Vehicle acoustics...

[Huricaine]

What can I learn about vehicle cabin acoustics? I know next to nothing. What causes cabin gain is it a harmonic thing? Why does mutual coupling work in Physics?

If you have any explainations on acoustics and waves when using ported/sealed subwoofers let me know as I am interested in the knowledge. The physics aspect.

[soho54]

Didn't you already have a thread sorta like this?

Cabin Gain is based on the fact that the sound waves can't escape in 360deg the way they want to. Boundary gain. Place the sub next to a wall or in this case a body panel, and the wave is reflected back by the wall instead of moving through it. This makes the SPL higher on the non-wall side than it would have been without the wall.

As the frequency gets lower the wavelengths get longer, so more and more "boundaries" are effectively right next to the enclosure, so gain increases. The lower the frequency the more constrained it is, until it is eventuality contained on all sides, and this is called pressure vessel gain, where there are no modes anymore.

Think of if like a flashlight, and its reflector. The boundaries focus all the "waste" pressure waves in the direction you need them.


Mutual coupling is when the sources creating the waves are close enough together that the waves work together with one another from the "relative origin" point out. The waves are in almost exact phase, so they build on one another and increase the waves strength/amplitude. I said "relative origin" as the drivers do not need to be right next to each other to couple, as it is again frequency based, and you can have coupling of low frequencies even with a good distance between two subs.

[Huricaine]

That thread was all cluttered with my babbling, this is more specific and easier i think.

So, two sources of exact same phase radiations will build a stronger one? It just always confused me the extra 3 db, so it is a phenomena that happens with waves together? Could you say that it is like water waves how they come close to eachother and then push away stornger?

See, i know that when you place a subwoofer to the rear wall/vehicle it is supposed to make the waves in phase because low freqs are 360 right? I figgured this was cabin gain?

I am not too smart with modes, standing waves etc even though iv'e been reading about it. I figured standing waves was the cause of cabin gain but im not sure of it all.

Any easy explainations via links etc to waves and the rest like resonation someone could give me would be great..

[buggsson]

If it's car acoustics your'e after, why not visit one or two of the car diyaudio forums that exist? When I thought that it was car audio that was my path, they was very helpful, just as this forum.

[Huricaine]

I go to them i just wanted the theory on it because alot of those sites don't like theory. Even here i see that people don't really seem to want to give me some theory explainations. It is not that i need help doing basic installs, it is that i want to know all about acoustics because it is a hobby to learn about what is going on and do the best i can when i do my setups. I just want to know all the possibilities and know what is pysically going on, it interests me to know.

[AllenB]

Why not expand to look at a cabin as a small room?

[hooligan72]

Cabin gain is definitley your friend in car. In general further towards the rear of the vehicle is best. This will avoid some of the out of phase cancellation that occurs with reflexions. Cabin gain can also lower your perceived F3 allowing for a very compact sub ( if you're not going for raw SPL ). It's very rare that I'll design an enclosure tuned lower than 40hz for the car.

[Huricaine]

Originally Posted by AllenB

Why not expand to look at a cabin as a small room?

My question? sure...

[Huricaine]

Okay so what about standing waves? how do they act in the small closed field? Arent they the reson that a closed field acts the way it does?

[Don Hills]

Originally Posted by Huricaine

Okay so what about standing waves? how do they act in the small closed field? Arent they the reson that a closed field acts the way it does?

Standing waves only have an effect above the lowest frequency that will form a standing wave in the space. The longest dimension of a typical car cabin is about 2.5 metres / 8 feet. The lowest frequency that will form a standing wave is about 70 Hz. Below that, the SPL in the cabin is entirely due to pressurisation - the difference in cabin volume betweeen "cone in" and cone out" position of the subwoofer cone, minus any air leaks.

Strictly speaking, the "cabin gain" due to pressurisation is there at higher frequencies too. It just doesn't show up at higher frequencies because it is proportional to cone movement, and the cone movement decreases with increasing frequency. It also gets swamped by the standing waves and reflections at higher frequencies.

For a "normal" speaker, the cone movement has to increase by 4 times with each halving of the frequency in order to maintain a flat response in open air or a large (compared to the speaker) room. This 4 times air displacement results in a 6 dB per octave increase in the "pressurisation SPL" when the speaker is in a sealed room. In order to maintain flat response in a car (a small sealed room), the frequency response of the speaker has to drop at the same rate: 6 dB per octave. This results in the cone displacement "leveling out" to a constant amount and thus resulting in a constant SPL.

You can see this for yourself with any speaker modelling software that shows you the cone displacement. Modelling a closed box that performs well in a car should show the excursion increasing as the frequency drops, then levelling off.

The design process to "get you in the ballpark" goes something like this:

- Take a sub, just about any sub. Preferably sealed.

- Measure its bass frequency response (below about 100 Hz, or below the lowest "standing wave" frequency of your car) in open air.

- Repeat the measurements with it placed in the location you want to use in the car. If the frequency response is "lumpy", try moving it about to find a better position.

- From the two responses, find the frequency where the response is up by about 6 dB in the car. This is your car's "6 dB cabin gain" frequency.

- In your design software, pick a driver and a closed box that is approximately the size you want to use. Set the power input to the speaker's max or the intended amp's max, whichever is lower.

- You now need to balance 2 factors.

1: The frequency response of the speaker will need to be about 6 db down at the "6dB cabin gain" frequency. Adjust the box volume to achieve this. If you can't get a smooth curve with no peak and with -6dB at the desired frequency, your driver is not suitable. Pick another one.

2: The driver should not exceed xmax at any frequencies. If it does, accept a lower amp power or use a driver with higher xmax.

- Examine the SPL figure. If it's more than you need, you can use a smaller driver (and likely a smaller box and /or amp.) The converse applies if the SPL is too low.

- Build. Measure again - open air vs in car. Repeat as required.

Note that this is just a "first approximation". You may want a non-flat bass response because the ear's sensitivity drops off at lower frequencies and SPLs. You may find that if your mids can do 100 dB SPL, your sub may need to hit 120 dB SPL at, say, 25 Hz to give a perceived "flat" response. It will depend on the levels you listen at and the type of music you listen to.

At least one speaker design program includes the ability to specify a "cabin gain" frequency and plot the resultant theoretical in-car response.
It's this Excel spreadsheet:
http://www.diysubwoofers.org/prt/ported.zip
The name implies that it is for ported enclosures, but you can specify sealed enclosures as well (set "fb" to 0).