The title basically sums it up. What is involved in the translation between sound waves and magnetic fields?
If you mean in a speaker, the magnet is just used to flex a paper cone.
That’s certainly part of it, but you really didn’t explain anything.
An electric current, varying in magnitude exactly as the frequency of the desired sound passes through an electromagnet. That magnet pulls on a metal element on a cone shaped piece of paper, and the paper vibrates at the desired frequency. The air is shaken by the paper cone, creating sound, which propagates into the room, or theater, reproducing the same sound that was recorded by a fairly similar, but reversed process when it was recorded.
Tris
Think of sound like a ripple on top of water. The only difference is that instead of it being the surface of the medium lifting and lowering, it’s contracting and expanding out in spheres. The hairs on the inside of our ears are then built to wave back and forth with rippling air and that sensation is sound. The vibrating paper cone just causes these air ripples.
Anything that is tensed tends to be good at vibrating, like a guitar string or a paper cone that’s being stretched taught by a magnet. If you play a sound at it, it will vibrate with the air to match the sound. And if you cause it to move exactly as it did when the sound was playing (for instance, by using a magnet to cause it to sway), it will cause the air to send out that same sound.
Well, no, this is exactly wrong with regard to speakers. You specifically do NOT want the paper cone to be taut, and indeed, it isn’t. Things under tension aren’t just good at vibrating, they have a property called resonance which means they tend to vibrate at a particular frequency. This is good in a guitar string, but very bad in a speaker cone. By allowing the cone to “float” and be driven only by the electromagnet formed by the voice coil, you insure that the speaker will be responding to the signal, and not trying to vibrate at its own natural frequency which would distort the reproduced sound.
You really didn’t ask anything.
People are guessing that you are asking about speakers. You could also be asking about electric guitar strings/pickups, microphones, or lots of other things. Better questions, better answers.
As far as magnetism in this sense, an electric guitar makes noise through a speaker through a magnetic pickup. The vibrating string causes a change in the magnetic field above the pickup which causes a change in voltage. From there, the signal is sent to an amp, then to the speaker to produce sound.
It’s not the hair in our ears that picks up the air rarefactions; it’s our eardrums that vibrate and send the corresponding electrical impulses to our brains.
Also, the speaker cone doesn’t oscillate only in frequency with the signal, but also amplitude.
I was going to go into detail, but the How Stuff Works link can do a better job.
Yeah, it doesn’t pick up chicks either.
Nitpick: I think that the reference was to the hair cells inside the cochlea.
I meant a plain ol’ non-electric guitar.
Hm, I’ll have to admit to being uncertain as to how sound reception is split between the ear drum and the cochlear hair cells. Can anyone be more specific but less technical than the Wikipedia?
The basic thing about magnetism is that a magnet exerts a force on things made from magnetic materials. If the things are moveable and if the magnetic vield is varying in strength and direction the thing will move in response to that field. When the thing moves it disturbs the air around it creating pressure waves. If the frequency of those waves is in our audible range we hear them as sound.
This applies to loud speakers, transformer core laminations, transormer coil wires and all such things.
Vibrations in the air move the eardrum. The eardrum moves the ossicles (the three tiny bones in your middle ear). The ossicles move the membrane covering the oval window into the cochlea. The vibrations of that membrane create vibrations in the fluid of the cochlea; the hairs of the hair cells are vibrated by the fluid. Hair cells in different places in the cochlea fire upon being vibrated by different frequencies. Oversimplifying, the cochlea looks a bit like a nautilus shell, curving in on itself; hair cells at the base (wide, open end, near the oval window) fire in response to high frequencies, while hair cells at the apex (the tip, at the center of the spiral) fire in response to low frequencies.
I know, but I figured as long as we were discussing magnetism, I might as well cover that tangent.
I thought it was the other way around. Tiny hairs, high frequencies, larger hairs, lower frequencies.
Tris
This is true. Sorry about my poor wording.
I was under the impression that the human hearing apparatus used mostly extrinsic tuning, where the factors that induce a given hair cell to respond to a particular frequency are external to the hair cell. There are animals that use intrinsic tuning - turtles, for instance, use hair cells with long stereocilia to detect low-frequency sounds, and short stereocilia to detect high-frequency sounds.
I am not aware of a significant intrinsic tuning component to human hearing. Each inner hair cell has a bunch of stereocilia, arranged in order of size, and interlinked. To my knowledge, there’s no significant difference between these bundles on a cell that fires in response to low frequency sounds and one that fires in response to high frequency sounds. The frequency-response properties have to do with the stiffness of the basilar membrane that the hair cells are attached to, which in turn varies with the location within the cochlea. Here’s a picture that makes it more clear.
