Few or no single loudspeaker drive units offer overall, high performance audio reproduction. The nearest contender is an ESL panel, which can work as the sole drive-unit for the kinds of music that have no loud, low bass content.

But to listen without restriction and risk of damage to every other kind of music, two, three or more drive units must be used to cover low, medium and high frequencies (which from 10Hz to 20kHz span a wavelength range of some 2000 fold !), and over the 120dB+ dynamic range required for high performance sound reproduction.

Matching levels
Often, the sensitivities (loudness) of the individual units that are optimum for each frequency band, differ. Commonly, the tweeter is more sensitive than the driver covering bass/mid frequencies.

If efficiency is unimportant, the mis-matched sensitivities (which would otherwise cause a uneven, ‘toppy’ frequency response) may be overcome by adding a series ‘padding’ resistor in line with the hf drive unit (tweeter).

Parallel connection
Alternatively, in touring PA and wherever else efficiency matters, or wherever high SPL capability is sought, an overall flat response may be attained by using two, parallel connected bass/mid drive-units. Applicability is always subject to coherence in the acoustic result, hence suitable mutual positioning of the paralleled drivers, so they work together.

If ‘ordinary’ drivers (i.e. electro-dynamic types), a 15 or 16Ω rating will be likely chosen, so the resultant load is about 8Ω, rather than 4Ω if the paralleled drivers were each the usual 8Ω.

Again always subject to coherence in the acoustic result (and thus suitable mutual positioning, generally closer than a quarter of the shortest wavelength), drive units or complete speakers can be paralleled across either a given amplifier; or when this runs out of drive-capability, across amplifiers ad infinitum that are driven with an identical signal, and have either identical, or acoustically justifiable different gains.

Despite the above, the fewer drive-units or speakers reproducing a given programme in a given frequency range, the better the sonic results. As is so often the case in high performance sound reproduction, least is best – if it is usable.

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