(posted 11Aug05 to 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
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
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
What is the primary source of distortion in your playback system when
you play a low E at 100 dB SPL?
John L Murphy