Recording - December 2014 - 40

We don't usually notice these changes when moving around
because each ear receives a different response, so what we perceive
is more of an average. A deep null in one ear is likely not present in
the other ear, and vice versa. And since all rooms have this property,
we're accustomed to hearing such changes and don't always notice
them. However, the change in response over distance is very real,
and it's definitely audible if you listen carefully. If you cover one ear
it's even easier to notice because then the frequencies missing in one
ear are not filled in by the other.
I'm convinced that comb filtering is at the root of people reporting a
change in the sound of cables and clocks when measurements imply that
an audible change is unlikely. If someone listens to their system using one
AC power wire, then gets up and switches cables and sits down again,
the frequency response heard is sure to be different because it's impossible to sit down again in exactly the same place. So the sound really did
change, but not because the cable made a difference!
With audio and music, some frequencies tend to be harsh sounding, such as the range around 2 to 3 kHz. Other frequencies are more
full sounding (50 to 200 Hz), and yet others have a pleasant "open"
quality (above 5 kHz). So if you listen in a location that happens to
favor harsh sounding frequencies, then change a wire or clock and
listen again in a place that suppresses the harshness, it's reasonable
to believe the wire change is responsible for the difference. Likewise,
applying dither might seem to affect fullness, even though the low-frequency response change was due entirely to positioning.
Proper test methods
It's common to read in audio forums about a "shootout" comparison of
preamps, sound cards, or sample rates. Often someone will record themselves singing or playing an instrument with one setup, then record themselves again for comparison. However, recording different performances
is not acceptable because the subtle details we listen for when comparing
gear-for example, a bell-like attack of a guitar note, or a glassy sheen
on a brushed cymbal-also change from one performance to another.
Nobody can play or sing exactly the same way twice. Nor can they
remain perfectly stationary, which is needed to ensure the microphone
captures the same sound, as explained above. Therefore, recording
different performances is not valid for testing wires, preamps, clocks,
or anything else, unless the difference really is large, such as between
a typical dynamic mic and a large-diaphragm condenser.
One useful self-test is simply closing your eyes while switching between
two sources with software. When I test myself blind using WAV files, I set
up two parallel tracks in Cakewalk SONAR, then assign the Mute switches for those tracks to the same group while the switches are in opposite
states. That is, one track plays while the other is muted, and vice versa.
Each time either Mute button is clicked, the tracks are exchanged. This
lets me switch from one track to the other without interruption or pops. I
put the mouse cursor over either track's Mute button, close my eyes, then
click a bunch of times at random without paying attention to how many
times I clicked. That way I don't know which version will play first. Then
I start playback, still with my eyes closed, and listen carefully to see if I
can really tell which source is which as I switch back and forth.
I've read claims that blind tests are inherently flawed, but in my
opinion that's just an excuse for not being able to pass the test! Blind
testing is the gold standard for all branches of science, and it makes
no sense that it's somehow invalid for assessing audio fidelity.
Some people claim the testing itself is stressful because they're put on the
spot to identify subtle differences, often in front of strangers, but this is easily solved using ABX software. An ABX program lets people test themselves
as often as they want, over a period as long as they want, in the comfort
of their own listening environment. The original ABX tester was a hardware
device that played one of two audio sources at random each time you
pressed the button. The person being tested must identify whether the "X"
40

RECORDING December 2014

currently playing is either Source A or Source B. After running
the same test, say, ten times, you'd know with some certainty
whether you really can reliably identify a difference. These
days several ABX testers that keep track of your selections over
time are available as freeware.
Whether using an informal blind test or ABX software, it's
important to understand a few basic requirements. First, the
volume of both sources must be matched exactly-to within 0.1 dB. When all else is equal, people generally pick the
louder (or brighter) version as sounding "better," unless, of
course, it was already too loud or bright. Indeed, people
sometimes report a difference even in an "A/A" test, where
both sources are the same!

Figure 1: This graph shows the low frequency response in
a room about 16 x 11 x 8 feet in size, at two locations
four inches apart. Even over such a small physical span,
the response changes substantially at many frequencies.

Figure 2: This shows the full-range response from the
same two measurements as Figure 1. At mid and high
frequencies the response difference four inches apart
is even more substantial than below 200 Hz.

Figure 3: This graph shows the low frequency response
of a bedroom-sized space before (red) and after (blue)
adding a number of bass traps.

Recording - December 2014

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