Miata Turbo Forum - Boost cars, acquire cats.

If the problem is a resonance at a given RPM, it's not specifically an issue with how "loud" the muffler is. This is not obvious because we feel the level of resonance is the issue, whereas the real problem is there is a resonance at all.

Anyone who has tried to install a subwoofer in a car of a home theatre will probably have dealt with the effects of room boundaries on low frequency sound. By changing the location of the sub, you can alter the resonance frequency. Similarly, by changing the shape of the room (which can be as simple as opening a door) you affect the resonance frequency.

In the case of the Miata, the area under the vehicle bounded by the axle, the fenders, the trunk bodywork, the rear fender, and the road comprise the "room". Open areas also comprise part of this "room".

There will be a number of natural reassurances in this area. This is vehicle-specific, both in terms of a Miata vs other cars, and in terms of your car vs other Miatas. If you've changed something, like perhaps a different rear bodywork, your situation will be more unique than in a stock Miata that many people may drive. Simply installing a different muffler that is not exactly the same size and shape of the stock muffler, or is physically located differently, will also change this.

A resonance is "excited" by a source of sound. A sound that, in free air (no boundaries on any side) may be at a certain level will be different in the area under the car, and will be different again in a different car. If it's at the natural resonance frequency of that area, it will be increased in level, perhaps by quite a lot.

When you have a resonance that is much louder at a given RPM, this is what is going on. Change the RPM changes the sound created by the muffler, which moves it out of the resonance frequency of the "room", and the level goes back to "normal".

There are, in reality, a staggering number of natural reassurances in a specific installation. To change a natural resonance, you change some physical aspect of the installation.

Because the change may not be for the better (it could create a bigger problem just as much as solving one) there is no one solution for every case.

A muffler like the Borla is going to have a certain family construction, which is going to create a certain family sound (set of frequencies). So, simply changing to a different brand muffler certainly will do something.

If you want to keep the Borla, you have to attack the problem as one of changing the resonance frequencies themselves. Changing the mount points of the exhaust will do that to a certain extent. Making existing mounts more or less rigid will also result in changes. You could dampen large panels with a coating or acoustic barrier or vibration suppression.

Finally, changing the physical aspects of the "room" will result in changes. You can move the muffler to a different location, for example. You can re-route pipes. You can change the shape of the room itself by, for example, installing an under-trunk tire mount system, which takes up room and changes the boundaries. Broadly speaking, making the room smaller raises the resonance frequency.

Raising the frequency may not sound like a great idea, but two things happen. One, it will be likely lower in volume; Low Frequency resonances are "louder". Secondly, it's less likely to penetrate the cab of the vehicle, especially if it's large body panels that are the a source of mechanical transmission. Even though the Miata is a ragtop, there will still be an advantage here.

Or, you can change the physical size of the muffler. Even if there was zero change in the sound of the muffler in this case, you have affected the volume and the boundaries of the "room", by taking up more or less space in it.

Cars produce a tremendous amount of low frequency sound, much of it subsonic (below our threshold of hearing, but not necessarily below our threshold of feeling). It's low frequency sound that is most likely to cause resonance problems.

It's important to realize that drastic changes may not be necessary. Small changes of a few inches can and will make a difference. The question is more one of are the changes enough, or do I need to make many small changes so they add up to the desired change.

Mounting changes: the pipe itself may be a source of resonance. Adding a mount on a long, unsupported pipe will raise the resonance frequency of the pipe. If the resonance of one part has a counterpart resonance elsewhere, they will be increased out of proportion to the individual resonances alone. So, the pipe may be exciting a resonance elsewhere. Avoid mounts that are multiples or fractions of the length of the pipe between existing mounts ... if you don't, this will increase the natural resonance that exists for your new pipe length between mounts.

Eg if you have an unsupported length of 3 feet, do not put a mount at 1.5 feet between them (factor of 2). Moving it a small amount, so that it's 1' 5" and 1' 7" may easily be enough.

If the car body itself is a source of resonance then isolate the mount with rubber, etc. This will avoid mechanically transmitting sound to the body, which will be excited at the resonance frequency and make that frequency louder.

The Mazda engineers almost certainly spent a great deal of time tuning the resonance of the stock muffler system. Auto engineers in every company since the 1950's have paid attention to exactly where the body to exhaust mounts are, and what materials to use to hang them, specifically to deal with exhaust resonance issues. When we change that, we throw that effort out. Since we don't have the benefit of Mazda's data (which might tell us what the natural reassurances are) we have to experiment.

I said earlier that the volume is not the problem, it's the resonance. If the sound level of the muffler is low enough, there won't be an excitation of the resonance frequencies to an extent that is bothersome. This does not solve the problem, it simply makes it irrelevant. If you want a louder muffler, you have to deal with the exhaust resonance issues (even if only by ignoring it).

A performance muffler does not need to be a small, low-restriction muffler. I don't remember who did the testing, but [some guy] determined that if you raise the internal volume of a non-restrictive box placed appropriately (length of pipe affects torque curve) to 8x the volume of 1 cylinder, it's seen by the engine as open air eg equivalent to open exhaust. Apparently placing a muffler downstream of that box does not affect the performance, if sound reduction is still needed. This was research done about 10 years ago when racing sanctioning bodies were moving to requiring mufflers on off-road racing classes that previously had no restrictions. That box would resonate like hell, though.