Updated: Tuesday, 21 February 2017

Selected Posts on Audio Topics
by John L. Murphy

Subject:   Sources of Distortion in Audio Playback Systems

(posted 11Aug05 to Madisound)

Hello Madisound!

As speaker designers we are concerned with achieving good clean bass reproduction. When we think of bass in the context of our recorded music it is often the bass guitar that stands out as the principle source of bass program content. So why not use it to test our systems?

One of the best ways of honestly revealing the real world distortions in a woofer system is to actually play a bass guitar through it with the treble fully cut at the instrument. Keep the level within the power handling range of the speaker and play a smooth sequence of notes up and down the scale in search of audio trouble. Typically the first things revealed will be rattles and buzzes related to the mechanical construction of the speaker system. The obvious rattles and buzzes will be embarrassing to most speaker builders who have never done this test before. Why even think twice about the distortion of the woofer if the cabinet buzzes are contributing distortion at the 20% level! Once you tame the cabinet distortion sources usually the next thing that you notice is the sympathetic rattles and buzzes around the listening room! Should we be concerned about these noise sources? Absolutely! After all, your playback "system" includes your complete sound system, speakers and playback environment. Who cares whether the distortion is coming from the woofer itself, the cable connector, cabinet hardware, or your Aunt Emma's urn of ashes on the mantle...distortion is distortion. And nothing reveals it like a solo bass guitar.

I call the sound from the bass that I use for speaker testing "mud ball" bass because you want to dial up a thick bass tone that is devoid of treble. On passive instruments like a Fender P Bass you just need to turn the tone control all the way down to get the mud ball test setting. If you leave the treble up you hear the treble from the bass (including the bass's own rattles and buzzes...but that is another story) and that tends to mask the distortions in the speaker under test.

Only after all the enclosure and room rattles are eliminated can we actually begin to analyze the distortions coming from the woofer itself... This is assuming we are not overdriving either the woofer or the associated electronics! Harmonic distortions from the speaker are usually quite subtle except when the driver is operated approaching its excursion limit. Often the most audible distortions are again from rubs and buzzes occurring within the woofer transducer itself. Sometimes there is just a single note or two that will trigger an audible noise or distortion.

With good quality drivers operated within their excursion limits the primary sources of distortion in the associated playback system are frequently NOT the drivers themselves, but rather originate with the enclosure and listening room. Do the test for yourself and see where the distortion comes from in your playback system. If you are not a bass player then there is a good chance you know one you could invite over to help you perform the test. Then again, a student bass can be had for around $150 at your local music store. Some people spend a whole lot more than that on audio test gear. A bass may not plot a pretty graph of THD vs. frequency but it will show you where the problems are in any playback system.

If you are doing this test for the first time you will probably be astonished at what you hear and will never think of speaker distortion in the same way again. It's funny to think of those poor misinformed audiophiles who obsess over .001 or .002% distortion from their power amp while actually listening to a playback system with 10% or more distortion from sympathetic vibrations around their listening room alone.

What is the primary source of distortion in your playback system when you play a low E at 100 dB SPL?



John L Murphy
True Audio

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