The theory behind sound generation from spinning objects

[Juan2g19]

Homework Statement

I am doing a physics research project for school and am struggling to find the relevant theory for my case.

Relevant Equations

NA

I am doing a research project investigating the relation between the rpm of an object and the frequency of the sound it generates. The set up is as follows; I am spinning a fidget spinner with a water pressure kit then recording its rpm and the frequency of the sound it creates from its fast spinning. I have collected results and everything's going well except for the fact that I can't find the theory behind this to explain why sound is generated. I have found many research papers on sound generated aerodynamically with plane engines, fans and wind turbines but they all talk about the effects of turbulence on the generated sound. This does not apply to me as I conducted the experiment when there was hardly any wind and the spider was clamped down without movement.

Does anyone have any knowledge in this area or any links that could be helpful? I'd really appreciate anything, thank you.

 

[haruspex]

they all talk about the effects of turbulence on the generated sound. This does not apply to me as I conducted the experiment when there was hardly any wind

Turbulence will arise from the relative motion of the surface of the spinner and the surrounding air.

 

[jedishrfu]

but if its cutting through the air even locally turbulence can be created.

From this article they describe the spinners as small gyroscopes:

https://interestingengineering.com/innovation/the-physics-of-fidget-spinners

so perhaps if you focus on a spinning gyroscope and how the spokes in the wheel cut through the air.

Here's more on gyroscopic sound effects:

https://gyroplacecl.com/gyroscope-sound-shattering-sound-unveiling-the-phenomenon/

 

[Juan2g19]

Ok thanks a lot for replying so quickly. I'll keep looking at what I found earlier that involves sound generation due to turbulence and try and find examples using gyroscopes then.

 

[nasu]

What do you mean here by "spider"? It's related to your water system?

 

[Juan2g19]

Sorry, I meant to say spinner. I am still struggling to find equations / relationships that could help me link rpm with generated noise frequency but now that I think about it there probably aren't many as the generated noise frequency depends on so many other factors than just the objects rpm right?

 

[haruspex]

Sorry, I meant to say spinner. I am still struggling to find equations / relationships that could help me link rpm with generated noise frequency but now that I think about it there probably aren't many as the generated noise frequency depends on so many other factors than just the objects rpm right?

Whatever the source of noise, its frequency is likely to increase in proportion to the rotation rate. E.g. if there are N vertical struts around it and it rotates at r rotations per unit of time then I would expect a frequency of Nr.
An exception might be noise coming from ground contact.

 

[berkeman]

The set up is as follows; I am spinning a fidget spinner with a water pressure kit then recording its rpm and the frequency of the sound it creates from its fast spinning. I have collected results and everything's going well except for the fact that I can't find the theory behind this to explain why sound is generated.

How are you using a "water pressure kit" to spin the fidget spinner? That sounds like a noisy way to spin the fidget to me. If you want to investigate the sound generated by the spinning fidget (or anything else), you should minimize the noise from other sources. Use a quiet DC electric motor, for example.

As to how the sound is created, just think about the sound created by waving a manila folder once past a microphone. The microphone picks up a single "woosh" sound. Wave the folder at 2 times per second past the microphone, and you get that many "woosh" sounds per second. You can analyze the "woosh" sounds (using an FFT or whatever) if you want more information about the air turbulence, but fundamentally the sound is the driving function plus harmonics and turbulence.

https://www.amazon.com/dp/B071Z872X1/?tag=pfamazon01-20