Reliable

All right first I'm calling out Gernby (you may have already considered this yourself?)

I'm looking for opinions on this and help, maybe some of you are physics experts?


So I have a Berk 3" header back exhaust, I'm looking to eliminate the 110-116hz drone emitted at 3400rpm - cursing speed. This is the frequency of the engine 3400rpm / 60 seconds * 2 = 113.3Hz.

I have learnt that if you put a Helmholtz Resonator on a pipe you can cancel out desired frequencies as demonstrated in this video: Youtube

Also here is a video that explains why the Helmholtz Resonator vibrates at a particular frequency Youtube

So using the Formula below to calculate for 112Hz. (Why 112Hz this is the frequency I found in real life testing while in the car and outside the car using a spectrum analyser)



I get the following: (using V=374200mm as the speed of sound at 70c) dimensions for the resonator

112.265Hz = 374200 / 2 * 3.141592654 √ 13.202543130000002 / 2495.280651* 14.890000000000004

Plan


This will package nicely beside the existing rear muffler on the Berk, so will T in just before the rear muffler.

3D view


Profile

fernando.

This is genuinely intriguing.

I'll be following this discussion.

Gernby

I never experimented with helmholtz chambers, but I did read about them. They are effective, but hard to perfect. It's likely that the required dimensions will be somewhat different than the calculated values. One parameter in particular that is hard to determine is the right value to use for speed of sound, since the temperature of the exhaust gas is EXTREMELY variable AND decreases rapidly from the front of the pipe to the rear. That's why the exhaust drone will be slightly different on some days than others. It's also hard to predict what the temperature of the exhaust gas will be inside the chamber, since that will be determined by many factors.

IF I was going to try what you are talking about, I would move forward as if I knew it was going to require multiple tries to get it right. I would also use the cheapest materials possible, so that I didn't burn up money on material that was destined for the garbage. Once I got it right, then I would build it again using stainless steel.

Reliable

Do you believe I am missing a parameter in the formula or is there some other inaccuracy? or is this in relation to the temperature humidity comment below?

This is my main concern too. There are factors like temperature, humidity and the mass of the exhaust gas which is different than air due to the fuel in it, all these factors will effect the speed of sound.

I used a infrared thermometer to take some measurements immediately after 10 minutes of really hard driving:

External temperature of the exhaust pipe where the resonator would T in: 60c
External temperature of rear muffler which the Helmholtz would be attached to: 55c
Temperature inside rear muffler: 90c


Taking a guesstimate for likely temperature inside the Helmholtz chamber I chose 70c, maybe it will be wrong or write, what do you guys think?


Running the calculations on temperatures ranging from 50c to 90c the resonant frequency's range from approximately 108Hz to 117Hz when applied to the dimensions of the proposed Helmholtz resonator. If I was to assume this was my only variable I would just have to vary my cruising speed between approximately 3300rpm and 3500rpm. (New Zealand has a speed limit of 100kph so that works out ok.)

ryuken

You probably have thought of this already but looking at the last picture are you going to leave a slight gap between the resonator and muffler so that they don't rub against each other due to thermal expansion?

Reliable

Good point I had not thought about this.

I was thinking of tacking them together as a means of supporting the Helmholtz resonator, if they were both made out of Stainless steel to you think the expansion would be an issue?

ryuken

There probably won't be an issue. If there is a difference in temperatures between the resonator and muffler, there may be some stress in the welds of the brown pipe, but I don't think it's bad enough to cause failure. Can someone else chime in?

Gernby

I believe you are missing at least 10 parameters in the formula, but probably 30 or more. If you google read about accoustic theory for resonators and chambers, where they talk about the theoretical properties compared to actual observations, they are always somewhat different.

Here is an example.
http://www.ricardo.com/Documents/Dow...n_mufflers.pdf

Gernby

tacking the resonator to the muffler would likely result in cracks. Most of my "dingle" prototypes had a loop of pipe on the driver side, which I tacked to the muffler, and they always broke. It's due to the variation in temperature across the loop, which caused the metal to expand at different rates. This put a lot of stress on the "loop", so it would crack at the weakest point.

GOGPWS2K

I actually have some experience with these resonators on the s2000. First you need to figure out the rpm range where drone occurs. Then figure out the length of these pipes needed for that drone range to be eliminated.

However, the problem is that there are so many variables that come into play. Thickness of the Pipes, size of the pipes, and length.