Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: YetiWoodz on 27/01/2020 19:22:32
-
So I want to put a helmholtz resonator on my 2 stroke bike engine to remove a certain frequency. My question is, Does the diameter of the main pipe need to be the same as the diameter of the hemholtz resonator? Also does it matter what angle the resonator intersects with the main pipe?
-
Also, it says these resonators have a narrow range of frequencies that it cancels. How narrow is it? Will it be able to cancel 220Hz to 253Hz?
-
220Hz to 253Hz?
Are you able to control the speed of your 2-stroke bike so that the engine always operates between 6600rpm and 7590rpm?
-
220Hz to 253Hz?
Are you able to control the speed of your 2-stroke bike so that the engine always operates between 6600rpm and 7590rpm?
Yeah, I can control it. I just want to know what the range is that the h resonator would cancel.
-
220Hz to 253Hz?
The thing about a resonator is that it... resonates!
How well it resonates is governed by the "Q-Factor" (Q for Quality).
- With a high Q-Factor, it will build up a lot of energy in the resonance, but only over a very narrow frequency range
- With a low Q-Factor, it will resonate over a wide frequency range, but with very little energy at any given frequency
- The Q-Factor is governed by how much energy is lost from every cycle
So you could use a high-Q resonator at 220Hz
- Or, you could take the opposite approach, and use a colander-type muffler with very low Q, but which operates over a very wide frequency range (eg 100Hz to 500Hz)
If you want to use the resonator for cancelling noise, then you lose all of the stored energy on every cycle, so the Q-Factor is zero. Fortunately, the resonator is also recharged on every cycle, so it still works.
You can work out the effectiveness at other frequencies with a fixed-length pipe using a back-of-the-envelope calculation like:
- If the central resonance is at 236.5Hz (halfway between 220Hx & 253Hz)
- The the wavelength at 220Hz will be 7.5% longer
- The degree of cancellation at 220Hz will be sin(27°) = 45%
- When you turn this into the logarithmic scale used by human hearing, it is only 3dB, which is audible in an A:B comparison, but not enough to stop someone complaining to the sound police if you start it up at 7am on Sunday morning...
So why not use a colander-type muffler, which works at all frequencies?
See: https://en.wikipedia.org/wiki/Q_factor
-
Ok, thanks a lot. Can you send me a link for a colander type muffler? I can't find one. Unless you mean a hemholtz resonator shaped like a colander?
-
Also, does a hemholtz resonator really act as an expansion chamber and defeat the need for a reed valve? I saw that info on a website. Can I put both a hemholtz resonator and expansion chamber and reed valve on without any of them negatively affecting each other?
-
Have a look inside a muffler - you can see the pipe with holes stamped in it:
Or if you have more time:
If you are into reading, you could try:
https://www.acoustics.asn.au/conference_proceedings/AAS2005/papers/34.pdf