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?

zen

A moment of Zen, Fridays at the shop



got to love being at the church of the wholly dub

Mondays around the shop ,for the greater zen

Most people hate Mondays. that must suck
Our hours of 'open to the public' are tue through fri 6 am to 6 pm. Mon and Sat are by appt only.
This might seem odd, but it makes since from a operational perspective .Airkooled Kustoms is a restoration
shop. Our core cliental are nut and bolt restorations. That means every nut/bolt and component is eather restored to better than factory or replaced with new, and then detailed. We do do service work, build suspensions as well as build one off kustom trikes , but primarily it ocd restorations . And for that to happen
planning, timing, and part prep is critical.
  The four day work week is a good thing for us, on the personal side, 3 days off..... can't beat that, for the staff here it is a bonus day to take care of life in gen, doc appt, kids , yard work, long weekend at the beach,  less day in traffic, one less day gas cost.
 On the professional side the type of work we do lends  itself to a longer day. Before you get in an up roar
the staff is set to 10 hrs per day 4 days a week, and can set there own hrs, with in reason. I , the evil corporate owner, am the only one who puts in insane hrs. most days I am in the shop by 430 am and I try to get out by 5 p.m., at least for dinner. Plus I have 4 acres worth of yard to mow.
Sorry digressing a bit
 The longer day provides the craftsmen to lose himself in his work, For Zens  sake ,

 Mondays provide me with time to get caught up, on projects, paperwork, cleaning the shop, you would not believe just how dirty the shop gets, I know "Its a shop" but DAMNNNNNNN.
anyway
 back to pushing the broom