The masking threshold is the sound pressure level of a sound

Sound

Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

 needed to make the sound perceptible in the presence of another noice, called a "masker". This threshold depends upon the frequency

Frequency

Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

, the kind of masker, and the kind of sound being masked. The effect is strongest between two sounds close in frequency.

In the context of audio transmission, there are some advantages to being unable to perceive a sound. In audio encoding

Audio codec

All codecs are devices or computer programs capable of coding or decoding a digital data stream or signal.The term audio codec has two meanings depending on the context:...

, for example, better compression

Data compression

In computer science and information theory, data compression, source coding or bit-rate reduction is the process of encoding information using fewer bits than the original representation would use....

 can be achieved by omitting the imperceptible tones, thus requiring fewer bit

Bit

A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...

s to encode the sound and reducing the size of the final file.

Applications in audio compression

It is uncommon to work with only one tone

Pitch (music)

Pitch is an auditory perceptual property that allows the ordering of sounds on a frequency-related scale.Pitches are compared as "higher" and "lower" in the sense associated with musical melodies,...

; most sounds are composed of multiple tones. There may be many possible maskers at the same frequency. In this situation, it is necessary to compute the global masking threshold using a high resolution Fast Fourier transform

Fast Fourier transform

A fast Fourier transform is an efficient algorithm to compute the discrete Fourier transform and its inverse. "The FFT has been called the most important numerical algorithm of our lifetime ." There are many distinct FFT algorithms involving a wide range of mathematics, from simple...

 via 512 or 1024 points to determine the frequencies that comprise the sound. Because there are bands that humans are not able to hear, it is necessary to know the signal level, masker type, and the frequency band before computing the individual thresholds. To avoid having the masking threshold under the threshold in quiet, one adds the last one to the computation of partial thresholds. This allows computation of the signal-to-mask ratio (SMR).

The psychoacoustic model

The MPEG audio encoding process leverages the masking threshold. In this process, there is a block called "Psychoacoustic model". This is communicated with the band filter and the quantify block. The psychoacoustic model analyzes the samples sent to it by the filter band, computing the masking threshold in each frequency band using a Fast Fourier transform. The number of points used depends upon the MPEG layer. Using these thresholds, the signal-to-mask ratio is determined and sent to the quantifier. The quantifier assigns more or less bits in each block based upon the SMR. The block with the highest SMR will encode with the maximum number of bits

BITS

BITS or bits may refer to:* Plural of bit* Background Intelligent Transfer Service, a file transfer protocol* Birla Institute of Technology and Science, a technology school in Pilani, Rajasthan, India, with campuses in Goa, Hyderabad, and Dubai...

.