Loudspeaker

A loudspeaker is a device which converts an electrical signal into sound.

Dynamic Loudspeakers

Its traditional design is in two parts, the most obvious being a fibrous semi-rigid cone. Attached to the apex of the cone is a copper coil of fine wire, 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 to the coil it becomes an electromagnet. The magnetic field of the voice coil interacts with the magnetic field of the permanent magnet causing the whole semi-rigid cone (diaphragm) to oscillate and reproduce sound at the frequency of the applied electrical signal.Where 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 makes the speaker components return to a neutral point after moving. 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.

Other Dynamic Loudspeakers

Another common design dynamic loudspeaker uses a small dome instead of a cone. this design is typically used for tweeters and sometimes for mid-range speakers.

The Ribbon Loudspeaker consists of a thin metal film suspened in between two magnets. The electrical signal is applied to the 'ribbon' which vibrates creating the sound. Ribbon loudspeakers can be very fragile but recent designs have the metal film print on a strong lightweight material for reinforcement.

Other technologies

Other technologies can be used to convert the electrical signal into an audio signal. These include piezoelectric and electrostatic.

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. Generally through the use of filters only appropriate signals are applied to the various units.

Enclosures

A loudspeaker is commonly mounted in a 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 postive phase sound waves from the front of the speaker. The result of this is cancellation and so the efficiency of the speaker is 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. 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 loudspeakers compliance, and, with the correct loudspeaker, this will improve the efficiency and frequency response of the speaker.

Other types of enclosure attempt to improve the low frequency response of the loudspeaker by various means. These include the 'bass reflex' cabinet, the 'transmisson line', the 'Tapered Quarter Wave Pipe' (TQWP), and 'horn loading'.

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.

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 material used in the actual loudspeaker, such 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 electrical watt measured at one metre from the loudspeaker. Loudspeakers are inefficent transducers. Only about 1% of the electrical energy put into the speaker is converted to acoustic energy. The remainder is converted to heat.

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) 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 loudpseakers, because sound at audible frequencies cannot be focused in such a narrow beam.

This technology was first released in 2003 by the American Technology Corporation (ATC) (http://www.atcsd.com/) under the trade name HyperSonic Sound.

See also sound reproduction, electronics Flat panel technology 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 pannels to act as sound sources. These can then be either made in a neutral colour and hung up walls where they will merge in, 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.