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Thread: Question re near-field low-frequency measurement

  1. #16
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    Hi Ian,

    thank you for your reply.

    I am cutting out some of your responses that I have a question on or I do not understand at all, so the back-and-forth does not turn into a mess. The forum software is not the best on such a manner of conversation.

    Re. 1

    IM: "Yes. You can however manipulate the QES + some series resistance Rs like adding a series 0.2 -.033 ohm resistor and a parallel shunt resister like 80 ohms down to 39 ohms on the woofer."

    I am actually aware of the series resistor technique, I used it in shaping the slope of the response. But I am not sure what the parallel shunt resistor is supposed to do. I will ascertain whether the software I use - Unibox - will let me add a parallel resistor.

    IM: "You also need to consider how your going to cover the first two octaves. The sealed 515 system will yield a damped mid bass but it has a relatively limited Xmax. Without the support of a bass reflex port or vent the 515 will have subjective limitations on its dynamic output below 100 hertz."

    It is my understanding that the room characteristics can help with the lower octaves. And, yes, the limited Xmax will prevent using Linkwitz transform.

    In this regard, I have difficulty to understand the concept of the claimed "subjective limitations on its dynamic output below 100 hertz" (not only by yourself). Here is my, perhaps incorrect reasoning. Since I am fortunate to live in a quite environment, my listening level is on overage 75 - 80 dB. A well recorder (classical) music has about 15 dB dynamic range, so the peaks will be 90 - 95 db. In my experience, apart from some organ works, 1815, Also sprach Zarathustra, and the like, there is not much content under 30 - 35 Hz. if the model is realistic, there is about 3 dB reserve. What am I missing?

    When I had Bruce Edgar's Titan folded mid-horn that, in my room, did not produce much below 35 Hz, I did not seem to have missed much. Perhaps I am not a golden ear unlike other audiophiles.

    I can always build a near-field subwoofer.

    Re: 2.

    IM: "A driver volume will not have a perceived effect in a 120 L enclosure."

    I estimated the driver volume and bracing at about 10 l, so perhaps I should have attached a 110 l enclosure (no fill considered). But there is about 1 dB difference, so in a big picture, you are correct.

    IM: "In your situation l think it’s best to try and validate your woofers TL parameters within a model (l can do that for you)."

    I would like to learn, how to do that myself, you are already wasting enough time on me. In fat, we had already moved away from the original question, which is fine with me, but I hate to abuse your generosity.

    I am rather lucky, because my T/S are withing about 5% of the published data.

    Re. 3

    IM: "My recommendation is to acquire the Dayton DATS3 woofer tester or attempt TL measurements with REW and an audio interface. See the REW help guides online."

    O.K., I will re-measure with the REW.

    IM: "In the scheme of things the group or class in which your 515 belongs on the Qts continuum is more important overall. In other words be mindful of what it’s intended purpose is."

    I know, all my previous inquiries met with a response that it will not work the 515B was meant to be a horn driver. Maybe I should just forget it, sell the Altec and concentrate on the TAD. Or, reacquire the Titans.

    Kindest regards,

    M

  2. #17
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    I think if you can to allow me to follow your progress please use this online website.

    It’s proven to be concise and accurate.

    There are a number on closed box and bass reflex calculators and explanations of what it all means in the context of these TL parameters.

    http://www.mh-audio.nl/Calculators/CCC.html


    What seems to be unclear are the actual TL parameters of your drivers AND the desired box volume you need. These are the unknowns.

    There are also the Great Plains Audio versions of the 515 drivers. I would take any online opinions with grain of salt because in general they are an opinion of an opinion that have no ownership of validating your questions.

    With your own measurements if everything is within 5 % of the original driver data that l have attached don’t even worry about it.

    A 1-2 db measured close mic near field measurement is not going to prove or validate anything. A breathe of air pr a truck driving past is going to blow all that to shit.

    Forget about uni box for the moment. You’ve got too many unknowns in the mix at the moment.


    Start here. Confirm your data within 5 %.

    http://alteclansingunofficial.nlenet...ele-Small.html

    Go here and compare the classic QTC box 0.706 with other boc real volumes and your subjective evaluations.

    http://www.mh-audio.nl/Calculators/CCC.html

    If you play around with this it will all make a lot more sense.
    Don’t get bogged down in the absolute details or opinions or references to a a quote in a book. It’s not going to help you.

    Don’t worry about losses , volume of the driver ect at this point. One inch fibreglass will add 5-10% volume and generally compensate for the volume of your bracing and the driver. Half filling the box loosely with fibreglass will increase the volume more to another 20-30 % possibly. I would not do that yet because it will only confuse your results.

    Put a partition on the box if yob want to try a QTC 0.707 design. I would do that as it will help you determine if your 515 driver is a suitable candidate for your project.

    Incidentally l have a pair of Acoustic Elegance 15M Apollo drivers. They were also a potential candidate for this project.

  3. #18
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    Hi Ian,

    thank you for the link, but it caused me to essentially give up. When I entered the (published) parameters for the TAD1601, the http://www.mh-audio.nl/Calculators/CCC.html yields:

    Vb=92.36 l
    F3=58.23 Hz
    Fb=58.22 Hz
    Qtc=0.707

    Unibox with the same parameter yields:

    Vb=141 l
    F3=49.92 Hz
    Fb=49.92 Hz
    Qtc=0.707

    If the difference in the recommended volume is 50 %, then what is the point of doing any simulation.

    Just to increase the frustration, I entered the Vb=141 l to the web-based calculator with the result:

    Vb=141 l
    F3=59.57 Hz
    Fb=49.91 Hz
    Qtc=0.606

    and entering Vb=92.36 l into Unibox:

    Vb=92.36 l
    F3=57.96 Hz
    Fb=46.99 Hz
    Qtc=0.845

    There is no hope.

    Kindest regards,

    M

    BTW, the TADs were measured on a ground plane and not near-field.

    M

  4. #19
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    Ah Ha.

    All is not lost. Please one walk around the mountain.

    All these simulations are based on a set of assumptions.

    I have not used Unibox but it is possible it has many variables at play such as QL losses, Fill/No fill

    With the Tad published data for the 1601A l got the following

    Fs 28 hertz
    Qes 0.36
    Qts 0.34
    Vas 307 L

    Sealed box simulation online simulation

    QTC = 0.707

    Vb = 92.36 L
    FB = 58.23 hertz
    F3 = 58.22 hertz

    Sealed box online simulation

    I got the same results as you did for Vb = 141 L

    Referring to the online instructions this simulation does not take into consideration volume of bracing or the driver or Fill.

    For what it’s worth l got the same result for the Qtc on two other online calculators.

    I then put in a full set of TL parameters and checked them in a model.
    The result was dependent on the entered box losses lts interesting that QL loss in a sealed enclosure has the opposite effect from a bass reflex system.

    The below simulations were Qtc = 0.707
    where QL = 15. (Small losses)
    Vb = 82.8 L
    FB= 53 hertz
    F3= 61.8 hertz

    where QL = 7 (Average losses)
    Vb = 71.5 L
    FB= 57 hertz
    F3= 62.6 hertz

    where QL = 3 ( large losses)
    Vb = 48.6L
    FB= 67 hertz
    F3= 70 hertz

    Complex simulations using full set of TL parameters (above) below
    I would need to contact the developer of my App to ask if fill is included in this simulation.

    It’s a bit abstract but normally the effect of fibreglass fill is to add virtual volume because of this frictional nature. But the thing would fill be regard as a box loss in sealed enclosure?

    Regards you last simulation in Unibox for comparison l got the following results

    Vb 92 L
    FB = 51 hertz
    F3 = 61.8 hertz
    Qtc = 0.646 (the difference on the curve is only slight)
    QL = 7

    What this suggests is that the Unibox simulation is not accounting for QL in the same way. This maybe something you can adjust.

    I can’t attach anything here at the moment. If pm me l can email you my graphics off the curves.

  5. #20
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    Question

    My general feedback on the original Altec 515B version having looked more closely at the driver is that it’s designed for a bass horn. It’s a efficient driver, very low Mms, low Xmax and very low Qts all point to a bass horn.

    That thread beyond Ariel was interesting at the time. However l think it a pipe dream concept and a lot of the claims are very subjective. I bought my AE 15M drivers for that project and l still haven’t got around to it.

    Since then there have been real technical advances in loudspeaker motors and honestly the JBL 2216nd is light years ahead these feather weight Altec drivers. Yes they require far more power but they have lower distortion under real operational conditions. It’s like comparing a Ranger Rapter to Suzuki Jimny. The BL curve is incredibly linear and as a result it’s almost a perfect transducer. You can EQ it out to your liking with Dirac and you will be on cloud nine.

    If you want a massive horn in your room go for it.

    I have my Teflon base plate on and bring on flame throwers.

  6. #21
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    Hi Ian,

    thank you for the reply.

    Not only I walked around the mountain, but I did practice some TIG welding for the calming effect due to the required concentration.

    I have re-read your simulation-related reply, but I am not quite sure how to interpret it. I will try to p.m. or e-mail you, after all nobody is interested in my whining.

    Regarding your last reply, yes as I noted the driver was meant for, and used, for horn. As much as I love the horn sound I cannot afford the space for one, so I will sell the drivers. Just a cursory check reveals that they are rather sought after.

    Kindest regards,

    M

  7. #22
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    Hi Ian,

    in response to your reply in the thread https://www.audioheritage.org/vbulle...560#post445560, let me first reiterate, where I currently am.

    You and I had discussed my goals for the speaker and measurement and modelling of a low-frequency enclosure. Based on our discussion I had again modified the enclosure. As my room is rather crowded, I could perform only the discussed near-field measurement, but to my surprise, the measurement is close to the model (F3, F10 within few Hz).

    I paired the enclosure with a small 11 in diameter OS wave-guide, which has poor loading for the 1 in compression driver, forcing a 1000 Hz cross-over. I when listening to the combination at close distance, I noticed a change in the sound in vertical plane, and switching a 1000 Hz signal between the enclosure and the horn revealed (subjective) difference in the perceived tone. Thus, I would still like to use a larger compression driver and a suitable wave-guide to move the cross-over point to between 600 Hz - 700 Hz, as discussed.

    Hence my inquiry about the availability of 1.5 in wave-guides.

    Referring now back to the wave-guide inquiry. Since the 4367 wave-guide is no longer available (and I prefer wave-guides with more even horizontal and vertical coverage anyway), I started to look at the M2. Considering the slight differences between the different measurements, the general trend appears to be about 5 dB difference between 700 Hz and 600 Hz. If one were not concerned with the highest achievable SPL, would it not be reasonable to equalize the wave-guide so that the plateau from about 700 Hz to 4000 Hz would be lowered by 5 dB? that would then enable to achieve my goal.

    I understand that there may be other issues, e.g., pattern matching as you mentioned, but I think that I know how to deal with those.

    Kindest regards,

    M

  8. #23
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    What you need to know is the sensitivity of your 515 woofers?

    I don’t recall specifically but the M2 with a 2451 has a sensitivity around 110 db at 700-4000 hertz.

    For a passive crossover:
    The hump is equalised with an LCR filter

    In order to achieve the lift in the response there needs to be a voltage attenuation immediately after the crossover filter. By then bypassing the voltage attenuation with a capacitor C and a series resistor R the high frequencies will appear to lift.

    But it is the frequencies below 4000 hertz that are really being attenuated.

    The limitation is the difference in the overall sensitivity of the M2 horn and your 515 woofer?

    Your Tad 1601 would certainly be a candidate for your low frequencies with a steep low pass filter.

  9. #24
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    Hi Ian,

    thank you for your reply.

    I am currently using a TAD 1601A in the enclosure and if my calibration can be relied on the sensitivity is about 95 dB/W/M.

    I do follow your explanation regarding the equalization, but I fail to understand the sensitivity issue. As I am bi-amplifying can the difference not be accounted for by setting different gains for the high-frequency amplifier and the low-frequency amplifier?


    Quote Originally Posted by Robh3606 View Post
    Hello

    Well if you look at the curve it's fairly linear until 700Hz where it quickly starts to roll off. When it starts to roll off the phase is changing as well. You are better off going for a 750-800 Hz.

    Rob
    Hi Robh3606,

    thank you for your reply.

    Yes, that is the general observation I made in response to Ian. However, as I understand the phase change can be compensate (i) by equalization of an amplitude, provided that at that frequency the CD/M2 is still a minimum phase or (ii) by FIR filter that allows independent phase and magnitude management.

    Kindest regards,

    M

  10. #25
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    Yes that is correct. The issue only applies to a passive crossover network.

    You can either use your FlR dsp or a passive EQ after the FIR crossover.

    Note: The Tad 1601a is not designed for operating at 1,000 hertz crossover point. It was specifically purposed for the Tad systems using a 6th order low pass filter at 650 hertz.

    In hindsight had this been more known (awareness) in the other thread then the discussion would have been more specific.

    The M2 waveguide is worth trying out. But being diy you are going to have to do some practical investigatings of your own to satisfy yourself that it meets your expectations.

  11. #26
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    Hi Ian,

    Quote Originally Posted by Ian Mackenzie View Post

    Note: The Tad 1601a is not designed for operating at 1,000 hertz crossover point. It was specifically purposed for the Tad systems using a 6th order low pass filter at 650 hertz.
    I know. There is a gentleman marco_gea on the diyaudio forum, who is very knowledgeable about all the TAD Exclusive speakers and he posted about the cross-over. Saying that I wanted to measure the enclosure and there was some doubt about near-field measurement with the ALTEC due to an opening in its dust-cap. I recon that I could replace it, but I really hate to deal with the fiberglass.

    Quote Originally Posted by Ian Mackenzie View Post

    In hindsight had this been more known (awareness) in the other thread then the discussion would have been more specific.
    Yes, but I did not want to present much details in the other thread as not to be off-topic; ironically we ended here again.

    Quote Originally Posted by Ian Mackenzie View Post
    The M2 waveguide is worth trying out. But being diy you are going to have to do some practical investigatings of your own to satisfy yourself that it meets your expectations.
    Indeed, but the admission ticket (currently $269 + tax + shipping).

    Kindest regards,

    M

  12. #27
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    Do you have a 1 " driver that can bolt on to a 2344?? If you do you may want to try that. It's going to be similar. If you already have 1.5 inch drivers you may want to try PTH1010's as they have 100X100 and are relatively inexpensive. The only issue is how far up your 15's can go. If anyone near you has a stock 4430 /4435 have a listen.

    As much as I liked the 2235's I always tended to keep them below 300 Hz and used the 10's specifically the 2122/2123 to cover 300hz to 1.5 K where the directivity of the 10 and the directivity of the 100x100 were well matched. To me that combined the best of the classic 4 ways like the 4344 and the 4430.

    As Ian says you need to see what works for you. You have DSP which is an advantage I didn't have as I went through my "upgrades". I am an analog active and passive/analog active crossover based in my experience. I never made the leap to DSP as experiences I did have with auto-EQ were awful in comparison.

    One of these days I have to give it another go!

    Rob
    "I could be arguing in my spare time"

  13. #28
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    Hi Rob3606,

    thank you for your reply.

    As Ian noted above, the TAD 1601A that is currently mounted in the enclosure had been crossed-over at about 650 Hz in the TAD Exclusive series of speakers, and the ALTEC 515B, which I understand could be crossed higher will likely not work in the enclosure.

    As I mentioned above, the small 11 in OS wave-guide does not load the 1 inch driver below 1000 Hz, and another wave-guide that I have B-52 PHRN-1014, likewise, is good only above 950 Hz.

    I live in the middle of nowhere, and none of my audio hobbyist friends have any wave-guides/horns that would work. So, it seems that I am out of options.

    Kindest regards,

    M

  14. #29
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    Lightbulb

    Yes l actually met Marco through Rich in London back in 2018. I enjoyed listening to Marco’s system.

    Regards

    Ian

  15. #30
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    Hi M,

    I think you might be able to use your 15M woofers because they are easily be used up to about 1500 hertz. You could therefore use the wave guide Robert S has recommended with a 1.5 inch compression driver.

    This really is a good driver and will work in your Tad enclosure tuned from 35-40 hertz.
    It’s a very fast, clean and punchy driver with a smooth mid in my experience.

    An f3 of 40-45 hertz is readily obtainable in 5-7 cu ft .

    Below are some amateur test results l found. Note the flat impedance curve.
    The port tuning was only 32 hertz hence the drooping ground plane measurement.

    I have a pair of these woofers and could assist you if needed.
    Attached Images Attached Images      

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