In the "old days", it was pretty obvious ... the mufflers were stuffed with stuff ... whatever.
Hopefully one of our sponsors who makes these systems will jump in but I think it is exactly that, depending on gas flow speeds more or less sound makes it through the pipe. Sound is echoing down the pipe meaning bouncing off the walls of the pipes so if those walls have holes some portion of those sound waves will go through the holes and be absorbed into whatever materials are beyond.
Chow,
There are many ways to make a muffler. Whether or not the pathway is straight through or routed through baffled chambers, the amount of perforation on the tubing that the gasses pass through, whether or not acoustical packing material is used, and the presence of a balance pipe are all variations you may find, among others.
I'll try to break down some basic principles as simply as possible in regards to performance mufflers:
In the case of a perforated tube, the amount of holes, size of the holes, and pattern of the perforation will all play a role in how the sound volume and tone are affected. The holes are escape routes for the gasses and accompanying sound waves. The cavity into which they escape can have packing material or not inside of it. Either way, this escape route will reduce the output sound volume of the muffler. Typically, if the cavity into which the gasses escape has packing like steel wool or other material, a softer and quieter sound will be made by the muffler.
Even the Glass Packs you refer to work this way. A perforated tube with a fiberglass like material wrapping this tube to capture sound waves and muffler the
exhaust
noise. The use of fiberglass to do this was new at the time, hence the name Glass Packs.
The engine makes a louder sound volume the higher you rev it and the harder you use it (load), and you will notice that sound increases even if there is a muffler in place. So the ability for the perforation to provide muffling is also dependent upon the operating condition of the engine. While there are some fluid dynamic effects going on depending on how much
exhaust
flow is going through the muffler, basically, the harder you use your engine, the more noise it makes, regardless of how the muffler is designed.
The fine tuning of the actual sound volume produced and how it sounds to the ear is dependent on the factors like I mentioned above: perforation design, whether or not packing material is used and what type is used, the presence of a balance pipe, etc.
I'd be happy to discuss our design further, but bear in mind that some of our choices are the secret to our success, so I can't reveal everything!
Thank YOU!
good explanation!
Mufflers work on a couple of different principles. The first is the ideal gas law (Van der Waals is more accurate for air) that for this discussion is P = 1/V and all others factors are constants. This is important because the "muffler" itself is larger in volume than the intake /
exhaust
pipe. This creates a differential drop of pressure once it hits the muffler, which allows diffusion of the escaping
exhaust
gases.
The holes / perforations (or baffles depending on muffler) are simply there to create resonance (thus a "resonation chamber" which mufflers are commonly referred to as..) in the sound waves to nullify the sound via interference. This is how your anti-noise headphones work, by knocking out sound waves by introducing sound waves that are out of phase...cancelling out the sound. Once the sound waves hit the muffler (resonance chamber) it expands, bounces off of some medium, then reflects back into the stream...some of those waves are cancelled, some escape unhindered, etc. You know what the end product sounds like.
Glass paks and other materials are used to adjust the absorption / reflection factors. You can use anything in a muffler, frankly. Bananas, cotton pants, or whatever, each would have different properties of sound reflection and make the muffler louder / softer / resonate, or who knows what...
Side note: There's a neat technology on industrial engines that use a direct reflection silencer (muffler) to enhance engine performance. It's essentially a muffler without the perforations and the size / dimensions are "precisely" tuned for the engine. When the
exhaust
gas comes out it 'reflects' off the far end and rebounds directly back at the cylinder chamber in a timed and precise manner, causing a back pressure that "reloads" the cylinder from the
exhaust
side creating much more fuel efficiency / power. This obviously only works on industrials that run at a set RPM, which they all do...
Paul
A decent video on mufflers is here:
http://auto.howstuffworks.com/muffler.htm
Not that I am a muffler expert, but I don't believe the content of that video is accurate.
Think of a handgun silencer. The explosion happens 6 inches up stream, it is NOT letting the explosion out slowly like in his balloon example. It is "active noise reduction" of the silencer (muffler) that reduces the sound, not diffusion of the explosion by letting the
exhaust
gas escape slowly...
Just my view, of course...
Paul
