Measuring headphones

[Al Bregman <BREGMAN@xxxxxxxxxxxxxxxxxxxx>]

Dear List,

A colleague of mine,  André Achim, at the University of Quebec in Montreal
is doing a project studying the interfering effects of tinnitus on hearing.
He asked me about how to calibrate headphones with the precision acceptable
to audiology journals.  He is.

If anyone has advice on this, rather than replying to me would you kindly
e-mail him at the following address:
  : achim.andre@uqam.ca

Thanks,

Al

----------------------------------------------------------------------------
-----------

Is a surrogate head required to calibrate an auditory threshold
evaluations system, and why?

I am setting up a system to assess and compare auditory threshold to
pure tones modulated by different amplitude envelopes for frequencies up to
16 kHz. My PC-based system implements a Von Békésy audiograph using the
DirectX technology.

I selected Sennheiser HD-615 circumaural dynamic headphones, rated for -3dB
from 10 to 30000 Hz, and am about to calibrate the whole system in SPL
units.  I understand that the interest is in the pressure received at the
ear drum and that a flat plate coupler [used by Al Bregman] would miss the
resonance characteristics of the ear canal.  I do not have a realistic
artificial head coupler.

My question is: Wouldn't it be quite satisfactory to calibrate the system
with a flat plate coupler and than correct this calibration for the
resonance characteristics of a representative ear canal?  I presume that the
required correction would simply be a difference curve, in dB, previously
obtained in calibrating the same earphones with a flat plate and with a head
model.  If this is reasonable, can anyone provide me with the gain
characteristic of the
ear canal as a function of frequency (i.e. the difference curve between
calibrations with the two coupler types) for a fairly wide range of
frequencies?  If this appears unreliable, can anyone explain why this
approach should be avoided, and what approach to use with my Sennheiser
circumaural headphones?

Please reply to: achim.andre@uqam.ca

----------------------------------------------------------------------------
----

Albert S. Bregman
Emeritus Professor
Dept of Psychology
McGill University
Montreal, QC
Canada H3A 1B1

Tel:  +1 (514) 398-6103
Fax: +1 (514) 398-4896