracted from my website:

    “Overall bass efficiency is likely due to the proximity of each driver to the three way corner (masonry walls and cement ceiling) such that over their entire bandwidth (below 110Hz) the Altec drivers are radiating into 1/8 space, i.e., the three rigid acoustic boundaries are within 1/8 of a wavelength for frequencies below 140Hz, which makes the apparent efficiency of the drivers about 9dB higher than if they were radiating into free space. (aprox 3dB for each boundary, ignoring absorbs ion and transmission losses.)

    Furthermore, the dimensions of the room are such that for all frequencies lower than 35Hz, the shortest dimension, the 8ft ceiling height is less than 1/4 of a wavelength, and for all frequencies below 20Hz, the medium dimension of 14ft. will be less than 1/4 of a wavelength, and finally, for all frequencies lower than 15hz, even the longest dimension of 18.75 feet will be less than 1/4 of a wavelength. As the frequency drops below 35hz, each of the room boundaries will move through from the range of beyond 1/4 wavelength, down to less than 1/8 of a wavelength (at the frequencies of 17.5Hz, 10Hz, and 7Hz respectively). In other words, for frequencies below 140hz, the ceiling is within 1/8th of a wavelength, and augments the sound pressure level by almost 3dB, when the frequencies go below 35Hz, the floor is also close enough to start contributing to the SPL of the sound, and this increases as the frequency is lowered to 17.5Hz, where the floor is 1/8th of a wavelength from the driver, and the phase of the waveform is nearly identical, regardless of height, and the SPL will get almost a 3dB boost below 17Hz. Likewise, for frequencies below 20Hz, the side walls start adding in phase, reaching maximum contribution below 10hz (another 3dB), and the longest dimension contributes below 15Hz, ramping to 7.5Hz at which point the room will be in complete isophase condition.

    Of course wave fronts still propagate throughout the room at the speed of sound for these low frequencies, however the rate of change of pressure amplitude will be so slow, and the wavelengths so large, that each point in the room will appear to have identical pressure amplitude, and phase for these low frequency waveforms. At frequencies below 7Hz, essentially all six of the room boundaries will be closer than 1/8th of a wavelength, and this will yield an extra 9dB of gain, for a grand total of 18dB extra SPL when the corner placement is included. The natural progression across the 1/4 to 1/8 wavelength (zero dB to +3dB increase) combined with the staggered dimensions deliver a gradual ramping up of room gain, which mimics the missing boost below 20hz. The trick is particularly effective in the current room, because of the full masonry construction of the four walls, and the uncommon use of prefabricated concrete floor and ceiling, such that the low frequency reflection coefficient is very near unity. “


    Jack Bouska
    Attached Images Attached Images    

  4. #34
    Senior Member Flodstroem's Avatar
    Join Date
    May 2004
    Location
    Norway
    Posts
    401
    Hello Jack
    Im amazed and impressed. What a great work. I must confess that I was hit by a lightning and couldnt move from my seat before reading all your posts till the end (all night long) I have read all your posts with great interest and everything you write ("say") seems to be so obvious to me that I think I must try some of your ideas and suggestions.

    Though I have a lot of and also some similar drivers as you have been using in your project Im going to try to build a similar audio stage. But maybe not as complete as yours because of the costs involved and I also lack most of your skills regarding technical education etc.. (Im a biologist)

    But my first step will be to buy some audio measuring equipment (and I have already done some buying) so I could do my own empirical verifications on how different parts really do behave acoustically in my home and also as an instrument to verify my acoustically calculations.

    Next step will be to buy a Behringer DCX2496 digital crossover for to be able to set different types of filter slopes etc.

    When reading all your posts I couldnt find any information regarding how you built those round "hat-boxes" for your speakers (for the 1401ND, and the 2123H). I am aware of that, they was made out of two boxes, one in another, with air in between them for to not get acoustic resonances from standing waves from the inside to the outside. But how did you built them?

    My own drivers from the beginning will be two 2245H, two 2012H, two 2440 (with 2445 Ti diaphragms) and two, either 2421 or 2405s´ I dont have any 1401ND but could use two 2215H as a start.

    Have planned to build a large ML-TQWT pair but this (yours) approach seems to better fit the very restricted WAF-factor in our house.

    In DiyAudio Forum Im involved (and has been) in the construction of a Krell KSA100mk-II Clone and I plane to build 4 mono blocks for the purpose: 120W/8 ohms, ca 220W/4 ohms (120W pure class A each). Including my two NAD, 6 x 30W (3 x 90W bridged) I will end up with a power source of ca 1.4 kW to start with. This is not enough so I plan to sell those NADs´for to be able to build four or six GB300 (Gregg Ball) MOSFETs´power amps later on. Total power source will then be ca 2.6 the twice of what I will have from the start.

    And yes, luckily I have a lathe for the turning the two horn shapes.

    Regards
    Flodstroem

  5. #35
    Member jack_bouska's Avatar
    Join Date
    Aug 2004
    Location
    Calgary, AB, Canada
    Posts
    83

    Construction of the "Impact Stack" post 1 of 2

    Quote Originally Posted by Flodstroem View Post
    I have read all your posts with great interest and everything you write ("say") seems to be so obvious to me that I think I must try some of your ideas and suggestions.
    Thanks for the feedback, I hope my comments on system philosophy and horn design will be of some use to you in your speaker building projects.

    Also, apologies to the Lansing heritage membership for my recent inactivity. For the last three months, it seems my spare time is measured in negative numbers! I seem to be inordinately busy at work, with my evening and weekend free time consumed by preparation for some unexpected external commitments to the European Association of Geoscientists and Engineers (see:
    http://www.eage.org/index.php?Menu_Code=DLPCourseDetails&EVS_Id=197&Ac tiveMenu=85&Opendivs=s19,s36
    and
    http://www.eage.org/index.php?Menu_Code=SCPCourseDetails&EVS_Id=191&Ac tiveMenu=85&Opendivs=s19,s41

    Quote Originally Posted by Flodstroem View Post
    When reading all your posts I couldn't find any information regarding how you built those round "hat-boxes" for your speakers (for the 1401ND, and the 2123H). I am aware of that, they was made out of two boxes, one in another, with air in between them for to not get acoustic resonances from standing waves from the inside to the outside. But how did you built them?
    A picture is worth a thousand words, so rather than write another series of long posts (which I still cant afford the time for), I am including a series of photo's taken during the construction of the speakers. (in this, and a subsequent post.)

    The pictures are numbered and captioned, and should be mostly self explanatory. If you have specific questions, or want more info related to some aspect of the process, you can post your question with the photo number as reference, and I can comment appropriately.
    Jack
    Attached Images Attached Images      

  6. #36
    Member jack_bouska's Avatar
    Join Date
    Aug 2004
    Location
    Calgary, AB, Canada
    Posts
    83

    Construction of the "Impact Stack" post 2 of 2

    Post number two (of two)
    The pictures are numbered and captioned, and should be mostly self explanatory. If you have specific questions, or want more info related to some aspect of the process, you can post your question with the photo number as reference, and I can comment appropriately.
    Attached Images Attached Images      

  7. #37
    Senior Member Flodstroem's Avatar
    Join Date
    May 2004
    Location
    Norway
    Posts
    401
    Jack, thanks for the excellent pictures of your hard-work. Amazingly. Yes it was worth more than "thousand words" and it looks as it could be possible for me to built a system like yours.
    Maybe not exactly due to I lack some of the parts and also, it must fit our listening room too (and the WAF factor not mentioned).

    My first question (of the 101 I have -no just kidding):

    What the h.....l is "RAPIDOBAT", newer heard of it before. Could you explain it in details

    Regarding the construction of the speaker boxes this is absolutely clear for me by now, very smart solutions.

    For the moment Im doing a research for to how to find the parts and materials for to build the boxes. I think this is the basics start for me. Then when I have found out I could start to order materials and some hardwares too that is not to find in my garage and working shop.

    Regarding your design, you are using two 14", two 10" one 2" (2441) and one 1" (Tad 2002). Was your intention to build a MTM- concept or was there any other reason for to use two 14 and two 10" per side? loobings etc..?

    My system at the very first stage would be to use one 15" (2215H), one 10" (2012H) one 2" (2445 Ti) and one 1" (2421) /or 07(2405) per side. Also, two 2245H for the two subs.

    I dont know if its possible to build a system out of what I have by now but Im planing to buy some more speakers, alternative would be to sell some and buy other models that is a better match than those I have planned to use.

    Thanks.
    Regards
    Flodstroem

  8. #38
    aust-ted
    Guest

    Inspirational stuff

    Jack, You have done a grand job on this project and your write up is inspirational. Having been out of action for a while i just read it and will attempt some JBL 2441 waveguides.

    I am thinking of a simpler system than yours with dual JBL 2235s in large reflex boxes (already in use), a single JBL 2123 (I only have a pair), a single JBL 2441 and a JBL 2405. I also have a DEQX acquired after advice from Mr Widget (Thanks Mr Widget, have never regretted taking up that excellent advice).

    I have recently acquired a wood lathe and expect to be in a position to construct horns using templates much as you have described on this thread after some further tuition at the end of this month.

    I am proposing using the DEQX to xover the 2123, 2441 and 2405 with restricted box design to assist the low end cut-off of the 2123. Have not worked out the xover for the 2235s but will probably try both passive and active. Will use a 4 amp configuration. The DEQX provides a lot of flexibility in the xovers and equalisation for the 3 top speakers but my limited budget does not permit me to purchase a second one for the bottom end. Any comments on this would be welcome.

    Jack I have a question which arises from your work. I note you are xovering the 2441s at 1KHz using a modified oblate spheroid with a tractrix mouth to save space. You have also given advice on the benefits of xovering it higher at 1.2KHz with a 2202H to "relieve low frequency related stress on the 2445". Sounds like good advice that I had not previously thought about.

    I had been thinking about trying to extend the 2441 down to 500Hz as they work in some JBL horns or even trying the JBL phenolics down lower but I suppose with the 2123s there is no need to stress the 2441s.

    Is there any software available to generate your own oblate spheroid waveguides with different cutoffs like there is for tractrix? As I propose to only use a single 2441 I will not have the need to use the modified profile you used. I am thinking of making a number of different ones partly to hone my turning skills, to achieve a higher WAF but also to experiment to see which one gives the best from a sound perspective.

    Regards
    Ted
    Last edited by aust-ted; 01-05-2008 at 03:09 AM. Reason: correct typos

  9. #39
    JBL 4645
    Guest
    Quote Originally Posted by Flodstroem View Post
    Hello Jack
    Im amazed and impressed. What a great work. I must confess that I was hit by a lightning and couldnt move from my seat before reading all your posts till the end (all night long) I have read all your posts with great interest and everything you write ("say") seems to be so obvious to me that I think I must try some of your ideas and suggestions.

    I’m kinder feeling immobilized I’m in perpetual daze. I can grasp some of the posts the rest is like light-years over my head.

    I like the Altec 3182, wow looked it up on pdf file nice. I looked around eaby, nope not one single Altec 3182 sub around.

    Do you still run the same system as this thread is few years old now and I’d be greatly disappointed if you not running it, after spending great deal of attention to detail, that is a labour of love for ones Hi-Fi.

    I can imagine the tightness in the low end supporting the mid and high range with great strength of authority.

    Well done Jack

  10. #40
    Senior Member Flodstroem's Avatar
    Join Date
    May 2004
    Location
    Norway
    Posts
    401
    Quote Originally Posted by aust-ted View Post
    Jack, You have done a grand job on this project and your write up is inspirational. Regards Ted
    Quote Originally Posted by JBL 4645 View Post
    I’m kinder feeling immobilized I’m in perpetual daze. I can grasp some of the posts the rest is like light-years over my head.
    Well, I felt the same way when first reading those hieroglyphs by the excellent writer Jack

    But later I have come to understood more and more about his concept
    Due to costs etc there have been a long journey to come to the position that Im in at right now: I could build a similar concept only with some "small" substitutions of the drivers
    The components I intend to use and what I have in hand are those:

    2 x 2245H
    4 x LE14H-1
    4 2123H (and/or 2012H)
    2 x 2440
    2 x 2421
    some Rapidobat tubes (hehehe)
    also 2 x DCX2496

    Amps: some "small" building project:
    1 Gregg GB300D (that you Ted might know anything about)
    6 x Krell KSA100 mk-II clones.

    including a great lathe for the Tractricx horns
    Flodstroem

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
  •