720 Chapter 24
24.2 The Interrelation of Components ..........................................................................
24.2.1 What Loudspeakers Look Like to the Amplifi er
There is a tacit presumption that most amplifi ers can comfortably drive any “ reasonable ”
loudspeaker. Beyond whatever is “ reasonable, ” discomfort may occur to all parties. To the
power amplifi er, which is nothing but a loudspeaker driver, the most salient information
about any loudspeaker it is expected to drive, and the stress that may engender, is that
loudspeaker’s impedance.
24.2.1.1 Impedances
A speaker’s nominal impedance is commonly (and oversimplistically) described by
a single round fi gure, usually 15, 8, or 4 ohms for the majority of moving coil drive-
units. With ribbon drive-units, or whenever several drive units are paralleled to increase
handling or coverage, lower impedances of 3, 2, or 1 ohms or even less are the norm.
With electrostatic and piezo (hf) drive-unit types, the load impedance can be higher, but
are also more or predominantlycapacitative (like a capacitor) across the audio range.
This can be far more taxing to the amplifi er.
24.2.1.2 Low versus High Impedances
At this juncture it is helpful for those unfamiliar with electronics jargon to grasp a
counterintuitive fact: that the lower impedance, the heavier the (current) loading on the
amplifi er. To remember this and that 4 ohms is harder to drive than 16 ohms, think of hill
slopes: a l-in-4 hill is far harder to drive or climb up than a l-in-16, that is, an impedance
in ohms is the reciprocal of the relative loading. Remember also:
A low impedance demands more current, and less signal voltage is needed for a given
current.
A high impedance requires more signal voltage to be driven with a given current.
24.2.1.3 Variation versus Frequency
Loudspeakers ’ impedances nearly always vary over the frequency range of use. Figure
24.8 shows how a nominal 8-ohm, 15 ” bass drive-unit typically varies from 5.5 ohms at
450 Hz, peaking up to about 40 ohms or so, at the mechanical resonant frequency, which
typically lies between 20 to 120 Hz for a bass driver. Here it is 31 Hz. Together, the drive-
unit and the speaker enclosure largely determine this. Impedance also rises to a maximum
at (and beyond) the highest usable frequency.