Online Encyclopedia
Loudspeaker
A loudspeaker is a device which converts an electrical signal into sound.
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Dynamic loudspeakers
A Dynamic Cone Loudspeaker
The traditional design is in two parts, a fibrous semi-rigid cone and attached to the apex of the cone is a coil of fine wire (usually copper), called the voice coil or moving coil. The coil is oriented coaxially with a permanent magnet where one pole is outside the coil, whilst the other is within the axis of the coil. When an electrical signal is applied, it causes electrons to flow through the coil. As these electrons move through the magnetic field of the permanent magnet, they experience Lorentz force which causes the coil and whole semi-rigid cone (diaphragm) to oscillate 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.
As well as the magnet, the voice coil and the cone, dynamic cone speakers 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.
The moving coil principle was patented in 1924 by two Americans, Chester W. Rice and Edward W. Kellogg. There is some controversy in that an application was made earlier by the Briton Paul Voigt but not granted until later. Voigt produced the first effective full range unit in 1928, although using electromagnets rather than permanent magnets, and he also developed what may have been the first system designed for the home.
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.
Woofers and tweeters
Because of effects such as resonance and various inertial effects, a single loudspeaker is not usually used to cover a wide range of frequencies: instead, a number of specialized units are employed. When there are several units they are wired together using crossover circuits, which allocate different frequency bands to the different units. See subwoofer, woofer, mid-range, tweeter. Through the use of filters, only appropriate signals are applied to the various units. Passive crossover circuits take a full-frequency, full-power signal from an amplifier and send the appropriate frequencies to each unit. They are generally found within the loudspeaker enclosure. Active crossovers split the signal before amplification; once split, the signal is sent to several amplifiers. Each amplifier powers one or more loudspeakers for a specific frequency range. Some manufacturers advertise their loudspeakers as "2-way","3-way", etc. This refers to the number of units in each enclosure. In other words, frequencies split two ways will have a tweeter and woofer/mid-range loudspeakers.
Enclosures
A loudspeaker is commonly mounted in an enclosure (or cabinet). The major role of the enclosure is to prevent the negative phase soundwaves from the rear of the speaker combining with the positive phase sound waves from the front of the speaker. The result of this is cancellation and interference patterns , causing the efficiency of the speaker to be compromised.
The most straightforward mount for a loudspeaker is a flat board of infinite size with infinite space behind it. Thus the rear soundwaves cannot cancel the front soundwaves. A shortage of infinite boards means that the enclosures must use other techniques to maximise the output of the loudspeaker (called loading).
A variation on the 'infinite baffle' is to place the loudspeaker in a large sealed box, filled mostly with foam or wadding. This is commonly referred to as an 'infinite baffle' as it approximates the ideal. Following on from this is a smaller sealed box, or an 'acoustic suspension' enclosure. In this configuration the air trapped inside the box acts as a spring and lowers the loudspeaker's compliance, and, with the correct loudspeaker, this will improve the efficiency and frequency response of the speaker.
Other types of enclosures attempt to improve the low frequency response or overall efficiency of the loudspeaker by using various combinations of reflex ports (these enclosures may be referred to as vented enclosures, the interior of such enclosures tends to be lined with fiberglass batting for absorbtion), transmission lines and horns. The 'Tapered Quarter Wave Pipe' (TQWP) is an example of a combination of transmission line and horn effects.
Enclosures play a significant role in the sound production, adding resonances, diffraction, and other unwanted effects. Problems with resonance are usually reduced by increasing enclosure rigidity, added internal damping and increasing the enclosure mass. Diffraction problems are addressed in the shape of the enclosure.
Variations on the dynamic loudspeaker
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 small (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 in all directions (as is usually desired). Making the moving component in the form of a dome rather than an inverted cone also helps direct sound evenly in all directions. The ideal shape would be a sphere that enlarges and contracts; the dome moving forwards and backwards provides a very simple approximation of this ideal shape.
The Ribbon Loudspeaker is another design. This consists of a thin metal film ribbon suspended in 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 recent designs 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.
There have been many attempts to reduce the size of loudspeakers, or alternatively to make loudspeakers 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. One problem with flat panel technology is that resonances in the panels are difficult to control, and this can lead to considerable distortion in the reproduced sound. Some progress has been made, 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.
A final unusal design was exemplified by the Ohm model "F" speakers. These speakers mounted a single relatively-conventional dynamic speaker vertically at the top of the cabinet, but a large truncated cone was then mounted on top of the conventional cone. As this assembly moved up and down, the truncated cone created the effect of a cylinder that changed diameter. This created a very effective omnidirectional radiator for mid- and high-frequency sound (although it suffered the same "planarity" effect as ribbon tweeters) while the conventional cone effectively radiated bass frequencies in an omnidirectional fashion.
History
The quality of loudspeaker systems until the 1950s was, to modern ears, very poor. Developments in cabinet technology (e.g. acoustic suspension) and changes in materials used in the actual loudspeaker, such as the move away from simple paper cones, led to audible improvements. Paper cones (or doped paper cones, where the paper is treated with a substance to improve its performance) are still in use today, and can provide good performance. Polypropylene and aluminium are also used as diaphragm materials.
Efficiency
The sound pressure level that a loudspeaker produces is measured in decibels (dB(SPL)). The efficiency is measured as dB/W/m - decibels output for an input of one nominal watt measured at one metre from the loudspeaker. Loudspeakers are inefficient transducers. Only about 1% of the electrical energy put into the speaker is converted to acoustic energy. The remainder is converted to heat.
Other technologies
Other technologies can be used to convert the electrical signal into an audio signal. These include piezoelectric, electrostatic, and plasma arc loudspeakers.
Converting ultrasound to audible sound
A transducer can be made to project a narrow beam of ultrasound that is powerful enough (100 to 110 dB(SPL)) to change the speed of sound in the air that it passes through. The ultrasound is modulated, which means that it consists of an audible signal mixed with an ultrasonic frequency. The air within the beam behaves in a nonlinear way and demodulates the ultrasound, resulting in sound that is audible only along the path of the beam, or that appears to radiate from any surface that the beam strikes. The practical effect of this technology is that a beam of sound can be projected over a long distance to be heard only in a small, well-defined area. A listener outside the beam hears nothing. This effect cannot be achieved with conventional loudspeakers, because sound at audible frequencies cannot be focused in such a narrow beam.
There are some criticisms of this approach. Anyone or anything getting in the path of the beam will disrupt the signal, and there are limitations on how loud and deep they currently play.
This technology was originally developed by the US (and Russian) Navy for underwater sonar in the mid-1960's, and was briefly investigated by Japanese researchers in the early 1980's, but these efforts were abandoned due to extremely poor sound quality (high distortion) and substantial system cost. These problems went unsolved until a paper published by Dr. F. Joseph Pompei of the Massachusetts Institute of Technology in 1998 (105th AES Conv, Preprint 4853, 1998) fully described a working device that reduced audible distortion essentially to that of a traditional loudspeaker.
The technology, termed the Audio Spotlight http://www.audiospotlight.com/ , was first made commercially available in 2000 by Holosonics http://www.holosonics.com/ , a company founded by Dr. Pompei.
See also sound reproduction, electronics
Cables and wireless
The cables that are used to connect the loudspeaker to the amplifier will have an impact on the sound quality. There is disagreement on the extent of this effect, some claiming that it is not audible in short lengths of cable. There is agreement that the cable should be as short as practical, and have a large diameter. A higher speaker impedance also results in lower losses in the cable.
[1] http://www.svconline.com/mag/avinstall_designer_cables_critical/ This article discusses cable effects on the audio signal.
So-called wireless loudspeakers are becoming popular in many applications, including home theater. Despite its name, however, the unit is really a wireless receiver, amplifier and loudspeaker in a single box. Inside the box, the loudspeaker is connected to the amplifier using conventional wires.