Sound Recording Solutions
Sound Card Recorder
Powerful voice activated microphone recorder for Windows. Click here to learn more.Loudspeaker
A loudspeaker, or simply speaker, is an electromechanical transducer which converts an electrical signal into sound. The term loudspeaker is used to refer to both the device itself, and a complete system consisting of one or more loudspeaker drivers (as the individual units are often called) in an enclosure. The loudspeaker is the most variable element in an audio system, and is responsible for marked audible differences between systems. The traditional design is a semi-rigid paper fibre cone and a coil of fine wire (usually copper), called the voice coil attached to the apex of the cone. A "gap" is a small circular hole, slot or groove which allows the voice coil and cone to move back and forth. The coil is oriented coaxially inside the gap made with a permanent magnet. The gap is also where the magnetic field is concentrated. One magnetic pole is outside the coil, whilst the other is inside the voice coil. In addition to the magnet, voice coil, and cone, dynamic speakers usually also include a suspension system to provide lateral stability and make the speaker components return to a neutral point after moving. A typical suspension system includes the 'spider', which is at the apex of the cone, often of 'concertina' form; and the 'surround', which is at the base of the cone. The parts are held together by a chassis or basket. When an electrical signal is applied, a magnetic field is induced by the electric current in the coil which becomes an electromagnet. The coil and the permanent magnet interact with magnetic force which causes the coil and a semi-rigid cone (diaphragm) to vibrate and reproduce sound at the frequency of the applied electrical signal. When a multi-frequency signal is applied, the complex vibration results in reproduction of the applied signal as an audio signal Driver cones may be constructed of a variety of materials, including paper, metal, various polypropylenes, and kevlar. Baskets must be designed in order to preserve rigidity and are typically cast or stamped metal, although injection-molded plastic baskets are becoming much more common in recent years. The size and type of magnets can also differ. Sometimes, larger and more powerful magnets are associated with higher quality speakers. Tweeters are subject to a unique set of variables and parameters; their design and construction is extremely variable. Despite marketing claims, lighter and more rigid cones do not always sound better. The weight and damping of the cone in a dynamic speaker should be appropriate for the characteristics of the rest of the driver and enclosure in order to produce accurate sound. A woofer is a loudspeaker capable of reproducing the bass frequencies. The frequency range varies widely according to design and hence while some woofers can cover the audio band from 50 Hz to 3 kHz, yet others may only work up to 1 kHz. A mid-range loudspeaker, also known as a squawker is designed to cover the middle of the audio spectrum, typically from about 200 Hz to about 4-5 kHz. The distinction between woofers and mid-ranges is blurred however since many woofers can operate up to 3 kHz. These are used when the bass driver (or woofer) is incapable of covering the mid audio range. Mid-ranges typically appear where large (>16 cm or 8") woofers are used for the bass end of the audio spectrum. A tweeter is a loudspeaker capable of reproducing the higher end of the audio spectrum, usually from about 1 kHz to 20 or 35 kHz. A full-range speaker is designed to have as wide a frequency response as possible. These employ an additional cone called a whizzer to extend the high frequency response. A whizzer is a small, light cone attached to the woofer's apex around the dust cap. However, the whizzer might not be necessary if the diaphragm is stiff and light enough. There exist full-range drivers with pure titanium diaphragm which are capable of reproducing a frequency range from 50 Hz to 20 kHz and higher without the whizzer cone, and also capable of avoiding the break-ups in the highest frequency range. These drivers are often quite small. Typically 2" to 5" in diameter. A subwoofer driver is a woofer optimised for the lowest range of the audio spectrum. Modern speaker systems often include a single speaker dedicated to reproducing the very lowest bass frequencies. This speaker (and its enclosure) is referred to as a subwoofer. A typical subwoofer only reproduces sounds below 120 Hz (although some subwoofers allow a choice of the cross-over frequency). Because the range of frequencies that must be reproduced is quite limited, the design of the subwoofer is usually quite simple, often consisting of a single, large, down-firing woofer enclosed in a cubical "bass-reflex" cabinet. Subwoofers often contain integrated power amplifiers that may incorporate sophisticated feedback mechanisms to assure the least distortion of the reproduced bass acoustic waveform. The very long wavelength of the very low frequency bass sounds reproduced by the subwoofer usually makes it impossible for the listener to localize the source of these sounds. Localization starts to happen above the 60Hz point. Because of this phenomenon, it is usually satisfactory to provide just a single subwoofer no matter how many individual channels are being used for the full-spectrum sound. For the same reason, the subwoofer does not need a special placement in the sound field (for example, centered between the Left Front and Right Front speakers). It can instead be hidden out of sight. Placing it in the corner of a room may produce louder bass sounds. A subwoofer's powerful bass can often cause items in the room or even the structure of the room itself to vibrate or buzz. Extended periods of high volume bass can cause items throughout a room to "walk" on a flat surface until they fall off. Amplified subwoofers frequently accept both speaker-level and line-level audio signals. When teamed with a modern surround sound receiver and full range speakers, they are typically driven with the specific LFE (low frequency enhancement) output channel (the ".1" in 5.1, 6.1, or 7.1 specifications) provided by the receiver. This is because most full-range speakers are incapable of delivering the acoustic power required by the LFE in movies or in some cases, music. When used with speakers that do not reproduce low frequencies well, a subwoofer will often be configured to reproduce both the LFE channel and all other bass in the system, the latter being referred to as "bass management". A loudspeaker is commonly mounted in an enclosure (or cabinet). The major role of the enclosure is to prevent the out-of-phase sound waves from the rear of the speaker combining with the positive phase sound waves from the front of the speaker, which would result in interference patterns and cancellation causing the efficiency of the speaker to be compromised, particularly in the low frequencies where the wavelengths are large enough that interference will affect the entire listening area. All speakers have two wires, and in a multi-speaker system these must be connected from the source of the signal (the amplifier or receiver) to the speaker's input terminals with the correct polarity, or phase. If both sets of wires for left and right (in a stereo setup) are not connected in phase, the speakers will be out of phase from each other. In this case, any motion one cone makes will be opposite to the other. This type of wiring error creates inverse sound waves that partially cancel out the sound of the other speaker. This does not cause silence, because reflections from surfaces diminish the effect somewhat, but results in a major loss of sound quality. The most prominent effect to the untrained ear is a loss of bass response. The second most noticed is an unsettling feeling. A similar effect is used in sound-cancelling headphones. The headphones produce the inverse sound waves of the external noise. The inverse sound waves and external noise cancel each other out and produce near silence. Speaker design is considered both an art and science. Adjusting a design is done with instruments and with the ear. Speaker designers use an anechoic chamber (essentially a room with soundproofing that inhibits any reverberation or echo) to ensure the speaker will perform the way it is intended to. Some developers (such as Bose) eschew the sole use of anechoic chambers in favor of specific standardized room set-ups designed to replicate likely real-life listening conditions. Some of the issues in speaker design are lobing, phase effects, off axis response and time coherence. Loudspeakers are rugged devices and can take some amount of abuse. However they do have limits and exceeding them by a large factor almost always causes permanent damage. The tweeters are usually the first to go under circumstances of abuse, since they have the lightest voice coil made of thin wire which easily melts if the temperature rises excessively. Tweeters are usually designed (and rated) keeping in mind that a typical music signal doesn't contain a lot of power or energy at the higher end of the audio spectrum. Thus a tweeter rated for 50 W is meant to be used with a 50 W amplifier only if the signals below the tweeter's lower operating frequency are filtered out. Thus, feeding a low frequency (or a DC) signal to a tweeter even though electrically it may be within the tweeter's specification may cause permanent damage to the tweeter. A badly clipping amplifier may also damage the tweeter despite a crossover, since a clipped waveform generates high-frequency harmonics which can contain sufficient power to heat up the tweeter's voice coil. Most woofers (and mid-ranges) can easily take up to 1.5 times or more power than what they are rated for - however this is dependent on the particular driver and the duration of the abuse or overload. Woofers will usually take a lot of power before burning out or suffering damage to their moving systems. Physical damage occurs if the signal causes the woofer's cone displacement to exceed the safe Xmech limits for prolonged periods. In rare cases, a very loud signal may cause the coupling between the parts of the woofer to simply give way. A large DC fed to the woofer may cause twisting or deformation of the voice coil such that it rubs against the pole-pieces or magnet. Electrical damage occurs when the voice coil burns out. The latter two typically happen when the amplifier dumps a large DC current into the speaker - a condition typical of a failing (or failed) amplifier. In all cases, replacement or full repair of the driver are the only options. A complication is the interaction of the speaker with the listening environment. This interaction affects the speaker's electromechanical behavior and thus the load it represents to the amplifier, making it difficult to predict the sound a given system will produce in its intended environment without listening tests. It has been theorized by some of the audiophile world that the perceived differences in sound between amplifier/loudspeaker combinations are in fact only differences in their interaction with their environment, rather than absolute differences in sound quality; and similarly, that any perceived differences in speaker cables, past a minimum set of specifications regarding resistance, inductance, capacitance, etc. are mainly due to advantageous interactions with a particular speaker-room combination. One problem with loudspeakers is that the essentially-planar form of most loudspeakers creates a soundwave that is somewhat directional, that is, the intensity of the sound produced varies depending on the listener's angle relative to the central axis of the speaker. This is especially a problem for high frequencies where the loudspeaker may be physically large compared to the wavelength of the sound being reproduced. A point source or a sphere that varies in size with the amplitude of the desired pressure wave would avoid this problem of beam-formation but is generally physically impossible or impractical. Several approaches have attempted to remedy this by approximating the sphere. Amar Bose of MIT spent many years trying to reproduce this spherical wavefront by constructing a one-eighth sphere covered in small drivers that would be situated in the corner of a room, thus mimicking one-eighth of a spherical wavefront emanating from that corner; in practice this idea never became workable, but Bose's experience with combining multiple small drivers in one loudspeaker cabinet gave rise to the popular Bose speakers which use multiple four-inch drivers, either to direct sound rearwards to reflect it from a wall behind the speakers, for home use, or to provide high power capacity when aimed directly at the listeners, for professional use. For high frequencies, a variation on the common dynamic loudspeaker design uses a small dome as the moving part instead of an inverted cone. This design is typically used for tweeters and sometimes for mid-range speakers. Because the wavelength of high-frequency sound is short (approximately 15 mm at 20 kHz), tweeters must have a physically small moving component or they will create a "beam" of sound rather than sending sound omnidirectionally (as is usually desired). Perhaps contrary to intuition, making the moving component in the form of a dome rather than an inverted cone does not help to direct sound evenly in all directions. The dome is used because it is an easily manufactured stiff structure - as anyone who has attempted to crush an egg the long way can attest to. The stiffness moves self resonances upward in frequency. The ribbon loudspeaker consists of a thin metal-film ribbon suspended between two magnets. The electrical signal is applied to the ribbon which vibrates creating the sound. The advantage of the ribbon loudspeaker is that the ribbon has very little mass; as such, it can accelerate very quickly, yielding good high-frequency response (although its shape is far from ideal). Ribbon loudspeakers can be very fragile but recently designed planar tweeters have the metal film printed on a strong lightweight material for reinforcement. Ribbon tweeters often emit sound that exits the speaker concentrated into a flat plane at the level of the listeners' ears; above and below the plane there is often less treble sound. The Ohm model "F" speakers invented by Lincoln Walsh feature a single driver mounted vertically as though it were firing downwards into the top of the cabinet, but instead of the normal almost flat cone, having a very-much extended cone entirely exposed at the top of the speaker. This turned normal speaker driver design problems on their head; whereas the normal problem with designing a driver is how to keep the cone as stiff as possible (without adding mass), so that it moved as a unit and did not become subject to traveling waves on its surface, the Ohm drivers were designed so that the entire purpose of the electromagnetic driver was to generate traveling waves that traversed the cone from the electromagnet at the top downwards to the bottom. As the waves moved down the truncated cone, the effect was to reproduce the omnidirectional soundwave, as with a cylinder that changed diameter. This created a very effective omnidirectional radiator (although it suffered the same "planarity" effect as ribbon tweeters for higher-frequency sounds) and eliminated all problems of multiple drivers, such as crossover design, phase anomalies between drivers, etc. However, in practice it was found necessary to use a very complex cone made up of various materials at different points along its length, in order to maintain the waveform traveling evenly. Actual digital speaker driver technology not only exists, but is quite mature, having been experimented with extensively by Bell Labs as far back as the 1920s. The design of these is disarmingly simple; the least significant bit drives a tiny speaker driver, of whatever physical design seems appropriate; a value of "1" causes this driver to be driven full amplitude, a value of "0" causes it to be completely shut off. (This allows for high efficiency in the amplifier, which at any time is either passing zero current, or required to drop the voltage by zero volts, therefore theoretically dissipating zero watts at all times). The next least significant bit drives a speaker of twice the area (most often, but not necessarily, a ring around the previous driver), again to either full amplitude, or off. The next least significant bit drives a speaker of twice this area, and so on. There are two problems with this design which led to its being abandoned as hopelessly impractical, however; firstly, a quick calculation shows that for a reasonable number of bits required for reasonable sound reproduction quality, the size of the system becomes very large. For example, a 16 bit system to be compatible with the 16 bit audio CD standard, starting with a reasonable 2 square inch driver for the least significant bit, would require a total area for the drivers of over 900 square feet. Secondly, since this system is converting digital signal to analog, the effect of aliasing is unavoidable, so that the audio output is "reflected" at equal amplitude in the frequency domain, on the other side of the sampling frequency. Even accounting for the vastly lower efficiency of speaker drivers at such high frequencies, the result was to generate an unacceptably high level of ultrasonics accompanying the desired output. In electronic digital to analog conversion, this is addressed by the use of low-pass filters to eliminate the spurious upper frequencies produced; however, this approach cannot be used to solve the problem with this digital loudspeaker, since it is the last link in the audio chain. There have also been many attempts to reduce the size of loudspeakers, or alternatively to make them less obvious. One such attempt is the development of flat panels to act as sound sources. These can then be either made in a neutral colour and hung on walls where they will be less noticeable, or can be deliberately painted with patterns in which case they can function decoratively. There are two, related problems with flat panel technology; firstly, that the flat panel is more flexible than the cone shape and therefore fails to move as a solid unit, and secondly that resonances in the panels are difficult to control, leading to considerable distortion in the reproduced sound. Some progress has been made using such rigid yet damped material as styrofoam, and there have been several flat panel systems demonstrated in recent years. An advantage of flat panel speakers is that the sound is perceived as being of uniform intensity over a wide range of distances from the speaker. Flat panel loudspeaker designs also work well as electrostatic loudspeakers. A newer implementation of the Flat Panel involves the panel and an "exciter", such as the NXT technology. Some speakers are electrostatically driven rather than via the usual electromechanical voice coil, thereby giving a more linear response; the disadvantage, however, is that the signal must be converted to a very high voltage and low current, which can be problematic for reliability and maintenance as they attract dust, and develop a tendency to arc, particularly where the dust provides a partial path; the point where the arc occurs often becomes more prone to arcing, as carbon builds up from the burned dust. Home cinema systems generally include multi-driver systems. 'Multi driver' refers to any speaker system that contains two or more separate drive units, including woofers, midranges, tweeters, and sometimes horns or supertweeters. Many multi driver systems use a bass reflex, or ported, design. These incorporate a small hole, (called a port), in the speaker cabinet to allow the low frequencies generated by the rear of the woofer cone to escape from the cabinet in phase with that radiated from the front of the cone. This improves the bass response of the system.
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