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Discussion Starter · #1 ·
Hello diyma!

I've been doing research on cabin gain in vehicles and there seems to be all sorts of different information on the effects of the volume levels of certain frequencies in smaller spaces where the wavelength of the frequency is longer than the space it is occupying. Apologies if I am confusing the concept of cabin gain and something else, as it's a little tricky to understand.

I am on this journey because I'm looking to downsize my vehicle's subs to something a little more practical in terms of space, and upgrade in terms of the sound. I currently have 2-12" subs in a sealed box with 1.25 cubes per chamber, and it's just a bit too much sub for what I need nowadays. They also, unfortunately, are high qtc drivers, so they are very peaky across their range require a lot of EQ to get the flat, deep response I really like. I'm replacing these two subs with a single 10" Alpine X-W10D4 woofer.

I really love deep bass ~30hz, and I wanted something that sounded nice and even in output as I swept between ~80 down ~27hz. I initially went in modeling a box in WinISD that would give me the flattest response from the woofer within that range, and ended up with this design in a ported box.

Rectangle Slope Font Parallel Screenshot


Now, this was great to me! It's tuned to 27.1 hz, has an extremely flat response with a -3db point at ~24.5hz, and I'd save space over my 2 12" subs and have a killer flat response that can get as deep as I need it to without rolling off early and thus saving me from requiring lots of eq! However, what I didn't take into account was the cabin gain. From what I have read on DIYMA posts, cabin gain is easily calculated for a sealed enclosure using linkwitz transform in WinISD, BUT! It apparently doesn't work the same for a ported enclosure. At least, according to this DIYMA post, a ported enclosure is a dipole, which in their example, doesn't add any cabin gain at 30hz, unlike a monopole which will add 12 db at 30hz. Now, I might just be confused and reading this wrong, but based on that post, there won't be any cabin gain from a ported enclosure.

But based on my research, a ported box != a dipole and they are two different designs. Also, according to SVS, both types of enclosures experience room gain. Their sealed subs roll off at 32-35hz to take advantage of the average room gain in a home to create a flat response. Their ported subs, however, do not have a frequency roll off like this, and remain fairly flat until ~20hz or less in a quasi-anechoic environment, but in a smaller room, they experience room gain, so to combat this and prevent uneven peaky sound from their ported enclosures, they include room-gain compensation controls to flatten out the sound.

So, I modeled the sub in a sealed enclosure and with approximate measurements of the longest path inside my SUV, I included a cabin gain simulation using linkwitz transform to see how my frequency response would be for BOTH ported and sealed enclosures.

Slope Rectangle Plot Font Line


Oh no! In both ported and sealed examples, assuming cabin gain for both, I've got a less-than ideal curve for the outputs! It seems that in both cases, cabin gain will cause the lowest frequencies to dominate the higher ones! It seems cabin gain might make my flat responding ported sub to sound much more progressively louder as it gets deeper. But so does the sealed box.
The Color Key for the curves is
Black - Ported (no cabin gain)
Orange - Ported with cabin gain
Green - Sealed (no cabin gain)
Blue - Sealed with cabin gain

You may ask why I originally modeled a ported enclosure vs just going with a sealed enclosure. My main reasons for doing so were the calculated flat response down to 30hz (prior to this journey into understanding cabin gain), the fun in the hobby of designing and building the enclosure (I'm excited to build something with more complexity!), the increase in output vs sealed (I'm concerned with the loss in output going from 2 12's to a single 10), power requirements (lower for the same output), and less potential for excursion related distortion at lower listening levels (less required excursion to achieve similar output).

I think I might be misunderstanding how cabin gain affects ported enclosures, and where I think I might be misunderstanding is the whole dipole argument. Is it just acting as a dipole BELOW the tuning frequency of the port, where the sub would essentially be acting as if it's in free-air? Also, with the frequency response of human hearing being less at the deepest frequencies, would a bump in the volume levels in both sealed and ported from cabin gain result in a peak sound at sub 40 hz, or to my ears, even with the higher db levels, would sound "flat" due to the decreased sensitivity our ears have at those frequencies? This is assuming that cabin gain also affects ported enclosures. Are my concerns overblown?

Thanks all for any help! Searching through this forum has been helpful but I still am confused a bit! Cheers!
 

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Have a microphone? Measure your current subwoofer response in car and do a ground plane measurement. The difference would be the transfer function aka cabin gain.

I'm unaware of the dipole issue but I guess that makes sense below tuning. But you're playing a ported box below tuning anyway.

Also why not model the 12s vs the ported 10.
 

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Discussion Starter · #3 ·
Have a microphone? Measure your current subwoofer response in car and do a ground plane measurement. The difference would be the transfer function aka cabin gain.

I'm unaware of the dipole issue but I guess that makes sense below tuning. But you're playing a ported box below tuning anyway.

Also why not model the 12s vs the ported 10.
I actually did model the old 12's, but boy they aren't great. They have a heavy bump in response centered around ~63hz and in order for them to sound relatively flat, I needed to use my amplifier's band reject filter centered around the peak, and the head unit's parametric eq is boosting 31hz as much as it can. The response hasn't been measured to verify it's a relatively flat response curve, it's just adjusted to get as close as I can by ear in the driver's seat.

Here's the model for the current 12's in their box with net internal volume at 1.021ft^3 in red, and the ported 10" sub as the thinner orange line with cabin gain factored in

Rectangle Slope Plot Font Parallel


I understand that I can try and EQ the 10 just as I have done the 12's, but I'm trying to avoid eq'ing as much as I can. I don't have a DSP, only the head unit's 9 band parametric eq, so I want to have it sound as good as I can given the space I am putting the sub into so what adjustments I need to make are still doable with the limited adjustments I have available. I suppose I could try and record how the current 12's sound in the driver's seat as is, and see how the curve looks, to see if the low end bump from cabin gain is going to need as much EQ cut as I think, or if the bump will actually compensate for any lack of sensitivity to those frequencies my ears may have. Granted, the 12's have some dips and peaks in response that I'd like to eliminate with the new sub, but perhaps I can take some of the idea of measuring the current subs as they are eq'd to see how the output graphs when it sounds as flat as it can sound by ear.

Then, I can model the driver to sort of fit that slope with cabin gain, that is, if the cabin gain does exist in the same capacity for ported subs above their box tuning frequency. I'm thinking it does, but I haven't quite found the answer to that yet.
 

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I think it safe to say that cabin gain applies to ported subs. So take your response, add the simulated cabin gain, and observe the response. If you're targeting a flat response and what to avoid EQ than run a sealed or better yet an IB subwoofer. I would go with a single 15. A SI SQL 15 can work in about 2cu ft which is probably around the same size as the gross 10" ported enclosure.
 

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Discussion Starter · #5 ·
I think it safe to say that cabin gain applies to ported subs. So take your response, add the simulated cabin gain, and observe the response. If you're targeting a flat response and what to avoid EQ than run a sealed or better yet an IB subwoofer. I would go with a single 15. A SI SQL 15 can work in about 2cu ft which is probably around the same size as the gross 10" ported enclosure.
Okay, sounds good! I have the simulated gain modeled in the orange line for the ported version. In that case, I think I'll run things in a sealed box and see if the output is satisfactory for what I need. I'm thinking it will be enough, given the gain I'm getting on the low end + the extra xmax this sub has over the 12's. Plus, I already think 2 12's is more sub than I need. I appreciate your suggestion for the 15! I already have the 10" sub though, so I'm going to work with this. Sealed will certainly save even more space, and if the sealed box just isn't enough output, I can consider the ported project later as something fun to do. I appreciate your input!
 
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