# Tag Info

770

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 spectral density of the sound and obtained the following results: I also recorded 5 seconds of me not dropping a coin 10 times to get a background measurement. In ...

306

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 frequency than a small tungsten sphere. A summary of everything that comes below - the result of several edits, and including a nice interaction with the ...

196

This effect is known as inharmonicity, and it is important for precision piano tuning. Ideally, waves on a string satisfy the wave equation $$v^2 \frac{\partial^2 y}{\partial x^2} = \frac{\partial^2 y}{\partial t^2}.$$ The left-hand side is from the tension in the string acting as a restoring force. The solutions are of the form $\sin(kx - \omega t)$, ...

176

We do. Normally the reflections are too quick to hear distinctly, and in an environment like a room they rapidly become diffused into a mush which a sound engineer would call reverberation. In larger spaces you can often hear distinct echoes as well or instead: a good way to play with this is to clap your hands (once) in a quiet hall: you will hear the ...

125

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 $$v=\sqrt{\frac{\beta}{\rho}}=\sqrt{\frac{2.2\ \mathrm{... 82 Sound doesn't go through walls? Please tell my neighbor. In electromagnetism, a medium has a property called an "impedance" which is related to the index of refraction and the speed of waves in the medium. At an interface between two media, the relative impedances determine how much of an incoming wave is transmitted or reflected, so that the entire power ... 75 Do low frequencies carry farther than high frequencies? Yes. The reason has to do with what's stopping the sound. If it weren't for attenuation (absorption) sound would follow an inverse square law. Remember, sound is a pressure wave vibration of molecules. Whenever you give molecules a "push" you're going to lose some energy to heat. Because of this, ... 74 This is because of the physiological differences in the functioning of the cochlea (for hearing) and the retina (for color perception). The cochlea separates out a single channel of complex audio signals into their component frequencies and produces an output signal that represents that decomposition. The retina instead exhibits what is called metamerism,... 74 The speed of sound increases with increasing pressure. Assuming ideal behaviour the relationship is:$$ v = \sqrt{\gamma\frac{P}{\rho}} $$or equivalently:$$ v = \sqrt{\frac{\gamma RT}{M}} $$where M is the molar mass. In a gun barrel just after the charge has gone off the gas is under very high pressure and very hot, so the speed of sound is much ... 71 But that still means the ear is only moving at one frequency at any given time No, it doesn't mean that at all. It means the eardrum is moving with a waveform that is a superposition of all the frequencies in the sound-wave it is receiving. Then, within the inner ear, hair cells detect the different frequencies separately. It is entirely possible for ... 70 A sonic boom is produced when a macroscopic object (say, roughly: larger than the average spacing between air molecules, \approx 3\,\mathrm{nm}) moves so fast that the air has no time to “get out of its way” in the usual way (linearly responding1 to a pressure buildup, which creates a normal sound wave that disperses rather quickly, more or ... 69 By popular demand (considering two to be popular — thanks @Rod Vance and @Love Learning), I'll expand a bit on my comment to @Kieran Hunt's answer: Thermal equilibrium As I said in the comment, the notion of sound in space plays a very significant role in cosmology: When the Universe was very young, dark matter, normal ("baryonic") matter, and ... 69 Hard though it is to believe, pH does have an effect on sound absorption in water. There are some reactions that are affected by pressure, that is pressure changes their equilibrium. One example is the equilibrium between boric acid and the borate ion:$$ B(OH)_4\,^- + H^+ \rightarrow B(OH)_3 + H_2O $$Increasing pressure pushes the reaction over to the ... 67 The answer to this question has significant overlap with my answer on piano tuning. There, I discuss how a thick wire has an extra restoring force, in addition to its tension, from its resistance to bending. This modifies the usual wave equation to$$v^2 \frac{\partial^2 y}{\partial x^2} - A \frac{\partial^4 y}{\partial x^4} = \frac{\partial^2 y}{\partial t^...

66

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 answer that I thought it better to write this as a separate post. I discovered two interesting things. First, when you spin a coin on a hard surface, it "rings" with ...

60

I'm not going to address the production mechanism,1 just the nature of the "sound" in this case. What you think of as the hard vacuum of outer space could just as well be seen as a very, very, very diffuse, somewhat ionized gas. That gas can support sound waves as long as the wavelength is considerably longer than the mean free path of the atoms on the gas. ...

58

In order to properly understand this without any unnecessary "controversy", let's break the whole process of sound generation and perception into 5 important, but completely separate parts. We'll then proceed to explain each part using a few different examples and pieces of derivative logic: Vibration of the vocal folds Transmission of energy from vocal ...

57

If for the 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 the source to the observer.

56

It's obviously not a sharp cut-off, but as a general guide sound waves cannot propagate if their wavelength is equal to or less than the mean free path of the gas molecules. This means that even for arbitrarily low pressures sound will still propagate provided the wavelength is long enough. Possibly this is stretching a point, but even in interstellar gas ...

54

On the most basic level, the answer is a flat no. The seven primary notes in an octave is specific to the western musical tradition. It's not entirely arbitrary as you say, but there are many other choices that could have been made, and there are other cultures who use fewer notes (e.g. pentatonic scales in blues music) or more (e.g. Indian classical music). ...

51

Because 16,000 is greater than 3. We only have 3 sorts of detector (called cones) in the eye, sensitive broadly to red, green and blue light. So a mix of red and green light excites the red and green cones. But yellow light, in between red and green, also excites the red and green cones, and the brain can't tell the difference. So it's not the mean of the ...

49

Ultrasound is used to break kidney stones and "burn" brain tumors. So no surprise that it can damage body parts, including inner ear. The effects of ultrasound on human body are multiple. One of them is heating due to absorption of the mechanical energy in the tissue and conversion to heat. This is used in physiotherapy as a desirable effect ...

48

The frequency of the recent experiment was in the audible range. The amplitude was off by unspeakable orders of magnitude. But yes, you would hear it (even in vacuum, if you were to survive). Yes, the GW are transverse (quadrupolar). But they do move things (they cause change in distances, that's actually how they detected them: the length of the 4km tube ...

47

Our eyes have excellent spatial resolution. We can tell the difference between objects only a fraction of a degree apart. This is possible due to both the construction of the eye and the fact that visible light has wavelengths that are tiny on our scale. Signals that arrive simultaneously can be independently detected. Our ears do not have this level of ...

46

The speed of sound in an ideal gas is given by $$a = \sqrt{\gamma R T}$$ Where $\gamma = \frac{C_p}{C_v}$, $R$ is the specific ideal gas constant and $T$ is the absolute temperature. Taking standard values for air, this makes a graph like this: The linear approximation is plotted by your formula, $a = 331\ \frac{m}{s}\ +\ 0.6 \frac{m}{sK} (T - 273\ K)$...

46

The hint given by the interviewer is a red herring. The limitation you're hearing has been part of the phone network since long before digital sampling had any part in the telephone system. And it applies even in a local phone call where the signal is never digitized. It is related to the fact that the connection from a land-line phone in your house or ...

46

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 ...

46

From the ideal gas law, we know: $$v_\textrm{sound} = \sqrt{\frac{\gamma k_\textrm{B} T}{m}}$$ Assuming that interstellar space is heated uniformly by the CMB, it will have a temperature of $2.73\ \mathrm{K}$. We know that most of this medium comprises protons and neutral hydrogen atoms at a density of about 1 atom/cm−3. This means that $\gamma = 5/3$, ...

46

The main issue in the setting of an orchestra or choir is the fact that no two voice or instruments maintain exactly the same pitch for any length of time. If you have two pure sine wave source that differ by just one Hertz, then the interference pattern between them will shift over time - in fact at any given point you will hear a cycle of constructive and ...

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