Loudspeaker

A loudspeaker, or simply speaker, is an electromechanical 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 more transducers in an enclosure. The loudspeaker is the most variable element in an audio system. The audible differences between speaker systems are considerable.

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.

Related Topics:
Phase - Interference patterns

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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).

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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 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 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.

Related Topics:
Compliance - Resonant frequency - Damping - Resonant peak - Thermodynamic - Adiabatic - Isothermal

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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.

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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 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, similar to a speaker driver but without an electrically activated voice coil, is present at the port to provide sufficient loading to the air.

Related Topics:
Reflex port - Bass reflex - Transmission line - Horns - Passive radiator - Voice coil

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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 (company) 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, so that the resulting wavefront from all drivers is coherent when it reaches the listener.

Related Topics:
Resonance - Diffraction - Wharfedale (company) - Sewer pipe - Coherent

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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.

Related Topics:
Woofer - Subwoofer - Acoustics - Lumped component - Loudspeaker topology

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