Perception of sound: Volume and perception of frequency, distorsion, Masking

The same sound is perceived as being more acute if the volume is increased considerably. The reasons for this phenomenon are not as yet still clear.

The sound of distorsion is more or less the sound you hear coming out of a electric guitar distortor. You probably know the sound we're speaking of- the trademark rock n roll distorted guitar sound that has made history (just to give you an example), but how is it generated? To understand it, let's again take a look at our sinusoid, this example being applicable to complex sounds [Time-Frequency representation ] . Let's presume that the output of a circuit upon which we'll apply an entering sinusoid cannot go beyond a certain value.

Distorsion

The output signal we'll get will be the one on the right-hand-side figure: the sinusoid whose head has been cut off. If we observe this wave form we can see the sudden transitions introduced by the cut, that generate frequencies that are higher than the sinusoid we mentioned. This shall be true with every sinusoidal signal component. Thus, at the distortor's output, the original signal shall be enriched with a whole set of high frequencies that depend upon the initial signal's frequency and that characterize the sound of analogical distorsion. To listen to the sound of distorsion we'll refer to the following sounds: the first taken from an electric guitar, and the second taken from the latter passed through a distortor:

Sound of a clean electric guitar  [Track 14]

Sound of a distorted electric guitar  [Track 15]

In this case the distorsion has been willfully applied to obtain an effect. Generally one should pay great attention to the circuit peaks we're using, so as to not distort the frequencies with over-high amplitude levels (for example the gain of a pre-amplifier present on mixer channels mustn't create an exceedingly high level in order not to saturate the circuits that find themselves downstream).

A frequency with a high amplitude can mask nearby frequencies that have lower amplitudes. This happens because frequencies that are close to each other are decoded by lashes that belong to the same critical band. This characteristic is widely exploited so as to create algorthythms of compression of audio data in digital format such as MP3's and ATRAC used in MiniDisc systems [The MiniDisc ] . Such algorhythms allow 5:1 compression rates to take place.