[scientist]

Actually, I want to ask different question based on this question.
First tell me what is sound.
Don't go too deep, a basic explanation will be okay.
If this solves my problem then it's well and good or else you will definitely solve it.

[JP]

Sound is basically variations in pressure moving through matter.  It carries energy with it that gets transmitted through these pressure variations.

[Don_1]

Ah! But are you not describing vibration. Even someone who is deaf may be able to feel some of these vibrations. But 'sound' is what our brain interpets those vibrations into.

'Tis the old question, 'if a tree falls in the forest and there is nobody there to hear it, does it make a sound?' I would venture the answer to that is no, but it does make vibrations.

[scientist]

So if a object vibrates at high speed(for audible sound) series of compression and depression will be formed and we will hear sound. Frequency will determine its pitch and amplitude will determine loudness.

Suppose only one object is vibrating. So for a given time there would be only one kind of sound i.e. for a given time freq will be 'x' but only 'x', as the vibrating object can not vibrate at two different speed at same time.

My question is, then how speakers make sounds of different instruments etc. at same time?

I hope the whole film/layer(i don't know the exact word) in the speaker moves simultaneously so there is no possibility  that different parts make different sounds.

Is that due to persistence of hearing(like vision) so speakers make different sounds very fast that we are not able to distinguish between them and we feel that it's coming all as one??

[scientist]

i hope this is not as simple as this

[RD]

I think the search-word you're looking for is psychoacoustics

[Spacetectonics]

'Tis the old question, 'if a tree falls in the forest and there is nobody there to hear it, does it make a sound?' I would venture the answer to that is no, but it does make vibrations.

Fare enough but!

 yes it makes sound ,but there is nobody to hear it!:)

[Pmb]

Ah! But are you not describing vibration.

The question was about sound and not vibration. They are related but not identical. For purposes of discussion I’ll use that given in the dictionary that comes with Microsoft Encarta

Sound – A vibratory disturbance, with frequencies in the approximate range of 20 to 20,000 hertz, capable of being heard.

Vibration – A rapid linear motion of a particle or an elastic solid about its equilibrium position.

Both sound and vibration pertains to music. Only vibration pertains to light.

Even someone who is deaf may be able to feel some of these vibrations. But 'sound' is what our brain interpets those vibrations into.

That is highly dependant on how one defines sound. I myself prefer not to define sound that way. I see no reason for it since sound has applications to things which are not related to the ear. It thus makes life easier to define sound in a much less restricted sense.

[JP]

So if a object vibrates at high speed(for audible sound) series of compression and depression will be formed and we will hear sound. Frequency will determine its pitch and amplitude will determine loudness.

Suppose only one object is vibrating. So for a given time there would be only one kind of sound i.e. for a given time freq will be 'x' but only 'x', as the vibrating object can not vibrate at two different speed at same time.

My question is, then how speakers make sounds of different instruments etc. at same time?

I hope the whole film/layer(i don't know the exact word) in the speaker moves simultaneously so there is no possibility  that different parts make different sounds.

Is that due to persistence of hearing(like vision) so speakers make different sounds very fast that we are not able to distinguish between them and we feel that it's coming all as one??

I think the concept that you're missing here and in the other thread on light reflection is that many waves are linear at low intensities.  This means that you can add two (or more) waves and the total wave is just the sum of those many waves.  So if you have several instruments playing at once, the vibration in the air is some complicated looking wave that's the sum of the waves from each instrument.  If you can record that complicated pressure wave and play it back, it will sound like many instruments being played at once.

The speaker can reproduce the sound of many instruments because it can wiggle in a way to recreate that complicated wave.

[David Cooper]

My question is, then how speakers make sounds of different instruments etc. at same time?

All the different sounds simply get combined into a single wave which dictates how the speaker moves in and out. Imagine a simple beep with a sine wave representing it. That wave gives a visual representation of what the speaker does: it moves outwards, slows down, stops, accelerates back in, passes the middle position, slows down, stops, accelerates back towards the middle position, etc.

Now imagine another beep with twice the frequency but at the same volume. To play this sound the speaker would move in and out twice as quickly. If the two sounds are combined, what happens then? Well, a lot depends on the alignment, but I'll describe one case. Let's say that the two sine waves are aligned such that the speaker starts maximally in, and with the two waves combined it will now be closer to twice as far in as it would be for playing just one of the two sounds on its own. It now accelerates outwards and passes the mid point, but there is now a conflict beween the two waves as one of them wants to go on pushing the speaker out while the other wants to pull it back in. The result will be that the speaker doesn't go as far out as it goes in, and it may even be pulled back towards the centre a little at the point where the longer wave beep wants the speaker to be furthest out.

The more sounds you combine, the more complex the movement of the speaker becomes, but it works fine because all it needs to do is recreate the same kind of movement that your eardrum will make when the sounds hit that, and your eardrum can't do anything more fancy than the speaker can.

The easiest way for you to see this in action would be to run a program such as Windows media player and to play a music file with some simple kind of music (ideally slow) and find the background display theme which shows the actual soundwave. You can pause it at any time, and what you see in that line will be a direct representation of how the speaker is moving.

Is that due to persistence of hearing(like vision) so speakers make different sounds very fast that we are not able to distinguish between them and we feel that it's coming all as one??

The sounds really are all combined into a single signal, so the brain has to do a lot of work to unpick them all from it to follow individual instruments or voices.

Edit: Actually, I'm not so sure about that last bit - most of the work may be done by the hairs in the cochlea if they are tuned to different frequencies.

[RD]

My question is, then how speakers make sounds of different instruments etc. at same time?

Many waveforms can be added together, and generally each waveform behaves as if it existed in isolation ...

... the superposition principle can be applied. That means that the net amplitude caused by two or more waves traversing the same space is the sum of the amplitudes which would have been produced by the individual waves separately.

https://en.wikipedia.org/wiki/Superposition_principle#Application_to_waves