MEASURING THE DRIVERS

Considered as a sequence of sounds over time, audio energizes a room with a complex jumble of fundamental tones and thousands of harmonics, augmented by room reflections and longer-term reverberation. This sounds like a recipe for aural confusion, but we learn instinctively from earliest childhood to distinguish direct sound from a source, person, or loudspeaker from the reverberant field. Without that skill, stereo perception in rooms would be impossible.

Audio listeners are accustomed to moderate amounts of distortion from a loudspeaker, usually low-order and generally innocuous. At higher sound levels, such errors are often masked by natural distortions present in our hearing process, to which we are more or less acclimatized.

Extant loudspeaker technologies promise lower distortion than the ubiquitous moving coil varieties, for example, ribbon, pseudoribbon, and electrostatic types. Full-size, high-efficiency horn designs typically employ a moving coil driver where the acoustically loaded sound-producing element needs to move very little, helping to minimize distortion. Frequently, the best of these technologies are capable of expressive musical dynamics that better convey the stunning excitement of live sound, although some also exhibit their own audible signatures and characteristic colorations.

This Purifi driver may well be immune from such considerations. Its midband third harmonic is claimed to average –76dB, or 0.015%, a very low level. I was interested to see whether I could hear the claimed quality differences, with its promised control of distortion at sensible but still useful loudness levels.

I made a few spot measurements on the Siren, primarily to make sure all was working as intended. I noted a slightly below average (and very slightly below spec) 86.5dB sensitivity with a nominal 2.83V input at 1m. I also saw useful indications of a particularly uniform frequency response: Over the crucial central region, 200Hz– 2kHz, it held to a very close ±2dB tolerance, well extended to 22kHz, –3dB, though with a steep falloff beyond. The heard and measured off-axis response was also promising. We’ll see what JA finds.

And what about distortion, bass to midrange? Purifi’s ambitious claims of exceptionally low distortion do seem credible. Where most loudspeakers measure 0.2%–0.4% distortion at accepted listening levels—say, 85 to 95dB/2.83V/m—this Purifi driver measures only a tenth that (–20dB) or even less. What’s more, this modestly dimensioned midwoofer was able to sustain a continuous sinewave up to 25W at 50Hz before entering mechanical overload, a comparatively massive power input. Here I noted some moderate second harmonic distortion but very little third—an important result since third is more discordant and thus more disturbing of timbre than second.

With a subjectively loud 1W, 500Hz sinewave, a dominant region of acoustic power not far removed from middle C, second harmonic distortion was low at 0.12% while the more important (more strident) third harmonic was well down at 0.011%, or –79dB, with no higher harmonics. At 2kHz, 2.83V, the second harmonic was at just 0.03% while third was 0.01%, exceptionally low for a mid/bass driver. No other artifacts were present with this signal even at my 0.003%, –90dB measurement threshold.

Could the Thrax horn tweeter match this exceptional midrange linearity? At 5kHz, a trace of harmless 10kHz second harmonic was found at 0.12%, while the potentially more annoying third harmonic was well below audibility at just 0.07%. Distortion continued to decrease substantially with decreasing power. 1V, 5kHz sounds quite loud in the room, and here, the second harmonic was a minuscule 0.013% with no other products present.

Plainly, these two drivers complement each other. For a typical midband spectral power maximum of 500Hz, the second harmonic from the mid/bass unit was truly exceptional, just 0.05% with no other artifacts of relevance. I suspect I heard this absence of distortion in the exceptional midrange transparency and natural timbre observed at normal loudness.

Some of these results are indeed superior to a number of good power amplifiers. I could not hear acoustic overload at any sensible volume level save in the low bass, and then only if I assaulted the Siren with heavy solo bass guitar below 45Hz, leading to some mild port noise.