Producing And Detecting Sound

• Type of physical science: Classical physics
• Field of study: Acoustics

When a vibrating source sets air molecules into vibration, a varying pressure wave having the same frequency as the source is produced. This varying pressure wave may be considered a sound if it is within the frequency range audible to the human ear—that is, from about 20 hertz to about 20,000 hertz. An acoustical receiver is any device, including the human ear, that detects sound waves. A microphone is an acoustical receiver that converts sound waves into electrical energy in the form of a time-varying voltage.

Overview

If a tree falls in a forest and there is no one present to hear it, is a sound produced?

Some people will say no; a sound is a subjective sensation produced in the brain by the auditory mechanism. Others will contend that sound, regardless of whether any device is present to detect it, is an objective phenomenon of nature. Questions such as this cannot be resolved by argument or logic because the disagreement hinges on the definition of sound, not on the nature of sound.

Scientists prefer that sound be defined objectively as a form of energy that exists whether it is heard or recorded.

Vibrations of material objects produce sound waves in the air. A plucked guitar string, with some assistance from the soundboard, produces an oscillating wave in the air that travels outward from this source with the same vibration frequencies as the source. If the frequency is between approximately 20 and 20,000 hertz, it can be heard as sound by an average human.

Sound can be defined as any propagating vibration of the air having a frequency within this range.

When a piece of iron is banged with a hammer, a disturbance is set up that produces a traveling wave pulse in the air. This wave pulse consists of a series of pressure condensations and rarefactions along the direction of the expanding wave. When a source vibrates continuously, a wave train of alternating condensations and rarefactions, having the same frequency, travels outward as a sound wave. Sound sources may be either free vibrators, which are abruptly set into vibration by an impulsive force, or forced vibrators, which are kept vibrating by a continuously applied force. The orchestral percussion instruments are examples of free vibrators, while forced vibrators include woodwinds, brass, and bowed string instruments. For free vibrators, the actual vibrating surface may be the source of sound, as is the case for drums, chimes, and xylophone bars. A relatively small vibrator, such as a piano string, may pass its energy onto a larger surface (the piano soundboard) so that its energy may be coupled to the air more efficiently. For the forced vibrators, the efficiency with which the vibrating source is converted to sound depends in large measure on the size of the vibrating surface. Most of the sound from bowed string instruments is radiated from their bodies. Wind instruments use a reed of one sort or another to interrupt an air stream so that a coupled air column may be set into vibration. The openings in the air column are the source of sound for these instruments, that is, the tone holes for woodwinds and the bell for brass instruments. The radiators of the human voice are the mouth and the nose. The electric guitar produces a weak acoustical signal, which is modified electronically and amplified in order to produce the varying voltage that drives a loudspeaker.

Although one cannot observe vibrating systems and sound waves directly, these phenomena can be detected and measured by means of acoustic receivers. The human ear is a very sensitive and intricate device that converts sound waves in the air into neural impulses in the brain. The range of sound pressures to which the ear can respond is truly remarkable. Not only can the ear withstand extremely intense sounds, but it can also respond to pressure variations that are so small that the eardrum is displaced by a distance smaller than the diameter of the air molecules striking it. Human hearing mechanisms also respond to sounds ranging in frequency from 20 hertz to 20,000 hertz, a range of ten octaves. Yet, the ear is more than a mere wide-range detector; it also acts as a sophisticated time and frequency analyzer, enabling one to analyze complicated sounds into their component frequencies and to listen selectively to multiple sound sources.

A microphone is an acoustic receiver that converts sound into a time-varying voltage that accurately represents the original air vibrations. A microphone operates as a sound wave impinges upon a diaphragm, forcing it to vibrate in exactly the same manner as the pulsating air wave. The diaphragm is coupled to a transducer that converts its mechanical motion into an electrical signal, preserving the form of the original pressure wave. Basically, four different types of transducers are used in microphones: carbon granules, piezoelectric crystals, capacitor plates, or wire coils in a magnetic field. By the 2020s, micro-electro-mechanical systems (MEMS) microphones were widely integrated into devices such as mobile phones, wearable devices, and hearing aids. The carbon microphone consists of a metallic cup filled with carbon granules and has a movable metallic diaphragm in contact with the carbon at the open end. When an electric current exists in the carbon, its magnitude will depend upon the electrical resistance that, in turn, varies with the force exerted on the granules by the diaphragm. Thus, as the diaphragm vibrates in response to a sound wave, a corresponding oscillating electric current is produced at the output.

Crystal and ceramic microphones utilize the piezoelectric effect to transduce the diaphragm vibrations into voltages, while the capacitive effect is used for condenser microphones. In ceramic microphones, the diaphragm is attached to one end of the ceramic material by a drive pin, and the other end is anchored to the microphone casing. When the diaphragm moves, the ceramic element is bent, thus inducing a voltage proportional to the deflection. A condenser microphone utilizes two parallel conducting plates, one of which is fastened to the casing while the other is fastened to the diaphragm. When the diaphragm vibrates, a corresponding oscillating voltage is produced on the fixed plate.

The dynamic microphone has a coil of wire attached to its diaphragm, and the coil is free to move between the poles of a permanent magnet. When the diaphragm moves, the coil and all the electrons in the coil move through the magnetic field. The electrons experience a force that produces a varying electric voltage across the ends of the coil, as described by the magnetic force law.

Applications

A loudspeaker provides a familiar example of an acoustic radiator and illustrates two important features. First, a relatively large radiating surface is required to radiate low-frequency waves efficiently. The sound power output of a loudspeaker depends on its surface area, the amplitude of its vibration, and the frequency. Since the vibration amplitude is limited for most speakers, an efficient woofer (low-frequency loudspeaker) must have a large diaphragm. Second, radiators do not radiate high-frequency sound uniformly in all directions. Low-frequency waves diffract around a loudspeaker and are thus radiated fairly uniformly around a room.

High-frequency sounds, however, are directional. They are radiated most strongly in the direction in which the tweeter (high-frequency loudspeaker) is aimed. This effect increases with increasing frequency. A well-designed high-fidelity loudspeaker--to increase the dispersion of high-frequency sound waves, includes several tweeters aimed in slightly different directions.

A loudspeaker input is a time-varying electrical signal fed into a voice coil that is situated in the magnetic field produced by a permanent magnet. The varying voltage in the coil causes the coil to experience a force (the magnetic force law) and, therefore, to move. The voice coil is attached securely to the speaker diaphragm, causing the diaphragm to vibrate the air in essentially the same manner as the original signal. Thus, a time-varying voltage is transduced into a sound wave by a process exactly opposite to that utilized in the dynamic microphone.

Carbon microphones were used widely in telephone transmitters because they are inexpensive. Although they have only a limited range of frequency response (300 hertz to about 3,500 hertz), this range happens to coincide with the frequency band, where the most important speech information is located. Ceramic microphones, being relatively inexpensive but offering a broader frequency range, are widely used in portable sound systems, tape recorders, and hearing aids. They possess a wide, albeit somewhat uneven, frequency response. When high precision and wide uniform frequency response are required, condenser microphones are often employed.

Although a high-voltage source is required for parallel plate condenser microphones, electret condenser microphones eliminate this requirement with only a slight decline in performance.

Electret microphones are also widely used in portable sound equipment systems and in hearing aids. Dynamic microphones are capable of high-power output, are rugged, and yield a broad frequency response over a wide dynamic range. The dynamic microphone, with its tolerance for both high-intensity sound levels and rough treatment, is employed frequently by popular musicians for both recording sessions and live performances.

Context

The broad scope of acoustics, as an area of scientific endeavor, is undoubtedly a result of the ubiquitous nature of acoustical radiation and its importance in all human activities.

Although the science of sound has not been considered an important subdivision of physics, its pervasive presence is subtly intertwined with most other branches of physics, engineering, and even seemingly unrelated sciences. Sound production is the essential ingredient in such areas as human speech, music, the sound recording industry, sound reinforcement technology, audiology, architectural acoustics, and the control of noise and vibration. Accurate sound detection is the basis for engineering electroacoustical devices for detecting everything from underwater sound to seismic waves in the Earth. Advances in telephony and audiology, as well as new loudspeaker technologies, have a strong association with the sensation of hearing.

The basic theory of the origin and reception of sound was proposed fairly accurately by the ancient Greeks early in the history of science. They recognized that sound emanates from a vibrating body, is transmitted through the air, and ultimately enters an ear to produce the sensation of hearing. Although somewhat nebulous, these ideas were the very epitome of clarity compared to contemporaneous ideas on light, heat, and the motion of solid objects. Since basic acoustic theory was spelled out correctly at an early date, later theoretical developments tended to be submerged by the broader science of mechanics.

Although Pythagoras, in the sixth century BCE, discovered the laws relating pitch to the length of a musical string, the actual association of pitch with frequency was not understood until Galileo’s explanations in “Discourses Concerning Two New Sciences,” first published in 1638. After reviewing Pythagorean notions relating the length of a plucked string to its pitch, he expanded this concept by expressing his hypothesis that the physical meaning of this relationship was to be found in the frequency of the plucked string. Friar Marin Mersenne (1588-1648) is given credit for the first published account of the correct theory of vibrating strings, whereby the pitch of a musical note could be inferred from the vibration frequency, which in turn was determined by the length, density, and tension of the string.

A complete mathematical description of the vibrating string was elegantly formulated in the eighteenth century by Joseph-Louis Lagrange, who also provided a mathematical understanding of musical wind instrument sounds and the tonalities of organ pipes. Throughout the eighteenth and nineteenth centuries, other scientists and mathematicians extended this work to solid bars, elastic plates, and membranes. Lord Rayleigh’s treatise The Theory of Sound, published in two volumes (1877 & 1878), synthesized all the basic theory and experimental work on sound production in one epic work.

Until the past hundred and fifty years, over the entire historical development of acoustics, the only sound detector of interest has been the human ear. After the relation between pitch and frequency had been established, researchers set out to determine the frequency limits of audibility. During the nineteenth century, K. R. Koenig made very detailed studies of the upper and lower frequency limits of audibility, while other scientists studied the related problem of the minimum sound level to which the ear will respond. Although devices to amplify the sound received by the ear, such as the ear trumpet, can be traced back hundreds of years, it was not until 1875—when Alexander Graham Bell built an early telephone transmitter—that the means for receiving sounds without human ears were even considered. Since then, the technological development of electroacoustic transducers has been rapid and prodigious. In the late twentieth century, a wide variety of microphones, spanning an even wider range of prices, became available for a multitude of purposes. One needs only to know the necessary quality and frequency range of a desired recording to find a suitable microphone, within budgetary restraints, for virtually any desired acoustical purpose.

Principal terms

CAPACITIVE EFFECT: when two parallel metal plates are electrically charged (one positively, the other negatively), the voltage between the plates will vary as the separation of the plates changes; thus, the mechanical motion of the plates can be transformed into a time-varying electric signal

FREQUENCY: the number of vibrations per second executed by an oscillating object; measured in hertz, equal to one vibration per second

MAGNETIC FORCE LAW: when a coil of wire carrying an electric current is placed in a magnetic field, the coil will be subject to a force that may cause the coil to be displaced; conversely, when a wire coil is moved through a magnetic field, the electrons in the wire will move because of a magnetic force, thus creating an electric current

PIEZOELECTRIC EFFECT: when certain crystals or ceramic materials are compressed or distorted, they acquire an electric voltage across the material that is proportional to the amount of compression or deflection

RADIATION: the emission or “giving off” of energy in the form of sound waves that propagate outward from a vibrating surface

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