What is masking effect?

Definition

Some sounds may no longer be perceived when combined with an additional sound: this effect is called masking. There are two forms of masking : spectral and temporal.

Spectral masking effect

Spectral masking refers to a sound component becoming inaudible due to other frequency components (simultaneous masking). Normal-hearing humans can hear every frequency component of a sound from the hearing domain if no other noise is present. However, the perception threshold of this component can change in presence of a masker. The masking effect can occur in several ways :

  • Tones masked by tones
  • Tones masked by broadband noise
  • Tones combination within a critical band (masking effects are stronger when both signals are within the same critical band.)

The masking efficiency varies according to both signals parameters (frequency, level, signal type), but also according to the listener.

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Application

The figure above highlights that within a noise due to a multi harmonic excitation (such as e-NVH in electrical machines) all frequencies are not necessarily perceived.

As an example, PWM can generate sidebands around multiples of the switching frequencies. As long as they are in the same critical band and lower in level, only the PWM fundamental frequency is heard and the sidebands are masked. However when the rotation speed increases the frequency spread becomes wider. Consequently they may leave the masking area and become audible. If the sidebands are in the same critical band, but their level is higher than the masker threshold, they also become audible and can generate roughness or fluctuation strength.

In xEV NVH applications, masking effect are also important when combining several noise sources such as e-motor whine due to magnetic forces, gear whine due to mechanical tooth forces, wind noise due to aerodynamic forces, and tyre noise due to mechanical friction forces.

Temporal masking effect

Temporal masking refers to a sound becoming inaudible due to the appearance of a sudden loud masker sound immediately preceding or following the first one. The time window for this kind of masking to occur is very short : approximately 20 ms before and 100-200 ms after the masker sound.

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This phenomenon comes from the difference in the duration of the cognitive process being effective in our brain to hear a signal. Softer sounds take longer to be heard than louder ones.

Sound quality

It is relevant to understand how sounds interact with one another and how the human ear perceive those interactions in order to improve sound quality. Psychoacoustic is the science dedicated to understanding the hearing perception linked with the physical parameters of a sound. For example, masking effects are taken into account in the loudness calculation.

Source

Herre, Jürgen & Dick, Sascha. (2019). Psychoacoustic Models for Perceptual Audio Coding—A Tutorial Review. Applied Sciences. 9. 2854. 10.3390/app9142854.

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