Am I Crazy?

How quiet can I make my Jeep? I want to hear my tunes but I may be nuts, because the Jeep is really, really loud on the highway thanks to off-road tires, a lift, 3" high flow exhaust, bad aerodynamics, and aging window and door seals. I can barely hear the stereo and bass response is all but non-existent above 40 mph. Most folks with 4x4's don't worry about noise, they just get a louder stereo.

But TC is supposed to be a best of both worlds kind of off-road rig... both highly trail capable and also really nice to drive on the highway, so let's see what I can get away with.

The Plan

The plan -- in the works for years -- is to incrementally add sound treatment to the vehicle and scientifically determine how much it helps using iSpectrum on my MacBook to do realtime spectrum analysis within the interior, and a Radio Shack analog sound level meter acting as both the meter and the iSpectrum mic.

Theory

So what is sound treatment and what sounds are we treating?

Silence -- or lack of noise, anyway -- in the car is helpful to sanity when driving long distances to Ouray and Moab and wherever else I go four wheeling and it's also key to a nice sounding stereo system. There's lots of sound to get rid of. Several types of sound exist in the cabin of your average car.

Resonating panels: As cars are made of fairly thin sheet metal, they tend to resonate at an audible frequency, either caused by your stereo, or by vibrations from the engine, road, tires. To reduce this type of sound, one must lower the resonant frequency of the metal by adding mass, and that is what sound dampening material does.

Reflected sounds: Sound waves behind your speaker can bounce off sheet metal in your car adding unwanted sound and coloring the music. Sound absorbing material placed behind the speakers reduces this effect.

Transmitted sounds: Sound barrier material prevents sound in one area from passing through sheet metal and entering into your cabin, such as engine noise passing through the firewall, or road and tire noise passing through wheel wells or floor.

There are also materials that insulate from heat and cold as well as absorb sound, and these can be placed on the floor, ceiling, and firewall. Since it gets fairly hot and cold here in Colorado, I'm seriously considering these materials, too.

A combination of sound dampening, sound barrier, sound absorption, and thermal/acoustical materials, applied in the right places, can tame a vehicle's noise and that's what I plan to do, measuring as I go.

Baseline Measurements

Before I got the spectrum analyzer software I used the sound level meter to measure in-cab noise levels. First, let's take a baseline reading in a relatively silent area: my office, with the hum of computers and not a lot else, then with speaker phone on during a conference call (conversational sound). Finally, the reading in the cabin of the FSJ running 65-75mph. In the latter test, the meter was in the 90dB range setting with A-weighting and slow response and was sitting in between the front seats on a cell phone mount attached to the center console (the location I'll keep using henceforth). Here's a good paper on noise, measurement, and sound levels.

Location	Sound Level (dB)
Office, quiet	55-60
Office, conf. call	60-80
FSJ, 65-75mph	80-90

Here are the results from iSpectrum. The first graph shows the spectrum graphs from idle. The second shows the same while driving on the highway at around 65-75.


Idle spectrum


Highway spectrum

Notice the jump of about 40dB from idle---when you can reasonably hear the stereo---to highway. Also note that the loudest frequency range is below 1kHz.

Right now all the door panels are out for some fixes. Restoring them should improve sound deadening / absorption somewhat, too.

Materials, Vendors

After much online research into the topic, several brands seem to be well worth considering: B-Quiet Ultimate or Damplifier Pro from Second Skin for sound deadening, B-Quiet V-Comp for sound barrier, Second Skin's Over Kill Pro for sound absorption, and B-Quiet Hiliner or Second Skin Heat Wave for thermo/acoustic application.

Another possible source for some of these treatments is RAAMat. There are several manufacturers out there, but my main concern has to be value and overall cost.

Treatment One

For the initial treatment, I've selected the new, butyl-based eDead V1SE2 from Elemental Designs at only $1.60/sq-ft. It's one of the cheapest butyl options I could find. (Butyl sticks in higher heat conditions than cheaper asphalt materials). I got it for less with an online coupon. The order arrived last week.

I got enough material to cover a part of the inside of the outer door skin as well as part of the rear quarter panel skins. Turns out the FSJ already has some asphalt mats on the door skins but that material is getting replaced with better quality material in greater quantity.

My new stereo is installed and I've applied limited treatment to the rear quarters, and a lot of treatment to the doors with a little more to do. So far I don't think it's made a huge difference. What did make a huge difference was switching out my worn Goodyear MT/R tires for a set of BFG Mud Terrains. The noise reduction was obvious the instant I drove down the road.

Look for sound measurements and a subjective report in the near future. Options for Treatment Two: additional coverage of the outer door skins, coverage of the inner door skins, or firewall / floor treatments.

I'll continue to update this page as I add sound treatments. Stay tuned! (Last updated 8/19/2008)