[Phoebe]

Phoebe asked the Naked Scientists:
   
If there are multiple sources of sound in a particular area - say 20 sources of air conditioning units, each creating about 60 Decibels of sound, does this create more sound (and a louder sound) in the same way that lighting multiple candles creates more light?

What do you think?

[RD]

Q. Do sounds add together to make louder sounds?

A. Yes, with the exception of anti-phase sound, as used in noise cancelling headphones.

[doppler1]

Nuff said

[Professor Mega-Mind]

...one frequency only...
IF the sound-wave peaks match up , then yes .
IF the peaks of one match the troughs of the other , then  no .
The second case results in what is now called "Noise-Cancellation" .
P.M.

[alancalverd]

If you have n sources of equal amplitude but random phases and frequencies, the total amplitude is  √n .

[Professor Mega-Mind]

...so , IF...
One has two identical speakers , in a perfectly sound-reflective room . Each emits an identical sound-wave , perfectly out of sync . All sound is perfectly cancelled out . Where does the energy put into the emitted sound-waves go ?
P.M.

[alancalverd]

Heat

[Professor Mega-Mind]

...most excellent !
Sounds (sic) like a good alternative  area heating mechanism , or very focused , internal-target heater . The medical and industrial applications could be many-fold .
 If thine appendix offends thee , cook it in situ , THEN pluck it out !
P.M.

[evan_au]

Each emits an identical sound-wave , perfectly out of sync . All sound is perfectly cancelled out

This is not correct.
The sound emitted by the speakers may be out of sync, but the speed of sound is not infinite.
So the points equidistant from the speakers will have cancellation, but points slightly away from equidistant will not have cancellation.

In fact, the points with zero amplitude will be balanced by the points with doubled amplitude.

in a perfectly sound-reflective room

You need to do this experiment in a perfectly sound-absorbing room (anechoic chamber).

Otherwise, the sound energy injected into the room will just build up forever, causing the speakers, amplifier and room to eventually self-destruct.
See: https://en.wikipedia.org/wiki/Anechoic_chamber

[Professor Mega-Mind]

...the limits of noise cancellation !
Your explanation actually brings in another possible use ; sonar for the blind . The return wave's characteristics would reveal the direction and distance to nearby objects . The subject would hear nothing , until an object approached , then they would hear what YOU said !
So now it's a room heater that also tells you what is in that room .
Unnecessary Zoom , unnecessary zoom !
D.H.

[evan_au]

sonar for the blind . ... . The subject would hear nothing , until an object approached

Some blind people already use sonar by clicking with their tongue and teeth.

They hear little reflection (maybe the path they are walking on) until they get near a pole, building, car, etc, when they hear a reflection.

The return wave's characteristics would reveal the direction and distance to nearby objects

I saw this blind man on TV walking along the street, successfully recognising cars, tree trunks and shrubs beside the path by the way they reflected sound. Here he is in Brugge...

[Professor Mega-Mind]

...on the same page...
If the blind person moves , or turns themselves , the return-patterns should change , thus indicating the object's direction/distance . It is a stereo effect .
P.M.

[Bored chemist]

All sound is perfectly cancelled out .

It isn't.

Where does the energy put into the emitted sound-waves go ?

In the limit it either escapes through, or is dissipated as heat in the loudspeakers.
Some will be lose by absorption in the air.

[Andrew Troup]

Phoebe asked the Naked Scientists:
   
If there are multiple sources of sound in a particular area - say 20 sources of air conditioning units, each creating about 60 Decibels of sound, does this create more sound (and a louder sound) in the same way that lighting multiple candles creates more light?

What do you think?

My recollection is that two sound sources, each producing (say) 60dB, will in combination amount to only 63dB, because of the logarithmic nature of the dB scale. And subjectively, two (say) musical instruments, identical and playing the same melody, are only slightly louder than one.

[Professor Mega-Mind]

Answer : No , adding together DIFFERENT sounds will NOT make any of those any louder .
However , considering that a sound is a unique frequency of crests and troughs , no one sound would be any louder , just the total noise recieved by the ear .
P.M.
Exemplification : Orcestras ; When one instrument plays , the hall is comparatively quiet . When another joins in , it's almost as quiet . If ALL join in , you can't even hear yourself think , despite the fact that no instrument is playing any louder than at the beginning .
*Can you hear me now ?!

[RD]

... If the blind person moves , or turns themselves , the return-patterns should change , thus indicating the object's direction/distance . It is a stereo effect ...

You don't need to move your head to tell where a sound is coming from:
the delay between the sound arriving at each ear tells you its bearing  ...

[Colin2B]

However , considering that a sound is a unique frequency of crests and troughs , no one sound would be any louder , just the total noise recieved by the ear .
P.M.

Would you mind rewriting this so that it makes sense.

[alancalverd]

Lots of interesting and confused physics going on here!

Simple pressure waves will add if in phase and subtract if out of phase. The amplitudes of non-coherent sounds add in quadrature (A = √(a₁² + a₂² +.....) so the roar of a crowd depends on the square root of the number of people shouting (it's a bit less, in practice, because the sources are dispersed).

Similar effect with an orchestra except that groups of instruments will be playing at pretty much the same  frequency, so you will get some constructive addition of the fundamental of the note they are playing. This can have a surprising effect on the listener's experience: a solo violin is identifiable by its "piercing" harmonics above a full orchestra but three well tuned violins in a studio band can sound like syrup (try some old recordings of George Melachrino's orchestra) and the more you add, the more each section sounds like a flute (almost pure sine wave).

Fairly complete noise cancellation is difficult in a large space, as has been pointed out above, because the intensity of the combined waveforms depends on position, but it's fairly easy  over a small area, which is why noise-cancelling headphones and microphones are very effective: you detect and electronically subtract the unwanted ambient signal from the output or input, so the pressure reaching the ear, or the waveform leaving the microphone, carries only the wanted signal.

What I always find astonishing is the ability of the brain to decode a single continuous pressure waveform into separate frequencies and identify musical instruments or  individual voices, even when the bandwidth is severely limited. And one question I'd like to investigate in my next life is whether somebody who had never heard the individual instruments would be able to decode the sound of a string quartet. 

[Professor Mega-Mind]

Damned good , Mr.A.C.Hertz !
You have a hella grasp of the intricacies of multiple-source sound interactions . I suppose if several instruments had the exact same frequency and were phase-matched , that would result in one sound changing amplitude , in direct proportion to the number of sources contributing at that moment . However , as you made clear , the tiniest diff. in frequency or phase or volume can reveal the different sources and their locations . So...rarely would an audience member be fooled by different instruments sporadically adding in to a group sound .
*Cheerio !
P.M.. 😎

[hamdani yusuf]

What I always find astonishing is the ability of the brain to decode a single continuous pressure waveform into separate frequencies and identify musical instruments or  individual voices, even when the bandwidth is severely limited. And one question I'd like to investigate in my next life is whether somebody who had never heard the individual instruments would be able to decode the sound of a string quartet.

It is the cochlea which act as a hardware "Fourier Transformer" which decomposes sound into its constituent frequencies. Its function is similar to glass prism or diffraction grating in spectroscopy.
https://en.wikipedia.org/wiki/Cochlea#Hearing

The hair cells in the organ of Corti are tuned to certain sound frequencies by way of their location in the cochlea, due to the degree of stiffness in the basilar membrane.[4] This stiffness is due to, among other things, the thickness and width of the basilar membrane,[5] which along the length of the cochlea is stiffest nearest its beginning at the oval window, where the stapes introduces the vibrations coming from the eardrum. Since its stiffness is high there, it allows only high-frequency vibrations to move the basilar membrane, and thus the hair cells. The farther a wave travels towards the cochlea's apex (the helicotrema), the less stiff the basilar membrane is; thus lower frequencies travel down the tube, and the less-stiff membrane is moved most easily by them where the reduced stiffness allows: that is, as the basilar membrane gets less and less stiff, waves slow down and it responds better to lower frequencies. In addition, in mammals, the cochlea is coiled, which has been shown to enhance low-frequency vibrations as they travel through the fluid-filled coil.[6] This spatial arrangement of sound reception is referred to as tonotopy.