# Audio sources doubt .

dexterdev
Audio sources doubt.....

hi all,
I dont know where to post this. sorry for that.

My doubt is that if there are 2 audio sources like one mp3 music file playing from a cd player and another speech signal coming from a radio. If I can hear both signals, what is the resultant audio signal which my ear hears. Does the mathematical operation done is addition of those signals and then hear it as a single sound or any other operation.

- Devanand T

Staff Emeritus

The ears (together with the brain) form a very sophisticated detector. You can easily separate signals based on their different originating directions, so you would be recognizing two different sources. Even were you to mix the signals so that they originated from the same location, your brain can readily separate them based on the different levels/frequency spectrum/content, etc., so again you would be hearing two different sources. (That explains why you can make out the words someone is speaking at a party, even when there are many other voices, some louder, all around.)

Maybe you could record one track of a musical hit on a CD, and another track of the same music on an mp3, and then play these synchronized and originating from the same point in a room. Then you may succeed in having the ears believe they are hearing only one source, that being the sum of those two.

The ears themselves sense the sum of the two signals; it's the brain that takes credit for performing all the complex processing, of course.

DragonPetter

hi all,
I dont know where to post this. sorry for that.

My doubt is that if there are 2 audio sources like one mp3 music file playing from a cd player and another speech signal coming from a radio. If I can hear both signals, what is the resultant audio signal which my ear hears. Does the mathematical operation done is addition of those signals and then hear it as a single sound or any other operation.

- Devanand T

The sound pressure waves into your ears are a single sound, and it is the superposition (like you said, it is addition) of all of the different sounds at any instant that they are arriving at your ear. With respect to NascentOxygen's post, the superposition coming in will be dependent on the phase and amplitude of each individual sound signal, and so there is still information that lets your brain/signal processor tell the difference between the sounds.

An interesting effect of this is comb filtering, where 2 sounds can add together at your ear, and, depending on where they came from, they can interfere with each other (they are out of phase). I think audio technicians and producers can use this to optimize their recording studios.
http://en.wikipedia.org/wiki/Comb_filter

A field of signal processing takes advantage of this comb filter effect in something called the cepstrum.

Last edited:
Gold Member

When you have two sine waves with frequencies that are close together, you can hear what's know as beats.

By varying the difference between these tones, you can vary the "beat frequency". Here are some animations with sound to see how this works.

Interference beats and Tartini tones

dexterdev

thanks for the replies guys