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About this Article
Written by: Andrew Turner
Written on: November 5th, 2000
Tags: electrical engineering, health & medicine
Thumbnail by: Davidbspalding/Wikimedia Commons
About the Author
Andrew Turner was an Electrical Engineering undergraduate at the University of Southern California in 2000. He is an audio recording engineer and works as an electrical engineer for the TMH Corporation.
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Volume I Issue I > Pyschoacoustics and Surround Sound Systems
Sound consists of variations in air pressure arriving at the ears. The human hearing system is capable of deciphering these relatively mundane and simple variations into a substantial amount of important information, much of which is relied upon for human survival. Since the hearing process is both psychological (associated with the brain) and acoustical (associated with the physics of sound), it is commonly referred to as psychoacoustics. This paper discusses the basics of the human hearing system and psychoacoustics. Following the aforementioned discussion, this paper specifically explores auditory localization; that is, the brain's ability to interpret sound (audio) as a three-dimensional image. By understanding how the ear-brain system works, the reader is capable of understanding how the seemingly complex surround-sound systems accomplish an immersive and sound auditory experience.

Introduction

The technology used in simulating surround sound systems makes use of human perception of sound in order to simulate specific sound effects. By understanding the working of the human auditory system, one can understand the engineering behind the design of various commercial sound systems.

Basics of Psychoacoustics with Respect to Localization

Psychoacoustics, or the study of human perception of sound, provides an explanation of how the human ear-brain system interprets sound and decodes information from a pair of receivers (ears) into a complete, 3-dimensional auditory 'image'. In the perception of sound, there are three general fields by which humans can arbitrate: pitch, volume (or loudness), and time. These three domains provide the essential information or cues necessary for localization, which is the process of determining where a sound came from in three-dimensional space. In order to understand the basics of human auditory localization, one must first understand the method by which sound is received through the ears, as well as the basic definitions of pitch, loudness, and time.

Anatomy of the Human Ear

The human ear is comprised of three main parts: the outer ear, the middle ear, and the inner ear. The outer ear is comprised of the pinna, the ear canal, and the eardrum (Fig. 1). The pinna assists in both directing sound into the ear canal as well as "encoding" the incoming sounds with directional information that the brain interprets. The ear canal carries this encoded information to the eardrum that vibrates according to the incoming sound pattern. The middle ear is mechanically connected to the eardrum by means of three bones: hammer, anvil, and stirrup. These bones move in accordance to the vibrating eardrum and are connected to the inner ear, where the cochlea (A5) translates the vibrations into neuron impulses that can be interpreted by the brain. The auditory nerve then carries the impulses to the brain where the impulses are interpreted [1].
Pickard/Wikimedia Commons
Figure 1: Anatomy of the human ear.

Quantification and Interpretation of Sound Based on Pitch, Loudness and Time

Pitch

The pitch of a sound is directly associated with its frequency. Each key or note on a piano has an associated pitch. As the keys are played from left to right, the pitch increases. The reason for this increase is related to the speed at which the strings inside a piano vibrate back and forth when struck by the individual keys. A string at a given tension and length will tend to resonate (or vibrate) at a specific, certain frequency. This frequency is known as the fundamental frequency. For example, the note "middle-C" has a fundamental frequency of 261.6 Hz (it vibrates 261.6 times a second).

Loudness

The second domain perceived by the ear-brain system is loudness, which is closely associated with acoustic pressure. Pressure is defined as a force over a given amount of area; pressure is measured in units of Newton of force per square meter of area. One Newton per square meter is equal to one Pascal of pressure. Acoustic pressure is similar to water pressure in pipes. High water pressure is present in fire hydrant pipes while low water pressure is present in shower pipes. The water from a fire hydrant can exert a large, powerful amount of force on anything in front of it because of its high pressure, while a shower's low pressure generally does not exert very much force. The force generated by a sound moves the eardrum and the various parts of the middle ear in accordance with its magnitude or power. Consequently, a high acoustic pressure that causes the eardrum to move a greater distance is a loud sound, while a low acoustic pressure is a soft sound.
Acoustic pressure is measured in decibels of Sound Pressure Level (dB SPL), and is calculated by the equation shown below. 20x10-6 (20 micro) Pascal is equal to 0 dB SPL which is approximately the quietest sound a human can hear. The "maximum" a human can hear is called the Threshold of Pain and is commonly referenced as 120 dB SPL, which is equivalent to 20 Pascal (100,000 times 20 micro Pascal). Exceeding 120 dB SPL is damaging to human hearing and may cause discomfort. As a general rule, an increase or decrease in Sound Pressure Level by 10 dB is approximately a doubling or halving of loudness, respectively.