Loudspeaker

When the electrical current from an audio signal passes through its voice coil—a coil of wire capable of moving axially in a cylindrical gap containing a concentrated magnetic field produced by a permanent magnet—the coil is forced to move rapidly back and forth due to Faraday's law of induction; this attaches to a diaphragm or speaker cone (as it is usually conically shaped for sturdiness) in contact with air, thus creating sound waves.

To adequately and accurately reproduce a wide range of frequencies with even coverage, most loudspeaker systems employ more than one driver, particularly for higher sound pressure level (SPL) or maximum accuracy.

[3] Alexander Graham Bell patented his first electric loudspeaker (a moving iron type capable of reproducing intelligible speech) as part of his telephone in 1876, which was followed in 1877 by an improved version from Ernst Siemens.

During this time, Thomas Edison was issued a British patent for a system using compressed air as an amplifying mechanism for his early cylinder phonographs, but he ultimately settled for the familiar metal horn driven by a membrane attached to the stylus.

Variants of the design were used for public address applications, and more recently, other variations have been used to test space-equipment resistance to the very loud sound and vibration levels that the launching of rockets produces.

The key difference between previous attempts and the patent by Rice and Kellogg is the adjustment of mechanical parameters to provide a reasonably flat frequency response.

[14] The Academy of Motion Picture Arts and Sciences immediately began testing its sonic characteristics; they made it the film house industry standard in 1955.

The ideal material is rigid, to prevent uncontrolled cone motions, has low mass to minimize starting force requirements and energy storage issues and is well damped to reduce vibrations continuing after the signal has stopped with little or no audible ringing due to its resonance frequency as determined by its usage.

A typical suspension system consists of two parts: the spider, which connects the diaphragm or voice coil to the lower frame and provides the majority of the restoring force, and the surround, which helps center the coil/cone assembly and allows free pistonic motion aligned with the magnetic gap.

Polyester foam, for example, is lightweight and economical, though usually leaks air to some degree and is degraded by time, exposure to ozone, UV light, humidity and elevated temperatures, limiting useful life before failure.

[23][24][25] Adjusting a design to improve performance is done using a combination of magnetic, acoustic, mechanical, electrical, and materials science theory, and tracked with high-precision measurements and the observations of experienced listeners.

[28] Because the intended range of frequencies is limited, subwoofer system design is usually simpler in many respects than for conventional loudspeakers, often consisting of a single driver enclosed in a suitable enclosure.

To accurately reproduce very low bass notes, subwoofer systems must be solidly constructed and properly braced to avoid unwanted sounds from cabinet vibrations.

Many subwoofer systems include integrated power amplifiers and electronic subsonic-filters, with additional controls relevant to low-frequency reproduction (e.g. a crossover knob and a phase switch).

[31] Used in multi-driver speaker systems, the crossover is an assembly of filters that separate the input signal into different frequency bands according to the requirements of each driver.

Passive crossovers can also be designed to compensate for undesired characteristics of driver, horn, or enclosure resonances, and can be tricky to implement, due to component interaction.

Some active crossovers, usually digital loudspeaker management systems, may include electronics and controls for precise alignment of phase and time between frequency bands, equalization, dynamic range compression and limiting.

The size of the throat, mouth, the length of the horn, as well as the area expansion rate along it must be carefully chosen to match the driver to properly provide this transforming function over a range of frequencies.

This is a hundredfold increase in output compared to a speaker rated at 90 dB sensitivity (given the aforementioned specifications) and is invaluable in applications where high sound levels are required or amplifier power is limited.

This typically causes monophonic material in a stereo recording to be canceled out, reduced in level, and made more difficult to localize, all due to destructive interference of the sound waves.

Sensitivity is usually expressed as the SPL (dBSPL by common usage meaning dB relative to 20 μPa) at 1 W electrical input, measured at 1 meter,[d] often at a single frequency.

Clapping one's hands in a typical empty room, without draperies or carpet, produces a zippy, fluttery echo due to a lack of absorption and diffusion.

In a typical rectangular listening room, the hard, parallel surfaces of the walls, floor and ceiling cause primary acoustic resonance nodes in each of the three dimensions: left–right, up–down and forward–backward.

It may be easier to imagine a tiny pulsating sphere, uniformly increasing and decreasing in diameter, sending out sound waves in all directions equally, independent of frequency.

As a simple example of the mathematical physics involved, consider the following: the formula for far field directivity of a flat circular piston in an infinite baffle is

Piezoelectric speakers have several advantages over conventional loudspeakers: they are resistant to overloads that would normally destroy most high-frequency drivers, and they can be used without a crossover due to their electrical properties.

Arcing remains a potential problem with current technologies, especially when the panels are allowed to collect dust or dirt and are driven with high signal levels.

Electrostatics are inherently dipole radiators and due to the thin flexible membrane are less suited for use in enclosures to reduce low-frequency cancellation as with common cone drivers.

Due to this and the low excursion capability, full-range electrostatic loudspeakers are large by nature, and the bass rolls off at a frequency corresponding to a quarter wavelength of the narrowest panel dimension.

This transformer also multiplies the capacitive load that is inherent in electrostatic transducers, which means the effective impedance presented to the power amplifiers varies widely by frequency.