# The theory behind sound generation from spinning objects

• Juan2g19
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.

Juan2g19 said:
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.

nasu
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.

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

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?

Juan2g19 said:
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.

nasu
Juan2g19 said:
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

## What causes sound generation in spinning objects?

Sound generation in spinning objects is primarily caused by the interaction of the object with the surrounding air. As the object spins, it creates pressure fluctuations and vortices in the air, leading to the emission of sound waves. Factors such as the object's shape, speed, and surface texture can significantly influence the characteristics of the generated sound.

## How does the speed of rotation affect the sound produced?

The speed of rotation has a direct impact on the frequency and amplitude of the sound produced. Generally, as the rotational speed increases, the frequency of the sound waves also increases, leading to a higher-pitched sound. Additionally, higher speeds can result in greater turbulence and more intense pressure fluctuations, which can increase the loudness of the sound.

## What role does the shape of the spinning object play in sound generation?

The shape of the spinning object is crucial in determining the nature of the sound generated. Aerodynamic shapes tend to produce less turbulent airflow and, consequently, less noise. In contrast, irregular or non-aerodynamic shapes can create more turbulence and complex airflow patterns, resulting in louder and more varied sounds. The distribution of mass and surface features like ridges or grooves can also affect the sound characteristics.

## Can the material of the spinning object influence the sound it generates?

Yes, the material of the spinning object can influence the sound it generates. Different materials have varying surface textures, densities, and elastic properties, which can affect how air flows around the object and how vibrations are transmitted. For instance, a rough surface can create more turbulence and noise, while a smooth surface may produce less sound. Additionally, materials that are more elastic can absorb and dampen vibrations, potentially reducing the sound output.

## Are there practical applications for understanding sound generation from spinning objects?

Understanding sound generation from spinning objects has several practical applications. In engineering, it can help in designing quieter machinery, such as fans, turbines, and propellers, by optimizing shapes and materials to reduce noise. In the field of acoustics, it aids in predicting and mitigating unwanted noise in various environments. Additionally, in the study of aerodynamics, it provides insights into the behavior of airflow around rotating bodies, which can be applied to improve the performance and noise characteristics of aircraft and automotive components.

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