ArticlesJanuary 2011Cabin Gain

Overview

This simple test demonstrates the response difference between placing a subwoofer system into a vehicle compared to its raw performance in a boundary free condition. From the cabin gain test we can compare the results of a system measured outside in a half-space environment to the measured response inside of a vehicle and determine what effects the confined interior space has on the output.

The Driver

For this example we used the TC Sounds LMS-5400 driver in a 120L sealed box. This driver demonstrates predictable well-behaved linear response in a sealed enclosure, so we can see exactly how the cabin affects the output. The base line of the test is with the microphone 2 meters away from the driver out in a field with the nearest objects at no less than 45ft. The input level is set to provide a voltage that will correspond to about 100 watts into the minimum system impedance between 10-200hz.

The Vehicle

The system was then placed inside a vehicle which happened to be a 2004 Jeep Grand Cherokee Overland and re-measured without changing any settings. The sub was placed with the driver facing back into the rear hatch at about 8 inches from it. The microphone was then placed in 3 separate places to measure the response: (1) the driver's head position, (2) the kick panel, and (3) in the middle of the dash near the windshield. These are about 8-10ft from the driver. The latter two are typical spots where spl is measured in competitions.

Analysis

Right off of the bat we can see a huge gain all the way down to the end of the measurement with as much as 38.5dB of added SPL from 7hz to 9hz. With this added gain it is easy to see how achieving very high dB numbers becomes possible relatively easily. At 60Hz, the LMS-5400 will produce 126dB with just 100 watts in the Jeep, a gain of about 24db over the outdoor measurement. It would take over 25,000 watts of power into the system to recreate that measurement at just 2m away in an outdoor environment! It doesn't take a physicist to figure out that the system will in no way handle that sort of power.
When it comes to producing ridiculous chest crushing bass, car audio has a major ace in the hole.
Free efficiency from boundary and pressure vessel gain is not to be underestimated! If I was trying to put up big numbers in this vehicle I would be looking at the 40-42hz area or the 56-60hz range (10hz would be fun and more challenging though). The moral of the story is --When it comes to producing ridiculous chest crushing bass, car audio has a major ace in the hole.

Of course, its not all bells and whistles

Looking at this from a perspective of accurate reproduction or as some like to call it, sound quality, we can see that the transfer function of the jeep also widely alters the nice predictable response of our sealed system, making it necessary to use some signal shaping to smooth out the response. Obviously, simply sticking a very good driver inside a car does necessarily yield a great response. It will be loud, it will have low distortion, but it won’t be necessarily linear in the frequency domain. However, it must be noted that compared to what we have seen from some room measurements the +/-8db response from 4-200hz is not a bad starting point!

At 65-70Hz, an area of response that is very heavily represented in typical modern music and in the kick drum fundamental range, we have a null in the driver seat which is not ideal considering we have peaks and a rising low end elsewhere. Below 40Hz or so it really makes little difference where the listener is and below 20Hz, the response is just about identical everywhere in the car which makes sense, considering how long the wavelengths are.

For sound quality, its important to understand that it is almost never likely you’ll have even bass response without some tweaking and active DSP, therefore a simple measurement rig and EQ equipment can go a long way into achieving your goal of ideal response or at least understanding what you like and don't like and why. Ideally for the driver's location, we would want to employ a shelving cut at about 50hz, boost the 65hz area a little, and perhaps cut the 90Hz area a bit. A product like the Dayton OmniMic is a simple to use, portable, one stop tool that is a great way to quickly step up from guessing what is going on with your system or relying on others to measure it for you, and start gathering hard data for yourself.

Conclusion

For those serious about sound quality we recommend you measure and know your frequency response because all vehicles will be different. For those dealing with SPL, it's equally important to know where the maximum peaks and positions are in the car in order to achieve your highest dB score. The difference between the top SPL drivers is probably less than 3dB and a few feet of difference in your car can easily be more than that! The difference is in the details. Whether you're going for sound quality or sound quantity, a simple measurement tool can be the best investment you make for your sound system.

Final Note

Subwoofers measured outside eliminate the car variable and we can more easily compare driver-to-driver or system-to-system. For that reason, the car audio subwoofers we measure will be done outside so we can more accurately demonstrate the drivers' or systems' strengths relative to each other without vehicle cabin effects inside data. We want to test bass drivers and systems, not vehicles.