Wikipedia:Reference desk/Archives/Science/2013 July 15

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July 15

"Hearing the wind"

When you crack a whip, you know how you "hear the wind"? If you threw a fastball and "heard the wind", how fast would an estimate be of that pitch? Albacore (talk) 02:45, 15 July 2013 (UTC)

...If you're gonna find the answer anywhere, it'll be at the Physics of Baseball webpage. In fact, you can even apply for a (competitive) fully-funded summer internship to elaborate on these studies! (Here's the summer 2004 final report).

The sound we hear when air rushes past our ears actually has very little to do with the net velocity of the air (or of any object that's moving through the air). Sound is the vibration of air - its volume is determined by the amplitude of the oscillation; and the timbre of the sound is determined by the waveform shape of the oscillation. A "whoosh" is pretty much white noise. Air speed doesn't really enter into things, at least not as a direct first-order term. So, bulk velocity of the air is a secondary concern; and thus velocity of the acoustic transducer (the ball, or the whip) is even further removed from the intensity, volume, and sound we actually hear - it influences the sound in an indirect and subtle way. In practice, you might be able to empirically measure and then deduce a relation between the velocity of a ball and the amplitude of the "whoosh," but that relationship will be a little bit tenuous, because you're essentially trying to be quantitative about a very noisy signal.

In a little bit plainer English: you can make a loud "whoosh" using a slow moving object or a fast moving object. You can also make a quiet "whoosh" using a slow moving object or a fast moving object. The speed of the object is not the main factor in the "whoosh." Nimur (talk) 05:55, 15 July 2013 (UTC)

There are two situations I can think of (which doesn't mean there are only two...) - a reed (instrument) which vibrates due to its own characteristics, creating a sound that depends (among many things) on the speed of the wind, and whipcracking where the object moves at the speed of sound and all that sound emitted piles up into a little sonic boom. Wnt (talk) 15:33, 15 July 2013 (UTC)

I agree that some non-aerodynamic objects make a lot of noise in little wind by creating turbulence and perhaps snapping in the wind like a flag, while other objects create very little noise even in high winds, since they only create laminar flow. However, this Q was about a baseball, so, since we've eliminated the variable of the object type, the relative wind velocity should correspond quite well with the sound level. The only other remaining significant variables should be it's spin, and the air pressure, temperature, and humidity. StuRat (talk) 18:36, 15 July 2013 (UTC)

Why do you assert these speculations as if they are facts? We don't get to arbitrarily decide what should happen in aerodynamics. Just because you think velocity should affect audible noise doesn't mean it does. Turbulent airflow is probably one of the least intuitive, most difficult-to-speculate about, impossible-to-describe-using-simple-first-principles-of-physics subjects known to humans. As an example: one of the papers on the website I linked shows experimental data indicating that the asymmetric flow separation due to the rotation of the ball contributes to turbulent airflow. And sometimes in totally the opposite direction from that which is predicted by ordinary flow separation theory. A small scratch on the ball's surface may have more impact than the net bulk translational or rotational velocity of the ball. In another experiment, the presence of a very tiny "raised wire" on an experimental sphere changed laminar flow into turbulent flow with R=30,000. StuRat, when you assert that velocity is the chief factor - yet you have neither experimental data or theoretical explanation why you believe that should be true, you are conducting pseudoscience. This is worse than being wrong - if your fact was only wrong, we could correct it and move on with our lives. But instead, you are asserting a claim without any evidence. Your methodology is profoundly unscientific. Nimur (talk) 22:34, 15 July 2013 (UTC)

Descriptions of turbulence like this are always so fascinating. If small scratches and extensions have such an impact, why can't we use it? I mean, why can't we fly a few kites per acre on one side of a hurricane at sea to turn it, or have a computer-controlled mesh of thin wires up- or down-wind of a windmill to increase its output by a significant factor? Wnt (talk) 01:43, 16 July 2013 (UTC)

So what's happening with a Bullroarer that produces the sound? HiLo48 (talk) 09:13, 16 July 2013 (UTC)

Bullroarers produce a predominantly humming sound due to the rapid rotation about the long axis as you whirl it round on the end of the string/cord, somewhat like a fan or propellor makes a humming sound. This rapid rotation moves air in pulsations - first towards any point and then away. Bull roarers also produce a bit of shot noise (white noise or "hiss") as does any surface moviing through air, due to the random impact of air molecules on the moving surface. 1.122.182.232 (talk) 13:18, 16 July 2013 (UTC)

(ec) Lots of things are happening, and I can't pretend to list all of them: but here are some interactions that I would pay attention to. Air is rushing over the surface of the object; the object's motion is constrained by a semi-taut vibrating string whose tension varies with the speed of rotation; vibrations of the object and the air couple to the string. When tension changes, the resonant frequency and the acoustic damping change. If you modify the toy, replacing the shaped peg with something else of equal weight, the tension in the string should be identical; if you whirl it at the same speed, you may observe a change in the tone, timbre, and intensity of the sound. If you change the type of string to some other material - say, from an ordinary string to a guitar string, the sound will be totally different! If you spin an object that resonates at a particular frequency, including a tube-shaped peg, you can produce an almost clean, tonal sound - like a whistle or a flute. If you throw the whirling contraption - even if the peg reaches the same airspeed - does it produce the same "roaring" or whistling sound? Does that sound only happen when the string is taut and the object is rotating? How about if you start by whirling and then release the string, allowing the object to fly at the same speed, under its own inertia? How quickly does it stop "roaring"? How would it sound if you got a friend to drive you down the freeway, and you (safely) held the peg out the window at 65 mph?

This apparatus would be a good candidate for some fun experimental acoustics (to be conducted outdoors). The aspiring scientist could even set up a microphone to record the results. How does the sound frequency and volume change when you whirl at different speeds, or throw the object? Does the rotation rate modulate the tone? (It should, that's the basis of vibrato in the old-fashioned Leslie speaker). Just watch out if you're recording with a "smart"-device to analyze the sound - many smart devices now use digital post-processing to "denoise" and normalize the recording to a constant volume. Experimental physicists need to know everything about their measurement-apparatus, to prevent themselves from drawing erroneous conclusions about their data. Nimur (talk) 13:29, 16 July 2013 (UTC)

Noisy ladybirds (ladybugs)

I've just read a technical document which described a component making a noise like a ladybird... I just want to make sure that this is as rediculous as I think it is, since I've never heard them make any noise at all. MChesterMC (talk) 14:32, 15 July 2013 (UTC)

Pretty sure I've heard them [in the UK] make a fluttering sort of buzzing noise when they fly, especially to get off the ground. --Dweller (talk) 14:34, 15 July 2013 (UTC)

Yep, at liftoff, or if they happen to fly right by your ear, you'll hear a whirring buzzing sound. I'd imagine a loose fan or something could make a similar, if much louder, noise. SemanticMantis (talk) 14:52, 15 July 2013 (UTC)

This was a suggestion for an indicator noise, and the other examples given were the chirp of a grasshopper or a cricket, so I'm pretty sure the writer was just not thinking about what they were writing. MChesterMC (talk) 15:59, 15 July 2013 (UTC)

River swelling

What's the physics behind this video: http://www.youtube.com/watch?v=8sEdgHH9F10&feature=youtu.be ? What causes the river to empty and swell like that? 65.92.5.24 (talk) 15:41, 15 July 2013 (UTC)

It's hard to see quite what's going on without a better understanding of the geography, but when a ship moves in a narrow channel it can produce a soliton wave (a solitary wave of compression and then rarefaction which can retain its identity for a remarkable distance). Although solitons occur in all kinds of circumstances, they were first scientifically described when generated by canal boats. -- Finlay McWalterჷTalk 15:48, 15 July 2013 (UTC)

Perhaps Bernoulli effect is helpful? (not sure about that) Wnt (talk) 16:02, 15 July 2013 (UTC)

It's a small tsunami. That article explains the physics. Looie496 (talk) 16:14, 15 July 2013 (UTC)

Similar waves are sometimes created by tidal conditions called a tidal bore; the Severn Bore in England is a great favourite with surfers and kayakers. Alansplodge (talk) 17:14, 15 July 2013 (UTC)

A ship that size displaces an enormous amount of water. I calculate that a "Panamax" sized tanker displaces around four million cubic feet of water - and that one looked much bigger than that. As the tanker moves a distance equal to it's own length, four million cubic feet of water has to move out of it's way and somehow travel around the sides and beneath the vessel to fill in the "hole" it leaves behind the stern. Water is essentially incompressible - so it can't compress and decompress around the ship. So at typical tanker speeds of around 20mph (30 feet per second) - a 950 foot long Panamax ship covers it's own length in about 30 seconds, so the water has to flow around it at about 130,000 cubic feet per second! (Imagine filling and then draining, two olympic-sized swimming pools every second!)

When the vessel is close to land and in relatively shallow water, the water flow will be forced through narrow gaps beneath and on the shoreward side - so the speed of flow will greatly increase. It's not surprising then that water will be forced into (and then sucked out of) side-channels and inlets.

I'm not sure whether the Bernoulli effect is likely to kick in to a significant degree...but imagine if the small river in the video is connected to the larger channel with the ship in it somewhere off to the right of the camera. As the ship moved towards the point where it joins the ocean, the pressure ahead of the ship would build up, causing rapid water flow into the river - and as it passes, would cause a dramatic drop in pressure, causing rapid flow out of the river. At the point in time when the direction of flow reverses, you'd expect lots of turbulance and such - which would explain all of those big waves...but I'm not sure that's what's going on because the water level doesn't go up until after the flow direction reverses.

But if the small river connected to the ocean FAR to the left of the camera - then perhaps the initial flow towards the left is just the natural flow of the river and the temporary reversal is due to the pressure wave from the arrival of the ship - but delayed by a minute or two by the time it took for that wave to travel from the mouth of the river. We might expect an abrupt lowering of the water level sometime later as the pressure drops behind the stern of the ship - but maybe we don't see it because the video ends before that would have happened.

Similar arguments are possible if the camera person is standing on an island with the "river" connected to the ocean at both ends and the water was merely reacting to a build up of pressure before and after the ship.

I think that what we see in the video is consistent with any of the three possible connections between river and ocean...but without knowing exactly how the small river connects - it's hard to know for sure.

SteveBaker (talk) 13:58, 16 July 2013 (UTC)

• It's a wake, although the effect in the canal is similar to a tsunami or a tidal bore. Causing a wake like that is illegal in most costal water in the US. It can be avoided by slowing down, although the pilot will not want to. The gentleman should contact his local coast guard or environmental protection agencies. μηδείς (talk) 21:51, 16 July 2013 (UTC)

This is a small canal (no current) off the St. Clair River in the Great Lakes system. The boat is a 1000-footer. Rmhermen (talk) 02:11, 17 July 2013 (UTC)

Then it's a matter of international treaty. In New Jersey the ship's action would be illegal, and the owners subject to fine. μηδείς (talk) 02:21, 17 July 2013 (UTC)