highs and lows

this was written by a guy I know talking about sound and vibrations in cars

Hey guys, I saw this thread and thought I'd chime in. I'm an acoustician and av designer by profession and it seems there is alot of misunderstanding about how sound energy works in this thread. So, I thought I'd shed some light for you all... Praises to the OP for actually using an SPL meter, but I think the fundamentals of acoustics are not being acknowledged. There are only four ways to reduce noise level (i.e. make things quieter) for direct sound in a car cabin, and the term "direct" here is key: 1. Make the source quieter (not many ways to make your engine quieter here aside from a milder muffler). If it's wind noise you're concerned with, then replace all your weatherstripping the right way so that it's water-tight everywhere. From here on out, I'll assume you're talking about engine noise though. 2. Move the source further away from the listener (not practical or advised) 3. Block the Sound from entering the cabin with massive material (i.e. thick metal, not absorptive padding). 4. Reduce any noises caused by engine vibrations by reducing vibration. (ok, 5 would be to wear noise cancelling headphones). Notice how I did not mention absorptive padding in this list? This is due to a couple of reasons. Sound waves are made up of different frequencies: high frequencies have short wavelengths that are easy to absorb and low frequencies have long wavelengths that are difficult to absorb. By "easy to absorb," I mean that absorptive padding on the order of 1-6" thick will reduce the level of sound energy (probably above 1-2kHz) from being transferred through the material. By "hard to absorb," I mean that absorptive padding needs to be on the order of 1-10 FEET thick to reduce the sound energy. Because absorptive padding is very inefficient at absorbing low frequencies and low wavelengths diffract around just about anything it comes into contact with, it is not worthwhile or even possible to try to absorb low frequencies with absorptive padding that can fit within the cabin. So, when you guys add all of this sound deadener and padding to your car, you're effectively only reducing high-frequency energy. This is all well and good, but lower-frequencies are typically the ones that are the most annoying when you drive. As it is, your car is naturally absorbing the higher frequencies from the engine with the stock firewall, rear set, headliner, etc. If I had to wager a guess, those of you who have added lots of padding have reduced the high frequency noise in your car (i.e. it will sound dead inside with the car off), but you're still getting alot of muffled rumbling from the engine when it runs. This has to do with the ineffeciency of absorptive padding and also what is termed "direct" sound. Direct sound is sound that travels from the sound source to your ears without bouncing off of anything. Direct sound in the case of a car cabin can be considered sound that is reaching your ear once it's in the cabin without bouncing off of seats, glass, headliner etc. If you have a ton of padding and you're still annoyed by loud engine noise, odds are you have a lot of direct noise penetrating the engine bay into the cabin and the absorptive padding in the cabin is not absorbing it very well. I.e. if you're hearing a rumbling make it to your ears in the cabin, the padding within the cabin will only absorb reflections of that sound, not the direct sound. The direct sound is coming through your fire-wall, which is a more crucial point of sound penetration. If your firewall was sufficiently massive to block engine noise, you wouldn't really need much absoprtive padding on the interior of the cabin. So! Solutions to get rid of low-freq engine noise are few, costly, evasive, and perhaps expensive. I would recommend increasing the mass of the wall between the engine bay and the cabin. For instance, you could buy another firewall and weld it onto the existing one, but even this won't help much until you're about 4 layers thick. Also make air-tight any passages from the engine bay into the car. This includes heater ducts as they penetrate into the cabin. One thing that is frequently overlooked are the vibrations that cause noise. These are often significant and can be reduced by making sure your engine hangers/vibration isolator mount rubber is fresh, free-floating, and not short-circuiting. This will go a long way to transferring noise-inducing vibration to the frame of the car. Sorry to be the bearer of bad news, but keep in mind you have a series of explosions happening many times a second within 8 feet of you. Keeping that quiet is no easy task... So, if I had to recommend a quieter engine, I'd recommend taking the money you'd waste on sound "deadener" and buy a newer BMW, they are quiet. Scientific enough for you?