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Long-term DBT example

kerr> At the risk of "blindsiding" myself, how would someone set up a long term DBT?

hobby> will a short sbt be inferior to a short sighted test? <

kerr> Hard to rank them. Short tests are worthless.

Here's an example of a long-term DBT using an ABX comparator. This is from David Clark's AES article, "10 Years of ABX Testing".

3 TESTING THE DOUBLE-BLIND TEST

The sensitivity of the A/B/X test can be tested by comparing
it to a long-term listening session with infrequent switching and
low stress. Audio magazine encouraged the present author and
Lawrence L. Greenhill to undertake such a comparison in 1984.
Unfortunately, the results were never published. The experiment
used a fixed detection task of identifying whether or not the
audio was passed through a nonlinear circuit which generated 2.5%
total harmonic distortion on a sine wave. The nonlinearity used
(called "Grunge") generated a constant distortion, independent of
sine wave amplitude or frequency over a wide range. High amounts
of the effect produce an annoying "garbled" sound on complex
program material. The circuit is described in reference [1].
Two groups of audiophiles were used as subjects. Lawrence
Greenhill's Long Island based, The Audiophile Society (TAS)
provided the high-end oriented "golden ears." David Clark's
Southeastern Michigan Woofer and Tweeter Marching Society
(SMWTMS) provided the "engineers."

Two sets of tests were to be run with each group. The first
test was a group double-blind test of 16 trials comparing the
2.5% distorted signal to a bypass. As it turned out, the TAS
group refused to have the signal passed through the relays and
connectors of the ABX Comparator. A manually-patched 16-trial
pair-comparison test was used instead. They listened to a very
expensive sound system which was familiar to most of them. The
SMWTMS group used A/B/X testing and an unfamiliar sound system
and room. They were given a one-hour familiarization period
before the test began.

The second of the tests consisted of ten battery powered
black boxes, five of which had the distortion circuit and five of
which did not. The sealed boxes appeared identical and were
built to golden ear standards with gold connectors, silver solder
and buss-bar bypass wiring. Precautions were taken to prevent
accidental or casual identification of the distortion by using
the on/off switch or by letting the battery run down. The boxes
were handed out in a double-blind manner to at least 16 members
of each group with instructions to patch them into the tape loop
of their home preamplifier for as long as they needed to decide
whether the box was neutral or not. This was an attempt to
duplicate the long-term listening evaluation favored by golden
ears.

The results were that the Long Island group was unable to
identify the distortion in either of their tests. SMWTMS's
listeners also failed the "take home" test scoring 11 correct out
of 18 which fails to be significant at the 5% confidence level.
However, using the A/B/X test, the SMWTMS not only proved
audibility of the distortion within 45 minutes, but they went on
to correctly identify a lower amount. The A/B/X test was proven
to be more sensitive than long-term listening for this task.
A general conclusion from this exercise is that,
introduction of a near-threshold amount of audio distortion can
be detected in a short-term A/B/X test using an unfamiliar sound
system. The same amount of distortion is likely to be
undetectable in individual long-term tests on familiar sound
systems. Those who have participated in A/B/X tests agree that
the reasons for a low threshold are the ease and speed of
comparison and the focus on the detection task. A longer test
and a familiar sound system were not necessary to get the job
done.

This post is made possible by the generous support of people like you and our sponsors:
  Atma-Sphere Music Systems, Inc.  

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