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July 2010

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Debunking Spec-ology, Part 3

Distortion, Speaker Cables and Damping Factor—The seamy underbelly of making sound

by Karl Winkler

In the previous two installments of “Debunking Spec-ology”, I covered some very basic but very important topics including speaker and amplifier power ratings, Ohm’s Law, microphone characteristics and the basic idea that if specifications are missing or incomplete in audio equipment documentation, you should be worried about that particular product or supplier. In this current article, I will cover some additional considerations when connecting speakers to amplifiers. But first, let’s get one thing straight: Speakers = Distortion

Let’s face it – among the audio components we generally work with, loudspeakers introduce by far the largest amount of distortion. Unfortunately, this can not be seen by simply looking at the specifications for even the best professional loudspeaker systems. None of the speakers I looked at, including from Klipsch, JBL, Meyer Sound, etc. included a distortion measurement in their specifications. Part of this is because for an acoustic device, the operating characteristics are very heavily dependant on the acoustic environment—i.e. the room they are in.

But before we go much further, let’s define “distortion”. The American Heritage Dictionary, under entry #4, Electronics, states:

An undesired change in the waveform of a signal.

A consequence of such a change, especially a lack of fidelity in reception or reproduction.

This describes it very well, I think. And some types of distortion are inherent in certain design criteria—i.e. that smaller speakers, for instance, do not reproduce extreme low frequency information, and thus, by definition, show some distortion in the lows (mainly, a complete lack of low frequency extension). Then there is the less obvious type: that even within the speaker’s audio band, there are changes to the signal that we don’t want. Take a look at Figure 1 showing the distortion measurements for a high-quality loudspeaker (just the woofer in this case). Note that the overall THD (total harmonic distortion) plus noise is a seemingly mild .7% at 100 Hz. Amplifiers on the other hand, have distortion figures well below .1% and often below .01%!

A few decades ago, when amplifier power was expensive, speakers were made to be very efficient (see Spec-ology #2) so that they could maximize acoustic output for a given amount of power. Particularly, horn-loaded loudspeakers were chosen for this purpose. But horns introduce certain kinds of distortion related to the shape of the horn and the use of a compression driver. In the past 15 years, amplifier power has become much more affordable, and thus more and more efforts have been put in place to continue minimizing speaker distortion while still maintaining good efficiency. This has resulted in speakers that perform amazingly well for our purposes of sound reinforcement. In addition, more and more efforts have gone into processing the signals for those speakers via internal DSP and amplifier circuitry.

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Karl Winkler has worked in the professional audio industry for more than 15 years including as a touring mixer, a recording engineer and a technician. He is currently Director of Business Development for Lectrosonics, Inc. in Rio Rancho, NM.