11-13-09, 06:10 PM #1
Speakers: How do they produce different sounds simultaneously?
At first glance, this appears a basic question with a simple answer - I know speaker cones vibrate and move air to produce a sound.
What I'm confused about here though is the "simultaneous" aspect - how is it that a system with only one output (the physical driver/cone), only capable of vibrating at a single specific frequency at one moment in time, is able to produce what we perceive as all the instruments in an orchestra playing at once?
There's many articles on the net explaining how speakers work in general, but I haven't seen one which really explains this aspect in detail. I'm sure the answer is simple - perhaps it's more of a psychoacoustic effect than anything else, perhaps achieved by rapidly cycling through the frequencies of each instrument?
11-14-09, 09:25 AM #2
The sound you hear is the result of nerve endings in your ears reacting to the vibration of the tissue in which they reside; then converting that kinetic energy into electrical impulses; which then go to an area in your brain for decoding. It's fairly similar to your sense of touch, which is sensitivity to pressure, but it's decoded by your brain in a much different way.
That tissue vibrates because the air molecules adjacent to it are vibrating. (Or water molecules, if you happen to be swimming underwater.) Your question could be just as easily restated as: "How can a molecule produce different sounds simultaneously by vibrating at a single specific frequency at one moment in time?"
The answer is: It does not vibrate at a single frequency.
It would help you understand this if you know someone who can take you into a sound studio and show you sound waves on an oscilloscope. You will see that "sound" is not a nice clean sine wave with an easily visible frequency and amplitude. It's an incredibly complicated waveform: the mathematical sum of all the individual sounds that are picked up by the microphone.
If it's music you'll see a repeating pattern, although the shape of the wave may be so complex that the repetition might be difficult to spot. If it's conversation or ambient noise there won't be a repeating pattern, because the individual component waveforms don't have regular repeating shapes, i.e., they're not music.
Consider the way you tune a guitar. (I play bass guitar but it works the same way.) You finger the E string on the fifth fret and pluck it so it produces an A, at the same time plucking the open A string. If the instrument is in tune both strings produce the identical note and you hear an A -- air vibrating at 440Hz. But if the strings are slightly out of tune, one is vibrating at (say) 440Hz while the other is at (say) 432Hz.
If you capture this with a microphone and look at the waveform on an oscilloscope, you'll see a wave that is the sum of a 440Hz function and a 432Hz function. It will look somewhat like a note with the frequency of 436Hz (a slightly flat A), but with the amplitude slowly changing from one wavecrest to another. If you pan out the scale of the display so you can see several pulses at once, you'll see that the amplitude changes at a regular interval of 8Hz and the combined wave is perfectly regular, but more complicated than a single note.
And the reason I use this example is that you can hear this waveform. This is how we old-timers tune our instruments. (We didn't have these newfangled digital tuners, which IMHO are useless crap anyway). If you pluck two strings simultaneously that are an A note but 8Hz out of synch, you will hear an A, but you will clearly hear the amplitude (loudness) of the note increasing and decreasing at 8Hz (8 times per second). We call those "beats." Of course if the beats are much faster than 15-20 per second we can no longer discern them, and instead of hearing beats we just hear dissonance. If the strings are that far out of tune (after all, if the frequency drops to 411Hz, a mere 29Hz out of tune, it's reached the pitch of a genuine A-flat) we can hear the difference and get them closer by turning the pegs a little bit, then plucking them again and listening for the beats. Eventually we get them to the point that there are no beats and the strings are vibrating in unison, i.e., they are perfectly in tune.
The molecules in the air are doing exactly the same thing the cursor on the oscilloscope is doing, which is exactly the same thing your eardrums are doing, which is exactly the same thing the microphone is doing, which is exactly the same thing the loudspeakers are doing: Adding the individual sound waves mathematically to vibrate in one single but very complicated waveform.
Throughout this chain of events, from the moment two sounds are created in the same physical location all the way through the recording, playback and listening process, the molecules in the air, equipment, or your body are not "vibrating at a single frequency."
No precision is lost in this process. The molecules in the air, the microphone and the loudspeaker are capable of vibrating at frequencies up into the millions of Hz, easily adding sounds together and sending the aggregate waveform to your ears for your brain to deconstruct.
The grooves in an old vinyl record, under a microscope, look exactly like the sound waves on the oscilloscope. Like the air, microphone, loudspeakers and eardrums, the phonograph needle is also well able to vibrate in a complicated pattern that combines multiple frequencies of sound into a single waveform--reproducing sounds up into the high frequencies and down into the bass range that you can't hear. The same is true of magnetic recording tape and radio waves.
Digital recording media, of course, do not have quite this degree of precision. CDs can't match the sound quality of a brand-new vinyl LP or a 32-track studio tape, although the drop-off is very slight. For most of us the difference is--literally--inaudible, but some people, especially fans of symphonic music with its much greater loud-to-soft range than pop music, claim that they can hear it.
People who listen to their music on cellphones can't possibly hear the difference and those people really don't care anyway.
Last edited by Fraggle Rocker; 11-14-09 at 09:33 AM.
11-14-09, 03:46 PM #3
11-14-09, 03:54 PM #4
Yeah thanks Mr Fraggle Rocker for taking the time to reply with such a detailed post, that was exactly the info I was after.
11-15-09, 09:11 PM #5
So then, would it be fair to say that "the amplitude slowly changing from one wavecrest to another" is the "trick" (I know trick is a horrible term) our brains use to say to us "hey, there's more than one sound going on here".
For clarification: If we looked at a speaker (or any other sound source) and watched it pulse once a single time, then pulse again immediately afterwards and measured the interval between those 2 pulses, we would have the frequency for that "moment in time" right? If we took that frequency, and then extended it over time, I'm certain it would sound something like white noise, rather than discernable, individual sound sources.
So it must be the constant changes and fluctuations in that frequency "over time" which our brains are using to "decode" this information and reproduce the original waveform?
11-18-09, 08:11 AM #6
Even then, if you play two notes that are in harmony, i.e. their frequencies are in the ratio of small integers like low C (128Hz) and low G (192Hz), you'll see a complex wave with peaks and valleys that look irregular at first, but then if you pan out you'll see that it repeats exactly with a frequency of 64Hz. Yet your brain will not deconstruct the combination that way and you will not hear a 64Hz tone.If we took that frequency, and then extended it over time, I'm certain it would sound something like white noise, rather than discernable, individual sound sources.So then, would it be fair to say that "the amplitude slowly changing from one wavecrest to another" is the "trick" (I know trick is a horrible term) our brains use to say to us "hey, there's more than one sound going on here". So it must be the constant changes and fluctuations in that frequency "over time" which our brains are using to "decode" this information and reproduce the original waveform?
Acculturation is certainly involved. If there's one Neolithic tribe left unexposed to modern music (and that's unlikely in the age of cheap radios and recordings), their brains might not make sense out of the standard chromatic twelve-tone scale, or even the harmonic (white keys only) scale, or even the pentatonic (black keys only) scale.
And so is training. When I hear a chord, even one played on a synthesizer so all the notes have the same waveform, I hear three notes (or whatever number that chord has). I'm not sure the average listener with no musical training hears it that way.
And then there's something else I can't explain. Our modern instruments are tuned to a chromatic scale: the frequency ratio between any two adjacent notes is 2^(-12). So the ratio between an C and an G is not the 2:3 of the Pythagorean scale, which sets up lovely harmonics that fill the room with resonance if they're played together. It's some ugly irrational number that fills the room with dissonance! In fact if you use the old trick of tuning your guitar to harmonics, lightly touching the E string at the 12th fret and the A string at the 7th fret and plucking them carefully to listen for a perfect octave, it will be slightly out of tune chromatically. Yet when we hear a chromatically tuned A and E (or C and G) played together, we hear a perfect fifth: the simple waveform I described at the beginning of the post, which does not actually exist!
There's a lot of psychology in music. It's not all physics.
And, BTW, there is a tiny percentage of the human race whose brains cannot decode music at all. The condition is known as amusia. All I know about it is what I saw on one PBS program, but at the very least they can't decode pitch so they don't hear melody and harmony, consonance and dissonance. (This is not the same as "tone-deaf," which is merely lack of precision; tone-deaf people can tell if one note is higher than another and they can appreciate harmony.)
I don't know if they can decode rhythm and cadence, that would seem to be more fundamental and it allows us to march in unison, but the program didn't go into it. For that matter if the inability runs that deep you'd think they wouldn't even be able to appreciate standard poetry.
Considering that even other species react to music and can move in time to it, amusia seems like a disability that goes deep into our brain.
Last edited by Fraggle Rocker; 11-18-09 at 08:20 AM.
11-21-09, 11:10 PM #7
11-22-09, 08:46 PM #8
11-23-09, 01:38 AM #9
I found a pic that might help out here (it isn't mine though, just FYI). Forgive me if I screw up the terminology here, but yes, the speaker produces a single, alas complex waveform that is simultaneously representing a broad range of frequencies. The pic in the link shows how this is possible with two frequencies, represented by two simple sine waves (which would each sound like a single tone to us, like the sound of a TV when the emergency communication system is activated). When added, they make a compound waveform that represents both sounds. Were you to hear a speaker driving that compound waveform (and by driving I mean moving in accordance with that waveform, therefore vibrating the air as such) and you have good hearing, your brain would be able to process the signal and you could probably pick out the two notes occurring at the same time.
I'd go on, but Rocker's already hit the nail on the head, and very well I might add.
11-23-09, 05:40 PM #10
11-23-09, 07:52 PM #11
Some systems do so more accurately than others, of course...
11-23-09, 08:55 PM #12
11-25-09, 02:06 AM #13
Here are a few videos to help visualize:
It's also worth nothing that the brain does not decode sound as much as it interprets it, the decoding takes place in the ear where sound waves are converted to neural impulses. Our brains learn to interpret these impulses in much the same way we learn how to discern visual objects and colors, thus the psychological aspect of music that Fraggle referred to. It's very noticeable when you listen to music in a scale you're not familiar with.
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