Acoustics engineering: Automotive exhaust harmonics

fastline

I thought I would try to discuss this here since there seems to be a large misunderstanding in automotive forums and I want to learn the reality, not someone's theory.

basically, I have a car here in which we removed one of the many mufflers. Piping is constructed as 2-1-2. It converges to 1 near the engine, runs through the resonator muffler, out of it about 2ft, to a Y, then 2ft on each side going to the tips.

the overall dbs from this exhaust are not bad but there is a droning noise at 2200rpm. I assume this is hitting the "tuned" resonance if the exhaust system much like a tuning fork and making other sheet metals active in the car. I am not real sure if you could really "eliminate" the resonant frequency but can probably "change" the frequency of resonation?

I have read where some people have cross drilled a hole normal to the exhaust and added a "T" pipe of certain length with a closed end. Some indicate this works like adding pipe length to the system and raising the frequency but I have to wondering if this is actually working on reverse sine cancellation principles or reverse phasing? Basically tuning the length of the this T to reflex a certain frequency of opposite sine or phase in the exhaust note to cancel a certain frequency?

Am I off base here? Would adding mass or weights really help by reducing the resonant frequency?

jack action

Adding mass to your exhaust system will change its resonance frequency for sure. Manufacturer have been doing it for years and they wouldn't pay extra to add weight on a car just for fun. Here are some examples:








As for your tuning pipe, it will probably affect the sound of your exhaust system, but the droning noise may come from the vibration caused by the moving parts of the engine. So the tuning pipe wouldn't help in such a case (except for the added mass it procures, of course!).

There is still the tedious job of finding out how much weight and where to put it.

fastline

I did not even think about a moment arm added with a mass on the end! That could really help.

basically there is already a resonator muffler still in the system that was very likely tuned to kill most of the major problems from the engine. I am highly suspecting the active vibrations of just the piping that was added. If I plug off one of the 2 outlets since there is a Y going from 1 into 2, it shuts right up!! Like 50% noise reduction. I am trying to wrap my head around that to see if that is really telling me it is the pipe and not the actual acoustics from the engine.

I did try setting some heavy Al blocks on top of the pipes in the back with zero change. The moment arm though might be something to consider though. Thanks so much for posting those pics. That gives ideas.

One test I did was to use PVC pipe connected to one of the outlets and created a branch resonator with a T and cap. Comparing the 50C exit temp and 100hz idle freq, I used a length of 36" for the branch which is 1/4 of the wave length. I got a whopping .5db drop in noise and the frequency dropped to 80hz. I was really hopeful in the branch system but not sure if I needed more exit length from the branch to make it work right.

fastline

I did some testing with PVC by adding to the end of the pipe and then a T where I can add variable lengths of piping. I am not real sure if this will prove out everything here but at my calculated 36" long which is 1/4 of the wave length, it did very little. When I pulled it down to 12" on both sides, it totally go rid of the resonance between 1800-2300 but did add a very slight resonance at 1500 which is pretty much outside the operating rpm most of the time. Certainly promising.

I am, however, very "baffled" as to why 12" seems to work but the ideal 36" did not...