Aperiodic - what does it mean?
‘Aperiodic’ actually means ‘without periodic repetition’ and, in the case of a loudspeaker, defines a system which has optimally damped resonance control. In the case of a closed box or reflex ‘vented’ box loudspeaker the primary resonance of the speaker driver in the box is high ‘Q’, in other words as the frequencies fed into the speaker get close to the primary resonant frequency they easily excite resonant behaviour.
In a poorly damped system what you hear from this resonance is a ‘boom’, in fact many of the early reflex loudspeakers were called ‘boom boxes’. It is generally left to the amplifier to control this resonance, though some help through damping is applied by stuffing the cabinet with absorbent, by damping the speaker using the amplifier’s negative output impedance or ‘damping factor’. This demands considerable instantaneous current flow from the amplifier to apply a ‘braking’ effect to the drive unit as it starts to oscillate – all well and good if you have a high power amplifier but not ideal for low power or valve amplifier users.
The Aperiodic enclosure applies resistive control of airflow in and out of the cabinet to effectively help damp out the primary driver resonance. You can see the difference in Graph 1. This is a plot of driver impedance and is the load the amplifier ‘sees’. You can see the primary resonance at 70Hz in the closed box ‘blue line’ as a sharp, high ‘Q’ peak. The high impedance at this point shows that the drive unit needs very little input in order to produce a large output, i.e. the driver is resonating in its box.
Compare this to the aperiodic plot (red line).This is the same driver in our optimally damped aperiodic enclosure. The high Q peak at 70Hz has been dramatically reduced. The flattening of the impedance curve also means it is an easier load for the amplifier.
Now look at the difference in response curves (Graph 2). The blue line is the response in the closed box. Due to resonance it exhibits a relatively sharp knee in the response through the 80-100Hz region before it falls away. Compare it to the red line of the aperiodic system. The ‘knee’ is reduced and the bass response is given a little more subjective extension at very low frequencies, albeit with the loss of about one dB at 70Hz. A reflex ‘vented’ system would give greater LF extension but, again, you would be back to the highly resonant behaviour. The only drawback to the aperiodic system is that, at subsonic frequencies, cone movement is higher than that experienced in a closed box system. This won’t be of concern when using CD or other subsonically ‘clean’ sources, but turntable owners should be careful their arm/cartridge/suspension system is not prone to resonance itself and avoids exaggerating record warps (this warning applies equally when using reflex speakers).
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