I know this is kind of a lame question, but I wanted to know how loudspeakers are able to play so many different sounds at the same time. Like, in a song we can hear the voice of the singer and all the musical instruments at the same time. The sounds come from the speaker at the same time. How is the speaker even able to do it?


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    \$\begingroup\$ This is possible, because to a practical degree air, speaker transducers, and cables are linear - which is to say that independent inputs exactly sum. At signal levels where that starts to break down (such as where A + B becomes something like .9 A + .9 B, though mostly on the waveform peaks) you start to get distortion. \$\endgroup\$ – Chris Stratton Oct 10 '13 at 14:43

The real answer has nothing to do with how speakers work, but the fact that sound waves add in air. When you are listening to a live performance with a singer and instruments, you hear them all together then too. Your question about how a speaker can produce such a composite sound is no different from asking how your ear can hear more than one sound at a time.

Think of what a sound wave really is, which is small aternating pressure variations in the air. You can have multiple sound source, but at any one point in the air you have a single function of pressure over time. This pressure function contains the sum of all the pressures variations caused by all the sound sources.

This varying pressure is what your ears measure at two different points in space (since you have two ears). This sound pressure variation is also what a microphone measures and converters to a electrical signal. At any instant, there is a single pressure value, or a single voltage value coming from a microphone.

A speaker simply does the reverse. It receives a single voltage value at any instant, and produces a local air pressure variation accordingly. Those air pressure variations are ideally the same as measured by the microphone when the recording was made. Since the single-valued function of pressure over time contained the sum of all the sound sources that the microphone picked up, the speaker reproduces the same sum of signals. Your ears then hear the same sum of signals, which allows you to hear the singer and multiple instruments at the same time, just as you would if you were there when the music was performed live.

  • \$\begingroup\$ i knew this what you said actually what i am confused at that when two sound waves with two different pitch and wavelength combine in the air shouldn't the produce a totally different sound \$\endgroup\$ – Dimensionless Oct 10 '13 at 14:47
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    \$\begingroup\$ @Akash: No, it's still just the instananeous sum of pressures at any one location and point in time. Or another way to put it, the sum is a different sound, but our ears and brain are able to decompose the composite into the individual pieces. This is a perception issue though, not physics anymore. \$\endgroup\$ – Olin Lathrop Oct 10 '13 at 15:45
  • \$\begingroup\$ They do combine, but this is also true of sound produced directly by instruments, and the combination is "decomposable" by the listener. Likewise two identical sounds from different locations may interfere and cancel out in places. \$\endgroup\$ – pjc50 Oct 10 '13 at 15:46

The same way that air is able to transmit lots of different sounds to your ear at the same time.

For air the pressure waves from each source are effectively summed in the air.

For recorded sounds played electroonically it's the same, the sound engineer's mixer sums the pressure waveforms that have been transformed into electrical waveforms. The speaker receives a waveform that is a sum of the waveforms received by various microphones each of which (especially in a live setting) will have received a pressure waveform that is the sum of several sources.

The individual receptors in our ears are each tuned somewhat to specific frequencies and our brains are able to (subconsciously) separate out the sounds.

  • \$\begingroup\$ but there are no receptor's in ear's the sound waves are transformed back to electrical signal's in the ear which are then modulated by the brain as sound \$\endgroup\$ – Dimensionless Oct 10 '13 at 12:36
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    \$\begingroup\$ The ear's receptors are small cilia in the ear canal, some of these break off as you get older which is why older people can't hear high pitched sounds. \$\endgroup\$ – RedGrittyBrick Oct 10 '13 at 12:37
  • \$\begingroup\$ hmm i didn't knew about that one thank's \$\endgroup\$ – Dimensionless Oct 10 '13 at 12:40

It is amazing considering that the speaker cone might be moving in and out at (say) 100Hz yet a pure tone at 1kHz doesn't appear to be doppler shifted acoustically by the 100Hz. This is because, for each frequency in the range of a speaker, the cone vibrates at a different radius.

EDIT - I'm assured by Brian Drummond that the sentance below is wrong!: -

This means that 100Hz makes cone movements at a wider radius (and all the way round) whereas 1kHz vibrates a portion of the cone at a significantly smaller radius.

Hopefully, back to the truth: OK, speakers aren't perfect and there are some doppler effects but these tend to be one frequency doppler shifting a frequency that is close to itself and this isn't "heard" by the ear.

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    \$\begingroup\$ Drive units are intended to work in "piston" mode, below the frequency at which they would break up; i.e. radiating all frequencies from the entire area. (This is the intent; reality is not so perfect). Above that frequency range, a smaller drive unit (e.g. tweeter) is used, with a crossover splitting the input signal into separate frequency bands for each. From a single driver, Doppler is observable and measurable, but the displacement is usually small enough to keep Doppler more or less inaudible. \$\endgroup\$ – Brian Drummond Oct 10 '13 at 20:59
  • \$\begingroup\$ @BrianDrummond well I never!!! I was taught wrong back in the bad old days. I shall correct immediately. \$\endgroup\$ – Andy aka Oct 10 '13 at 21:03
  • \$\begingroup\$ Celestion many years ago(30+?) had a paper on laser doppler interferometry conclusively demonstrating drive units operating as pistons; and (other brands) failing to do so. I do know that when I got my hands on ceramic-stiffened tweeters that remained in piston mode above 20kHz, I had one sweet sounding pair of speakers... \$\endgroup\$ – Brian Drummond Oct 10 '13 at 21:11
  • \$\begingroup\$ @BrianDrummond I got taught about speakers in latin it was that long ago LOL - probably before the celestion experiment. I remember the teacher proudly pointing out doppler shifts and how the cone was shaped so that it resonated at different radii for different frequencies. He may have been misled or wrong but what is sooo surprising is that I've been in the (amateur) music business for quite a few years now and anyone who had anything to say about it tended to agree. Did speakers once produce sounds like I said above or is this just another load of mumbo jumbo I've swallowed? \$\endgroup\$ – Andy aka Oct 10 '13 at 21:18
  • \$\begingroup\$ It's possible that 1930s paper cones did that; not necessarily by design! But by the time Bextrene came along in the 1970s - or even the 1960ish Leak Sandwich (alu skin; foam core) stiffness was key, to try for piston operation. By the time of Colloms' book (my copy from 1980) piston operation was orthodoxy, and he quotes James Moir's 1974 work on Doppler. So it must have been a while ago... \$\endgroup\$ – Brian Drummond Oct 10 '13 at 21:33

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