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Distortion

epos encore 50 lf thd2 Close

Bass distortion (Epos Encore 50 loudspeaker)

Green - bass unit. Yellow - port.

epos encore 50 thd1 copy

Mid-band distortion (Epos Encore 50 loudspeaker).


WHAT IT TELLS US

Superficially, a distortion measurement tells us how clean and free of the 'coarseness' of distortion a loudspeaker will be. However, with loudspeakers the picture is a little complicated. Generally, except at bass frequencies below 100Hz, distortion hovers around 0.3% (see the analysis above) and comprises second and third harmonics. This is a subjectively benign characteristic, meaning distortion is, seemingly, not a major issue in loudspeaker sound quality. Except that electrostatics typically give less than 0.1% distortion and measure better in this area our results show, suggesting distortion even at these levels may be of consequence. It is not a major factor in sound quality though.


kingsound-prince-ii-lf-thd1

Bass  distortion  (Kingsound Prince II electrostatic).


kingsound-prince-ii-thd1

Mid-band distortion (Kingsound Prince II electrostatic).


The Epos Encore 50, whose distortion performance we show at top is a modern floorstander that works well and gives representative result against which other loudspeakers can be judged when reading our reviews. Just bear in mind the Encore 50 is large, expensive and has plenty of cone area. A small budget loudspeaker will inevitably produce more bass distortion, around 4% at least.


It is common for peaks of a few percent to exist here and there across the frequency band of our swept distortion measurements, usually due to mechanical effects, loosely termed “rub & buzz” (there is a proper measurement for this). Correlating peaks with perceived sound quality, in the absence of obvious buzz for example, has proved difficult in our listening tests, unless the distortion is severe.


Distortion rises at bass frequencies, often to 10% or so, and even to 20%. Ports commonly distort more than drive units. Much of this is what is known as ‘bass doubling’ and comprises second harmonic that, when severe (10% or more), subjectively lightens timbre slightly by transferring energy from the fundamental and into the second harmonic. However, large cone loudspeakers (12in and 15in bass drivers) consistently produce less bass distortion, often below 1%, than normal size domestic loudspeakers (3%), so this may well be a measure of that “relaxed” bass quality attributed to large loudspeakers. Certainly, low bass distortion is no bad thing and around 3% or less from the bass unit and 5% or less from the port, at 40Hz, is to be hoped for.


HOW WE MEASURE IT


rs-upl


Rohde & Schwarz UPL  distortion analyser.


In a room distortion must be measured ‘near field’, the mic positioned within millimetres of the cone, to avoid standing waves which severely disrupt results. This methodology relates distortion to specific drive units; for system response an anechoic chamber must be used. We measure at a modest sound pressure level of 90dB at 1m, which translates to 110dB at the cone. Our Rohde & Schwarz UPL analyser is stepped over 100 points from 20Hz to 100Hz, measuring bass unit then port, and from  100Hz to 6kHz, measuring bass/mid unit or midrange unit. As most bass/mids and mids cut off progressively above 3kHz, little is to be expected above this frequency. Our plots show true relative distortion in percent, rather than showing harmonic level relative to the frequency response curve (as in Clio, etc), and thus give a very clear picture of distortion trends and loudspeaker behaviour in this area.

 

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