Any electronic device that increases the power of an electrical signal whose vibrations are confined to the audio frequency range—the range that can be perceived by the human ear—is an audio amplifier. All devices that transmit, record, or otherwise electronically process voice signals employ audio amplifiers. Voice-recognition or voice-synthesis systems, communications or eavesdropping devices, hearing aids, entertainment systems, talking toys, are examples of devices containing audio amplifiers.
The need for amplification. Acoustic or sound waves are longitudinal pressure waves (i.e., waves that cause molecules to oscillate along the wave's line of travel rather than across it) in air, water, or any other medium. A sound is said to be in the audio frequency range if it is not too high or low in frequency to be heard by the human ear. Audio sound waves may be converted by microphones into electrical signals for analysis, transmission, or recording. Electrical signals can also be converted by speakers into audible sound waves. Microphones and speakers are both transducers, that is, devices that convert energy from one form (e.g., electrical) into another (e.g., acoustic) or vice versa. Audio amplifiers are required with both microphones and speakers.
Input amplification. Amplification of the signal produced by a microphone—often termed preamplification—is necessary because the electrical signal that can be derived directly from sound waves impinging on a microphone is weak (i.e., on the order of .01 V or less; for eavesdropping applications, much less). Input signals of such low amplitude must be amplified before they can be processed in either analog or digital circuits.
In analog circuits—circuits that process smoothlyvarying electrical quantities—there is a always a certain amount of random electrical activity or "noise." This noise is mixed with any information signal processed by the circuit, corrupting it. Amplifying a weak input, such as that from a microphone, before it mingles with circuit noise makes the noise problem manageable. Furthermore, all analog circuits that lack amplification (passive filters, transmission lines, etc.) experience signal loss; that is, they dissipate energy. A weak signal fed into a circuit that does not contain amplification will, therefore, quickly disappear, making amplification necessary in most analog circuits. Finally, amplification provides electronic isolation between the signal being amplified and the result of the amplification process; among other gains, this simplifies the circuit-design process.
If an audio signal is to be processed using digital circuitry (as is often the case today), a digital signal (i.e., on-off, high-low signal that can represent signal magnitudes symbolically) must be derived from the analog input. This conversion is performed by a device termed an analog-to-digital converter. For reasons ultimately deriving from the atomic properties of semiconductors, a typical analog-to-digital converter requires an analog input signal with an amplitude variation on the order of several volts. A low voltage signal must therefore usually be amplified before being digitized.
Output amplification. Wherever human ears are the ultimate destination of a signal it is necessary to drive a physical sound-making device at the output. Here audio amplification is needed for a reason complementary to that which applies at the input: the signal power needed to drive an output device (e.g., speaker or headphones) is greater than that conveyed by the signals processed throughout the circuitry of a typical electronic device, whether analog or digital. An audio amplifier is thus found at the output as well as at the input of almost every system handling signals in the audio range.
Applications. The number of audio amplifier designs that have been produced over the last century is probably in the hundreds of thousands. Such devices are a ubiquitous feature of modern life, and are found in computers, telephones, radios, high-fidelity audio systems, all military voice-communication systems, many appliances, and even toys.
Audio amplifiers can be miniaturized for placement in headsets, mobile phones. In applications where small size is at a premium, as in hearing aides and espionage applications (bugs and "wires"), they may be ultraminiaturized. At the high-power end, audio amplification drives public-address systems, speaker systems, and (potentially) weapons. Research is being conducted by several countries, including Russia and the U.S. (through its Low Collateral Damage Munitions Program), into the use of highly amplified sound as a weapon; frequencies in the infrasonic, audio, and ultrasonic ranges are all being considered for use against human beings. Though acoustic weapons are sometimes assumed to always be in the nonlethal category, sound can be irritating, painful, or fatal, depending on its intensity and on the efficiency with which its energy is coupled to the body.
Loud music has repeatedly been used as a psychological weapon in siege situations (e.g., by the U.S. Army against former Panamanian dictator Manuel Noriega in 1989, by cult leader David Koresh against police in 1993, and by Peruvian police during the hostage crisis at the Japanese Embassy in 1997) and as an instrument of torture. Specially-designed acoustic weapons can induce, among other effects, vomiting, choking, spasms, incontinence, thermal burns, intolerable sensations in the chest, injury to internal organs, and hearing damage. The latter is considered a serious drawback in antipersonnel applications, as hearing loss caused by intense sound is often partly or wholly permanent. Like laser weapons designed to blind (which have been outlawed by recent international agreement), acoustic weapons designed to deafen would violate international humanitarian law. Further, they would be vulnerable to obvious countermeasures, such as earplugs. Indeed, some scientists are skeptical about the possibility of developing reliable, affordable weapons of any kind from sound. However, research and development are proceeding. Military and security applications of high-intensity sound currently under development in the U.S. or elsewhere include the following
1. A device projecting "acoustic bullets," baseball-sized pulses of low-frequency (10-Hz) sound over distances of hundreds of yards, scalable in intensity from painful to lethal.
2. Multisensory grenades emitting disorienting light flashes, painfully loud sounds, and possibly disagreeable odors.
3. A ship-mounted system to disable crewmembers of nearby vessels (e.g., prior to boarding by Coast Guard personnel).
4. The "directed-stick radiator," an audio frequency, battery-powered weapon that could be clipped to a rifle. It fires acoustic bullets with a range in the tens of feet.
5. A helicopter-mounted nonlethal weapon emitting painfully loud sound in the audible range, with a reported (but unlikely) range of 1.2–6 miles (2–10 km).
6. Acoustic-beam weapons designed to cause discomfort: intended for embassy defense, denial of access to sensitive facilities, crowd control, and other miscellaneous antipersonnel uses.
It is unlikely that such devices will see widespread application or that, if they do, they will replace ordinary lethal weapons such as firearms. Due to the tendency of sound waves to diffuse with distance, the unpredictability of their effects on individual persons at sub-lethal levels, and the extremely high power requirements (megawatt range) for lethal levels, acoustic weapons are likely to remain a military curiosity. Audio amplification will thus remain ubiquitous in communications devices and rare in weaponry.
Further Reading
Books
Jones, Dwight V., and Richard F. Shea. Transistor Audio Amplifiers. New York: John Wiley & Sons, 1968.
Periodicals
Altmann, Jürgen. "Acoustic Weapons-A Prospective Assessment." Science and Global Security no. 9 (2001): 165–244.
Electronic
Roxana, Tiron. "Acoustic-Energy Research Hits Sour Note." National Defense Magazine. August 21, 2001. <http://www.nationaldefensemagazine.org/article.cfm?Id=746> (December 13, 2002).
