question about how cabin volume effects SPL

[spencer321 10+ year member]

is it true the less cabin volume you have to fill the easy it is to score higher?

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[Cameron_B 10+ year member]

Yes, in most cicumstances (doors/windows) sut.

 

[skmfkr 5,000+ posts]

no

 

[Cameron_B 10+ year member]

no

Wow, maybe I am misimformed?

If this is the case, why do people do walls?

 

[skmfkr 5,000+ posts]

it all depends on the vehicle obviously and how everything is installed

for example i had a reg cab truck with a 12 in it, and a civic coupe with a similar 12 then put the same 12 in a hatchback

the hatch was louder than the civic coupe and the civic coupe was louder than the reg cab truck. so in order from loudest to not. the cabin was also bigger

and walls usually help with cancellation i believe and allow for a bigger box

 

[John_E_Janowitz 10+ year member]

The simple answer is that the smaller and more well sealed the cabin, the more you are pressurizing the air, and the more energy there is. This however doesn't necessarily mean you have more audible output. There is much more energy at the lowest frequencies than at the higher frequencies.

The following image is from some cabin gain studies a few years ago. The green graph is with all windows up in a buick regal. The light blue is with all windows down and yellow is with the 2 passenger windows down:

In this case, looking at the audible range from 20hz on all the way up to 100hz, the levels are louder with the windows down at all points. However if you take into account the output below 15hz where there is more energy, there is clearly still more energy in the vehicle with all windows closed.

John

 

[Ed Lester 10+ year member]

Yes, it is easier to achieve a higher SPL with a smaller cabin volume.

Now if your box and setup is not tuned to compensate, like what skmfkr is describing, there may not be an increase in score.

Hatchbacks can get loud because of the loading effect that the open cargo area has, but smaller hatches do much better than larger hatches but are tuned at a much higher frequency.

Look at CRXs and Pandas doing 156-160s in Street classes vs explorers doing 155 max in street class.

Also vehicles with walls do much better when they have a smaller cabin. Look at the Seat Terra in SS 1-2 doing 169s and a Jeep Wrangler with a similar setup in SS 1-2 doing 165s.

Then look at extreme cars where one of the tricks is to build the cabin as small as possible to build pressure as easily as possible.

Alan Dante's Volvo has the smallest passenger side cabin that I have ever seen and he does 180 dB.

Its rocket science. When you have 2 pressure chambers with a pressure generator in between, the pressure will build the greatest in the smaller of the 2 chambers.

It is much easier to move 10 cubes worth of air molecules than it is to move 20 cubes of air molecules, but it will have to be at a higher frequency.

 

[spencer321 10+ year member]

wow i had no idea this shit was that complicated. i was just curious if my standard cab pickup was at an advantage or disadvantage because of the small cabin.

 

[Ed Lester 10+ year member]

If you do a cut through you will have an advantage.

Otherwise, you dont have enough room for a good enclosure.

 

[skmfkr 5,000+ posts]

another thing with the reg cab truck is even you crack the driver side door, and play low freqs it had a huge gain, same with windows down with like 45hz or so. but in the civic and honda fit, it lost pressure when it wasnt sealed up

 

[spencer321 10+ year member]

If you do a cut through you will have an advantage.Otherwise, you dont have enough room for a good enclosure.

i got 6.5 cubic feet below the window line... thats not enough? i could take it to the roof and wall it off, gets me 10cubes... i would "hope" that enough but maybe its not?

 

[spencer321 10+ year member]

another thing with the reg cab truck is even you crack the driver side door, and play low freqs it had a huge gain, same with windows down with like 45hz or so. but in the civic and honda fit, it lost pressure when it wasnt sealed up

yeah my mustang and my prelude both sounded louder windows up doors closed not "big" noticable but with the truck... dear god if you have it cranked to what you think loud is with the windows up doors closed, and then you roll down a window its UNBEARABLE rolling the windows down or opening a door makes a HUGE difference

 

[Ed Lester 10+ year member]

i got 6.5 cubic feet below the window line... thats not enough? i could take it to the roof and wall it off, gets me 10cubes... i would "hope" that enough but maybe its not?

depends on the subs, may not be enough.

Also, the shape and depth of the box is very important for SPL, and so is port design.

I have never seen 6.5 cubes in a single cab truck unless it is a shallow, wide box and that isnt too good.

 

[spencer321 10+ year member]

depends on the subs, may not be enough.Also, the shape and depth of the box is very important for SPL, and so is port design.

I have never seen 6.5 cubes in a single cab truck unless it is a shallow, wide box and that isnt too good.

well, 60 wide, 11deep 24 high

 

[Rashaddd 5,000+ posts]

When the size of the box starts to match or be bigger than the size of the effective cabin, it is in many ways like you are reversing the two, and the cabin becomes the enclosure, as it will dampen and load the cone(s) more than the box will. Have you ever tried sticking a meter in a port or inside a box?

Granted, your actual box is alot more air tight and better built to generate acoustic energy than any part of your cabin is, but you get the idea.