how do ears depict sound from places?

michael_w

As I was walking home today a question popped into my mind.

How do your ears tell your brain that a sound is coming from you from the front or the back of you? It's kinda obvious if its from left or right but from the front or rear kinda made me think...

RuSsMaN

Why? The scientific explanation is fairly complex. How about this:

We live in a 4-dimensional world (or least we are AWARE of 4).

(space) - height, width, depth - and time. Both our optic and auditory senses can detect all four, if both the l/r are working properly - if not, space dimensions can get fuzzy.

Just as something appears 'wide', something can also sound 'wide'.

Cheers,
Russ

michael_w

Hmm.. thanks for the response. It sort of clears things up but yet opens so many more questions.

Frank Z

Copied from http://www.kemt.fei.tuke.sk/Predmety/KEMT320_EA/_web/Online_Course_on_Acoustics/hearing.html


DIRECTION PERCEPTION
The brain is able to detect the relative direction of a sound using the following mechanisms;

Interaural delay
Depends entirely upon time delays between similar excitement levels in each ear. The distance between each ear can be taken to be about 150mm. This means that there exists a vertical plane, running through the centre of the head, within which sound reach each ear simultaneously.

The Effects of the Pinnae
These are designed to collect frontal sound and reflect it down the aural canal. Sound entering from above and behind must have been diffracted by the pinea and, as a result, slight spectral changes to the sound will have occurred (more on diffraction later).

Subtle Head Movements
These assist in determining the height of a sound source in the medial plane (as described above).

Basically your brain is able to determine direction by the delay in excitement (reception of the sound) within each ear.

BlueMDPicker

Natural selection - the ones who couldn't discern sound behind them never made it into the gene pool.

michael_w

Sound entering from above and behind must have been diffracted by the pinea and, as a result, slight spectral changes to the sound will have occurred

Thats what I was looking for. Thanks for the link and info.

Frank Z

Your welcome.

Mjr7531

The time difference of a sound receive between both ears gives you a general directions, try this,
Have someone clack spoons around your head behind you, point to where you heard it, have them verify,
then,
put a paper tube completly over one ear and have your friend clack spoons together and point to where you heard it, have them verify, it's a nifty simple experiment.

RuSsMaN

Nice read Frank.

Cheers,
Rooster

brody05

covered this at uni a while back (actually quite a while but still have my notes) and here is a copy/paste from another thread some time ago about the old hearing thing, it may help fill in a little more detail without getting into the big words.....

A person determines direction of sound by two mechanisms, 1: the time lag between the entry of sound into one ear and into the opposite ear and 2: the difference between the intensities of the sounds in the two ears, the first of the 2 methods works best at at frequencies below 3000 cycles per sec and the intensity mechanism operates best at higher freq because the head acts as a sound barrier at these freq.
The time lag mechanism works best out of the two because it does not depend on on extraneous factors but only on the exact interval of time between the acoustical signals.
Lower frequencies are harder to place because the lower stuff depends to a great extent of loudness, there are many variables but lets say an average adult relies on sound and vibration thresholds. Low freq soundwaves travel all the way up the basilar membrane as opposed to medium and high freq, brain neurons are are activated by specific sound freq and as low freq stimulate the entire basilar membrane at 200 cylcles per second it is difficult to differentiate between low sound freq betwwen 20 and 200 cycles. This however has no bearing on the first of the two theories only the second but it does not stop the brain from placing bass at 22hz or above, just limits one of the two mechanisms. At a level of 73.8 DB the vibration sensors are stimulated at 1 hz (prickling and tactile threshold).

In saying that at a moderate level of 72 DB and above I and others are more than capable of telling the difference where the sound is coming from to a certain degree regardless of the freq most speakers are able to reproduce right up to 10,000 cycles.

Brody

michael_w

Thats great Brody, thanks for all the information. I just keep learning more and more about this with every post . Mjr7531 I haven't had a chance to try the experiment cause I've been pretty busy but I'll try it when I get some time.