Auditory masking

Masking effects (also called masking ) effect in the human ear, that man can not perceive or only with reduced sensitivity in a certain noise frequency components.

Principle

So the hearing is for example not able to simultaneously perceive very soft sounds in the medium frequency range with very loud bass. The bass mask the middle here. The minimum level to be perceived by the middle of this, in this example depends on the level of the bass signal and the frequency separation between bass and Mittenton.

The picture shows the mode of action of masking effects is shown. For example a 1 kHz tone with a sound level of 80 dB is present, then a 2 kHz tone of 40 dB no longer be perceived. That is, the 2 kHz tone may be omitted without a person is listening to the difference. Occurs along with a 1- kHz tone of 80 dB 2- kHz tone of 60 dB, one can perceive both sounds. But you can transfer this 2- kHz tone with very poor quality: self- noise of 40 dB can not be perceived by humans.

Cause

These masking effects are caused by the mechanics of the human inner ear. In the inner ear, the basilar membrane is made ​​to vibrate by the sound. Each pitch results in a different location of the basilar membrane the response, that is to particularly strong movement. These movements are sensed by nerve cells, which are distributed over the length of the basilar membrane and result in auditory sensations of different pitches.

The mechanics of the inner ear is structured so that high tones directly to resonances and thus the excitation of nerve cells at the beginning of the basilar membrane. After the resonance point they are greatly attenuated and no longer influence the authorities responsible for lower notes nerve cells. Low frequencies must but only " walk along " the entire length of the basilar membrane, before they lead to resonance and excitation of nerve cells and before they are damped. This means that even nerve cells for high and medium pitches noticed the bass vibrations. Midtones must be so strong in the presence of low sounds at least that they " drown " the co- excitation by the bass.

History

In 1894 the physicist Alfred Max Mayer first described overlap effects between high, silent violin tones and deep louder wind instruments in symphony orchestras. The first systematic experiments on masking were carried out in 1924 by R. L. Wegel and C. E. Lane at Bell Laboratories. The musicologist Johann Sundberg 1977 examined the effect of masking effects on the assertiveness of the singing voice against the orchestra. In recent times, these phenomena play a role in the development of audio formats.

Application

In methods of lossy audio data compression, such as MP3 or Ogg Vorbis, such masking effects are deliberately exploited to filter out frequency components that are inaudible due to masking at the time, for this moment, or frequency ranges that are partially masked with lower quality (ie h to transmit at a lower data rate ).

For the measurement of perceived loudness masking effects play a significant role. For here on is described which nerve cells are stimulated by a noise at all. The sum of all nerve impulses reflects the perceived loudness.

Calculation

Slope of the left flank ( lower frequency masking )

See: Bark

Slope of the right flank ( concealment of higher frequencies )

With: center frequency: the frequency level