Loudspeaker Enclosures 713without 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 10 H z
to 20 kHz span a wavelength range of some 2000-fold!), and over the 120-dB dynamic
range required for high-performance sound reproduction.
24.1.4.1 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/midfrequencies. If effi ciency is unimportant, the mismatched sensitivities (which
would otherwise cause an uneven, “ toppy ” frequency response) may be overcome by
adding a series “ padding ” resistor in line with the hf drive-unit (tweeter).
24.1.4.2 Parallel Connection
Alternatively, in touring PA and wherever else effi ciency matters, or wherever high SPL
capability is sought, an overall fl at 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) are used, a 15 or 16 Q rating
will be likely chosen, so the resultant load is about 8 Q rather than 4 Q if the paralleled
drivers were each the usual 8 Q.
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 amplifi er or, when this runs out
of drive capability, across amplifi ers ad infi nitum that are driven with an identical signal
and have either identical or acoustically justifi able different gains. Despite this, the fewer
drive-units or speakers reproducing a given program 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.
24.1.4.3 Why Crossovers?
When two or more drive-units are used to cover the audio range, it is usually very
important that they only receive program over their respective, intended frequency
ranges. Program at other frequencies must be omitted, as it will usually degrade a
drive-unit’s sonics by exciting resonances and subharmonics. At moderate to high drive
levels, physical damage is also likely. The “ frequency division ” or “ frequency conscious