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In this second part of these Sound Theory classes, we have dealt with the human perception of sound:

We are capable to hear air pressure variations thanks to our eardrum, that vibrates and transmit its movements to a set of three little bones, which in turn provoke longitudinal waves in our cochlea, that are transformed in electrical impulses by the movement of the cilia. This process is known as transduction.

Due to the physical characteristics of our ear canal, we do not perceive all frequencies as equally loud: we tend to feel low and high frequencies softer, while we are more sensitive to middle frequencies in the range of 2KHz to 4KHz. Using weighted dB values gives us a better indication of the perceived loudness.

Our hearing can be damaged in many ways by exposure to high sound pressure levels, and the damages can be temporary or permanent, depending on the length of the exposure and the kind of pressure exercised on our eardrums.

Also, the perceived loudness depend more on the average peak values of a sound wave (RMS) rather than on single transients.

In localising a sound source, our brain interprets the sound qualities transmitted by both ears and gives us a sense of direction: the qualities that concur to this are Interaural Amplitude Differences (IAD), Interaural Time Differences (ITD) (and consequently phase differences), and frequency content differences.

We have also seen how in some cases we hear things differently than what they are: the most common situations being the mask effect, the Doppler effect, beats, and the Haas effect.

Eventually we have seen how the audible spectrum is divided in octaves and their relation to sound intensity. We have also compared white noise (all octaves at the same amplitude) and pink noise (all octaves whose amplitude is weighted according to their intensity, so that we feel them equally loud).

This is the end of the Sound Theory lessons, now you have the theoretical knowledge necessary to understand the nature of sound. We are going to use this knowledge applying it to the most common audio processing techniques in part 3. Now, it?s time to talk about how Sound Theory relates to music, in the next part: music theory.

Do not forget that the ultimate purpose of this course is to help you develop trained ears! The theory is useful for you to understand why things happen, but the practice is vital, as in any other activity. Take a lot of tests in the free tests section, it is the only way you can progress!!!