Loudspeaker Enclosures 723of the latter, extra parts may be added to create a conjugate crossover, which makes the
overall loading look resistive, but also absorbs power.
24.2.1.5 Static versus Transient
Conventionally, impedance values are taken after applying a steady and repetitive test
signal (e.g., continuous sine wave) and allowing a few moments for the recovered signal
amplitude that represents impedance to settle. In the short term, impedance can be
considerably lower. At worst, it is possible for an ordinary dynamic (moving coil) type
of loudspeaker to demand current as is it had 1/6th of its nominal impedance. In other
words, an 8-ohm speaker can sometimes look like 1.4 ohms. This will not happen all
the time or even very often, nor for very long at a time; however, for high-quality sound
reproduction, and not forgetting that music involves repetition, the possibility must be
allowed for.
24.2.1.6 Acoustic Contribution
The impedance (load) characteristics of drive units can be affected by cabinet air leaks,
and also by refl ections in the room, hence the positioning of the enclosure. Horn-loaded
drive-units are usually the most sensitive to this.
The upshot is that most loudspeaker loads are a wide variable, not just between different
models and types, but depending on program dynamics and excitation frequencies.
24.2.2 What Speakers Are Looking For
The fact that most loudspeakers do not employ conjugate impedance compensation, and
so they have impedance curves that vary “ all over ” with frequency, means that for high
performance, the amplifi er kind the speaker needs to see is a “ voltage source. ”
24.2.2.1 Why Voltage?
The signal voltage must be almost unaffected (ideally far below say 1% change) whether
the speaker is connected or not, regardless of whether it’s drawing 50 milliamps or 50
amperes. That means a “ stiff power source, ” alias a low impedance or “ high-current-
capable ” source.
If the source impedance isn’t low (enough), then as the speaker’s impedance varies
with frequency, the change will be superimposed on its own frequency response as a
tonal aberration. A source impedance that is almost as high as speakers ’ own minimum