814 votes

Can I compute the mass of a coin based on the sound of its fall?

So, I decided to try it out. I used Audacity to record ~5 seconds of sound that resulted when I dropped a penny, nickel, dime, and quarter onto my table, each 10 times. I then computed the power ...
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322 votes

Can I compute the mass of a coin based on the sound of its fall?

If you have the dimensions and material of an object, you can compute both the mass and the normal vibration modes. Just the mass is not enough - a large paper "coin" will have a different fundamental ...
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  • 116k
93 votes

Intuitively, what actually is the cause of resonance?

For intuition I find it easier to start with a regular pendulum. Imagine a steel ball on a string hanging down. If you give it a push, it will start to swing back and forth. Now if you, while the ...
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  • 9,877
68 votes

Can I compute the mass of a coin based on the sound of its fall?

This is not an advertisement. Under the rubric of "do try this at home", I wanted to share one more thing that I discovered after writing my previous answer - but it is so unrelated to that ...
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  • 116k
49 votes
Accepted

Why do string instruments need hollow bodies?

Yes, the room has a lot of air, but most of it isn't in direct contact with the vibrating string. In order for the string to make much sound, it needs to transfer some of its energy to the surrounding ...
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  • 9,675
42 votes

If sound passes through material, vibration is produced. So are electromagnetic waves produced too?

A sound wave passing through a medium (e.g. air) indeed displaces molecules by a distance of a few nanometers. It seems reasonable that it should also displace the atoms, and thus electrons and ...
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35 votes
Accepted

Can the vibrations from the Earth affect gravitational wave detectors?

Summary Yes they can. False positives arising from the acoustic sources you name are ruled out by seismological analysis and the examination of correlation between the separate gravitational wave ...
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31 votes
Accepted

In counting degrees of freedom of a linear molecule, why is rotation about the axis not counted?

The energy levels of a diatomic molecule are $E = 2B, 6B, 12B$ and so on, where $B$ is: $$ B = \frac{\hbar^2}{2I} $$ Most of the mass of the molecule is in the nuclei, so when calculating the moment ...
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30 votes

Intuitively, what actually is the cause of resonance?

If you have ever learned how to swing on a swing set then you have directly applied resonant forcing. By moving your body in the right way in time with the swinging motion, you can add more energy to ...
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  • 3,330
28 votes
Accepted

Why does a large train cause the ground to shake?

There's a nice article on exactly this subject in this PDF file. Summarising from the article: the vibration arises because the track is not completely smooth and the train wheels are not perfectly ...
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24 votes

What causes this strange noise in a pair of walkie-talkies?

It's called feedback . Here is what happens: When Alice presses TRANSMIT, it turns on the microphone in her radio and hence begins to transmit any noise that hits the mic. With Bob's radio on RECEIVE, ...
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20 votes

If all harmonics are generated by plucking, how does a guitar string produce a pure frequency sound?

Usually a guitar does not produce a pure tone/frequency. If so, its sound would be very close to a diapason. The difference between noise and a musical tone is not that a tone is made by a unique ...
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18 votes

Can I compute the mass of a coin based on the sound of its fall?

I don't mean to take anything away from the previous great answers, but the "simple and to the point" answer is, a very qualified, yes. By qualified, I mean one must know the coin's composition, ...
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  • 2,295
18 votes

Non-resonant but efficient frequencies

This is a subtle issue! Your intuition is correct (a driving at $f_0/2$ should be very effective) even though the graph seems to contradict this. The reason is that the graph displays the response to ...
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17 votes

Why do harmonics occur when you pluck a string?

When you release the plucked string, its shape is momentarily triangular: tied down at the ends and pointed at the location of your finger. But the solutions to the wave equation are not triangle ...
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  • 70.2k
16 votes

Why does a metal block make a shrill sound but not a wooden block upon hammering?

Is it that the wooden block vibrates with lesser frequency than the metal block? If so, why is that? 'Yes', to the first question. Metal is stiffer than wood and produces higher frequencies (...
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14 votes

In counting degrees of freedom of a linear molecule, why is rotation about the axis not counted?

Just an addition to John Rennie's answer. The equipartition theorem can only be derived in classical statistical physics. In quantum statistics it is not correct. For each degree of freedom there is a ...
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  • 23.6k
14 votes
Accepted

Does plucking a guitar string create a standing wave?

Yes, plucking a guitar string does create standing waves, but... No, plucking a guitar string does not create a standing wave, as the sum of standing waves is in general not a standing wave (thanks ...
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  • 11.6k
14 votes
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Why does a metal block make a shrill sound but not a wooden block upon hammering?

The metal block has a relatively low level of internal damping, however the wooden block has a high level of internal damping: Much of the energy imparted to the wooden block is dissipated internally ...
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  • 2,632
14 votes
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If all harmonics are generated by plucking, how does a guitar string produce a pure frequency sound?

Human perception is involved here because when you humans talk about noise this generally means a sound that is aperiodic. However the tone produced by a guitar will be something like: $$ A(t,x) = \...
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14 votes

Vibration in a guitar when playing two strings

The first image shows a string oscillating at its fundamental frequency $f$. The second image shows a string oscillating at $2f$. The third image shows a string (or the wooden soundbox of a guitar ...
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14 votes
Accepted

If sound passes through material, vibration is produced. So are electromagnetic waves produced too?

If it passes through a piezoelectric material, it may generate a measurable voltage and current, some fraction of which will be radiated. For most materials, however, there won't be a detectable ...
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14 votes

Why does glass, in spite of being amorphous, often break along very smooth surfaces?

What you describe is the cleavage type of fracture of polycrystalline material. The surfaces are smooth, but the micro anisotropy due to the several grains can be seen in a scanning electron ...
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11 votes

Why sound does not heat up the air?

Sound waves do generate changes in temperature because the propagation of sound is an approximately isentropic process. Keep in mind though that changes in static temperature can very well occur ...
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  • 3,625
11 votes
Accepted

Why don't metals damp vibrations?

Damping implies a loss mechanism. In liquids, where molecules move freely in close proximity, this loss mechanism is a transfer of momentum from one molecule to another. In pure crystalline metals, ...
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  • 116k
11 votes
Accepted

How does a guitar work?

The string oscillations are mainly transverse (a standing wave). The string motion causes the tension to oscillate thus applying a varying force on the guitar top through the bridge and saddle. The ...
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  • 11.9k
11 votes

Why do string instruments need hollow bodies?

The soundboard is the crucial component in acoustical amplification of string vibrations. The strings pass over the bridge, which is solidly attached to some part of the soundboard. A vibration at the ...
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11 votes
Accepted

Why does plucking a string produce harmonics instead of a pulse?

It does produce a pulse. However, the speed of sound in a string is very large so the pulse reaches the ends of the string very quickly. This means that in a second many reflections occur and you only ...
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10 votes

Why sound does not heat up the air?

Put more simply: sound waves are attenuated as they propagate through air (this is more easily measured for very short wavelengths, e.g. ultrasound). This means they lose energy - which is turned into ...
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