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Loudspeaker

Closeup of a loudspeaker driver
Closeup of a loudspeaker driver

A loudspeaker is a device which converts an electrical signal into sound. The term is used to refer to both the transducer or driver itself, and a complete system consisting of one or several transducers in an enclosure.

As an electromechanical device, the speaker is the most variable element in an audio system, the electronic components' designs having been standardized long ago so that the differences between them are minimal, compared to those between speaker systems. Further complicating things is the interaction of the speaker with the listening environment, which is, of course, widely variant. Furthermore, this interaction affects the speaker's electromechanical behavior and thus the load it represents to the amplifier, making it even more difficult to predict the sound a given system will produce in its intended environment without actual listening tests. It is often theorized by those who are skeptical of at least some of the audiophile world that the perceived differences between amplifiers are in fact only differences in their ability to control a given speaker-room interaction in a pleasing manner, 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.

Dynamic loudspeakers

Image:Loudspeakerconstruction.png

Cross Section of 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. (During the early days of loudspeaker-equipped radios, permanent magnets with sufficient strength were a rarity, and an electromagnet was often used to provide the stationary magnetic field. This winding often did double duty by serving as a choke coil in the power supply). When an electrical signal is applied, a magnetic field is induced by the electric current in the coil which is effectively becoming an electromagnet. The coil and the permanent magnet interact to each other with magnetic 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.

Wall-mounted loudspeaker
Wall-mounted loudspeaker

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 known as the drivers of a speaker. These drivers 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 drivers. Passive crossover circuits take a full-frequency, full-power signal from an amplifier and send the appropriate frequencies to each driver. 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. Most manufacturers advertise their loudspeakers as "2-way","3-way", etc. This refers to the number of frequency bands into which the incoming source signal is split. For instance, a 2-way design splits the incoming signal into two bands with the tweeter handling sound above a certain frequency (known as the crossover point) and one or more combination woofer/mid-range speakers handling all frequencies below that. A 3-way design will have 2 crossover points with separate tweeter, mid-range, and woofer drivers. Lowpriced speakers typically have a very minimal crossover design, consisting of a small capacitor in series with the tweeter in order to attenuate the lower frequencies, and simply relying on the woofers inherent inability to reproduce high frequencies.

The nature of speaker design is considered both an art and science. Tweaking a design is done not only by instruments but also with the human ear. Many speaker designers will spend countless hours both in and out of an anechoic chamber (essentially a room with soundproofing that destroys any reverbation or echo) to ensure the speaker will perform the way they intended to. There are many issues in speaker design, a few of which include lobing, phase effects, off axis response, time coherence among others. In addition to the number of crossovers, another often advertised specification is the order of the crossover or also called the crossover slope. An example would be a speaker with the published spec (2-way, 3rd order). Essentially, no crossover stops frequencies exactly at a crossover point. Rather the process is a gradual slope. The order of a crossover refers to how abrupt the slope is. Higher order crossover networks slope more sharply than lower order networks so therefore a first order network will have a more gradual split than a second order. A first order network filters at 6 dB per octave, a second order at 12 dB per octave, and a third order at 18 dB per octave. Generally speaking, there are more commercial third order designs in the consumer audio market. The characteristics (and expense!) of a a driver must also be taken into consideration. Drivers can be made of paper, metal, various polypropylenes, or more exotic materials such as large speaker manufacturer Bowers & Wilkins, generally known as B&W, which expounds the use of kevlar. In addition, the driver's basket must be designed in order to preserve rigidity and can be be cast or stamped. Finally, the size and type of magnet can also differ. Generally, the more powerful the magnet, and consequently bigger, the better. Tweeters are treated differently in that there are other variables and designs to consider, such as ribbon speakers (see below) which are beyond the scope of this article. All the principles described above are only the tip of the iceberg pertaining to the design and implementation of speakers.

A less-expensive alternative is to use a single loudspeaker unit that contains two cones and a mechanical cross-over. This is usually implemented by placing a very small cone directly over the voice coil and coupling the larger cone to the voice coil with a mechanically-compliant material (or making the larger cone itself mechanically compliant). In this way, the small cone (usually referred to as a whizzer cone) is driven by all frequencies including the treble frequencies while the larger cone is only driven by the bass and midrange frequencies. In many modern speakers, a small piezoelectric tweeter (see below) is used instead of the whizzer cone.

Subwoofers

Modern speaker systems often include a single speaker dedicated to reproducing the very lowest bass frequencies. This speaker is referred to as a subwoofer. A typical subwoofer only reproduces sounds below 100 Hz (although some subwoofers allow you to choose 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 infinite baffle. 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. 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. The structural integrity of the room is important; a subwoofer's powerful bass can often cause items in the room or even the structure of the room itself to vibrate or buzz.

Powered 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 effects) 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".

Enclosures

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 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.

The ideal mount for a loudspeaker would be a flat board of infinite size with infinite space behind it. Thus the rear soundwaves cannot cancel the front soundwaves. An 'open baffle' loudspeaker is an approximation to this - the transducer is mounted on a simple board of size comparable to the lowest wavelength to be reproduced. However, for many purposes this is impractical and 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. This is commonly referred to as an 'infinite baffle' as it approximates the ideal. The loudspeaker driver's compliance, i.e. the stiffness of the suspension of the cone, determines the resonant frequency and damping properties of the system, which affect the low-frequency response of the speaker; the response falls off very sharply below the resonant frequency. The designer trades off bass response for flatness; the larger the resonant peak in the bass, the lower the speaker will seem to reproduce, but the more over-emphasized the resonant frequency will be. The box must be large enough that the internal pressure caused when the driver cone moves backwards into the cabinet does not rise high enough to affect this. The box is usually filled loosely with foam or wadding, converting the speaker's thermodynamic properties from adiabatic to isothermal, and giving the effect of a larger cabinet.

Following on from this is the 'acoustic suspension' enclosure, which rather than using a large box to avoid the effect of the internal air pressure, uses a smaller, tightly sealed box (typically designed with a very small rate of leakage so that internal and external pressures can slowly equilibrate over time, allowing the speaker to adjust to changes in barometric pressure or altitude). In this case, the true suspension of the driver's cone is the air trapped inside the box which acts as a spring with very close to ideal behavior rather than the mechanical suspension of the speaker driver, which for this application must be very weak, just strong enough to keep the cone centered in the absence of any signal. The drawback of these speakers is their low efficiency, due to the loss of the power absorbed inside the cabinet.

Other types of enclosures attempt to improve the low frequency response or overall efficiency of the loudspeaker by using various combinations of reflex ports to transmit the energy from the rear of the speaker to the listener; these enclosures may be referred to as vented enclosures, bass reflex, transmission lines or horns. The interior of such enclosures are also often lined with fiberglass batting for absorption. The 'Tapered Quarter Wave Pipe' (TQWP) is an example of a combination of transmission line and horn effects. Sometimes a passive radiator, similar to a speaker driver but without an electrically activated voice coil, is present at the port to provide sufficient loading to the air.

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. One speaker manufacturer, Wharfedale, addressed the problem of cabinet resonance by using two layers of wood with the space between filled with sand. Home experimenters have designed speakers built from concrete sewer pipe for similar reasons. Diffraction problems are addressed in the shape of the enclosure; avoiding sharp corners on the front of the enclosure for instance. Sometimes the differences in reaction time of the different size drivers is addressed by setting the smaller drivers further back, so that the resulting wavefront from all drivers is coherent when it reaches the listener.

Enclosures used for woofer and subwoofer applications can be adequately modelled in the low frequency range (approximately 100200 Hz and below) using acoustics and the lumped component model. For the purposes of this type of analysis, each enclosure has a loudspeaker topology.

Variations on the dynamic loudspeaker

One problem with loudspeakers is that the original soundwave is usually radiating outwards in a spherical wavefront, which will reach both ears; this is difficult to replicate with the usual, essentilly planar loudspeaker designs as it is difficult to create either a point source for the sound or a sphere that varies in size with the amplitude of the desired pressure wave. Several different 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 mimicing 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 utilize 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 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 dome moving forwards and backwards provides a very simple approximation to the ideal shape of a sphere that enlarges and contracts.

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. Flat panel loudspeaker designs also lend themselves to being 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 low current, which can be problematic for reliability and maintenance.

Another unusual design was exemplified by the Ohm model "F" speakers invented by Lincoln Walsh. These speakers 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. See more details here (http://www.donlindich.com/uniquespeakers.html).

The most exotic speaker design is undoubtedly the plasma arc loudspeaker, using electrical plasma as a driver [1] (http://www.plasmatweeter.de/home.htm), once commercially sold as the Ionovac[2] (http://www.belgaudio.com/ionophone.htm). Since plasma has minimal mass, but is charged and therefore can be manipulated by electrostatic signal, the result is a very linear output at frequencies well higher than audibility. As might be guessed, problems of maintenance and reliability for this design tend to make it very unsuitable for the mass market; the plasma is generated from a tank of helium which must be periodically refilled, for instance. A lower-priced variation on this theme is the use of a flame for the driver (http://www.madsci.org/posts/archives/feb98/888372043.Ot.r.html), flames being commonly electrically charged (the electron, which carries the charge in an electrical current, is defined as a negative charge only because the repulsion of positively charged candle flames from a positively charged electrode and towards a negative charge was misunderstood as the flame being blown by a flow of charge carriers from one electrode, which was therefore labeled as positive, needlessly complicating the life of beginning engineering students). Unfortunately, the recent marketing of plasma displays as high-end television sets and computer monitors has caused the me-too labeling of many speakers as "plasma" which have nothing whatsoever to do with plasma[3] (http://www.proaudiosuperstore.com/electrovoice-plasma-speakers.html), much as the advent of digital audio cause the marketing of a large number of "digital" headphones and speakers.

History

The current concept of loudspeaker was created in 1898 by Oliver Lodge, an English scientist.

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.

Piezoelectric speakers are most commonly found as tweeters in low-cost speaker systems. The have several advantages over conventional loudspeakers for this use:

  • With no voice coil, there is no electrical inductance to overcome so it is easy to couple high-frequency electrical energy into the piezo transducer.
  • They can be physically small yet powerful, leading to good dispersion of the treble sounds.
  • They are relatively immune to overloads that would burn out the voice coil of a conventional loudspeaker.
  • Because they appear to be capacitive, they often don't require an external cross-over network; they can simply be placed in parallel with the relatively-inductive woofer/midrange loudspeaker(s).

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.

There are currently two devices available on the market that use ultrasound to create an audible "beam" of sound: the Audio Spotlight and Hypersonic Sound. See AudioSpotlights.com (http://www.AudioSpotlight.com/) for more information. This site includes: an introduction to this technology, history of development, product specifications, product comparison, latest product news and case studies with important tips on how to use these devices for the best results.

See also sound reproduction, electronics

Home cinema speakers

There are various different channel formats evolving for home cinema speaker systems. They include :

  • 5.1 channel sound. This requires:
    • Left, center, and right front speakers
    • Left and right rear speakers
    • A subwoofer (which is counted as ".1" channel because of the narrow frequency band that it reproduces)
  • 6.1 channel sound is similar to 5.1 but there is an actual center rear channel
  • 7.1 channel sound in home theater is identical to 6.1 except that it has two rear center speakers. In SDDS, 7.1 is the same as 5.1 but adding center-left and center-right speakers in the front of the listener for better audio positioning.

Cables and wireless

The cables that are used to connect the loudspeaker to the amplifier will have an impact on the sound quality, which is often a subtle loss of definition and detail. There is disagreement on the extent of this effect, some claiming that it is not audible in short lengths of cable, say below 1 m (3 ft). There is agreement that the cable should be as short as practical, have a sufficient cross-section of the conductors and low inductance. A higher speaker impedance (e.g. 8 Ohms vs. 4 Ohms) also results in lower relative losses in the cable, however some speaker manufacturers claim the 4 Ohm speakers to be dynamically superior. Due to the impulsive nature of music and the complex electrical and mechanical characteristics of speaker systems (including the frequency crossover), the major cable effects are momentary dynamic losses which may be more audible to the trained ear than measurement results with steady-state test signals suggest.

This article (http://www.svconline.com/mag/avinstall_designer_cables_critical/) discusses cable effects on the audio signal.

Cable Nonsense (http://www.verber.com/mark/cables.html) This newsgroup message from a speaker manufacturer is very informative about issues of speaker cables.

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.

Floor standing

Home cinema speaker generally includes floor standing systems

Loudspeaker manufacturers

For an almost complete list of manufacturers of loudspeakers, see The Audio Circuit (http://www.audiocircuit.com)

See also

External links

  • The Audio Circuit (http://www.audiocircuit.com) - Information on and user reviews of loudspeakers, headphones, amplifiers, and playback equipment
  • Wireless Speakers (http://www.wireless-speakers.org/) - all about wireless speakers and headphones
  • Audio Express magazine, (http://www.audioxpress.com/) do-it-yourself audiophile magazine incorporating the former Speaker Builder magazine
  • Voice Coil magazine, (http://www.audioxpress.com/magsdirx/voxcoil/index.htm) "The Periodical for the Loudspeaker Industry"


 

 

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