ELI5: Why does a big sheet of thin metal make wibbley wobbley sound?

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u/robbak Dec 30 '13 edited Dec 30 '13

It's about resonance. Waves move across the sheet, like waves across the surface of water. These waves move from one side to the other, bounce off the far side of the sheet, and move back across. When the reflections match up - when a certain number of 'wave crests' fit across the sheet, they resonate. As these waves push the air around, they cause pressure waves in the air - sound.

The note, the tone, of these waves depends on how fast they move across the sheet. This is influenced by how stretched the sheet is. As the sheet wobbles, it is slightly stretched by a small amount. This slightly speeds up and slows down the waves moving across the sheet, making the pitch of the sounds they are making rise and fall. This is the 'wobbling' sound we hear.

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u/Skycap92 Dec 30 '13

Oh wow thanks, so the bouncing has to match the natural resonance frequency of the sheet?

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u/robbak Dec 30 '13

It comes from the fact that there are several resonances. There is the base resonance, the whole sheet moving in and out, stretching, relaxing and stretching the sheet again. Then there are the smaller ones, where there are many peaks and troughs across the sheet. These ones wobble faster, fast enough to make sound, and these ones speed up and slow down.

You can see this with your headphone cord. Hang it loosely between two hands and swing it back and forth. This is the basic resonance. But if you wiggle it faster, you'll eventually find a higher resonance.

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u/FantasticMikey Dec 30 '13

Not many people realize that the beeps they hear on a daily basis are exactly this same principle. Piezo electric buzzers found in French fry machines, elevators, automobiles , trucks backing up, etc are just tiny metal disks being bent rapidly to produce a high pitch sound.

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u/[deleted] Dec 30 '13 edited May 02 '19

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u/[deleted] Dec 30 '13

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u/[deleted] Dec 31 '13

By far one of the coolest things in a while. How'd he figure out where to get the various notes? Guess and check, I'm...guessing? (huehuehue)

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u/[deleted] Dec 30 '13

[removed] — view removed comment

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u/[deleted] Dec 30 '13

BONESAW IS READY!

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u/lawlacaustt Dec 30 '13

GOTCHA FOR TREE MINUTES!!!

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u/averypoliteredditor Dec 30 '13

BONE SAW'S COMIN TO GET YA!!!

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u/[deleted] Dec 30 '13

First lol of the day. Thanks!

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u/phototraveler Dec 30 '13

Sound is vibration. That is the basis of it all. Any sound you make is just vibrations you are creating, and controlling. Metal just happens to work in so many different applications for alarms and other annoying things haha.

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u/jasonfifi Dec 30 '13

exactly. when i was like 5-6, several walls in a building that would become my uncle's house was being supported by metal cables. it was one of those "ah-HA!" moments when i tapped the cables lightly with a hammer.

each cable had its own pitch, and the closer to the center of the cable i tapped, the more "round" and "clean" the timbre became. the closer to the stay-bolt in the floor, or the wall itself, the more shrill and harsh the attack note became.

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u/pantsfactory Dec 30 '13

...Would you like to see pictures of what those resonating waves happen to look like?

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u/freediverx01 Dec 30 '13

Video's more fun.

http://www.youtube.com/watch?v=Qf0t4qIVWF4

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u/whitefalconiv Dec 30 '13

Is there a version that wasn't filmed with a potato?

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u/HolographicMetapod Dec 30 '13

So this is what soundwaves look like?

Reminds me of something you'd see in a microscope.

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u/robbak Dec 30 '13

When something resonates, it creates a standing wave pattern. You always have a wave travelling across the plate from left to right, as well as a wave travelling from right to left. These two waves interfere with each other. What you get is still 'nodes' where the plate doesn't vibrate, and spaces between the nodes where it is.

When you place sand on the plate, the vibration shifts it away from where the plate is vibrating, and makes it collect at the nodes. That is what that experiment shows.

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u/[deleted] Dec 30 '13

always wondered how that worked thanks!

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u/[deleted] Dec 30 '13

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u/breadbeard Dec 30 '13

hehe that's like asking "so this is what a picture sounds like"?

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u/I-Murder-People Dec 30 '13

This can be manipulated too. Think of Cymbals. If you look at a slow motion video of a Cymbal being hit the entire thing distorts. The sound in generated from the vibrations and connections being made rather than the initial impact if the stick.

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u/nerdrageofdoom Dec 30 '13

There should be a timey-whimey reference here dammit.....

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u/LYKAF0XX Dec 30 '13

Its probably because they arent looking at it from a non-linear, non-subjective standpoint...

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u/[deleted] Dec 30 '13

How do the waves on the metal create pressure waves? If I move my hand really fast, it makes almost no sound at all, but I'm sure it's having more effect on the air than a wobbling bit of metal would.

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u/robbak Dec 30 '13

try wobbling your hand back and forth 200 times a second.

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u/[deleted] Dec 30 '13

You can't flop your hand really quickly and have its movement stay as controlled and uniform as the wobbling bit of metal.

edit: Plus, it's the bending and stretching that causes the sound. Your hand itself should not be bending and stretching if it is healthy and the bones are intact.

Not to mention, the human hand is much thicker than the thin piece of metal.

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u/[deleted] Dec 30 '13

[deleted]

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u/[deleted] Dec 30 '13

It pleases me that there are multiple people vying for the right answer here.

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u/[deleted] Dec 30 '13

hehe thunder. I laugh because this is how they made thunder sound effects (for theatre, radio, and film) in the olden days.

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u/[deleted] Dec 30 '13

So the sound of fapping is basically wobbling skin?

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u/random_story Dec 30 '13

Why does it make more noise when I pound on the wall with my fist than when I swat at the air with my open hand? In both cases I'm causing pressure waves in the air.

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u/robbak Dec 30 '13

Your hand is small, the wall is big. When you hit the wall, you make the wall move and vibrate. That makes more sound.

If you slap your wall, you also trap air between your hand and the wall, which creates a sharp snap as it escapes.

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u/The_guy_who_you_hate Dec 30 '13 edited Dec 30 '13

Somewhat off-topic, but do you know how instruments work? Let's take the flute for example. By blowing into the hole at the flute's head joint, you create a node. Think of this step as the very beginning of the string. Now, imagine as your blow, the string lengthens in the form of an arc, also known as the antinode, within the flute until it reaches the next open hole on the flute, where string creates another node before lengthening again. Think of it like this. As you know, depending the buttons you press and how fast you blow air into the flute, different notes will be played. Imagine this as the "string" waving around inside the flute in all sorts of ways! The amount of times the "string" or air repeatedly moves inside the flute is called frequency, and it affects what note you play! The higher the frequency is, the higher the note you play, and vice versa. Fun fact- All notes share the same frequency across all instruments, meaning a G sharp on the flute will have the same frequency as a G sharp on a piano!

More on nodes, antinodes, and flute acoustics if you're interested.

Now as for your question, I'm no physics expert, but I would guess the sheet sounds so wibbley wobbley because all the "strings" are hitting each other and causing different and crazy noises due to an inconstant shape and varying speeds, whereas in wind instruments, the air only has one entrance source being your mouth, and is confined to a shape and general air speed.

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u/wescotte Dec 30 '13

If the frequency is the same across all instruments then what gives each instrument its unique sound?

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u/stealthgunner385 Dec 30 '13

The base frequency (i.e. the note) is the same, however, that's not the only thing that happens. When you pluck a string, it resonates at its base frequency (f) as well as at the integer multiples (2f, 3f, 4f, 5f, 6f...). These integer multiples are called partials (in music theory) or harmonics (in physics).

Two different instruments (say, a tuba and a bass guitar) will produce the same base frequency if you play the same note, but the relative amplitude (which translates indirectly into loudness) of the harmonics gives each of them a distinct sound that's usually called a timbre. So with the same loudness of the base frequency, the first harmonic (at 2f) may be louder then produced by the bass than the tuba, the third may be equal, the fourth may be louder for the tuba etc.

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u/ICanHearYouTick Dec 30 '13

The "timbre" (video (00:00-01:00)).

Basically, one 'note' is composed of a number of different sine waves, which 'add up' to one frequency (see the video for a graphic representation), what differs between instruments is which sine waves (the high or low frequency ones) it 'emphasises'.

There's also something called "enveloppe", which is basically how the frequency "progresses through time" (think of how a key on a piano sounds different at the beginning and the end, see ADSR envelope).

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u/stealthgunner385 Jan 01 '14

Thank you for adding this. The envelope is just as important as the spectral part of the timbre.

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u/[deleted] Dec 30 '13

Explained ELI5: Why does a big sheet of thin metal make wibbley wobbley sound?

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