Auditory Perception

A New Synthesis

Richard M. Warren, Arnold P. Goldstein, Leonard Krasner

• Publisher: Pergamon
• Year: 2013
• Print ISBN: 9780080259574
• eBook ISBN: 9781483148144
• Topic: Psychology
• Subtopic: History & Theory in Psychology
• Index: Psychology

Auditory Perception: A New Synthesis focuses on the effort to show the connections between key areas in hearing. The book offers a review of classical problems, and then presents interpretations and evidence of this topic. A short introduction to the physical nature of sound and the way sound is transmitted and changed within the ear is provided. The book discusses the importance of being able to identify the source of a sound, and then presents processes in this regard. The text provides information on the organs involved in the identification of sound and discusses pitch and infrapitch and the manner by which their loudness can be measured. Scales are presented to show the loudness of sound. The relationship of hearing with other senses is also discussed. The text also outlines how speech is produced, taking into consideration the organs involved in the process. The book is a valuable source of data for research scientists and other professionals who are involved in hearing and speech.

1

Sound and the Auditory System

This chapter provides a brief introduction to the physical nature of sound, the manner in which it is transmitted and transformed within the ear, and the nature of auditory neural responses.

THE NATURE OF AUDITORY STIMULI

Understanding hearing requires an understanding of sound. Sounds consist of fluctuations in pressure which are propagated through an elastic medium, and are associated with displacement of particles composing the medium. When the substance conducting sound is air at a temperature and pressure within the normal environmental ranges, compressions and rarefactions are transmitted at a velocity of about 335 meters per second, regardless of their amplitude (extent of pressure change) or waveform (pattern of pressure changes over time). Special interest is attached to periodic sounds, or sounds having a fixed waveform repeated at a fixed frequency. Frequency is measured in Hertz (Hz) or numbers of repetitions of a waveform per second (thus, 1,000 Hz corresponds to 1,000 repetitions of a particular waveform per second). The time required for one complete statement of an iterated waveform is its period. Periodic sounds from about 20 through 16,000 Hz can produce a sensation of pitch, and are called tones. For reasons to be discussed shortly, it is generally considered that the simplest type of periodic sound is a sine wave or pure tone (shown in Fig. 1.1A) which has a sinusoidal change in pressure over time. A limitless number of other periodic waveforms exist, including square waves (Fig. 1.1B) and pulse trains (Fig. 1.1C). Periodic sounds need not have simple, symmetrical waveforms: figure 1.1D shows a periodic sound produced by iteration of a randomly generated waveform.

Fig. 1.1 Waveforms and amplitude spectra. The periodic waveforms have line spectra, and the nonperiodic waveforms have continuous spectra or band spectra. See text for further discussion.

The figure also depicts the waveforms of some nonperiodic sounds: white or Gaussian noise (Fig. 1.1E), a single pulse (Fig. 1.1F), and a short tone or tone burst (Fig. 1.1G).

The waveforms shown in figure 1.1 are time-domain representations in which both amplitude and time are depicted. Using a procedure developed by Joseph Fourier in the first half of the nineteenth century, it also is possible to represent any periodic sound in terms of a frequency-domain or spectral analysis in which a sound is described in terms of a harmonic sequence of sinusoidal components having appropriate frequency, amplitude, and phase relations. (Phase describes the portion of the period through which a waveform has advanced relative to an arbitrary reference.) A sinusoidal tone consists of a single spectral component as shown in figure 1.1A. The figure also shows the power spectra corresponding to the particular complex (nonsinusoidal) periodic sounds shown in figures 1.1B, 1.1C, and 1.1D. Each of these sounds has a period of 1 millisecond, a fundamental frequency of 1,000 Hz (corresponding to the waveform repetition frequency), and harmonic components corresponding to integral multiples of the 1,000 Hz fundamental as indicated.

Frequency analysis is not restricted to periodic sounds: nonperiodic sounds also have a spectral composition as defined through use of a Fourier integral or Fourier transform (for details see Leshowitz, 1978). Nonperiodic sounds have continuous rather than line spectra, as shown for the sounds depicted in figures 1.1E, 1.1F, and 1.1G.

As we shall see, frequency analysis of both periodic and nonperiodic sounds is of particular importance in hearing, chiefly because the ear performs a crude spectral analysis before the auditory receptors are stimulated.

While figure 1.1 shows how particular waveforms can be analyzed in terms of spectral components, it is also possible to synthesize waveforms by adding together sinusoidal components of appropriate phase and amplitude. Figure 1.2 shows how a sawtooth waveform may be approximated closely by the mixing of only six harmonics having appropriate amplitude and phase.

Fig. 1.2 Synthesis of a complex waveform through addition of harmonically related sinusoidal components. The approximation of a sawtooth waveform could be made closer by the addition of higher harmonics of appropriate amplitude and phase. Source: From Perception and the Senses by Evan L. Brown and Kenneth Deffenbacher. Copyright 1979 by Oxford University Press, Inc. Reprinted by permission.

The range of audible amplitude changes is very large. A sound producing discomfort may be as much as 10⁶ times the amplitude level at threshold. Sound level can be measured as power or intensity as well as amplitude or pressure: power usually can be considered as proportional to the square of the amplitude, so that discomfort occurs at a power level 10¹² times the power threshold. In order to span the large range of values needed to describe the levels of sound normally encountered, a logarithmic scale has been devised. The logarithm to the base 10 of the ratio of a particular sound power level to a reference power level defines the level of the sound in Bels (named in honor of Alexander Graham Bell). However, the Bel is a rather large unit, and it is conventional to use a unit 1/10 this size, the deciBel (or dB) to express sound levels. The level in dB can be defined as:

dB = 10 log₁₀ I₁/I₂

where I₁ is the power level of the particular sound of interest, and I₂ is the reference level expressed as sound power. DeciBels can also be calculated on the basis of amplitude or pressure units using the equation:

dB = 20 log₁₀ P₁/P₂

where P₁ is the relative pressure level being measured, and P₂ is the reference pressure level. The standard reference pressure level is 0.0002 dyne/cm² (which is sometimes expressed in different units of 20 microPascals), and the level in dB measured relative to this standard is called Sound Pressure Level (or SPL). Sound level meters are calibrated so that the numerical value of the SPL can be read out directly. There is another measure of sound level, also expressed in dB, called Sensation Level (SL), which is used occasionally in psychoacoustics. When measuring SL, the intensity corresponding to the threshold of a sound for an individual listener is used as the reference level rather than the standard physical value employed for SPL, so that dB SL represents the level above an individual’s threshold. Since SL is used relatively infrequently, dB will always refer to SPL unless otherwise specified.

To give some feeling for intensity levels in dB, the threshold of normal listeners for a 1,000 Hz sinusoidal tone is about 6 dB, the ambient level (background noise) in radio and TV studios is about 30 dB, conversational speech about 55 dB, and the level inside a bus about 90 dB.

Experimenters can vary the relative intensities of spectral components by use of acoustic filters which, in analogy with light filters, pass only desired frequency components of a sound. A high-pass filter transmits only frequency components above a lower limit, a low-pass filter only frequencies below an upper limit. Band-pass filters (which transmit frequencies within a specified range) and band-reject filters (which block frequencies within a specified range) are available. Filters are specified in terms of both cut-off frequency (the frequency at which the filter attenuation reaches 3 dB), and the slope, or roll-off, which is usually expressed as dB/octave beyond the cut-off frequency (an increase of one octave corresponds to doubling the frequency). Filter types are shown in figure 1.3.

Fig. 1.3 Characteristics of filters. Low-pass, high-pass, and band-pass filters are shown, with filter slopes (dB/octave) and cut-off frequencies (frequencies at which there is a 3 dB reduction in intensity) illustrated.

OUR AUDITORY APPARATUS

The Outer Ear and the Middle Ear

It is convenient to consider the ear as consisting of three divisions. The outer ear (also called the pinna or auricle) is shown in figure 1.4. It appears to contribute to localization of sound sources by virtue of its direction-specific effect on the intensity of certain frequency components of sounds, as will be discussed in a later chapter. The human pinna is surrounded by a simple flange (the helix) which is extended considerably in some other mammals to form a conical structure functioning as a short version of the old-fashioned ear trumpet. These ear-cones increase the sensitivity of such animals to high frequency sounds when pointed toward their source by controlling muscles, as well as providing information concerning azimuth of the source.

Fig. 1.4 The outer ear (other names: pinna and auricle). The major anatomical features are shown.

After the acoustic transformation produced by reflections within our pinna, the sound passes through the ear canal (or external auditory meatus) which ends at the eardrum or tympanum as shown in figure 1.5. This canal is more than a passive conduit. Its length is roughly 2.5 cm, and it behaves in some respects like a resonant tube, such as an organ pipe. The effect of this resonance is to amplify frequencies appreciably (5 dB or more) from about 2,000 through 5,500 Hz, with a maximum amplification of about 11 dB occurring at about 4,000 Hz (Wiener, 1947). The pressure changes at the end of the canal cause the tympanum to vibrate. This vibration is picked up and transmitted by a chain of three small bones or ossicles located in the middle ear. The first of these bones, the malleus (or hammer) is attached to the tympanum, and its movement is transmitted to the incus (or anvil) and thence to the stapes (or stirrup). The stapes is connected to the oval window at the base of the fluid-filled cochlea. This window lies at the boundary of the middle and inner ears. The passage of sound through the cochlea is shown in figure 1.6, and will be discussed subsequently.

Fig. 1.5 Diagram of the entire ear. The outer, middle, and inner ear are shown, along with adjacent structures. Source: Adapted from P.H. Lindsay and D.A. Norman, Human Information Processing: An Introduction to Psychology. (2nd ed.) (New York: Academic Press, 1977).

Fig. 1.6 Conversion from air-borne to liquid-borne motion by the ear. Source: Adapted from P.H. Lindsay and D.A. Norman, Human Information Processing: An Introduction to Psychology (2nd ed.) (New York: Academic Press, 1977).

The middle ear permits the air-borne sound to be converted to liquid-borne sound without the great loss which would otherwise occur. When sound in air impinges directly upon a liquid, a loss of about 30 dB (99.9 percent of the power) takes place, with most of the sound energy being reflected back into the air. Three physical principles act to increase the efficiency of the transmission of sound by the middle ear: (1) the curvature of the tympanum (which is somewhat conical in shape) causes it to act like a more efficient mechanical transformer (Tonndorf & Khanna, 1972); (2) the chain of three ossicles acts like a lever with a small mechanical advantage; and (3) the force applied to the larger area of the tympanic membrane, when transmitted to the much smaller area of the footplate of the stapes embedded in the oval window, produces a considerable mechanical...

Table of contents

• Cover image
• Title page
• Table of Contents
• Pergamon Titles of Related Interest
• Copyright
• Dedication
• Preface
• Chapter 1: Sound and the Auditory System
• Chapter 2: Spatial Localization and Binaural Hearing
• Chapter 3: Perception of Acoustic Repetition: Pitch and Infrapitch
• Chapter 4: The Measurement of Loudness and Pitch
• Chapter 5: Perception of Acoustic Sequences
• Chapter 6: Perceptual Restoration of Missing Sounds
• Chapter 7: Speech
• Chapter 8: The Relation of Hearing to Other Senses
• References
• Author Index
• Subject Index
• About the Author