# Why does sound propagate in all directions from source?

Recently I've read about sound wave propagation. Let's look on loudspeaker. If it creates sound waves by pushing and pulling the air in front of it , how we can hear sound when we are behind the loudspeaker or above and under? How does sound travel in all directions from loudspeaker's membrane?

• Our ears are surprisingly well tuned. A sound wave 10 times as powerful sounds twice as loud to our perception. That's why 2 people talking don't sound twice as loud as one. You need about 10 people talking for that. I don't know the ratio of sound waves ahead of the mouth vs behind it, but even if it's 50 or 100 to 1, there's still enough backwards traveling sound-waves that you should be able to hear someone from behind without too much trouble. hyperphysics.phy-astr.gsu.edu/hbase/sound/loud.html – userLTK Oct 31 '16 at 15:05
• Notice that it is much louder in front of the speaker, even though you can hear it from and angle. – psitae Oct 31 '16 at 17:19

Reflections from hardish surfaces and diffraction. The housing and the object on which the loudspeaker stands will also vibrate and produce sound waves.

• May I add that: in fact, you can notice that the intensity of the sound is really different depending on whether you are standing in front or behind the loudspeaker. – valerio Nov 23 '16 at 17:19
• I believe it is the case that, even in an anechoic chamber, sound can be heard even directly behind the loudspeaker. This is, of course, highly frequency dependent. – Alfred Centauri Dec 9 '16 at 2:31

@Farcher's answer say it all in a nutshell. Basically, the dominant factor behind the sound reaching behind the loudspeaker, is reflection from surrounding objects, and the vibration of the loudspeaker itself(not very clear sound). Anothet important factor, is diffraction which is common to waves. The sound waves diffract around the corners of the soundbox, producing disturbances behind the box as well. One more thing: the picture you gave shows a sort of sound 'beam'. In general, the wave is much more spread out even at the box membrane.

• This is a comment on Farcher's answer. It's not an answer. – garyp Oct 31 '16 at 17:35
• It was too big for a comment, so i put it separately. I fully acknowledge it is part of @Farcher's answer. – Lelouch Oct 31 '16 at 17:39

It depends on the geometry and vibrational mode of the vibrating source generated. It can be spherical, cylindrical or plane wave fronts.

Use turning fork as an example, we can regard it as a quadruple source (see the link here) and also animations for vibrational mode of tuning fork.

If the membrane diameter is small compared with the wavelength in air (which is usually the case with loudspeakers) then the sound will travel omnidirectionally. Think of extending your curves showing the sound wave, into full circles.

With low frequencies diffraction is much easier - it is as though the box is not really there at all.

## protected by Qmechanic♦Dec 9 '16 at 7:07

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