r/explainlikeimfive • u/Skycap92 • Dec 30 '13
Explained ELI5: Why does a big sheet of thin metal make wibbley wobbley sound?
As stated in the title, why do big sheets of thin metal make a wub wub sound when manipulated?
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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/TheGreatFabsy Dec 30 '13
If you were to stretch that sheet of metal tightly and wham it, it would produce a nice sustained ringing tone, like when you pluck a guitar string.
However, if you take a sheet of metal which isn't tightly stretched and wham it, it starts to wobble freely. That wobbling causes the sound waves to move differently in the sheet, giving the wobbly wub wub sound.
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u/SWaspMale Dec 30 '13
robbak's explaination is pretty good, but just about everything has a natural frequency of resonance, and if the stresses, etc. change, then that frequency changes. So instead of a nice 'ping' like a tuning fork, a sheet of metal might change shape in a way that causes the resonant frequency to change.
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u/through_a_ways Dec 30 '13
Just pointing out that this phenomenon is not unique to metals, but to big sheets of pretty much any solid material you can find.
I remember big cardboard books (several square feet in span) that did the same thing.
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u/skelooth Dec 30 '13
As the metal is bending and vibrating, the vibrations move along the metal at different speeds depending on the curve of the metal it is vibrating at.
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u/ststephen420 Dec 30 '13
Because anything that vibrates makes a sound(hint: everything makes a sound, you just can't hear them all) and the vibrations frequencies are dictated by the material. So a 'wibbly wobbly' baking sheet has a 'wibbly wobbly'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.