Psychoacoustics

In many applications of acoustics and audio processing, you need to know what people hear . The sound that air pressure waves form can be accurately measured with modern equipment. However, understanding how these waves are received and displayed in our brain is not easy. Sound is a continuous analog signal that (assuming that air molecules are infinitely small) can theoretically carry an infinite amount of information (since there are an infinite number of oscillations containing information about amplitude and phase).

Understanding the processes of perception allows scientists and engineers to focus on the capabilities of hearing and not to take into account the less important capabilities of other systems. It is also important to note that the question “what a person hears” is not only a question about the physiological possibilities of the ear , but in many respects also a question of the psychology of perception .

A person nominally hears sounds in the range from 16 to 20 000 Hz . The upper limit tends to decrease with age. Most adults cannot hear sound above 16 kHz. The ear itself does not respond to frequencies below 20 Hz, but they can be felt through the organs of touch .

The volume range of perceived sounds is huge. But the eardrum in the ear is sensitive only to pressure changes. The sound pressure level is usually measured in decibels (dB). The lower threshold of hearing is defined as 0 dB (20 micropascals), and the definition of the upper limit of hearing refers rather to the threshold of discomfort and then to hearing , contusion , etc. This limit depends on how long we listen to the sound. The ear can carry a short-term increase in volume up to 120 dB without consequences, but long-term perception of sounds with a volume greater than 80 dB can cause hearing loss. ^[one]

More thorough studies of the lower limit of hearing have shown that the minimum threshold at which sound remains audible depends on frequency. The graph of this dependence is called the absolute threshold of audibility . On average, it has the area of ​​greatest sensitivity in the range from 1 kHz to 5 kHz, although with age, the sensitivity decreases in the range above 2 kHz.

The curve of the absolute threshold of hearing is a special case of the more general - curves of the same loudness, isophones : sound pressure values ​​at different frequencies , at which a person feels sounds equally loud. The curves were first obtained by Fletcher and Munson ( H. Fletcher and WA Munson ) and published in 1933 in the work " Loudness, its definition, measurement and calculation " ^[2] . Later, more accurate measurements were performed by Robinson and RS Dadson ^[3] . The curves obtained vary considerably, but this is not an error, but different conditions for the measurement. Fletcher and Munson used headphones as a source of sound waves, and Robinson and Datson used a front-mounted speaker in an anechoic room.

The Robinson and Dutson measurements formed the basis of the ISO 226 standard in 1986. In 2003, the ISO 226 standard was updated taking into account the data obtained from measurements during 12 new international studies.

There is also a way to perceive sound without the involvement of the eardrum - the so-called microwave acoustic effect , when pulsed or modulated radiation in the microwave range affects the tissues around the cochlea , causing a person to perceive different sounds. ^[four]

In certain cases, one sound can be hidden by another sound. For example, a conversation near the railroad tracks may be completely impossible if a train passes by. This effect is called a disguise. A weak sound is said to be masked if it becomes indistinguishable in the presence of a louder sound.

There are several types of masking:

• By the time of arrival of the masking and masked sound:
  • simultaneous (monoural) masking
  • temporary (non-simultaneous) masking
• By the type of masking and masked sounds:
  • pure tone pure tone of different frequency
  • pure tone noise
  • Speech clean tones
  • speech monotonous noise
  • speech impulse sounds, etc.

Simultaneous Disguise

Any two sounds while listening at the same time affect the perception of the relative volume between them. A louder sound reduces the perception of the weaker, even to the disappearance of his audibility. The closer the frequency of the masked sound to the frequency of the mask, the more it will be hidden. The masking effect is not the same when the masked sound is shifted lower or higher in frequency relative to the masking sound. Low-frequency sound masks high-frequency. It is important to note that high-frequency sounds cannot mask low-frequency sounds.

Temporary disguise

This phenomenon is similar to frequency masking, but there is a masking in time. When stopping the supply of masking sound masked for some time continues to be inaudible. The masking time depends on the frequency and amplitude of the signal and can reach 100 ms. Under normal conditions, the effect of temporal masking lasts much less.

In the case where the masking tone appears later than the masked one, the effect is called post-masking. When the masking tone appears earlier than the masked one (this is also possible), the effect is called pre-masking.

Post-stimulation fatigue

Often, after exposure to loud high-intensity sounds, a person's hearing sensitivity is sharply reduced. Restoring normal thresholds can last up to 16 hours. This process is called a “temporary shift in the threshold of auditory sensitivity” or “post-stimulus fatigue.” The threshold shift begins to appear when the sound pressure level is above 75 dB and accordingly increases with increasing signal level. And the greatest influence on the shift of the threshold of sensitivity have high-frequency components of the signal.

See the Missing fundamental article .

Sometimes a person can hear sounds in the low-frequency region, although in reality there was no such frequency. This is due to the fact that the oscillations of the basilar membrane in the cochlea are not linear, and oscillations with a difference frequency between the two more high-frequency ones can occur in it.

This effect is used in some commercial sound systems in order to expand the range of reproducible low frequencies if it is impossible to adequately reproduce such frequencies directly, for example, in headphones, mobile phones, low-budget speakers (acoustic systems), etc.

Psychoacoustic models of hearing allow high-quality compression of a signal with loss of information (when the recovered signal does not match the original one), due to the fact that they can accurately describe what can be safely removed from the original signal — that is, without significant degradation of sound quality. At first glance it may seem that it is unlikely that this will provide a strong signal compression, but programs using psychoacoustic models can reduce the size of music files by 10-12 times, and the difference in quality will not be very significant.

These types of compression include all modern lossy compression formats:

• MP3 is almost the same as Musicam , which is used for digital audio broadcasting in some countries, and is considered a more professional compression algorithm (also known as MPEG-1 Layer 3).
• Ogg vorbis
• Wma
• AAC
• Musepack
• ATRAC - used in MiniDisc format and in some Sony portable MP3 players

• Perception
• Feeling
• Psychophysiology
• Acoustics
• Musical acoustics
• Musical psychology

• Helmholtz G. The doctrine of auditory sensations as a physiological basis for the theory of music. Per. with him. SPb., 1875.
• Aldoshina I. Basics of psychoacoustics. Oborongiz., Moscow, 2000.
• Stumpf, K. , Tonpsychologie, 1883, Bd. 1, 1890, Bd. 2 ("Psychology of musical perception").
• Meyer MF , Contributions to a psychological theory of music (1901).
• Meyer, M. , The Musician's Arithmetic (1929).
• Meyer, M. , How we hear: How tones make music (1950).
• Roederer and psychoacoustics of music. NY: Springer, 1975
• Howard D., Angus J. Acoustics and psychoacoustics. Oxford: Focal Press, 2001.

• Fundamentals of psychoacoustics. Hearing disguise
• Fundamentals of psychoacoustics. Auditory thresholds
• Psychoacoustics