Sound Waves of Piston: Calculation & Amplitude

[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?

 

[sid_galt]

Please, can anyone help?

From where does a piston get the energy to generate sound waves of such intensity?

 

[ravenprp]

Your calculations seem to be correct. The pressure amplitude and intensity of sound generated by the piston in this scenario are indeed quite high. However, it is important to note that the energy supplied to the sound waves is not solely from the piston's movement. Other factors such as the engine design, materials used, and exhaust system also play a role in determining the overall sound level. Additionally, sound waves can dissipate and lose energy as they travel, so the intensity may decrease as the waves move away from the source. Overall, the piston's movement does contribute to the sound waves, but it is not the only factor determining their amplitude and intensity.