To further complicate matters, human beings do not hear all frequencies equally efficiently. Or to put it another way, if you play a range of different frequencies to a person all at exactly the same volume, the person will not "hear" them at the same volume, especially if those frequencies are played at a low volume. Have you noticed that when you listen to piece of music at a low volume the low frequencies like the bass drum & bass guitar seem less noticeable, as you turn the music up you then hear these frequencies more clearly in relation to the other instruments playing.

In 1933 two scientists - Harvey C. Fletcher & Wilden A. Munson - carried out experiments relating to this & came up with the "Fletcher Munson Equal Loudness Contour" - a graph detailing how human's experience different frequencies at differing volumes.

FletcherMunson

If you look at the bottom contour, this represents the threshold of audibility. Or the quietest volume a person can hear. You will notice that the low frequencies (20 to 60 Hz) have to be played much, much louder in order to be heard than for instance frequencies at around 3000 to 4000 Hz. What can we deduce from this? Well, at very quiet levels human's ears are most efficient at around 3 to 4 kHz & fairly inefficient at low frequencies. Also human's do not hear very high frequencies (5kHz and above) well at low volumes.

Once, the volume levels are turned up, human hearing becomes more efficient in the lower frequencies this is demonstated in the graph as you can see the top contour is noticeably less wavy in the lower frequencies.

You can test this out for yourself by going to http://www.phys.unsw.edu.au/jw/hearing.html. Here, with a decent set of headphones you can try out Fletcher & Munson's theories for yourself. As a sound engineer this has great relevance & you should now realise that when mixing a piece of music it is vital that you listen at various different levels because the relationship between different frequencies changes depending upon what volume you are listening at.

3. VELOCITY

This is the speed at which sound travels in air - 340 metres per second.

4. WAVELENGTH

This is the actual distance in a medium (typically air) between the beginning & the end of a wave's cycle. In the waveform below, the red arrows show the wavelength of the sound wave.

 

Wavelength

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