Sound Waves of Piston: Calculation & Amplitude

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SUMMARY

The discussion centers on the calculation of sound waves generated by a piston in an engine with an 86mm stroke operating at 8000 RPM. The frequency of the sound waves is determined to be 133.333 Hz, leading to a wavelength of 2.57 meters based on the speed of sound at 343 m/s. The pressure amplitude is calculated to be 30425 Pa, resulting in a sound intensity of 1097104 W/m², equivalent to 180 dB. The user seeks clarification on the energy source for such high-intensity sound waves produced by the piston.

PREREQUISITES
  • Understanding of basic physics principles, particularly sound wave propagation.
  • Familiarity with piston mechanics and engine operation.
  • Knowledge of pressure amplitude calculations in acoustics.
  • Basic grasp of decibel levels and sound intensity metrics.
NEXT STEPS
  • Research the relationship between piston stroke length and sound wave frequency.
  • Learn about the bulk modulus and its role in sound wave calculations.
  • Explore the physics of sound intensity and its measurement in decibels.
  • Investigate energy transfer mechanisms in piston-driven engines.
USEFUL FOR

Engineers, physicists, and acoustics specialists interested in the dynamics of sound generation in mechanical systems, particularly those involving pistons and engines.

sid_galt
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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?
 
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Please, can anyone help?

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

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