Masking effect

Masking effects (also known as concealment ) have the effect in the human hearing that the human being can not perceive certain frequency components in a noise or only with reduced sensitivity.

principle

How masking effects work

For example, the hearing is not able to perceive very quiet tones in the middle frequency range at the same time when the bass is very loud. The basses mask the mids here. In this example, the minimum level from which these mids are perceived depends on the level of the bass signal and the frequency spacing between the bass tone and the midrange.

The picture shows how masking effects work. For example, if a 1 kHz tone with a sound level of 80 dB is present, a 2 kHz tone of 40 dB can no longer be heard. That is, the 2 kHz tone can be omitted without a human hearing the difference. If a 2 kHz tone of 60 dB occurs together with a 1 kHz tone of 80 dB, both tones can be heard. But you can transmit this 2 kHz tone with very poor quality: Even background noises of 40 dB can no longer be perceived by humans.

root cause

These masking effects are due to the mechanics of the human inner ear. In the inner ear , the sound makes the basilar membrane vibrate. Each pitch leads to resonance at a different point on the basilar membrane , i.e. H. too strong movements. These movements are sensed by nerve cells that are distributed over the length of the basilar membrane and thus lead to hearing sensations of different pitches.

The mechanics of the inner ear are structured in such a way that high tones lead to resonances directly at the beginning of the basilar membrane and thus to the excitation of nerve cells. After the resonance point, they are strongly attenuated and no longer affect the nerve cells responsible for lower tones. However, low frequencies must first “run along” the entire length of the basilar membrane before they lead to resonance and excitation of the nerve cells and before they are attenuated. This means that nerve cells for high and medium pitches also perceive the bass vibrations. In the presence of low tones, medium tones must be at least so strong that they "drown out" the co-excitation caused by the bass.

history

In 1894 the physicist Alfred Max Mayer first described the overlap effects between high, soft violin tones and louder deep 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 . In 1977 the musicologist Johann Sundberg investigated the effect of masking effects on the ability of the singing voice to assert itself against the orchestra. More recently, these phenomena play a role in the development of audio formats .

application

With methods for lossy audio data compression , such as MP3 or Ogg Vorbis , such masking effects are specifically exploited to filter out frequency components that are currently inaudible due to masking, or to reduce frequency ranges that are partially masked with lower quality (i.e. with lower data rate ).

The masking effects play an essential role in measuring the perceived loudness . This is because it describes which nerve cells are actually stimulated by a sound. The sum of all nerve excitations reflects the perceived volume.

calculation

Edge steepness of the left edge (concealment of lower frequencies)

{\ displaystyle S _ {\ mathrm {l}} = 27 \, {\ text {dB / Bark}}}

See: Bark

Edge steepness of the right edge (masking higher frequencies)

{\ displaystyle S _ {\ mathrm {r}} = \ left (24 + {\ dfrac {0 {,} 23 \, {\ text {kHz}}} {f _ {\ mathrm {m}}}} - 0 { ,} 2 \ cdot {\ dfrac {L} {\ text {dB}}} \ right) \, {\ text {dB / Bark}}}

with : center frequency,: level of the frequency ${\ displaystyle f _ {\ mathrm {m}}}$ ${\ displaystyle L}$

literature

Thomas Görne: Sound engineering . 1st edition. Fachbuchverlag Leipzig by Carl Hanser Verlag, Munich a. a. 2006, ISBN 3-446-40198-9 .
Gustav Büscher, Alfred Wiegelmann: Little ABC of electroacoustics . 6th edition. Franzis, Munich 1972, ISBN 3-7723-0296-3 .
Hubert Henle: The recording studio manual: practical introduction to professional recording technology . 5th edition. Carstensen, Munich 2001, ISBN 3-910098-19-3 .