# Sound waves of piston

1. Oct 3, 2005

### sid_galt

I posted this in general physics forum but in retrospect I think I should have posted it here.

Since piston compresses and decompresses a gas, it must be generating sound waves.

Lets take a small engine with piston stroke 86mm, RPM 8000. An 8000 RPM means it is making 133.333 revolutions per second. Thus the frequency of the pressure waves or sound waves being generated by the compression and decompression is 133.333 Hz. Taking the speed of sound to be 343 m/s, the wavelength is 2.57 m.
Assume that the chambers length is so long that the compression ratio turns out to be less than 1 and has negligible effect on the speed of sound.

pressure ampitude = bulk modulus * wave number * displacement amplitude = speed of sound^2 * density * wave number * displacement amplitude =
343^2 * 1.23 * 2*pi/2.57*0.086 = 30425 Pa.

Intensity of sound = pressure amplitude^2/(2*speed of sound * density) = 1097104 W/m2 = 180 dB.

Isn't this a rather large amount of energy supplied to the sound waves? Or am I going wrong somewhere with my calculations?

2. Oct 3, 2005