# Tag Info

2

For general audio programming or playback, 96kHz or 192hHz is simply useless. Indeed, the Nyquist theorem tells you that a signal can be exactly reproduced given that the sample rate is greater than the highest frequency contained in the original signal. The "excuse" of the slope of analog filter required after digital to analog conversion is no longer ...

1

because the higher the sampling rate is the sloppier the (annalogue) filtering preceding the sampler can be to reduce the aliased noise/interference

1

The usual way that this phenomenon is described is when you hold a sea shell up to your ear, you can hear the sea What actually happens, according to this link, is that the vessel acts as a resonator / reflector for ambient noise - sounds that already present in the environment are amplified and stand out more. The "you hear your blood" myth is just ...

1

For practical purposes, we can assume that the disappearance of a 1.2 cubic cm (1.2 ml) object gives a waveform that's very similar to the sudden appearance of a 1.2 ml object, except for the sign of the resulting pressure wave. Now we do have a simple means to create that effect: setting off gunpowder will suddenly produce a lot of gas. You'd need just a ...

0

I believe your question is perhaps ill posed. I confess, I don't understand what you mean by "intensity decreases much faster" at r=1 inch vs r=1 meter. The equation shown governs the intensity at a given distance: $$I=\frac{S}{4\pi r^2}.$$ All this says is, for a sound source of intensity S, the intensity is inversely proportional to the distance ...

4

Acoustic waves travel through a medium (air, water, metal, etc), there is no known medium through which light travels Both the speed of sound and the speed of light have fixed values regardless of the speed of their source Acoustic waves can be longitudinal (in gases) or transversal (in solids) whereas light is only transversal. You can measure acoustic ...

1

Acoustic Wave is a wave in which motion of one atom causes motion of another atom because it is lying next to it. Light is change in electric or magnetic field which further causes changing fields.

0

Acoustic waves need a medium through which to travel. Light does not.

0

Acoustic waves are longitudinal waves. Light is a transversal wave, hence not an acoustic wave.

1

The following is not my own research, but taken from Randall Munroe' wonderful what-if "Glass Half Empty" where he describes a glass of water, bottom half filled with water, top half filled with vacuum (or: nothing). (edited to exclude other two glasses) But what if the empty half of the glass were actually empty—a vacuum? (Even a vacuum arguably isn’t ...

3

I suggest a totally different approach. But it's only a partial approach with much guesswork, too. The ear is able to perceive 20 µPa. (at 2 kHz). Of course you could calculate some pressure changes at the closing void, but these actually have nothing to do with the sound pressure at your ear drum. Let's do some energy calculations. 20 µPa at 1 cm² area at ...

3

Kudos to the question-asker for thinking about everything they read! :-) I was pleased to note that the author of the previous answer mentioned "audible" means "audible" to the human ear. Note also that "audible" also depends on the frequency a bit...generally-speaking, as humans, for high-frequency sounds we need them a little more intense if we're going ...

39

Sound intensity is measured on the dB scale, which is a logarithmic scale of pressure. The "threshold of hearing" is given by the graph below: which tells you (approximately) that 0 dB is about "as low as you go" - the "threshold of hearing". Note that sound signal drops off with distance - we will have to take that into account in what follows. If you ...

0

This only a partial answer. Your understanding of acoustics needs to be enhanced a bit: for one thing, an "audible sound" means there are frequencies our ear responds to. This means roughly 20 Hz to 20 000 Hz . Now, if we assume (I know, I know :-) ) that the parchment left a vacuum in its place of some small but specified volume, and we assume a ...

0

if there is nothing or no one around to hear it Given that trees/forests don't exist in isolation we would have a very difficult time finding a tree that falls with absolutely no creature capable of perceiving sound within a reasonable distance. Go find a forest completely devoid of birds, rodents, snakes, insects, fish etc. Don't forget a 2+km buffer ...

0

Put a microphone & recorder near the falling tree. No one was around to hear the tree fall. Yet you can play the recording and provide evidence of sound.

8

It depends on your definition of "a sound". If a sound is not a sound unless it is perceived as a sound (that is, processed in the auditory system of a sentient being), then the answer is "no". If a sound is a coherent disturbance in the pressure distribution of the air, and this disturbance propagates through the medium "at the speed of sound", then the ...

0

It's difficult to tell without seeing the whole thing, but ... If it is ultra(low)-sound it means that the driver for your horn should be very large - like over 15-inch to produce high volume with small displacement of the driver. Small displacement of the driver is critical for producing (in normal working conditions) vibrations of air - which are wave - ...

1

Short answer: in phase radiates more in the far-field. But read the long answer. Long answer: this actually depends on the frequency and the distance between the sources. For a wavelength significantly larger than the distance between the sources, the pressures emitted by each source simply add up when in-phase; and they cancel when out-of-phase. But what ...

0

For all practical purposes, in the Earth's atmosphere, terminal velocity is less than the speed of sound in water by a large margin ie around 15x. So water will cushion an impact. That Mythbusters episode left something to be desired - accuracy. The world record for a shallow dive into 30cm of water is about 11m in height, meaning an impact velocity of ...

2

To first order, the speed of sound is not affected by pressure. Pressure waves can be shown to fulfill the D'Alembert wave equation $(c_S^2\,\nabla^2 - \partial_t^2)\psi=0$ where the wavespeed $c_S$ is given by: $$c_S = \sqrt{\frac{K}{\rho}}$$ where $K$ is the bulk modulus of the medium in question and $\rho$ its density. Now, for an ideal gas, the bulk ...

2

I suppose you could say this is cheating, but you could surround the object emitting the sound with a perfect vacuum. Sound waves are vibrations in a medium; because a perfect vacuum has nothing in it, it cannot "conduct" (for lack of a better word) sound waves. You could attempt to levitate the object with magnets; because of Earnshaw's theorem, the setup ...

0

Resonance occurs when small amounts of energy accumulate because the rate at which they are being added matches the the frequency of the system itself and the phase. The classic example is pushing a child on a swing. If you add energy, ie push, at a frequency that matches the fundamental frequency of the pendulum motion of the swing the amplitude slowly ...

0

Your two proposed approaches are the ones usually followed by researchers in acoustics. There is a third one that would be an experimental approach. For example you could place a microphone inside the tube and move it to a large number of positions to construct a sound pressure map. The first approach you mentioned is comparable to a lumped elements model ...

1

There is a "universe" where the speed of sound is greater than the speed of light (or at least the speed of electromagnetic wave propagation), and that is inside conductors. In a conductor, the EM wave velocity is $$v = c \left(\frac{2 \omega \epsilon_0}{\sigma}\right)^{1/2},$$ where $\sigma$ is the conductivity and $\omega$ is the angular wave frequency. ...

2

Sound is a pressure wave, and the generation of a pressure gradient requires atoms/molecules to move to create a density difference. No particle can move faster than light, so it's impossible to create a pressure gradient that propagates faster than light. Nathaniel's argument that sound waves could travel faster than light in a system like a Bose Einstein ...

4

We have to be careful about what we mean by "speed of light". It can mean two things: the speed at which light travels, which I'll write as $s_{light}$, and the maximum speed at which anything can possibly travel, which is written $c$. In our universe, in a vacuum, $s_{light}=c$, as far as we know. Now, no information can ever be transmitted faster than ...

-1

We all "know" nothing can travel faster than light [...] When referring to "the speed of light (in vacuum)" in the context of relativity we know definitely that we mean "signal front speed"; and the notions of "signal" and "signal front" are considered definite and unambiguous as well. [...] a universe where sound travels faster than light? By ...

3

Let's make the following assumptions: One needs to add $1$ log / $30$ min. to keep the fire burning, One log weight approximately $1$ kg. Under these conditions, with your 50% efficiency estimation, the power consumed by the fire is: $15$ x $10^6$ joules x $0.5$ / $1800$ seconds $=~ 4.2$ x $10^3$ W (which is consistent with the estimated heat output ...

0

I think you've got it right. The air space above the liquid creates a chamber that supports resonance at specific frequencies, similar to a flute. Since the chamber may be oddly shaped, it will in general support multiple frequencies or a range of frequencies (unlike a flute, which is designed to resonate at only one frequency). However, since at least ...

0

It would appear to be correct. An example is a supersonic bullet fired over your head from a few hundred metres distance. You hear the crack of the bullet's shock wave as it passes, the sound of the round fired and (if your hearing is till up to it) the kind of sucking sound the bullet makes as it travels downrange. I imagine that a better setup would be to ...

3

What you are probably observing is a stick-slip phenomenon (sometimes called a relaxation oscillator - think chalk on blackboard). The whole point is that high modulus ("not flexible enough" == high modulus) is just what you need to get such high frequencies: a small displacement must give rise to a large force. I must say I'm surprised you can do this by ...

1

The reason behind the hissing sound is that the temperature of the water droplet is much lower than the hot surface. As soon as the water droplet's base touches the hot surface it quickly evaporates but still the top part of the droplet is in liquid state and there is an opposition to th water-vapour coming from below. As the water-vapour couldn't vertically ...

1

The movement of air around the blades of the rotor produces a "white noise" due to turbulence. However, the "white noise" is actually "colored" and a certain band of frequencies predominates. As the blade approaches you there is a Doppler shift toward a higher frequency, and as it turns and goes away from you the Doppler shift reverses. It is this ...

4

First, assume a spherical helicopter... A helicopter isn't a sphere, or even close to one. Consequently, you get complicated acoustic effects where the pressure waves from the blades reflect off other parts of the helicopter, such as the tail boom. You also get intermittent reflection of the tail rotor sound off the main rotor blades, intermittent ...

2

Actually you do hear a constant whoosh (although often drowned out by engine noise) when you are directly below the helicopter. The tips of the blades cause a wave to propagate outward at the speed of sound. This wave does not have the same strength in all directions. If you are a distance away laterally, then you hear these waves in succession produced ...

-2

Sound is different speeds of air coming to your ear resulting in different air pressure. When a blade is coming towards you it generates a higher air pressure, than when it is going away from you. Therefore the flapping sound. Imagine if you could be exactly above or under the rotor. Then the different air pressures will equalise each other and you would ...

24

In start-up and hover each blade produces more or less constant sound. But the sound is attenuated by distance and may not be the same in all directions. Therefore you hear it differently depending on the blade's position relative to you. So as the blades rotate, the sound you hear pulsates because the blades alternately get to positions where you hear them ...

2

It is generally caused by poor piloting technique. Blade-slap occurs when the helicopter is allowed to slowly drift downward while the pilot is still applying significant power. When blade-slap occurs, the pilot should either stop the descent, or lower the collective to enter a more positive descent.

3

The amplitude/intensity of a sonic boom (in Earth's atmosphere) is dependent on the change in pressure across the shock wave. This should make sense, as the intensity of a sound wave is dependent upon its pressure relative to quiet periods. We also know that the ratio of the downstream to upstream pressure is proportional to the square of the Mach number. ...

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