SPL Theory

Basic SPL Knowledge

 
Energy cannot be created or destroyed. All the energy from the audio system is either reflected off the walls staying in the vehicle, absorbed by the walls turning in to heat, or transmitted outside and lost. The more energy that is kept inside the vehicle, the more internal SPL there will be. Each source adds its own effects to the vehicle. There is cancellation and reinforcement dependent on phase angle, frequency, and distance from the source to the point of reference.

 

(SPL total) - (SPL losses) = (SPL avg inside)

SPL Increasing Techniques
 
if you can hear sound outside of the car you are wasting power. There is something that can be done about power loss coming from sound energy transmission to the outside of the car, and the absorption of sound energy by panels in the car. Reducing the air space to 1/2 in the car will give a 3 Db boost compared to the original volume. Increasing the air space by 2 times will take 3 Db away compared to the original volume. By adding a layer of dynamat you might get around 2 - 4 db of spl gain. If you added a 4-inch thick steel reinforced concrete wall the spl gains would be much more. 

In increasing the rigidity of the outside panels there is less sound energy loss due to absorption of sound energy in the car, and less sound energy loss due to transmission outside the car. 
The less the outer panels move in and out the less sound energy is transformed in to heat.

There will be points that are lesser in spl and points that are greater in spl. These are from the frequency interactions with the vehicle. You can force these points to certain places with

the easiest way to find the max spl frequency is to use a function generator. Sweep a sine wave up and down until you find the frequency of max spl output.
any weak point in the parameter of the volume(the car) will lose energy through it.

Equations

 

10 * log ([new wattage ] / [wattage now] = db of power difference

Cabin gain has three major frequency points in the vehicle. Find the distance between the front and back, the distance between the two sides and also, the top to bottom.

Speed of Sound = 1100 Ft per Sec

Wavelength = 1100 / Frequency

Frequency = 1100 / Feet

Frequency and Length

 
Frequency
Length
20
55
30
36.6
40
27.5
50
22
60
18.3
70
15.7
80
13.7
90
12.2
100
11