# Tag Info

7

Yes, the sound can be reversed. Thanks to JiK, we have this animation (Python source code) of a supersonic jet moving forwards that can illuminate what is going on: The red circle represents the first sound produced by the object, the blue circle the second sound produced by the object and the remaining (black) circles representing the sounds produced ...

1

As you stated, the observer would not hear anything as the plane approached him. If fact, he still would not hear anything until the plane had past by since it takes some time for the sound to travel from the plane to the observer. In answer to your question; no, you would not hear the sound backwards since you still hear the sound in the same sequence as it ...

2

An equation of state is a relation of state variables: $$p=p(\rho,\,T,\,\mu,\,\alpha)$$ where $\mu$ is chemical composition and $\alpha$ the acentricity (itself dependent on $\mu$), the other variables take their normal meaning. There are some equations of state where the dependencies on these variables is non-linear (e.g., the Peng-Robinson eos), so ...

8

The maximum speed of sound is the speed of light - the maximum speed at which "information" can be propagated. This will occur for an equation of state that satisfies $P = \rho c^2$, where $P$ is the pressure and $\rho$ the density. Such an incompressible equation of state may be approached in the cores of neutron stars due to the strong nuclear force ...

2

Sound travels fastest in less compressible materials. But it is also affected by the state of the material, specifically its temperature. As a mechanical wave, sound must overcome the inertia of the material in which it travels. Higher temperatures mean greater kinetic energy in the molecules which carry the compression wave, and therefore less inertia ...

7

The speed of sound is a function of the compressibility of materials and their density: $$c=\sqrt{\frac{E}{\rho}}$$ Where $E$ is the bulk modulus (sometimes written as $K$) and $\rho$ the density. Compressibility itself depends on the material; for instance diamond, with relatively low density (3.52 g/cm3) and very stiff covalent bonds, has a high speed of ...

0

To keep things simple, let's talk about plane acoustic waves in one dimension. If we solve the wave equation in one dimension , we find that the acoustic pressure as a function of space and time is of the form $$P(x,t) = Ae^{i(kx -\omega t)}$$ where $A$ is the maximum amplitude, $x$ and $t$ are the displacement and time respectively, $\omega$ is the ...

0

You may consider reading about Aharonov-Bohm effect. This is one of those cases, where the phase of the wave function, in sum with the electromagnetic 4-potential, is extremely important, as it gives different physical results. This effect was also checked experimentally, so it is not a pure theoretical abstraction.

1

Some thoughts on the subject: The key difference between a microphone and an antenna is that the microphone is sealed from the back - it senses a pressure difference between the front and the back of the membrane regardless of the extent of that pressure region. If you have a small membrane that is not sealed from behind, then at low frequencies it will ...

0

The Question is basically "how Quickly"? It's definetly not "instantly", and there is some delay, which causes movement inside the fluid and it must take some time before viscous forces kills these movements. And as they are, by nature exponential, means the lower the velocity, the smaller the losses and thus the time can be considerably different according ...

2

A microphone is a transducer that converts variations in air pressure from sound waves into electrical signals. Air pressure varies as the wavefront passes into the diaphragm (or the ribbon, or the condenser) of the microphone. The diaphragm needn't be as long as the wavelength, as it senses the wave from a "head-on" perspective rather than "looking at it" ...

1

Microphones transform the pressure wave of sound to an electric signal. The wavelength of the sound wave tells us the distance over which the wave's shape repeats itself in space. The frequency measures the changes in the medium in time. As the sound wave passes, the molecules of the microphone vibrate in place ,according to the frequency, like a harmonic ...

1

No. A sonic boom is an acoustic disturbance caused by supersonic flow over an aircraft's surface. Supersonic flow creates a discontinuous shock boundary that emanates from the aircraft surface and the shock wave propagates behind the aircraft with a large amount of energy, however dispersive as it travels through the atmosphere. Resonance does require an ...

5

If we look at the sonic boom as a $\delta$-function, where we have a really loud sound for a really short time, then it will be able to excite all frequencies at the same way. You can actually compute this by showing that $$\delta(t)=\frac{1}{2\pi}\sum_n e^{int},$$ which show how the $\delta$-function is actually composed of all frequencies. Then it's ...

0

As the humidity of the air increases, its density decreases, so the fan blades will have an easier time passing through the air. I doubt the fan blades move muchy faster, because they are synched to the motor, but perhaps the decrease in force to pass through the air leads to a louder fan. What have you observed?

4

A graph should clarify the relationship between two quantities, requiring the least amount of mental effort on the part of your audience. If you are trying to show the change of density as a function of position along a wave, you should plot position along one axis, and density along the other. Whether you use vertical deflection as a measure of density or ...

0

Actually the pitch obtained by striking a coffee cup is a function of how full the cup is with coffee - the fuller the cup, the lower the pitch. You can convince yourself of this with a simple experiment just by changing the amount of coffee (water will work) and tapping the outside of the cup with a spoon. But when you stir a cup of coffee with a fixed ...

1

In fact, that's bloody complicated if you would go to the details. Generally, the initial guess can be made from this formula: $$f = St\frac{U}{d}$$ where $U$ is the flow speed, $d$ is distance between the edge (labium) of the whistle and a narrow canal from which the air goes. Constant $St$ is called Strouhal number and for these kind of systems is ...

2

I would go for this: Imagine the bottom of the cup as a saw. The noise or chattering of the spoon jumping on the sawteeth is higher the faster spoon moves. Those "sawteeth" on the cup bottom are very small, but the principle is the same. Therefore the faster stirring the higher pitch.

0

If we assume that you're talking about a broad sound wave traveling through a large, homogenous medium (air, water, rock) then normally, no: the pressure waves that are sound involve only motion along the direction of the sound's travel. You can wonder if the higher-pressure zones would tend to push the particles of the medium sideways, but remember that ...

3

That video is very poor in one aspect: particles in the sound field doesn't move "horizontally" nor "vertically". Really, the proper word is "longitudinal motion" and you are in fact asking about "transversal motion". In basic description, the air is considered to be an ideal fluid. Therefore no shear stress is possible and hence no transversal motion as ...

10

If you have any kind of solid material, it will become a little bit thicker as you compress it, and thinner as your stretch it. This means that a "one dimensional" wave traveling longitudinally down a rod will in fact cause some lateral motion. The ratio of displacements in the perpendicular direction is obtained from the strain (relative displacement of ...

6

The situation you are describing is an example of Fresnel diffraction (or near-field diffraction). In general, when a wave propagates every point of the wave front can be thought of as its own source of waves traveling in all directions (called Huygens construction). It turns out that neighboring point sources along an infinite straight wave front reinforce ...

1

You will benefit by finding some tutorials on wave theory. In brief, assuming a spherical wavefront from the emitter, you are correct there's no direct path to the receiver. However, the edge of yourabsorber there causes diffraction (Huygen's principle), so thatsome of the sound wave (energy) will make its way to the receiver. You can see a demo of this, ...

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