This is one of the most asked questions in car audio, and the reality is that the answer is not brands, or speaker size, or any of that, it all boils down to physics. It’s all math and that greatly simplifies the solution. You need to know the following-
Subwoofer Sensitivity or SPL rating. This is typically in the mid to upper 80s, such as 86dB.
You need to know the actual output of your amplifer. If your amp came with a test sheet showing the actual output at the ohm level you are using, that is perfect. If not, you will need to calculate it. We will do another article on testing amp output.
Here is the worksheet- (click on the image in the gallery for a printable version)
Decibel work sheet
Using the sensitivity rating, we can calculate within +-3 dB the spl for any given system at any given power.
add 3 dB per doubling of cone area
ie 2 subs, 3 dB, 4 subs, 6 dB
take sub sensitivity, with dB per sub added
and calculate 3 dB added per doubling of power starting at 1 watt\meter
Next we add in Transfer Function
add 12 dB for a standard car\truck
add 16 dB for a hatchback car
Now we figure in box type
for a sealed box, 0 dB, sealed offers no dB gain.
for a ported box, 3-9 dB depending on tuning frequency and air ripple. the higher the ripple, the more gain but at a cost of sound quality. A typical qood quality pre-fab box with a slot port yields a 6dB gain. Plain round ports without flares will lower your gain. Large slot ports up your gain but reduce the sound quality.
for a bandpass box, 6-12 dB again depending on the tune freq. and ripple. A typical good quality pre-fab bandpass will net you 9dB gain. Plain round ports without flares will lower your gain.
For example, a 12″ Rockford Fosgate subwoofer has a sensitivity of 86 db at 1 watt/1 meter, and is capable of handling 300 watts.The sub is installed in a ported enclosure, so we are going to use a the lower output rate for the ported box at 3 dB. Powering the subwoofer is a Rockford Fosgate Punch 300-1, which is rated at 300 watts to a single 4-ohm load. All of this is installed in a 2006 Honda Civic Si, which is a hatchback and gets a boost from the 16 dB transfer function. Using the formula, we get the following results-
86+ 24 (at 256 watts, it is close, but you could add in an extra dB if you want) + 16 (trans function) + 3 (box gain) = 129 dB estimated output. This estimation will be within +\- 3 dB of the measured output. This is simple physics, and it works.
That is all there is to calculating the potential SPL of your system. These numbers are based on test tones, not dynamic music, which will lower the SPL in most cases. The maximum SPL will be achieved using a test tone near the tuning frequency of the enclosure if ported or bandpass. In sealed enclosures, the peak frequency will vary on a few factors, but will most likely peak near the vehicle’s resonant frequency.