Cabin gain question (car related)

[bolland83]

Does anyone here know how to calculate cabin gain in a car? I'm having a hard time wrapping my head around the math. I'm trying to find a good target F3 for a sub in a scion tc. It doesn't have to be exact, I'm just looking for a somewhat flat response and don't know where the cabin gain starts to take affect. I realize there's no way to get anything flat in a car without the use of a digital eq, but I'm just looking to add a bit of bass without making a one note wonder. I'm thinking about going with something cheap like the Dayton DVC line.

[johnnyrichards]

There is no way to accurately predict cabin gain. There are a lot of factors at work, but (this is off the top of my head) cabin gain kicks in at 2x the wavelength of the longest dimension in your vehicle. Say it is 12ft (~40Hz) cabin gain would start kicking in around 70-90 hz.

In the theoretical world, shooting for an F12 1/2 that of the frequency at which cabin gain starts kicking in should yield flat bass to nearly 0Hz.

In the real world, cabin gain is never 12db/octave. My Alero kicks in somewhere around 85 Hz but my rough measurements indicate more like a 8db gain below that frequency, and it is not flat. My subwoofers are a pair of Infinity Reference 8" in a small, sealed enclosure with an F3 of around 50 hz. Very satisfying bass, but a car presents all the same problems as a living room for recreating bass and then some. A judicious amount of EQ and tinkering with the crossover frequency can go a long ways. I cross my subs over at 80Hz using the 3rd order built into my amp and the 4th order built into my deck. This very steep slope has helped out tremendously as well.

[billfitzmaurice]

Cabin gain begins at the frequency where the longest room dimension is 1/2 wavelength. With a perfectly sealed and structurally solid room the gain is 12dB/octave, though real world results are usually less. Cars specifically configured for SPL competitions fill the door panels etc with anything from injected foam to concrete, to stiffen and seal the structure. The latter tends to have a slightly deleterious effect on gas mileage.
Some JBL autosound woofer data sheets show SPL charts with and without cabin gain.

[Steve]

According to John Murphy at True Audio,the corner frequency for cabin gain is figured by dividing 565 by the longest dimension of the car interior(in feet).There are several examples on his web site.

Steve

[rudyjakubin]

Does anyone here know how to calculate cabin gain in a car? I'm having a hard time wrapping my head around the math. I'm trying to find a good target F3 for a sub in a scion tc. It doesn't have to be exact, I'm just looking for a somewhat flat response and don't know where the cabin gain starts to take affect. I realize there's no way to get anything flat in a car without the use of a digital eq, but I'm just looking to add a bit of bass without making a one note wonder. I'm thinking about going with something cheap like the Dayton DVC line.

For my son's truck I used an F0 of 70 or 80 - I ferget but it was around there. I measured the furthest point under the dash to the rear of the cab. I used 2 TangBand 10" shallow woofers in a .75 cu. ft. Sealed box. Came out perfect..
http://www.linearteam.org/forum/view...cabin+gain#p13

I used WinIsd to get the box size and adjusted the F3 to get to 70 or 80.

[billfitzmaurice]

According to John Murphy at True Audio,the corner frequency for cabin gain is figured by dividing 565 by the longest dimension of the car interior(in feet).There are several examples on his web site.

Steve

It's the same equation. 565FPS is 1/2 the speed of sound, so this arrives at the 1/2 wavelength frequency.

[Sydney]

http://www.linearteam.org/forum/view...cabin+gain#p13

I've yet to find a technical paper on "Cabin Gain" ( like AES ); only discussion on automobile sound forums as above.
The 4 ways of "treating the car" require assumptions ( like windows up ).
It seems that determining what particular model ( like "IRREGULARLY STUFFED AND LINED, rigid, LEAKY, VENTED" or one of the others ) is applicable isn't clear.
One of the variables in the model is how well the cabin can be pressurized.
I've noticed; speaker cabs ( designed with anticipation of a 12db cabin lift in a tight vehicle ) sound bass "lite" without it - like used in a much larger room or taken outdoors at a campsite etc.

[davidl]

I think most car subs can be designed the following way:

Use a sealed box and aim for a QTC of .8 to 1
Use a driver with a healthy xmax of 10mm or more
FC should be around the 60 to 70 Hz range

I once had a "lowly" 8 inch driver with 10mm of xmax in a sealed box with a FC of 60Hz and QTC of .8 in a Honda CRX and it was MORE than enough for satisfying DEEP bass and not boomy in the least.

[rudyjakubin]

I've yet to find a technical paper on "Cabin Gain" ( like AES ); only discussion on automobile sound forums as above.
The 4 ways of "treating the car" require assumptions ( like windows up ).
It seems that determining what particular model ( like "IRREGULARLY STUFFED AND LINED, rigid, LEAKY, VENTED" or one of the others ) is applicable isn't clear.
One of the variables in the model is how well the cabin can be pressurized.
I've noticed; speaker cabs ( designed with anticipation of a 12db cabin lift in a tight vehicle ) sound bass "lite" without it - like used in a much larger room or taken outdoors at a campsite etc.

Sheese, you want technical!!!!
http://www.linkwitzlab.com/images/gr...losure-spl.gif

With ported boxes you have two resonance modes to contend with.

It didn't matter too much if the windows were down or doors opened.
We'd sit in the yard having a few beers and listen to the tunes. Sounded great.
Problem with subs that I've listened to in other peeps cars was the BLOOM and the one note bass. You get the right F3 for the cabin length in a sealed box and your good to go.
For SPL with the 2 - 10's with a 300 watt amp was way more than adequate. The Pioneer head unit with the EQ button and bass controls made it way easy to set for the mood you're in.

[rudyjakubin]

I think most car subs can be designed the following way:

Use a sealed box and aim for a QTC of .8 to 1
Use a driver with a healthy xmax of 10mm or more
FC should be around the 60 to 70 Hz range

I once had a "lowly" 8 inch driver with 10mm of xmax in a sealed box with a FC of 60Hz and QTC of .8 in a Honda CRX and it was MORE than enough for satisfying DEEP bass and not boomy in the least.

The FC 60 and .8 would fit the small car.

[Sydney]

Sheese, you want technical!!!!

Actually not at all
I thought designing for a particular cabin gain was over the top, and I've seen cabin gain promises attributed to a non-cabin situations.
FWIW: I followed the "Killer Car Stereo on a Budget" ( Daniel L. Ferguson's book ) recommendations and built & used the active variable Q adjustment to fine tune.
Using 2 Madisound 8's: I did notice the difference in a fully loaded vehicle vs 1 passenger.

[isaeagle4031]

I typically use a couple of measurements. The overall longest distance and the distantce from the drivers headrest and the center point of the sub. If going with a ported or horn-loaded type enclosure I will also measure from the mouth to the drivers location.

[ErinH]

Quite honestly, I take my home subwoofer, measure it outside in free space, then put it in the car, in various positions and measure it. This gives me cabin gain but also FR with the enclosure in various positions.


Something I've been wondering, and brought up on another forum, is for guys running IB; we seal the trunk wall so we separate front/rear waves. Do the subs "see" both volumes, or only one, when considering the "enclosure" loading? If only one, is it the smallest volume?
Now, the same question but for cabin gain. Is the longest distance only one section of the car (the trunk or the cabin), or is it the whole length of the car's enclosure. I would err to the logic of it seeing only one as, again, we do a very thorough job of making sure we seal off the trunk wall so that you have no way for the front/rear waves to interfere with each other.


I may have to futz around with some measurements this weekend.

I tell you one thing I have found is that various changes in the car (ie: windows down, or trunk open, or rear seat folded down, or ... , or any combination of the aforementioned) results in a shift in the final alignment (qtc), which certainly proves to me that the qts /=/ qtc in the car environment. Testing was done with the wt3 one day when I got curious.
From a realistic standpoint, this makes perfect sense as it's not true "infinite baffle" or vas, for that matter. However, people really like to argue otherwise.
I like that John @ AE actually posted why he made his IB15a (car audio version) with a Q of x (can't recall, though I think it's 0.41) so that the final in car alignment would be closer to 0.7.

- Erin

[bolland83]

Thanks guys, I think I can get close enough to were I want to be with the info posted. Again, it's not a critical install, just adding a little bump to a factory system.