14 votes
Accepted

Playing cymbals in vacuum

When you strike cymbals (or any other object) you give them a certain amount of energy. If the object is stiff and light (like cymbals) this energy will cause the object to vibrate. The vibrations ...
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  • 33.9k
13 votes

Sound waves adding up

Since the phases are random, the waves do not add coherently... but neither do they cancel coherently. Furthermore, loudness is really a measure of the intensity of the sound. Thus, if we consider a ...
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10 votes

Eyes shut, can a passenger tell if they’re facing the front or rear of the train?

I think you might be able to distinguish the direction of motion by turning sideways and listening for the apparent motion of the clickety-clack sounds and vibrations from the carriage wheels (...
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9 votes

Eyes shut, can a passenger tell if they’re facing the front or rear of the train?

I don't think this question as posed is really well defined. But here's an attempt at an answer. In the real world the answer is yes. Assuming the train is longer in the direction of motion (as is the ...
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  • 34.3k
9 votes
Accepted

Confusion with Doppler effect problem

Consider, by analogy, a small, fast boat sitting almost motionless in the open ocean. It's moving just fast enough to maintain steerage way, and keeps moving slowly with a following sea. The waves ...
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  • 10.2k
7 votes

Confusion with Doppler effect problem

insipidintegrator wrote: "But when the speed of the detector exceeds that of sound, the sound waves which are subsequently emitted by the source will never be able to reach the detector. " ...
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  • 7,757
7 votes

Eyes shut, can a passenger tell if they’re facing the front or rear of the train?

Yes, because the train predictably hits bumps/gaps in the track. As you travel on a train, you hear first one set of wheels then the other then the other going from the front of the train to the back ...
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  • 1,019
6 votes

Eyes shut, can a passenger tell if they’re facing the front or rear of the train?

The simple answer is no, you would not be able to know. In fact you would not even be able to know that you are traveling at a constant speed. To you, the train could be completely still (assuming ...
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  • 848
5 votes

Confusion with Doppler effect problem

There are four distinct phases for the situation in question: Subsonic movement As the detector moves ever faster away from the source, the frequency of the waves drops because the waves move slower ...
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4 votes

Eyes shut, can a passenger tell if they’re facing the front or rear of the train?

I never seen a train overtaken by another train in a parallel adjunct track lane. Usually there are trains bypassing opposite direction to each other from parallel track lanes. I think it would be ...
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  • 3,186
4 votes
Accepted

How to derive the energy density of a sound wave?

Notes: I believe that the answer you are trying to obtain is incorrect. First, if you are looking at total energy density, you should not have the factor of 1/2. Alternatively, you could be looking ...
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  • 1,392
4 votes
Accepted

Is there an electromagnetic analogue of thermoacoustics?

Yes, you are describing a waveguide which is a pipe that is used to conduct microwaves. It is possible to build standing electromagnetic waves inside such a pipe, giving rise to the klystron and ...
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3 votes

Why can thin sponge lining soundproof earmuffs but not walls?

It's because the effectiveness of different noise-abatement treatments depends on the frequencies that the noise source contains, and whether the objective of the treatment is to absorb reflected ...
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3 votes
Accepted

Resonant cavity for sound waves

The equation for the resonant frequencies are correct. To get to that equation you indeed need to impose the boundary conditions such that at the boundaries the wave vanishes, which is at $x=y=z=0$, $...
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  • 869
3 votes

Eyes shut, can a passenger tell if they’re facing the front or rear of the train?

Bumps and gaps are asymmetric. They make the cart jump up AND BACK and then, at slower rate, return to its equilibrium speed and direction. So you will detect rapid accelerations back and slower ...
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  • 6,093
2 votes

Why solid makes sound upon hitting?

Here is a simplified answer. Let's say you strike a piece of wood with a hammer. The hammer possesses kinetic energy and momentum and some or all of these will be transferred to the wood, a process ...
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2 votes
Accepted

How can we derive an equation for double slit interference without the approximations made?

For near-field effects, it may be better to think about the full field rather than just path differences. Based on you wording of "point sources" and the fact that you are using speakers, I ...
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  • 1,392
2 votes

Is upper-atmospheric lightning loud?

To hear means, someone needs an ear sensitive enough. As you pointed out, density is the problem. Single atoms or molecules might carry a lot of energy, but there are so few, that your ear might not ...
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2 votes
Accepted

Why the amplitude of monopole solution in Helmholtz equation is complex?

If there is a single monopole it does not matter whether $A$ is real or not but if you have two or more sources then their relative phases, and thus the phase of $A$, do matter. The same holds if the ...
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  • 9,700
2 votes

Bottle Marimba calculations?

The bottles are essentially balloons under tension. Kuo, Hunt and Lister calculated the frequency of a spherical shell to be the following "simple" formula: $$ \omega^2_n = \frac{A\left(1+\...
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2 votes

Sound waves adding up

You can compare this to taking the mean of a random sample of mean zero random variables. Some observations will be positive, and some will be negative, and so there will be a lot of canceling. ...
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1 vote

Maximum oscilations of air molecules

What you are most likely referring to is an acoustic standing wave, and asking whether the air molecule velocity is highest at pressure nodes. The short answer to this is, yes, the air molecule ...
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  • 2,832
1 vote

Clapping-like sound from large planar object hitting a surface

The basic idea is simply that you need to expel air of volume $\pi h R^2$ through the periphery of area $2 \pi h R$, implying that for a given $\dot{h}$ there's an outflow proportional to $R$ and a ...
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1 vote

Bottle Marimba calculations?

Most important: First, I would suggest, WEAR SAFETY GLASSES. If a small piece of plastic embeds itself anywhere in your body, the damage will almost always be reparable, unless it's in your eye. It ...
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  • 2,832
1 vote

Help me find a material with low acoustic impedance but high wave velocity

It looks like you want to maximize the ratio between wave velocity and acoustic impedance. Let say we call it "p". But this is just the inverse of density. The stiffness is irrelevant. ...
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  • 6,568
1 vote

Help me find a material with low acoustic impedance but high wave velocity

Low acoustic impedance comes from a low product of density and stiffness. High wave velocity comes from a high ratio of stiffness to density. So, your requirements favor low density but conflict on ...
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  • 4,103
1 vote

Does a tower bell ringing prevent thunderstorms?

The energy scale of thunderstorm is much larger than that of a bell (most other devices built by humans), so it is unlikely that such devices can affect thunderstorms in a controllable way. A single ...
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1 vote

Why are traditional telephone lines limited to about 3-4 kHz bandwidth?

Telephone company operators used to physically patch one wire to another to make a dedicated line from end user to end user. Certainly parasitic capacitance had a limiting effect. But today, ...
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