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For instance, it
is not uncommon to present speech materials at a level 40 dB above the
patient' threshold. If my patient's threshold is 20 dB HL then the signal
would need to be presented at 60 dB HL to achieve my goal of presenting
at 40 dB above the threshold. Since dB SL uses the patient's threshold
as the reference value I could state that I was presenting the test at
a level equal to 40 dB SL. If my next patient had a threshold of 45 dB,
I would need to present the test at 85 dB HL (threshold = 45 + 40 - 85).
Since it is not uncommon
to need to describe values relative to an individual's threshold, the
use of the dB SL scale has become quite widespread. Using the above speech
testing example, we could define a rule for the test that states that
the speech signal should be presented at 40 dB SL. By definition, then,
the tester would know to obtain the individual's threshold and then present
the test at 40 dB above this threshold level.
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