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## Hot answers tagged acoustics

119

Water is compressible (nothing can be completely incompressible). Treating water as incompressible is just a (usually very good) approximation. Therefore, longitudinal waves are possible. Wikipedia reports the bulk modulus to be about $2.2\ \mathrm{GPa}$. This puts the speed of sound in water at about $$v=\sqrt{\frac{\beta}{\rho}}=\sqrt{\frac{2.2\ \mathrm{... 82 Sound doesn't go through walls? Please tell my neighbor. In electromagnetism, a medium has a property called an "impedance" which is related to the index of refraction and the speed of waves in the medium. At an interface between two media, the relative impedances determine how much of an incoming wave is transmitted or reflected, so that the entire power ... 74 The speed of sound increases with increasing pressure. Assuming ideal behaviour the relationship is:$$ v = \sqrt{\gamma\frac{P}{\rho}} $$or equivalently:$$ v = \sqrt{\frac{\gamma RT}{M}} $$where M is the molar mass. In a gun barrel just after the charge has gone off the gas is under very high pressure and very hot, so the speed of sound is much ... 56 If for your purposes of your task you can treat water as uncompressible, then you can also assume that sound propagates instantly in it. Indeed there will be no sound waves in this case, the movement will be just propagated instantly from source to the observer. 38 Deflagration means that the combustion moves through the fuel slower than the speed of sound in the fuel. It doesn't say anything about the speed of the resulting gas, or how it compares to the speed of sound in that gas (and the speed of sound in solids is generally higher than that of gasses). The gas that is released from the combustion isn't at ... 38 Wikipedia gives a pretty much straightforward answer. In an ideal gas, the speed of sound depends only on the temperature:$$ v = \sqrt{\frac{\gamma \cdot k \cdot T}{m}} $$So it neither decreases, nor increases with altitude, but just follows air temperature as can be seen in this graph: 37 Humans hear the correct perceptive signal for a sound wave of that frequency. We really can't say much more than that. The psychology of acoustics are very complicated and could fill volumes. It's closer to say we have cells which act resonant at a specific frequency. Our brain identifies which cells are resonating at any point in time, and constructs ... 34 Sound is a pressure & velocity wave in fluid medium, i.e. air. Air molecules wiggle back and forth and bump into other air molecules so they wiggle too so you have a whole chain of wiggling air molecules. The jet engine moves air molecules A LOT, hence it's extremely loud. As the sound moves away from the jet engine the energy disperses over a larger ... 24 Actually this effect has been discovered in 1932 with light diffracted by ultra-sound waves. In order to get observable effects you need ultra-sound with wavelengths in the μm range (i.e. not much longer than light waves), and thus sound frequencies in the MHz range. See for example here: On the Scattering of Light by Supersonic Waves by Debye and Sears in ... 24 So obviously the audible frequency is twice the envelope Sorry, that's wrong. If you play two tones (say 440 Hz and 267 Hz), you simply hear two tones at two different frequencies and you have two excitations at different spots on the basilar membrane and two different sets of nerves firing. You don't hear the envelope at all, they just sound like two ... 20 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 frequency, but there is a continuum between a pure tone (one frequency) and noise (all frequencies, not only multiple of a fundamental, without any regular pattern ... 17 vibration is only a one dimensional motion This is not generally true. As a trivial example, one could the movements of water in a pond where a few small rocks have been tossed. The motion is definitely a wave behavior, and could even be called vibration, but it is most definitely not one dimensional. Another potential example would be the vibrator on ... 16 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 (higher pitch). This follows from the wave equation (here in one dimension):$$u_{tt}=\frac{E}{\rho}u_{xx}$$E is Young's Modulus and \rho the material's ... 16 Human perception is involved here because when you humans talk about noise this generally means a sound that is aperiodic. However the tone produced by a guitar will be something like:$$ A(t,x) = \sum_{i=0}^\infty A_i \sin(n\omega_i t - k_i x) $$i.e. a superposition of the frequencies f, 2f, 3f, etc. The function A(t,x) is periodic in time with ... 15 Musical instruments generally produce sound waves at a collection of frequencies, even when playing a single note. For instruments which make distinct pitches, these frequencies are roughly multiples of a fundamental frequency, which determines the perceived pitch. The perceived brightness is determined by the strength of higher frequencies, in the kHz range.... 15 Since sound travels as longitudinal waves, sound waves should only be able to propagate in a medium through compressions and rarefactions. However, water, as a liquid, is generally treated as an incompressible fluid. The underwater environment is characterized as an inhomogeneous medium, such inhomogeneity is due to point-to-point changes in underwater ... 14 The answer by niels nielson is much more useful than my answer. But just in case you really do want a rough estimate of how much power is emitted as sound... According to [1] (references are listed at the end), a sound level meter is a hand-held instrument with a microphone that measures the sound pressure level (SPL). I'll assume that this is the kind of ... 14 The speed of sound in a gas is given by \sqrt{ \dfrac {\gamma \,P}{\rho}}= \sqrt{\gamma \, R \, T} where the temperature, T, is in kelvin, \gamma is the ratio of the specific heat capacities of a gas at constant pressure and constant volume and R is the specific gas constant. With increasing altitude there is a decrease in the density but also a ... 13 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 as heat and deformation, also the higher frequencies are damped more than the lower frequencies (it acts like a low pass filter). So the wooden block will ... 13 The phenomenon you're asking about is known as octave equivalence. It's a hard-wired thing in the human ear-brain system. (We know it's hard-wired because it's present without musical training and is true cross-culturally.) Notes that differ in frequency by a factor of 2 (or a power of 2) are perceptually similar, and may be mistaken for one another, even by ... 13 Sound waves are just pressure oscillations; when they strike a surface they are either reflected, transmitted, or absorbed. When they're transmitted, you'll hear them on the other side. According to Wikipedia, regarding acoustic absorption: Deformation causes mechanical losses via conversion of part of the sound energy into heat, resulting in acoustic ... 13 Can two waves (like sound or electromagnetic waves) interfere head-on? Yes. When waves add in a superposition it is called interference. Two waves heading towards each other with have interference. suppose they are out of phase with each other and thus interfere destructively, where does the energy of the waves go? It depends on what you mean by "... 12 One horsepower represents 746 watts. A refrigerator motor develops (typically) 1/4 to 1/3 horsepower of which only a tiny fraction of wattage is dissipated as vibratory noise. The leakage of heat into the refrigerator through its walls is a far more significant loss mechanism than noise generation. By the way, the front-most rubber feet of a refrigerator ... 12 The answer depends on the kind of motion you are thinking of. As PhysicsDoc points out, random motion at the molecular level averages out such that only a net effect will actually move the eardrum. For larger scale movement, we need to understand a bit about how hearing works. Perception of sound depends upon the movement of tiny hairs in the cochlea, ... 12 It does. There's an important concept in soundproofing called flanking noise. There are various sources, but the most common is having sound transmitted through some kind of solid structure (walls, roof struts, door and window frames, etc.) into the soundproofed area. This is a major problem for soundproofing, and it's one of the hardest problems to solve ... 11 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 string engage the air very little (as is evident on an electric guitar without amplification). This is because the acoustic wave impedance of the air does not ... 11 Human ears are evolved to furnish a good impedance match between sound waves traveling in air, and the nerve array inside your ear that turns vibrations into electrical impulses. This means that the greatest possible amount of sound wave energy will be conveyed to those nerves, across the greatest possible range of different frequencies. The characteristic ... 11 I think the simple answer here is resolution. Generally when imaging with waves (including light) the limit to resolution is a length that is similar to the wavelength, \lambda. If f is the frequency and c is the speed of the wave then the wavelength is given by$$\lambda = {c \over f}  so the higher we make $f$ the smaller $\lambda$ becomes ...

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Sound wave is not a transverse wave, as you thought. That means the vibration and the direction of propagation for sound wave are parallel. And the vibration is caused by difference in air pressure at different places. To the question "how I can listen to it" thats because the pressure difference propagates toward your ear and force your eardrum to vibrate.

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First, the cabin is quieter because the fuselage walls are designed to limit the transmission of sound from the engines. Second, on most commercial aircraft, the engines are suspended beneath the wings, which block the noise from the engines before it can strike the fuselage.

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