Psychoacoustics

Psychoacoustics, the study of the physical effects of sound on biological systems, has been of interest since Pythagoras first heard the sounds of vibrating strings and of hammers hitting anvils in the 6th century BC, but the application of modern ultrasonic technology has only recently provided some of the most exciting developments in medicine. The domain of psychoacoustics can be divided into two different fields of observation:

  • the ability of hearing to assess the physical characteristics of sounds;
  • the ability to detect variations.

Both in the first case and in the other, the concept of threshold is fundamental. In biology and psychology, this term generally refers to the minimum value of intensity of stimulation necessary for a certain biological or psychological response to occur.

When, in psychophysics, the interest is directed to determine the difference between perceptible stimuli and non-perceptible stimuli, we talk about absolute threshold; when, instead, we want to determine the minimum appreciable variation, we talk about differential threshold. In practice, however, more than the absolute value of the variation is the percentage ratio of this with the value of the intensity of the initial stimulus, as, moreover, stated by Weber’s law: “a stimulus must be increased by a constant fraction of its value because the difference begins to become perceptible”.

As in all biological phenomena, the values obtained from the measurements have statistical significance. In this case, they are the average of the responses of a listening group made up of a certain number of subjects chosen according to the criteria of homogeneity (especially with regard to age) and placed in the same experimental conditions.

In order to obtain results more easily quantifiable, in psychoacoustic research are used mainly pure sounds, in this sense called tones, as well as noises appropriately calibrated as, for example, the white noise.

Psychoacoustics is a part of experimental psychology concerned with auditory perception. In particular, psychoacoustics is concerned with the measurement of subjective quantities that relate to sounds: sound sensation (or subjective intensity), pitch, duration, timbre, annoyance. The general problem of psychoacoustics is to establish the psychometric function that links the subjective quantity to the corresponding objective quantity; for example, the psychometric function of sound sensation is that which links this quantity to the sound pressure level.

Several measurement methods are used for this purpose. For the evaluation of fixed ratios given a sound, or a noise, of given objective characteristics (spectrum, duration, sound pressure level), one makes the subject listen to successive pairs of sounds or noises, the first fixed, the second different from the first only in sound pressure level; Vary the level of the second until the subject judges it to have a sound sensation in a given ratio (1/4, 1/2, 2/1, 4/1) with the first; then repeat the measurement assuming as a fixed sound the result of the previous measurement, and so on, thus obtaining a succession of levels to which correspond sensations that are, for example, 8, 4, 2, 1, 1/2, 1/4, 1/8 …

For the relative evaluation of sensations we make the subject listen to two sounds or noises of different characteristics, attributing to the first a conventional value, for example 10, of sensation, and asking the subject to indicate with a number the sensation due to the second; then we repeat the measurement by changing the level of the second, and so on, obtaining the psychometric function.

For absolute evaluation of sensations, the subject is made to listen to two to five sounds that differ only in level, and asked to indicate the corresponding sensations with numbers. For equi-repartitioning, given two sounds A and B that differ only in level, with a very high difference, the subject is asked to listen to the two sounds with a third intermediate sound of equal characteristics, adjusting it until the corresponding sensation for the subject is intermediate between the other two; the measurement is repeated between each of the two extreme sounds A, B, and the intermediate sound M, and so on. Thus, a succession of objective sound pressure level values is obtained to which corresponds a scale of equidistant sensations. Since the aim of psychoacoustics is to obtain average psychometric functions, the measurements are repeated for a sufficiently large number of subjects (at least 10) and then the data obtained are statistically processed.

In many applications of acoustics and sound signal processing, it becomes strictly necessary to know how sound is perceived by a human being. Sound, whose acoustic stimulus is composed of pressure waves propagating through the air, can be accurately measured using sophisticated equipment.

However, understanding how these waves are received and converted into thoughts within our brain is not to be underestimated: sound is a continuous analog signal that (approximating to zero the volume of air molecules) can theoretically carry an infinite amount of information (there being an infinite number of carrier frequencies, each containing information about amplitude and phase). Identifying the peculiar characteristics of auditory perception allows scientists and engineers to concentrate, for the analysis and design of acoustic instruments and equipment, on the truly audible components.

It is also important to emphasize that what we “hear” is not only a physiological consequence related to the conformation of our ear, but also has psychological implications.

Psychoacoustic phenomena of temporal order

The auditory system is also characterized by a certain inertia that is manifested both at the beginning (attack duration) and at the end (extinction duration) of an excitation. In the generation of sound, that is, there is always a transition time necessary to overcome the inertia of the sound source and bring it from its resting state to its normal regime of vibration (transient, attack), and vice versa (transient of extinction). The dynamics of these phenomena are consistent with the nature and mechanics of the sound source, and they occur in a very short time. To psychoacoustic effects, below 30 ms all attack transients appear to be of equal duration, and the same is true for extinction transients in the range between 100 and 300 ms, depending on whether the intensity is weak or strong.

The duration of sounds influences the evaluation of intensity and pitch. For a sound to be perceived, it must have a minimum duration, which can be shorter the higher its intensity. However, reducing the duration also reduces the loudest sensation.

In order for the pitch of a sound to be perceived, it must have, independently of its pitch, a minimum duration of 10 ms. Below this value the sensation is that of a noise impulse. It is also verified that the sensation of sound persists for about 150 ms even after the sound excitation has ceased. Moreover, there is a tendency to overestimate the duration of short sounds and underestimate that of long sounds.

Psychoacoustic phenomena of spatial order

The spatial identification of a sound source is called stereophony. The recognition of the direction of origin of a sound is due to the existence of the two auditory apparatus, right and left, which can receive the same signal from two different positions.

When the sound source is in a frontal position with respect to the listener, the sound wave reaches the two ears at the same time, with the same phase and the same intensity, and the equality of the two trains of nerve impulses originating from the two cochlear apparatus is interpreted by the cortical centers as coming from a central direction.

When, on the other hand, the two ears are in an asymmetrical position with respect to the source, the same signal reaches them at different times and with different phase and intensity, giving rise to two sets of nerve impulses that, integrated in the brain, are interpreted as coming from different directions.

Distance, on the other hand, is evaluated on the basis of the experiences of the subject and, more easily than direction, can be subject to error (think, for example, of the effects of listening to voices in normal conditions, above wind and below wind).

There is also a differential threshold for directionality in the sense that the ability to detect the displacement of a sound source or to identify the directions of two point sources is null below about 6° horizontal and 8° vertical.

While intensity does not seem to affect the identification of the origin of sounds when the length of the perceived waves begins to exceed the distance between the two ears by reducing the effects of phase difference, the ability to recognize spatial decreases progressively until low frequencies where it becomes problematic.

In stereophony, wave reflection phenomena are of great importance. Even greater is the intentionality of listening, which allows, for example, to identify a person speaking in the midst of a crowd, selecting his voice among many others; an operation that is not possible, on the contrary, with monaural listening.

Applications of psychoacoustics

Psychoacoustics is applied today in many fields: from computer engineering to acoustic engineering, passing through defense in relation to the possibility of developing and using acoustic weapons that can cause injury or death. Of course, it is also applied to music, where musicians and artists continue to create new acoustic sensations, breaking the traditional perception of real sound. It is also used in education, medicine, and marketing.

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