Helmholtz Resonators?

[SloMo228]

Is there anyone running with a Helmholtz resonator on their exhaust? We had a tough time passing noise in the '06 competition, and now we've made the decision to haul off to Italy in September, so I'm looking for fixes to put on the '06 car to get it through. I just don't have time to design an exhaust for this car with all the other work and testing that's going on. Anyway, I've been looking at Helmholtz resonators because they can be tuned to provide significant destructive interference at specific frequencies. I'm thinking that if I build a Helmholtz tuned for 367Hz (Our engine's fundamental frequency at 11,000 RPM), I can take a nice-sized chunk out of our dB rating at the noise test. Am I totally wrong in my thinking here? I know there are other frequencies going on in the exhaust note, but wouldn't the fundamental frequency be the best one to go after?

[Jersey Tom]

I would have thought the fundamental would be an octave lower than that..

[SloMo228]

It could be. I'm not entirely sure of the correct way to calculate this. What I did was this:

We need to test at 11,000 RPM. This equates to 11,000/60 = 183.333 revs/sec. From there, I divide by 2 to find the number of exhaust pulses per second per cylinder, then multiply by 4, since we run a 600RR and have 4 cylinders. Is this incorrect?

[Jersey Tom]

11000rpm... 183.3 rev/sec. Each revolution of the crank there are two exhaust events.. so yea you're right. 367Hz.

With regard to the resonator.. cool idea, trying to tune a anti-resonant peak right at that rpm. Don't know how you'd do it though.

[Maverik]

It could work, but if you already have an exhuast system designed for your powerband (I am assuming it is tuned) why go through the extra trouble of tuning a resonator that will more than likely help with sound at that rpm exactly, but compromise performance elsewhere? Would it have a noticeable effect if you weren't exactly on the right frequency when you are being tested? There's got to be a suitable muffler that can lower sound decibles to where you want, look at TA&M I think they had a car muffler on their car this year at west!

[Jersey Tom]

There were definately some teams with reasonably sized mufflers at East such that you could barely hear the car go by..

There were also ones that I'm amazed passed sound

[Ben Inkster]

FYI, a normal muffler with a perforated tube surrounded by packing is effectively a continuous arrangement of helmholtz absorbers. The ratio of perforation diameter to perforation spacing and perforated tube Diameter to muffler O.D is how you tune the Helmholtz frequency (along with the absorption coefficient of your packing material alpha).

However I do think that a single absorber in the exhaust can be used to reduce on frequency more specifically, this is what car manufactureres do on the intake to reduce induction noise for NVH reasons. But i would start by simply trying different muffler packing (increasing alpha).

Cheers
-Ben

[SloMo228]

The reason I'm looking into this (among other things) is the difficulty we had passing noise in the '06 competition. We tried several different mufflers, from the Akrapovic we started with, to last year's Yoshimura, to a car muffler, to a stock CBR600RR muffler. None of them worked. We repacked the Yoshi a few times, also to no avail. I believe the problem lay in our engine, as we replaced the stock pistons with new high-compression pieces. We dropped a valve partway through the competition and swapped engines to a stock 600RR and were able to pass sound easily after the swap. The simple solution would be to just keep the stock engine, but I'd like to use the high-compression pistons and the little extra power they provide.

By the way, thanks for the responses so far, they've been pretty helpful.

[Garlic]

It's very unlikely that the pistons contributed to a much higher noise level. I would say that whatever caused your failure was also the culprit for the high noise. A poor running engine can easily be much louder than a smooth running one, regardless of the compression.

As for resonators, just remember that you can also be noise tested at any RPM below the test level, and must pass. I'm sure that they'll be lenient, but if you are WAY louder 1000 RPM below the test level, people will notice, and you'll be retested.

[VFR750R]

For passing sound don't forget that several decibels can come from from the intake, so if you have an intake inlet out the back like I see some teams do, moving to one side of the car could help. Also logical placement of the outlet where the sound can leave and not bounce off the ground or a wheel back to the sound meter can help noticeably.

There is also condsiderable amount of noise that comes from the valvetrain, so you might want to consider things that could block the mechanical sounds of the engine from the meter, such as extra bodywork, shielding, ect. although this is extreme, and probably won't be much more then 1-2 dbs.

[Ben Inkster]

Posted June 22, 2006 05:19 PM
For passing sound don't forget that several decibels can come from from the intake, so if you have an intake inlet out the back like I see some teams do, moving to one side of the car could help. Also logical placement of the outlet where the sound can leave and not bounce off the ground or a wheel back to the sound meter can help noticeably.

There is also condsiderable amount of noise that comes from the valvetrain, so you might want to consider things that could block the mechanical sounds of the engine from the meter, such as extra bodywork, shielding, ect. although this is extreme, and probably won't be much more then 1-2 dbs.


Good point VFR750R! When UWA changed from a rear spaceframe engine bay in 2003 to an enclosed full monocoque in 2004, the sound level dropped significantly!

Slomo, there could definetly be a quick fix option with something like this.

-Ben