A hydrophone is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates an electric potential when subjected to a pressure change, such as a sound wave; some piezoelectric transducers can serve as a sound projector, but not all have this capability, some may be destroyed if used in such a manner. A hydrophone can detect airborne sounds, but will be insensitive because it is designed to match the acoustic impedance of water, a denser fluid than air. Sound travels 4.3 times faster in water than in air, a sound wave in water exerts a pressure 60 times that exerted by a wave of the same amplitude in air. A standard microphone can be buried in the ground, or immersed in water if it is put in a waterproof container, but will give poor performance due to the bad acoustic impedance match; the first hydrophones consisted of a tube with a thin membrane covering the submerged end and the observer's ear on the other end.
The design of effective hydrophones must take into account the acoustic resistance of water, 3750 times that of air. The American Submarine Signaling Company developed a hydrophone to detect underwater bells rung from lighthouses and lightships. The case was a hollow brass disc 35 centimetres in diameter. On one face was a 1 millimetre thick brass diaphragm, coupled by a short brass rod to a carbon microphone. Early in the war the French President Poincaré provided Paul Langevin with the facilities needed to work on a method to locate submarines by the echos from sound pulses, they developed a piezoelectric hydrophone by increasing the power of the signal with a vacuum tube amplifier. The same piezoelectric plate could be vibrated by an electrical oscillator to produce the sound pulses. In the war the British Admiralty belatedly convened a scientific panel to advise on how to combat U-boats. It included the Australian physicist William Henry Bragg and the New Zealand physicist Sir Ernest Rutherford.
They concluded. Rutherford's research produced his sole patent for a hydrophone. Bragg took the lead in July 1916 he moved to the Admiralty hydrophone research establishment at Hawkcraig on the Firth of Forth. The scientists set two goals: to develop a hydrophone that could hear a submarine despite the racket produced by a patrol ship carrying the hydrophone and to develop a hydrophone that could reveal the bearing of the submarine. A bidirectional hydrophone was invented at East London College, they mounted a microphone on each side of a diaphragm in a cylindrical case, when the sounds heard from both microphones have the same intensity the microphone is in line with the sound source. Bragg's laboratory made such a hydrophone directional by mounting a baffle in front of one side of the diaphragm. It took months to discover that effective baffles must contain a layer of air. In 1918 airships of the Royal Naval Air Service engaged in anti-submarine warfare experimented by trailing dipped hydrophones.
Bragg found it inferior to British models. By the end of the war the British had 38 hydrophone officers and 200 qualified listeners, paid an addition 4d per day. From late in World War I until the introduction of active sonar in the early 1920s, hydrophones were the sole method for submarines to detect targets while submerged. A small single cylindrical ceramic transducer can achieve near perfect omnidirectional reception. Directional hydrophones increase sensitivity from one direction using two basic techniques: This device uses a single transducer element with a dish or conical-shaped sound reflector to focus the signals, in a similar manner to a reflecting telescope; this type of hydrophone can be produced from a low-cost omnidirectional type, but must be used while stationary, as the reflector impedes its movement through water. A new way to direct is to use a spherical body around the hydrophone; the advantage of directivity spheres is that the hydrophone can be moved within the water, ridding it of the interferences produced by a conical-shaped element.
Multiple hydrophones can be arranged in an array so that it will add the signals from the desired direction while subtracting signals from other directions. The array may be steered using a beamformer. Most hydrophones are arranged in a "line array" but may be in two- or three-dimensional arrangements. SOSUS hydrophones, laid on the seabed and connected by underwater cables, were used, beginning in the 1950s, by the U. S. Navy to track movement of Soviet submarines during the Cold War along a line from Greenland and the United Kingdom known as the GIUK gap; these are capable of recording low frequency infrasound, including many unexplained ocean sounds. Communication with submarines Geophone Underwater acoustics Sonar Reflection seismology John. SOSUS. Retrieved January 28, 2005. Watlington, Frank. How to build & use low-cost hydrophones. Unknown. Hydrophone. Retrieved January 28, 2005. Unknown. Schlumberger Oilfield Glossary: Term'hydrophone'. Retrieved January 28, 2005. Onda Corporation.'Hydrophone Handbook'.
Report AIR 1/645/17/122/304 - National Archives Kew. Airship Hydrophone experiments. DOSITS—Hydrophone introduction at Discovery of Sound in the Sea orcasound.net—Live hydrophone streams from killer
In mathematics convolution is a mathematical operation on two functions to produce a third function that expresses how the shape of one is modified by the other. The term convolution refers to the process of computing it; some features of convolution are similar to cross-correlation: for real-valued functions, of a continuous or discrete variable, it differs from cross-correlation only in that either f or g is reflected about the y-axis. For continuous functions, the cross-correlation operator is the adjoint of the convolution operator. Convolution has applications that include probability, computer vision, natural language processing and signal processing and differential equations; the convolution can be defined for functions on Euclidean space, other groups. For example, periodic functions, such as the discrete-time Fourier transform, can be defined on a circle and convolved by periodic convolution. A discrete convolution can be defined for functions on the set of integers. Generalizations of convolution have applications in the field of numerical analysis and numerical linear algebra, in the design and implementation of finite impulse response filters in signal processing.
Computing the inverse of the convolution operation is known as deconvolution. The convolution of f and g is written f ∗ g, using an star, it is defined as the integral of the product of the two functions after one is shifted. As such, it is a particular kind of integral transform: An equivalent definition is: ≜ ∫ − ∞ ∞ f g d τ. While the symbol t is used above, it need not represent the time domain, but in that context, the convolution formula can be described as a weighted average of the function f at the moment t where the weighting is given by g shifted by amount t. As t changes, the weighting function emphasizes different parts of the input function. For functions f, g supported on only [0, ∞), the integration limits can be truncated, resulting in: = ∫ 0 t f g d τ for f, g: [ 0, ∞ ) → R. For the multi-dimensional formulation of convolution, see domain of definition. A common engineering convention is: f ∗ g ≜ ∫ − ∞ ∞ f g d τ ⏟, which has to be interpreted to avoid confusion. For instance, f ∗ g is equivalent to.
Convolution describes the output of an important class of operations known as linear time-invariant. See LTI system theory for a derivation of convolution as the result of LTI constraints. In terms of the Fourier transforms of the input and output of an LTI operation, no new frequency components are created; the existing ones are only modified. In other words, the output transform is the pointwise product of the input transform with a third transform. See Convolution theorem for a derivation of that property of convolution. Conversely, convolution can be derived as the inverse Fourier transform of the pointwise product of two Fourier transforms. One of the earliest uses of the convolution integral appeared in D'Alembert's derivation of Taylor's theorem in Recherches sur différents points importants du système du monde, published in 1754. An expression of the type: ∫ f ⋅ g d u is used by Sylvestre François Lacroix on page 505 of his book entitled Treatise on differences and series, the last of 3 volumes of the encyclopedic series: Traité du calcul différentiel et du calcul intégral, Chez Courcier, Paris, 1797–1800.
Soon thereafter, convolution operations appear in the works of Pierre Simon Laplace, Jean-Baptiste Joseph Fourier, Siméon Denis Poisson, others. The term itself did not come into wide use until the 60s. Prior to that it was sometimes known as Faltung, composition product, superposition integral, Carson's integral, yet it appears as early as 1903. The o
A megaphone, speaking-trumpet, blowhorn, or loudhailer is a portable or hand-held, cone-shaped acoustic horn used to amplify a person’s voice or other sounds and direct it in a given direction. The sound is introduced into the narrow end of the megaphone, by holding it up to the face and speaking into it, the sound waves radiate out the wide end. A megaphone increases the volume of sound by increasing the acoustic impedance seen by the vocal cords, matching the impedance of the vocal cords to the air, so that more sound power is radiated, it serves to direct the sound waves in the direction the horn is pointing. It somewhat distorts the sound of the voice because the frequency response of the megaphone is greater at higher sound frequencies. Since the 1960s the voice-powered acoustic megaphone described above has been replaced by the electric megaphone, which uses electric power and a folded horn to amplify the voice; the initial inventor of the speaking trumpet is a subject of historical controversy.
There have been references to speakers in Ancient Greece wearing masks with cones protruding from the mouth in order to amplify their voices in theatres. Hellenic architects may have consciously utilized acoustic physics in their design of theatre amphitheaters. A drawing by Louis Nicolas on page 14 of the Codex canadensis, circa 1675 to 1682, shows a Native American chief named Iscouakité using a megaphone made of birch bark; the text of the illustration says. Both Samuel Morland and Athanasius Kircher have been credited with inventing megaphones around the same time in the 17th century. Morland, in a work published in 1655, wrote about his experimentation with different horns, his largest megaphone consisted of over 20 feet of copper tube and could project a person's voice a mile and a half. Twenty years earlier, Kircher described a device that could be used as both a megaphone and for "overhearing" people speaking outside a house, his coiled horn would be mounted into the side of a building, with a narrow end inside that could be either spoken into or listened to, the wide mouth projecting through the outside wall.
Morland favored a tube-shaped speaking device. Kircher’s horn, on the other hand, utilized a “cochleate” design, where the horn was twisted and coiled to make it more compact. A papier-mache trumpet of special design was the Sengerphone. Additionally, in ruins of Tiwanaku are stones around the central place with holes shaped in megaphone's profile, their purpose is today unknown, but as local guards can show, it is possible to amplify human voice as it is loud enough to hear it across large area. The term ‘megaphone’ was first associated with Thomas Edison’s instrument 200 years later. In 1878, Edison developed a device similar to the speaking trumpet in hopes of benefiting the deaf and hard of hearing, his variation included three separate funnels lined up in a row. The two outer funnels, which were six feet and eight inches long, were made of paper and connected to a tube inserted in each ear; the middle funnel was similar to Morland’s speaking trumpet, but had a larger slot to insert a user’s mouth.
With Edison’s megaphone, a low whisper could be heard a thousand feet away, while a normal tone of voice could be heard two miles away. On the listening end, the receiver could hear a low whisper at a thousand feet away; however the apparatus was much too large to be portable. George Prescott wrote: “The principal drawback at present is the large size of the apparatus.” Since the 1960s acoustic megaphones have been replaced by electric versions, although the cheap, rugged acoustic megaphone is still used in a few venues, like cheering at sporting events and cheerleading, by lifeguards at pools and beaches where the moisture could damage the electronics of electric megaphones. An electric megaphone is a handheld public address system, an electronic device that amplifies the human voice like an acoustic megaphone, using electric power, it consists of a microphone to convert sound waves into an electrical audio signal, an amplifier powered by a battery to increase the power of the audio signal, a loudspeaker to convert the audio signal to sound waves again.
Although heavier than acoustic megaphones, electric megaphones can amplify the voice to a higher level, to over 90 dB. They have replaced acoustic megaphones in most applications, are used to address congregations of people wherever stationary public address systems are not available. Although electronic public address systems have existed since vacuum tube amplifiers were developed in the early 1920s, vacuum tube versions were too heavy to be portable. Practical portable electric megaphones had to await the development of microelectronics which followed the invention of the transistor in 1947. In 1954, TOA Corporation developed the world's first transistorized megaphone. Handheld versions are shaped like the old acoustic megaphone, with a microphone at one end and a horn speaker at the other, a pistol grip on the side, with a trigger switch to turn it on. In use, the device is held up to the mouth, the trigger is pressed to turn it on while speaking. Other larger versions hang from the shoulder on a strap, have a separate handheld microphone on a cord to speak into, so users can address a crowd without the instrument obscuring their faces.
A vast array of modern electric megaphones are available to purchase, characteristics like power, weight and the presence of alarms and shoulder straps all contribute to a consumer’s choice. The shape of the megaphone
A-weighting is the most used of a family of curves defined in the International standard IEC 61672:2003 and various national standards relating to the measurement of sound pressure level. A-weighting is applied to instrument-measured sound levels in an effort to account for the relative loudness perceived by the human ear, as the ear is less sensitive to low audio frequencies, it is employed by arithmetically adding a table of values, listed by octave or third-octave bands, to the measured sound pressure levels in dB. The resulting octave band measurements are added to provide a single A-weighted value describing the sound. Other weighting sets of values – B, C, D and now Z – are discussed below; the curves were defined for use at different average sound levels, but A-weighting, though intended only for the measurement of low-level sounds, is now used for the measurement of environmental noise and industrial noise, as well as when assessing potential hearing damage and other noise health effects at all sound levels.
It is used when measuring low-level noise in audio equipment in the United States. In Britain and many other parts of the world and audio engineers more use the ITU-R 468 noise weighting, developed in the 1960s based on research by the BBC and other organizations; this research showed that our ears respond differently to random noise, the equal-loudness curves on which the A, B and C weightings were based are only valid for pure single tones. A-weighting began with work by Fletcher and Munson which resulted in their publication, in 1933, of a set of equal-loudness contours. Three years these curves were used in the first American standard for sound level meters; this ANSI standard revised as ANSI S1.4-1981, incorporated B-weighting as well as the A-weighting curve, recognising the unsuitability of the latter for anything other than low-level measurements. But B-weighting has since fallen into disuse. Work, first by Zwicker and by Schomer, attempted to overcome the difficulty posed by different levels, work by the BBC resulted in the CCIR-468 weighting maintained as ITU-R 468 noise weighting, which gives more representative readings on noise as opposed to pure tones.
A-weighting is valid to represent the sensitivity of the human ear as a function of the frequency of pure tones, but only for quiet levels of sound. In effect, the A-weighting is based on the 40-phon Fletcher–Munson curves which represented an early determination of the equal-loudness contour for human hearing. However, because decades of field experience have shown a good correlation between the A scale and occupational deafness in the frequency range of human speech, this scale is employed in many jurisdictions to evaluate the risks of occupational deafness and other auditory problems related to signals or speech intelligibility in noisy environnements; because of perceived discrepancies between early and more recent determinations, the International Organization for Standardization revised its standard curves as defined in ISO 226, in response to the recommendations of a study coordinated by the Research Institute of Electrical Communication, Tohoku University, Japan. The study produced new curves by combining the results of several studies, by researchers in Japan, Denmark, UK, USA.
This has resulted in the recent acceptance of a new set of curves standardized as ISO 226:2003. The report comments on the large differences, the fact that the original Fletcher–Munson contours are in better agreement with recent results than the Robinson-Dadson, which appear to differ by as much as 10–15 dB in the low-frequency region, for reasons that are not explained. Fortuitously, the 40-phon Fletcher–Munson curve is close to the modern ISO 226:2003 standard, it will be noted that A-weighting would be a better match to the loudness curve if it fell much more steeply above 10 kHz, it is that this compromise came about because steep filters were difficult to construct in the early days of electronics. Nowadays, no such limitation need exist. If A-weighting is used without further band-limiting it is possible to obtain different readings on different instruments when ultrasonic, or near ultrasonic noise is present. Accurate measurements therefore require a 20 kHz low-pass filter to be combined with the A-weighting curve in modern instruments.
This is defined in IEC 61012 as AU weighting and while desirable, is fitted to commercial sound level meters. A-frequency-weighting is mandated by the international standard IEC 61672 to be fitted to all sound level meters; the old B- and D-frequency-weightings have fallen into disuse, but many sound level meters provide for C frequency-weighting and its fitting is mandated — at least for testing purposes — to precision sound level meters. D-frequency-weighting was designed for use when measuring high level aircraft noise in accordance with the IEC 537 measurement standard; the large peak in the D-weighting curve is not a feature of the equal-loudness contours, but reflects the fact that humans hear random noise differently from pure tones, an effect, pronounced around 6 kHz. This is because individual neurons from different regions of the cochlea in the inner ear respond to narrow ba
The decibel is a unit of measurement used to express the ratio of one value of a power or field quantity to another on a logarithmic scale, the logarithmic quantity being called the power level or field level, respectively. It can be used to express a change in an absolute value. In the latter case, it expresses the ratio of a value to a fixed reference value. For example, if the reference value is 1 volt the suffix is "V", if the reference value is one milliwatt the suffix is "m". Two different scales are used when expressing a ratio in decibels, depending on the nature of the quantities: power and field; when expressing a power ratio, the number of decibels is ten times its logarithm to base 10. That is, a change in power by a factor of 10 corresponds to a 10 dB change in level; when expressing field quantities, a change in amplitude by a factor of 10 corresponds to a 20 dB change in level. The decibel scales differ by a factor of two so that the related power and field levels change by the same number of decibels in, for example, resistive loads.
The definition of the decibel is based on the measurement of power in telephony of the early 20th century in the Bell System in the United States. One decibel is one tenth of one bel, named in honor of Alexander Graham Bell. Today, the decibel is used for a wide variety of measurements in science and engineering, most prominently in acoustics and control theory. In electronics, the gains of amplifiers, attenuation of signals, signal-to-noise ratios are expressed in decibels. In the International System of Quantities, the decibel is defined as a unit of measurement for quantities of type level or level difference, which are defined as the logarithm of the ratio of power- or field-type quantities; the decibel originates from methods used to quantify signal loss in telegraph and telephone circuits. The unit for loss was Miles of Standard Cable. 1 MSC corresponded to the loss of power over a 1 mile length of standard telephone cable at a frequency of 5000 radians per second, matched the smallest attenuation detectable to the average listener.
The standard telephone cable implied was "a cable having uniformly distributed resistance of 88 Ohms per loop-mile and uniformly distributed shunt capacitance of 0.054 microfarads per mile". In 1924, Bell Telephone Laboratories received favorable response to a new unit definition among members of the International Advisory Committee on Long Distance Telephony in Europe and replaced the MSC with the Transmission Unit. 1 TU was defined such that the number of TUs was ten times the base-10 logarithm of the ratio of measured power to a reference power. The definition was conveniently chosen such that 1 TU approximated 1 MSC. In 1928, the Bell system renamed the TU into the decibel, being one tenth of a newly defined unit for the base-10 logarithm of the power ratio, it was named the bel, in honor of the telecommunications pioneer Alexander Graham Bell. The bel is used, as the decibel was the proposed working unit; the naming and early definition of the decibel is described in the NBS Standard's Yearbook of 1931: Since the earliest days of the telephone, the need for a unit in which to measure the transmission efficiency of telephone facilities has been recognized.
The introduction of cable in 1896 afforded a stable basis for a convenient unit and the "mile of standard" cable came into general use shortly thereafter. This unit was employed up to 1923 when a new unit was adopted as being more suitable for modern telephone work; the new transmission unit is used among the foreign telephone organizations and it was termed the "decibel" at the suggestion of the International Advisory Committee on Long Distance Telephony. The decibel may be defined by the statement that two amounts of power differ by 1 decibel when they are in the ratio of 100.1 and any two amounts of power differ by N decibels when they are in the ratio of 10N. The number of transmission units expressing the ratio of any two powers is therefore ten times the common logarithm of that ratio; this method of designating the gain or loss of power in telephone circuits permits direct addition or subtraction of the units expressing the efficiency of different parts of the circuit... In 1954, J. W. Horton argued that the use of the decibel as a unit for quantities other than transmission loss led to confusion, suggested the name'logit' for "standard magnitudes which combine by addition".
In April 2003, the International Committee for Weights and Measures considered a recommendation for the inclusion of the decibel in the International System of Units, but decided against the proposal. However, the decibel is recognized by other international bodies such as the International Electrotechnical Commission and International Organization for Standardization; the IEC permits the use of the decibel with field quantities as well as power and this recommendation is followed by many national standards bodies, such as NIST, which justifies the use of the decibel for voltage ratios. The term field quantity is deprecated by ISO 80000-1. In spite of their widespread use, suffixes are not recognized by the IEC or ISO. ISO 80000-3 describes definitions for units of space and time; the decibel for use in acoustics is defined in ISO 80000-8. The major difference from the article below is that for acoustics the decibel has no
The metre or meter is the base unit of length in the International System of Units. The SI unit symbol is m; the metre is defined as the length of the path travelled by light in vacuum in 1/299 792 458 of a second. The metre was defined in 1793 as one ten-millionth of the distance from the equator to the North Pole – as a result the Earth's circumference is 40,000 km today. In 1799, it was redefined in terms of a prototype metre bar. In 1960, the metre was redefined in terms of a certain number of wavelengths of a certain emission line of krypton-86. In 1983, the current definition was adopted; the imperial inch is defined as 0.0254 metres. One metre is about 3 3⁄8 inches longer than a yard, i.e. about 39 3⁄8 inches. Metre is the standard spelling of the metric unit for length in nearly all English-speaking nations except the United States and the Philippines, which use meter. Other Germanic languages, such as German and the Scandinavian languages spell the word meter. Measuring devices are spelled "-meter" in all variants of English.
The suffix "-meter" has the same Greek origin as the unit of length. The etymological roots of metre can be traced to the Greek verb μετρέω and noun μέτρον, which were used for physical measurement, for poetic metre and by extension for moderation or avoiding extremism; this range of uses is found in Latin, French and other languages. The motto ΜΕΤΡΩ ΧΡΩ in the seal of the International Bureau of Weights and Measures, a saying of the Greek statesman and philosopher Pittacus of Mytilene and may be translated as "Use measure!", thus calls for both measurement and moderation. In 1668 the English cleric and philosopher John Wilkins proposed in an essay a decimal-based unit of length, the universal measure or standard based on a pendulum with a two-second period; the use of the seconds pendulum to define length had been suggested to the Royal Society in 1660 by Christopher Wren. Christiaan Huygens had observed that length to be 39.26 English inches. No official action was taken regarding these suggestions.
In 1670 Gabriel Mouton, Bishop of Lyon suggested a universal length standard with decimal multiples and divisions, to be based on a one-minute angle of the Earth's meridian arc or on a pendulum with a two-second period. In 1675, the Italian scientist Tito Livio Burattini, in his work Misura Universale, used the phrase metro cattolico, derived from the Greek μέτρον καθολικόν, to denote the standard unit of length derived from a pendulum; as a result of the French Revolution, the French Academy of Sciences charged a commission with determining a single scale for all measures. On 7 October 1790 that commission advised the adoption of a decimal system, on 19 March 1791 advised the adoption of the term mètre, a basic unit of length, which they defined as equal to one ten-millionth of the distance between the North Pole and the Equator. In 1793, the French National Convention adopted the proposal. In 1791, the French Academy of Sciences selected the meridional definition over the pendular definition because the force of gravity varies over the surface of the Earth, which affects the period of a pendulum.
To establish a universally accepted foundation for the definition of the metre, more accurate measurements of this meridian were needed. The French Academy of Sciences commissioned an expedition led by Jean Baptiste Joseph Delambre and Pierre Méchain, lasting from 1792 to 1799, which attempted to measure the distance between a belfry in Dunkerque and Montjuïc castle in Barcelona to estimate the length of the meridian arc through Dunkerque; this portion of the meridian, assumed to be the same length as the Paris meridian, was to serve as the basis for the length of the half meridian connecting the North Pole with the Equator. The problem with this approach is that the exact shape of the Earth is not a simple mathematical shape, such as a sphere or oblate spheroid, at the level of precision required for defining a standard of length; the irregular and particular shape of the Earth smoothed to sea level is represented by a mathematical model called a geoid, which means "Earth-shaped". Despite these issues, in 1793 France adopted this definition of the metre as its official unit of length based on provisional results from this expedition.
However, it was determined that the first prototype metre bar was short by about 200 micrometres because of miscalculation of the flattening of the Earth, making the prototype about 0.02% shorter than the original proposed definition of the metre. Regardless, this length became the French standard and was progressively adopted by other countries in Europe; the expedition was fictionalised in Le mètre du Monde. Ken Alder wrote factually about the expedition in The Measure of All Things: the seven year odyssey and hidden error that transformed the world. In 1867 at the second general conference of the International Association of Geodesy held in Berlin, the question of an international standard unit of length was discussed in order to combine the measurements made in different countries to determine the size and shape of the Earth; the conference recommended the adoption of the metre and the creation of an internatio
A microphone, colloquially nicknamed mic or mike, is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and public events, motion picture production and recorded audio engineering, sound recording, two-way radios, megaphones and television broadcasting, in computers for recording voice, speech recognition, VoIP, for non-acoustic purposes such as ultrasonic sensors or knock sensors. Several different types of microphone are in use, which employ different methods to convert the air pressure variations of a sound wave to an electrical signal; the most common are the dynamic microphone. Microphones need to be connected to a preamplifier before the signal can be recorded or reproduced. In order to speak to larger groups of people, a need arose to increase the volume of the human voice; the earliest devices used to achieve this were acoustic megaphones. Some of the first examples, from fifth century BC Greece, were theater masks with horn-shaped mouth openings that acoustically amplified the voice of actors in amphitheatres.
In 1665, the English physicist Robert Hooke was the first to experiment with a medium other than air with the invention of the "lovers' telephone" made of stretched wire with a cup attached at each end. In 1861, German inventor Johann Philipp Reis built an early sound transmitter that used a metallic strip attached to a vibrating membrane that would produce intermittent current. Better results were achieved in 1876 with the "liquid transmitter" design in early telephones from Alexander Graham Bell and Elisha Gray – the diaphragm was attached to a conductive rod in an acid solution; these systems, gave a poor sound quality. The first microphone that enabled proper voice telephony was the carbon microphone; this was independently developed by David Edward Hughes in England and Emile Berliner and Thomas Edison in the US. Although Edison was awarded the first patent in mid-1877, Hughes had demonstrated his working device in front of many witnesses some years earlier, most historians credit him with its invention.
The carbon microphone is the direct prototype of today's microphones and was critical in the development of telephony and the recording industries. Thomas Edison refined the carbon microphone into his carbon-button transmitter of 1886; this microphone was employed at the first radio broadcast, a performance at the New York Metropolitan Opera House in 1910. In 1916, E. C. Wente of Western Electric developed the next breakthrough with the first condenser microphone. In 1923, the first practical moving coil microphone was built; the Marconi-Sykes magnetophone, developed by Captain H. J. Round, became the standard for BBC studios in London; this was improved in 1930 by Alan Blumlein and Herbert Holman who released the HB1A and was the best standard of the day. In 1923, the ribbon microphone was introduced, another electromagnetic type, believed to have been developed by Harry F. Olson, who reverse-engineered a ribbon speaker. Over the years these microphones were developed by several companies, most notably RCA that made large advancements in pattern control, to give the microphone directionality.
With television and film technology booming there was demand for high fidelity microphones and greater directionality. Electro-Voice responded with their Academy Award-winning shotgun microphone in 1963. During the second half of 20th century development advanced with the Shure Brothers bringing out the SM58 and SM57; the latest research developments include the use of fibre optics and interferometers. The sensitive transducer element of a microphone is called its capsule. Sound is first converted to mechanical motion by means of a diaphragm, the motion of, converted to an electrical signal. A complete microphone includes a housing, some means of bringing the signal from the element to other equipment, an electronic circuit to adapt the output of the capsule to the equipment being driven. A wireless microphone contains a radio transmitter. Microphones are categorized by their transducer principle, such as condenser, etc. and by their directional characteristics. Sometimes other characteristics such as diaphragm size, intended use or orientation of the principal sound input to the principal axis of the microphone are used to describe the microphone.
The condenser microphone, invented at Western Electric in 1916 by E. C. Wente, is called a capacitor microphone or electrostatic microphone—capacitors were called condensers. Here, the diaphragm acts as one plate of a capacitor, the vibrations produce changes in the distance between the plates. There are two types, depending on the method of extracting the audio signal from the transducer: DC-biased microphones, radio frequency or high frequency condenser microphones. With a DC-biased microphone, the plates are biased with a fixed charge; the voltage maintained across the capacitor plates changes with the vibrations in the air, according to the capacitance equation, where Q = charge in coulombs, C = capacitance in farads and V = potential difference in volts. The capacitance of the plates is inversely proportional to the distance between them for a parallel-plate capacitor; the assembly of fixed and movable plates is called an "element" or "capsule". A nearly constant charge is maintained on the capa