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Some Psychoacoustics


Some Psychoacoustics

Psychoacoustics deals exactly with auditory perceptions and the way they are interpreted by the brain.Differentiation of strength and height, and the way of analyzing complex sounds and combinations of sounds are psychophysiological phenomenon. Understanding of this mechanism is the basis of the successful coding of sound and reducing sound data without getting a sense of great acoustic degradation.

That is why this type of coding is called psychoacoustic.

But let us first see what the physiology of hearing is.

Ear generally comprises three sections – the external, middle and internal.

The outer covers cochlea and the auditory canal, which focus and direct the received sound vibrations.

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The middle ear perceived, transmitted and amplified oscillation through the auditory membrane (eardrum) and three small bones – hammer, anvil and stirrup (the smallest bones in the human body!).

And the inner ear accept vibration and translates them into nerve impulses.

As we see, until reaching the sound to the inner ear we have a fully mechanical system, detecting and transfering the vibrations of the environment.

The interesting thing occurs at the moment at which the stirrup transmits the vibrations to the inner ear.

There’s something resembling a snail’s shell or shell filled with lymph fluid, along which there is separation membrane called the basilar. Inside the

cochlea the periodical movement is converted into a sound wave spreading in the lymph fluid medium. Accordingly, the basilar membrane separates two types of lymph by various chemical and electrical properties, present in the cochlea of the inner ear.

Along it is situated the so called. ORGAN OF CORTI, consisting of about 30 000 small stigmas associated with neuronal fibers.


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Upon stimulation of the lymph one side due to the sound that came out of the stirrup / last of the small bones of the middle ear, remember / membrane, figuratively speaking, began to wriggle changing along with the situation of the cilia.

Frequency localization is done in a very cleverly way. Because entire system has resonant fields in different locations along the basilar membrane bends are obtained which are dependent as regards the place of its excitation from the wavelength, respectively, from the frequency of the sound.

Higher sounds generate deformation in areas close to the beginning of the snail, and the lower – to the end.

 A shaking the the cilia is obtained in the area of the organ of Corti, leading to excitation the associated nerve endings.

This provides information to the brain about the height of the sound, as well as in certain complex sounds it provides an indication of the spectrum.

Actually thus have a resonance system in which the ear is capable to distinguish the height, the amplitude and the spectrum of the sound.  The brain, in combination with the inner ear acts as a spectrum analyzer with linear filters, managing through organ of Corti to carry out an analysis by the order of Fourier (FFT Analysis), something previously impossible even for computing machine.

In fact, the analogy with digital audio system is quite complete: the ear canal and eardrum are respectively the exterior and the membrane of the microphone, the inner ear and the organ of Corti in it are similar to the analog-to-digital converter, and the analyst portion in the brain, located in the auditory part of the cortex plays the role of the processor data and codec. Along with this system is quite perfect in amplitude ratio, it can capture and analyze even relatively very weak sounds.

This is an extremely simplified model of the effects of hearing aid, however, showed greater complexity and perfection of the system of perception and analysis of acoustic information.

Of course, it also has weaknesses. Because resonantly it is set mostly to medium high frequencies (about 3 kHz), it is more linear in this part of the spectrum. Ability of differentiation hearing and the thresholds of hearing fall in the border areas, ie hearing is by nature nonlinear, as we can see from the curves of the Fletcher-Munson,

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for which we have spoken. Moreover, the emergence of different auditory artifacts not present as sound phenomena in many cases is due to the characteristics and nonlinearity of hearing, particularly of the transformations in the inner ear. These are different distortions especially at high levels, and combinational tones nonexistent before that occur as auditory phenomenon in the inner ear, as evidenced.

Such is the feeling of “beating” the sound at adjacent frequencies as well as the occurrence of subjective harmonics in the processing of the sound from the hearing aid.

Furthermore, the excitation of certain areas of the sensory apparatus results in little or no perception of weaker stimuli adjacent to more strongly thereof.

This is a general psycho-physiological phenomenon. How can we illustrate it?

Personally, I immediately think of a film based on the novel of Dumasл So there Porthos / May was he / landing with work by a high fence and groaned: “The good thing about sciatica is that past him you forget about hemorrhoids …”

Here is an example of the masking function of a stronger sense effect …:-)


The Inner ear and the the brain subsequently tend to ignore certain sounds if they are perceived together with stronger than them in a common audible frequency domain.


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