Hearing and Psychoacoustics - High End audio perception

Psychoacoustics - Timbre and Pitch    ||    MP3 engineering and psychoacoustics

An audiophile definitely want to have the best quality and, as such, you pay the price for those products. But when you are fair to yourself, and make an A-B comparison of your high end product with one of those mainstream products, you will be surprised about the small differences, especially for amplifiers and CD-players. Where high end products are in the range of $1000-$5000 (Krell is even in $10000 range), mainstream products you buy for less than $500.

Besides the better sound processing, high end also deals with the suggestions people have with such components. This brings us to the area of psychology, and with this, the study of psychoacoustics. In this study external cues given to the listener can influence the way in which sound is perceived. On a more quantitative level this study includes the effects of psychoacoustic cues like loudness, timbre and pitch. The importance of this study to todays audio processing is becoming more important now, especially for internet and MP3 like formats. But also for the perception of audiophile sound it is crucial not to be ignorant for the psychological effects of how sound is processed by the brain and the peripheral auditory system.
For this reason, this site gives the reader some basic knowledge on this important phenomenon as psychoacoustics in relation to high end and, for example, MP3. It is up to the reader to decide on the effects of psychoacoustics for high end audio equipment, hopefully with the background knowledge of this site.

The study of hearing can be subdivided roughly into two areas of expertise. One is related to neurophysiology and the medical anatomy of hearing. The other to the physics and acoustics of hearing.

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Especially this last one will be elaborated on in this site. This study is often referred to as the study of Audiology (although this study also includes parts of medical science and psychology). It typically includes the effect of psychoacoustics and the physics of sound in relation to (measurable) hearing and perception. Here, one can think of loudness/intensity curves, hearing sensitivity, speech discrimination and dynamic range.

Psychoacoustics - Perception of Pitch and Timbre

Psychoacoustics can be defined as the study of the relationship between physical sounds and the brain's interpretation of them. Stated differently, psychoacoustics can be defined as the psychological study of hearing. Until recently, psychoacoustics has devoted more attention to the behaviour of the peripheral auditory system than to the details of cognitive processing. And especially the cognitive processing is something that draws conclusions on how certain audio components are perceived by the listener. In this, the effects of suggestion given by external cues (for example the price of a component) are very strong and difficult to ignore.
The discipline is a branch of psychophysics in that it is interested in the relation between sensory input stimuli (the sound or music) and the behavioural or psychological response that they provoke (how is the music perceived by the listener).

Psychoacoustic involve the perception of pitch, loudness, volume and timbre and especially pitch and timbre are decisive in high end audio. These parameters are important in the understanding of auditory perception. Typically these parameters are qualitative parameters and therefore subject to manipulation or suggestion.
Pitch is the subjective impression of frequency, in the same sense that loudness is the subjective sense of the amplitude of a sound. As such, pitch is a psychoacoustic variable, and the degree of sensitivity shown to it varies widely with people. Some individuals have a sense of remembered pitch, that is, a pitch once heard can be remembered and compared to others for some length of time; others have a sense of absolute pitch called perfect pitch.
Both are interesting when dealing with A-B tests of audio components. Favourite music of the listener falls in the category of remembered pitch; he is able to compare the sound of his music with the component under test.
Timbre is that attribute of auditory sensation in terms of which a listener can judge that two sounds similarly presented and having the same loudness and pitch are dissimilar. Put more simply, it relates to the quality of a sound. It is determined by the relative amplitudes of the harmonics present; the sound is "thin" or "rich".
Both pitch and timbre are perceptual cues not physical ones and vary per individual.

The smallest degree of pitch discrimination between two pitches depends on their intensity and frequency range. Under the best conditions, a person with good hearing can discriminate about 1400 different pitches, of which 120 are used in the western scale of equal temperament. The lowest pitch corresponds to the lowest frequency giving a sensation of, around 20 to 30 Hz. The highest pitch depends on the highest audible frequency, which varies with age and especially noise exposure, but lies generally in the range of 15 to 20 kHz with younger people.

The research results of psychoacoustics and audiology are currently being used in the digital audio area. The knowledge from this study is becoming more and more used to applications where good sound reproduction is required but where digital storage capacity, size and price is an issue like internet and MP3 data streaming. In the high end area the results of psychoacoustics are not explicitly used since audio reproduction need to be as pure as possible. Unfortunately, vendors of audio equipment do know how to abuse psychoacoustical cues to influence potential buyers on useless features.

Psychoacoustics in audio engineering

The pitch ascribed to a complex tone or sound may not necessarily correspond to a frequency that is physically present in the sound. For instance, if a spectrum consists of harmonics beginning with the second or higher harmonic, the sound will still be heard as having the pitch of the fundamental, called the periodicity pitch or the missing fundamental. Removing the fundamental causes the timbre to change, but does not affect the pitch.

Psychoacoustics and SACD

This aspect is interesting when judging the quality of SACD. From SACD it is said that the higher harmonics add realness to speech and music. If so, the pitch is changed indeed and new musical cues are added to the auditory perception of it which were not heard before. Of course, this is only valid when these higher harmonics are also present in the real live music composition. With the correct SACD recording these harmonics can become available at home audio reproduction. Also timbre can be changed with the correct SACD recording since the dynamic range of the SACD is increased. This introduces the possibility to give a more realistic perception of amplitude, say timbre.

Psychoacoustics and MP3

Interesting in this respect is the current research that is being adopted to find new ways of good audio sound reproduction with small bit sizes. A very good example of this is the MP3 music codecs. Although not relevant for high end music reproduction, MP3 is something to give a certain amount of attention. MP3 is especially to be used for portable systems or internet applications. The source material is compressed by stripping audible musical information in order to decrease the required data storage capacity. At reproduction the data is decompressed for further processing. Besides the bitrate used for sampling the data, psychoacoustic models are used to reduce the redundant information in music. The current codecs for MP3 use such optimized algorithms that only strips those sounds that are otherwise also not audible. 

The effect of masking and other related auditory aspects are used such as:
1) Traditionally, the auditory effect of amplitude or spectral masking is used in these algorithms; the pitch of a multitone sound is not changed when one of the tones masks the other tones. On this basis, the masked tone can be stripped from the source without affecting the auditory perception of it. In reality we experience that pitch and timbre is affected but that this depends on the model used. When the tones are close to each other the masking easily occurs, the effect diminishes when the tones are more separated.
2) In these models also the temporal resolution of the auditory system is used. This effect takes into account that tones that have a certain duration will also mask other tones even after the tone already stopped. It is an indication of how fast the auditory system can react to sequential tones. Also this enables the shrinking of data size.

In all these examples it must be noticed that quantitative relations in psychoacoustics are developed from statistical analysis and that results can vary from person to person. For example, the masking effect (spectral as well as temporal) can be very different. As a result, audiophiles and musicians often are able to listen more precise and notice differentations better than others. Their hearing system can be faster and the just noticable difference in tones better. They will perceive MP3 quality as being really inferior to uncompressed sounds while another perceives it as good quality.

 

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