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Thread: Testing 3,2,... (keywords: WinMLS, S2000, IB)

  1. #1
    Senior Member cvjoint's Avatar
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    Testing 3,2,... (keywords: WinMLS, S2000, IB)

    Book 1: Tease

    Abstract
    Is there cabin gain in convertible cars with the soft top up? Evidence from a Honda S2000 application gives a resounding yes!

    Results

    Figure 1


    What you are looking at: 5 samples of a sweep in WinMLS. Two samples are crap, these are the ones one the bottom. I will be very thankful if anyone can chip in as to why some measurements come out like this. The drivers are 3 Acoustic Elegance IB12 in IB. I put a 200hz LP filter on it, and the mids have something like a 600hz HP so they don't interfere. The subsonic was turned off on my P99 head. The Clarion XH7110 amp has a nodefeatable subsonic at 10hz from what I remember. Otherwise there was no EQ

    Methodology
    WinMLS on Windows7, M-Audio Transit audio card, Behringer ECM 8000 mic, M-audio Audio Buddy pre-amp.

    Testing was performed with me in the seat. The mic was in a different direction everytime about 4 inches from my nose. It doesn't seem to matter much at all if I move or if the mic moves or gets pointed in a different direction.

    Thoughts
    What else would you guys want to see? If anybody has setup tips for WinMLS I'm open to experiment. I'm hoping to do more than frequency response plots if I can.
    Build pic, more available upon request.
    Last edited by cvjoint; 03-23-2011 at 04:55 AM.

  2. #2
    Tester Extraordinaire ErinH's Avatar
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    How do you know cabin gain occurs? I'm not seeing evidence of such. The graph posted is a bit small, but the largest delta you seem to have in that passband (not accounting for the rolloff on the upper end) looks to be only 5dB. I'm not seeing any "gain" or overall rise in amplitude through the passband; just a nominal line (sans 80hz, which is likely a phase/wavelength issue but I don't know for certain). Look at 100hz and then 30hz. The delta seems to be only a couple dB, if that.

    The only way to really tell if you have real cabin "gain" is to compare what you go in the car to what you get with the subwoofer in free space. That's hard to do with IB. Of course, putting the mic right up next to the sub should give you a pseudo-free-space response (I think). that might give us a better indication of what the car's affect is, but I'm certainly not seeing anything that makes me think you have true cabin gain in these pictures.

    Cabin gain is caused by a subwoofer (or woofer) alignment in an enclosed volume itself... so basically, a box within a "box" (your car). With a sealed box/ported box/etc box, your trunk isn't completely sealed off so your box alignment is culprit to the entire car's overall volume.
    The gain should start at about the frequency = to the half wavelength of your car's enclosed space (trunk+cabin) and should rise at about 12dB/octave (unfortunately, I don't have the data or research to back this up without searching for it again). IE: If your car "enclosed space" is 6' long... 1116ft_per_second/6ft = 186_per_second/2 = 93hz.
    For most car's, it's probably closer to 10ft, which should result in a gain starting at about 56hz. That actually seems pretty common, too, if you look at the JBL cabin gain plot Andy posted on DIYma.

    Now, what I'm personally wondering and plan to start researching is how an IB alignment is affected by cabin gain. Does the IB alignment "see" the smallest volume (the trunk, typically) or the largest (the cabin)? I'd guess the trunk simply because it is the most limiting. The reason I think you'd split the two up is because you've essentially (assuming you did your best to do a good install) got two independent volumes (again, trunk and cabin) due to the separating baffle.
    If that's the case, then you'd probably have a length of about 4' or so, which should give a rise starting at 140hz. Well, I've not noticed this in my car, but I've also not ran them up that high to test this. I do, however, notice a good gain in response starting at about 60hz, which then tells me that it's "seeing" the entire car's volume. I just don't know if I trust that sole piece of data so I'm not going to say that there isn't 2 distinct volumes for the IB setup.
    If I assume that the smallest volume is the limiting factor, then you would expect to have cabin gain noticably in your plots, which you don't. That's causing me to question my theory of 2 different volumes.
    Taking it a step further, is it also possible the subs' "see" the trunk as the enclosure, and the car's cabin acts as the encompassing enclosure? Therefore, the cabin would be the driving factor in the gain, and let's say the cabin length is something like 6'... you get a rise at 93hz. I don't see this in my setups, so I don't really think it's the case, but it is another way of looking at it.


    Overall, I'd wonder if any rise you get isn't an affect loading moreso than cabin gain.


    Also, what is your response like with the top down? I'd like to see a comparison.
    Last edited by ErinH; 03-17-2011 at 11:56 AM.
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    Big Daddy Chad's Avatar
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    I'd like to say, that I feel that this is one of the cooler things I've ever seen done in a S2000.

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    Senior Member cvjoint's Avatar
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    I'm Chad approved super!!

    There are 3 reasons why I think there is cabin gain:
    1. the FR extends down to 25hz. No simulation of a sealed box would have that behavior, furthermore these subs should have a rather sharp rolloff given their efficiency.
    2. The output seems to drop like a rock after about 125hz, and also seems to settle off at about 200hz.
    3. The response in my Accord looked just like that, no dispute over cabin gain there.

    Delta DB from 30hz to 100hz: If the cabin gain extends all the way to 100hz then there should be no differential anyway. I prefer to compare say 30hz and 250 hz. That will be my next test, but one caveat is that my Clarion amp is chock full of nondefeatable bandpass filters. The cabin gain graph on DIYma you referred me to has a few cars with cabin gain at 100hz, I suspect my S2000 has the same transfer function as those, as opposed to the ones where the cabin gain is nonexistent at 100hz.

    That was a really good post on how cabin gain comes to be. I suspect that in both of my cars that I IBd only the cabin transfer function matters. In that case the S2000s cabin gain should start at about 120hz which is seems to be doing imo but the Accord should have started lower to the extra cabin length, yet it looks the same. If you ever find some more info on this do link me up, I'm interested.
    Measuring cabin gain: I was thinking of taking the baffle down and mounting two subs in a sealed box. The difference between the sealed box in car and out of car should give me cabin gain. The problem here would be fitting a sealed box in the trunk of an S2000 haha.

    Test with the top down: There is one problem with this as a "control" test. When the top folds down the cabin is gone, which is what we want. However, the top does fold right over the subs. That means the output has to go through a bunch of vinyl or go around it. The test will be post both of these effects happening, it's a noisy measure. I was thinking of holding down the top botton for half the time so that the top only manages to "open" but not yet fold, and test it that way. That could also work as a massive waveguide !!

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    Quote Originally Posted by ErinH View Post
    The only way to really tell if you have real cabin "gain" is to compare what you go in the car to what you get with the subwoofer in free space. <snip>
    Also, what is your response like with the top down? I'd like to see a comparison.
    I consider the two the same query. That is to say, "cabin gain" is the difference in LF response between the top-down and top-up configurations. In a targa-topped car thay may not hold as well, but in a classic ragtop it does.

    As to the question of whether cabin gain exists in a convertible. I've not measured it with the softtop in my Miata. Never thought to. Yes, it's there, but I've never measured it. Probably I should.

  6. #6
    Tester Extraordinaire ErinH's Avatar
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    I'm mixing up my terminology. When I say top down, I actually mean "with roof".

    I'm asking for top down response measurements just to compare to top up... to see if the lining has an effect and ultimately makes the car's cabin more "closed", thus giving a more noticeable cabin gain.

    With the top down (ie: no roof, and using your terms just for clarity) I wouldn't expect that you'd have much cabin gain if any at all (leads back to my previous post) but I would expect the enclosed volume (roof up) to result in cabin gain. I would expect so.


    Again, I'll just say that, the way I'm reading the graphs, I see no large deltas in FR as you typically do due to cabin gain. However, without having a baseline measurement, there's nothing for me to compare to.
    You mentioned a sealed sub... do you have an HT sub in your house that you can just plug in and go? If so, that's a perfect way to measure cabin gain effects as you can easily drop one in the car and measure it, and then measure it in free space. That's actually how I found the gain in my wife's civic so I could better model it in bass box pro. I would certainly be interested in the results.

    The drop in output past ~100hz - to me - is simply a drop due to natural rolloff (whether it be by the baffle or driver's themselves). I don't consider anything below that frequency a gain, per se, again, as it's almost flat (You have about a +5dB margin at most from what I'm seeing). With cabin gain, you should expect a pretty linear rise in amplitude; not a flat response. I don't want to keep arguing the same points, so I'll just say that the way you and I are viewing the delta between 30hz and 100hz is quite different. IMO, if you have cabin gain, you would certainly expect these two frequencies to be different, given that some math points you at typically having the rise start @ 1/2 wavelength of longest dimension, which as I mentioned above can depend on how we assume the box acts. If it does start at 125hz, like you believe it does, then you would be looking at a length of ~4.5ft. **That would probably be the trunk's longest dimension... interesting that this could go toward my thought of the 2 separate volumes. **
    Aaaaaaaaaaaaaaaanyhow, based on my more minimal experience, I'm going to err on the side that says the entire car's enclosed volume dictates this and *if you had a more normal car with hardtop* you would likely see that rise starting around 40-60hz given that the length would probably be somewhere around 9-13' (rough numbers).

    Now, here's a wild take on the subject and I'm not sure if you guys will follow me down this path...
    If you were to take the "enclosed" volume (the car's cabin) that the 'enclosure' (your sub/box) sees, then *in the case of no roof* your "enclosed" volume seen by the 'enclosure' is nearing infinity because your longest distance is ... air ... the environment around you. So, let's take this method.... let's say your longest distance is 100', as you have no roof and you're practically approaching infinity. 1116/100/2= 5hz. That says cabin gain would start at 5hz.

    No way to prove that at all... but a random thought.

    Guess we'll just have to see how your baseline measurement vs your roof up/down measurements turn out. But, it is kinda fun to think about it like that.
    Last edited by ErinH; 03-17-2011 at 04:37 PM.
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  7. #7
    Senior Member cvjoint's Avatar
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    We definitely both agree that 30hz output is almost the same as 100hz output. You take this as evidence that there is no cabin gain. But what if the cabin gain is +20db at 30hz and +20db at 100hz? That's why I was pointing out that some of the cabin gain measurements in that picture had this exact behavior. I agree, it's not the best test or the final verdict. It's a teaser for now!

    One note, these drivers should not roll off naturally after 100hz, maybe after 400hz. They have an LE of under .2. Since I can't test 10hz, the low bound, I will try to test out to 250hz, hopefully we start to see the natural output of the driver by then. Then there is the whole top down, an sealed box tests. I don't have an HT sub I can just plug in but I do have a dual large sealed box, it would take some work.

    Hmm, I was thinking, I could plug in the subs in the 4 channel amps, those don't have the 230hz LP built in.
    Last edited by cvjoint; 03-17-2011 at 04:53 PM.

  8. #8
    Tester Extraordinaire ErinH's Avatar
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    Yea, we're just looking at it differently.

    This is how I see it:
    Cabin gain typically has a rise of 12dB/octave. Per Andy's graph, you can see that in most every case, there is more than a +5dB rise overall from 100hz down to 30hz; in some extreme cases, approaching 20dB total. In your graphs, there is no general rise toward the low end, just a flat(ish) response. Thus, by definition, at least to me, I can't call this cabin 'gain'. There is, however, an abrupt rise, just like you see in his. I do believe this is some sort of natural filtering (either via the car or the sub/wall effect).
    http://farm4.static.flickr.com/3194/...d155d5ac_o.jpg

    Given that, I'm not inclined to say you're seeing cabin gain here. I'm not saying I'm right. I'm just saying, based off the commonly accepted definition of rise being a rise in output at xdB/octave, I don't believe you have any affect of cabin gain because you don't have a rise in output. And I really do think the sharp rolloff you're seeing is not a rise; I do believe it's more an artifact of some sort of rolloff. But, I'm not certain.

    HOWEVER, I am interested in exploring the potential for the 'smallest enclosure', aka the trunk, to be the limiting factor, and in this case it's probably about 4', which translates to around 150hz (ballparking it off the top).
    Additionally, I'd like to further explore the option I presented earlier: if there isn't 2 separate volumes, then what about the possibility of your open environment (again, approaching infinity for practical purposes) dictating the cabin gain. In this case, the 'gain' wouldn't start until a very low frequency. I gave 100' = 5hz earlier. That may not be dead on, but for all intensive purposes, it seems plausible.
    Last edited by ErinH; 03-17-2011 at 06:07 PM.
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  9. #9
    Senior Member cvjoint's Avatar
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    I'm more of an applied guy by trade. The common view of adding 12db/octave to mimic cabin gain is a theorist perspective. If there is a transfer function I like to have it's pdf at the db level. A theorist might come in and say, "well, I can approximate this to the nearest 5 db for the average case. It's nice because I can predict behavior." That's all good from a prediction point of view when testing is not available, but in this case it is. I don't care that I can't approximate it with a weibull distribution with some small degree of error if I can actually get raw numbers with an almost nonexistent degree of error. Look back at the graph, I think my car would look like the red or yellow or blue samples that have a huge bump at 100hz. I can't tell what car it is because they seem to have recycled their legend colors.

  10. #10
    Tester Extraordinaire ErinH's Avatar
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    I understand. I'm actually right there with you. I was never so hot with theory... I had to see it in action. I love some math, but I hate some calculus. ;)
    So, given that, I certainly understand what you're saying. However, I'm just not seeing anything that looks like a gradual rise over the subs' passband. That's all I'm getting at.

    Really, I don't want to seem like I'm arguing for the sake of arguing. If I had the means, I'd be outside in the garage right now measuring my own setup in many scenarios to answer some of the questions I have. But, I've got the little one tonight (taking a nap now) and my wife is at work, so no dice.

    Hopefully we'll get some more useful info out of your further tests. This is certainly an intriguing subject and I'm VERY curious how we might possibly viewing two separate enclosures when IB and how your top down (woohoo.... boobies!) tests may differ. I'm leaning to the side that says you're going to get different results which will likely mimic what we are used to calling "cabin gain".
    Last edited by ErinH; 03-17-2011 at 08:55 PM.
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