Speakers

Closeup of a loudspeaker driver

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.

History

Nikola Tesla is believed to have put electrically charged carbon dust in a cup-shaped device to create the first telephone loudspeaker. However, the first documented ^[1] device that might fit this description was created in 1881.

Alexander Bell patented the first loudspeaker as part of his telephone in 1876. This was soon followed by an improved version from Ernst Siemens in Germany and England (1878). The modern design of moving-coil loudspeaker was established by Oliver Lodge in England (1898). [2]

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, and he also developed what may have been the first system designed for the home, although using electromagnets rather than permanent magnets

These first loudspeakers utilized electromagnets because large, powerful permanent magnets were not freely available at reasonable cost. The coil of the electromagnet is called a field coil and is energized by direct current through a second pair of terminals. This winding usually served a dual role, acting also as a choke coil filtering the power supply of the amplifier which the loudspeaker was connected to.

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, led to audible improvements. For Example, 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. Additional improvements to loudspeaker technology occurred in the 1970s, with the introduction of higher temperature adhesives, improved permanent magnet materials, and improved thermal management.

Dynamic loudspeakers

Cross Section of a Dynamic Cone Loudspeaker

The traditional design is a semi-rigid paper fiber cone and a coil of fine wire (usually copper), called the voice coil attached to the apex of the cone. 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. 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.

Wall-mounted loudspeaker

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. Generally, 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.

Using Crossovers for various Drivers

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 drivers are employed. These drivers are often wired together using crossover circuits, which allocate different frequency bands to each appropiate units. Through the use of filters, only appropriate signals are applied to the various drivers. The passive crossovers are generally found within the loudspeaker enclosure.

There is a diference between active and passive crossovers. The active ones 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. The passive ones splits the signal after amplification.

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 example, a 3-way design will have 2 crossover points and separate tweeter, mid-range, and woofer drivers (the crossover point is when the sound frecuency go above a certain value and the crossover uses the tweeter to handle it.

Low-priced 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 woofer's inherent inability to reproduce high frequencies.

In addition to the number of crossovers, another often advertised specification is the crossover order, also called the crossover slope. 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, and so on.

A less-expensive alternative is to use a single loudspeaker unit that contains two cones and a mechanical cross-over. This is usually done 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 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 150Hz 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 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".

Tweeters

See tweeters

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 maximize the output of the loudspeaker (called loading).

Closed

A variation on the 'infinite baffle' is to place the loudspeaker in a very large sealed box. The loudspeaker driver's mass and 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 cabinet resonant frequency (Fcb). 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, pillow stuffing, fiberglass, or other wadding, converting the speaker's thermodynamic properties from adiabatic to isothermal, and giving the effect of a larger cabinet.

The 'acoustic suspension' enclosure, rather than using a large box to avoid the effect of the internal air pressure, uses a smaller, tightly sealed box. The box is 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.

bass reflex

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 also be referred to as vented/ported enclosures, bass reflex, transmission lines or horns. The interior of such enclosures are also often lined with fiberglass matting for absorption. The 'Tapered Quarter Wave Pipe' (TQWP) is an example of a combination of transmission line and horn effects. Sometimes a passive radiator or drone, similar to a speaker driver but without an electrically activated voice coil, is used instead of a reflex port to eliminate port turbulences, and provide a steeper rolloff below the drone's tuning frequency Fb. Reflex ports are tuned by amount of mass within the vent, using appropriate diameter and length to reach this point. Passive radiators are tuned by the mass (Mmp) of the diaphragm or cone.

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. The speaker manufacturer Wharfedale has 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, by leaning or stepping the front baffle, so that the resulting wavefront from all drivers is coherent when it reaches the listener. The Acoustic Center of the driver, or physical position of each driver's voice coil, dictates the amount of rearward offset to time-align the drivers.

Enclosures used for woofer and subwoofer applications can be adequately modelled in the low frequency range (approximately 100–200 Hz and below) using acoustics and the lumped component model. For the purposes of this type of analysis, each enclosure has a loudspeaker topology.

Phase or polarity

All speakers have two wires that must connected from the source of the signal (the amplifier or receiver) to the speaker's input terminals in 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 180 degrees opposite the other. This type of wiring error creates inverse sound waves which cancel out (to a degree) the sound of the other speaker. This won't cause silence because reflections from surfaces diminish the effect somewhat but resulting in a major loss of sound quality. The most prominent effect to the untrained ear will be a loss of bass response. The second most noticed will be 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.

Construction and Testing

The nature of speaker design is considered both an art and science. Adjusting a design is done with instruments and with the ear. Speaker designers will 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 of the issues in speaker design are lobing, phase effects, off axis response and time coherence.

Efficiency

The sound pressure level (SPL) that a loudspeaker produces is measured in decibels (dB). The efficiency is measured as dB/(W·m)—decibels output for an input of one nominal watt measured at one metre from the loudspeaker usually on the axis of the speaker. This is called the "sensitivity" rating. Loudspeakers are very inefficient transducers. Only about 1% of the electrical energy put into the speaker is converted to acoustic energy. The remainder is converted to heat. The main reason for this low efficiency is the difficulty of achieving proper matching between the acoustic impedance of the drive unit and that of the air. This is especially difficult at lower frequencies. The better the matching, the higher the efficiency. Large horn loudspeakers that used to be used in cinemas, were very efficient by todays hi-fi speaker standards. From a technical standpoint "sensitivity" is not the absolute reference of efficiency. As an example, a simple cheerleader's horn makes more sound output than the cheerleader does by herself, but technically the horn did not "improve" or increase the cheerleader's "efficiency". True or absolute efficiency is the ratio of "desired" output power divided by total input power.

Normal loudspeakers have a sensitivity of 85 to 95 dB/(W·m).
Nightclub speakers have a sensitivity of 95 to 102 dB/(W·m).
Rock concert, stadium speakers have a sensitivity of 103 to 110 dB/(W·m).

Current state-of-the-art loudspeakers can approach efficiencies of 70% or higher. This is partly due to a very high magnetic field and partly to a high amplitude displacement (speaker cone pumping in and out). The ratio of the sound output to the mass of the cone/coil combination grows significantly at high sound pressure levels i.e. above 140 decibels. In closed or small environments (such as cars or bedrooms) it is far more important to have a speaker with a high X max. (cone eXcursion maximum) as opposed to high (dB/(W·m)) rating. A higher X max. indicates that the driver can move a larger volume of air as power increases. A few top of the line woofers have a very low "sensitivity" rating i.e. 80 to 86 dB/(W·m)(sensitivity efficiency of 0.01% ). However at full power may achieve 160+ decibels at 20% to 40% "true" efficiency.

• video of 158 dB woofer at 80 millimeter amplitude 450 kB mpeg

As shown in this example, sometimes the speaker with the lower sensitivity rating outputs a far higher amount of acoustic watt output.

In general a higher quality speaker will have a higher sensitivity rating, larger and or heavier magnet, and a higher X max.

Specifications

Speaker specifications generally include:

Specifications label in the backside of a loudspeaker

• Speaker type: (full-range, woofer, tweeter or mid-range).
• Rated Power: (nominal or continuous or RMS power and peak or maximum short-term power)
• Impedance (4 Ω, 8 Ω, etc.).
• Baffle or enclosure type: (sealed, bass reflex, etc.) - finished systems only.
• Number of drivers (2-way, 3-way, etc.) - finished systems only.

and optionally,

• Crossover frequency: the frequency or frequencies where electrical filtering occurs.
• Frequency response: the measured or specified variance in sound pressure level over a range of frequencies.
• Thiele/Small (T/S) parameters: - individual drivers only.

Interaction with listening environments

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.

Variations on the dynamic loudspeaker

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 espcially 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). 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 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. See more details here.

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

Piezoelectric transducers, frequently used as beepers in watches etc., are often used as tweeters in cheap speaker systems. Computer speakers and portable radios are common examples. Piezos have several advantages over conventional loudspeakers when applied to such purposes:

• Piezoelectric transducers have no voice-coil, therefore there is no electrical inductance to overcome; it is easy to couple high-frequency electrical energy into the piezoelectric transducer, especially under the low-power, non-critical applications in which they are usually employed.
• Piezoelectric transducers are physically small yet powerful, leading to good dispersion, although the fidelity of such devices remains in question when it comes to critical listening.
• Piezoelectric transducers are resistant to overloads that would normally burn out the voice coil of a conventional loudspeaker.
• Because piezos comprise a capacitive load, they usually do not require an external cross-over network; they can simply be placed in parallel with the inductive woofer/midrange loudspeaker(s).

Plasma arc loudspeakers

The most exotic speaker design is undoubtedly the plasma arc loudspeaker, using electrical plasma as a driver [3], once commercially sold as the Ionovac[4]. Since plasma has minimal mass, but is charged and therefore can be manipulated by an electric field, the result is a very linear output at frequencies far higher than the audible range. 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 [5], flames being commonly electrically charged. 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 [6], much as the advent of digital audio caused the marketing of a large number of "digital" headphones and speakers, when all drive-units are analog in nature.

Digital speakers

Actual digital speaker driver technology not only exists, but is quite mature, having been experimented with extensively by