UK audio researchers have discovered two of what they suspect will be several subtle links between loudspeaker cross-over capacitors and the audio they produce.
One is the effect of equivalent series resistance (ESR), even when apparently swamped by other resistances. The other is phase change due to mechanical resonance.
Measuring audio performance, swapping between 4.7µF polypropylene and higher ESR electrolytic capacitors, “100 per cent of listeners could pick out the difference with only 20 minutes of training,” said Paul Dodds, a researcher at Wrexham-based capacitor maker ClarityCap.
The listeners were detecting tiny variations compared with the series resistor user to balance tweeter and woofer response. “The tweeter has a resistor of around 1O. This is three orders of magnitude bigger than the ESR,” said Dodds.
The phase change effect strays away from the standard capacitor electrical model and is related to sound produced directly by capacitors. Although they have not proved significantly microphonic in testing, some construction geometries produce mechanical vibrations that can be energised by the audio content.
“Resonance tends to be between 10kHz and 20kHz, within the range of a tweeter,” said Dodds. “At resonance you get a phase change which affects the tweeter, and sound from the capacitor.”
ClarityCap has developed an undisclosed technique to measure this phase-change behaviour. “We can get frequency and time parameters,” said Dodds
Continued listening tests have also begun to link audio language to these parameters.
“There is a definite bias to certain use of audio language: like ‘harshness’ and ‘brightness’. It is very, very tentative at the moment,” he said.
Conventional test gear is not detecting the phase changes Dodds has looked at. “It sounds like black magic, but it really isn’t,” he said. “It is just that the ear is better than £5,000 or even £10,000 test gear.”
ClarityCap makes capacitors for high-end speaker manufacturers and, together with audio specialists at the University of Salford, is 16 months into a two-year DTI-funded Knowledge Transfer Partnership, attempting to link audiophile expressions like ‘transparent’ and ‘brilliant’ to measurable parameters.
The aim is to be able to fine-tune custom products, for example when a speaker maker reports that a prototype sounds ‘muddy’.
The work is to be published in detail by the Institute of Acoustics.