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Chapter 3. Loudspeakers and Monitoring > Loudspeaker Powering Systems

Loudspeaker Powering Systems

Loudspeakers are powered either externally or internally. Externally powered speakers are called passive loudspeakers; internally powered speakers are called active loudspeakers. With a passive loudspeaker, an external amplifier feeds power to the speaker elements. With an active loudspeaker, power amplifiers are built-in, and each speaker element has its own amplifier.

Theoretically, a loudspeaker should be able to reproduce all frequencies linearly—that is, having an output that varies proportionately with the input. In reality, however, this does not occur. A loudspeaker that is large enough to generate low-frequency sound waves most likely will not be able to efficiently reproduce the high frequencies. Conversely, a speaker capable of reproducing the shorter wavelengths may be incapable of reproducing the longer waves.

To illustrate the problem, consider that many radio and television receivers contain a single speaker; even many stereo radios and recorders contain only one loudspeaker in each of the two loudspeaker enclosures. Because a single speaker has difficulty coping with the entire range of audible frequencies, a compromise is made: the long and the short wavelengths are sacrificed for the medium wavelengths that a single, midsized loudspeaker can reproduce more efficiently. Therefore, regardless of a recording’s sound quality, a receiver with just one loudspeaker cannot reproduce it with full-frequency response.

Crossover Network and Drivers

To widen the loudspeaker’s frequency response and make reproduction of the bass and the treble more efficient, the crossover network was created. Also, individual speaker elements, called drivers, were developed to handle the different physical requirements necessary to emit the long, powerful bass frequencies and the short, more directional treble frequencies.

The crossover network divides the frequency spectrum between the low and the high frequencies. The actual point, or frequency, where the bass and the treble divide is called the crossover frequency. A driver large enough to handle the low frequencies is dedicated to the bass, and a driver small enough to handle the high frequencies is dedicated to the treble. Informally, these drivers, or loudspeakers, are called the woofer and the tweeter. Low- and high-frequency drivers are contained in a single cabinet. The size of the drivers is related to the power output of the loudspeaker system.

If a loudspeaker system divides the frequency spectrum once, it is called a two-way system loudspeaker. The crossover frequency in a two-way system is in the neighborhood of 1,500 to 2,000 Hz. Lows from the crossover feed the woofer power amplifier, which is directly connected to the woofer; highs from the crossover feed the tweeter power amp, which is directly connected to the tweeter. A loudspeaker may have more than one driver for each frequency range.

A loudspeaker system that uses two crossover networks is called a three-way system loudspeaker, and one that has three crossover networks is called a four-way system loudspeaker. Three-way systems divide the frequency spectrum at roughly 400 to 500 Hz and 3,500 to 5,000 Hz. Four-way systems divide it at anywhere between 400 to 500 Hz, 1,500 to 2,000 Hz, and 4,500 to 6,000 Hz.

Passive and Active Crossover Networks

Two types of crossover networks are used in loudspeakers: passive and active. In a passive crossover network, the power amplifier is external to the speakers and therefore precedes the crossover (see 3-1). This design can create certain problems: output level at the crossover frequency is usually down by a few decibels; intermodulation distortion is more likely to occur, particularly at loud levels; and harmonic distortion may be heard in the high frequencies.

3-1 Passive two-way crossover system. (a) Layout. (b) Frequency response curves assuming crossover frequencies of 1,500 Hz.

In an active crossover network, the crossover precedes the power amps and operates at low power levels. In a two-way system loudspeaker, one amp receives the low-frequency signals and drives the woofer; the other amp receives the high-frequency signals and drives the tweeter. This is known as a biamplification, or biamped, system (see 3-2).

3-2 Biamped monitor system with an active crossover.

An active crossover network has several advantages over a passive one. It is usually less expensive; it reduces distortion; and, if distortion occurs in either the woofer or the tweeter, it will not be transferred to the other component. Power use is more efficient because a lower-power amp can drive the tweeter, which requires less power than the woofer. Transient response is better, especially in the lower frequencies.

A disadvantage of active loudspeakers is that if an amplifier fails, the entire speaker has to be serviced. It is not possible to substitute another amp, which can be done with a passive loudspeaker.

Active three-way system loudspeakers can be triamped, meaning that each speaker is driven by its own amp. Some active three-way systems may be biamped, in which case a passive crossover is added before the tweeter to create the three-way design (see 3-3).

3-3 Biamped monitor system with both active and passive crossovers.

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