probe microphones

[Bill Treurniet <bill@xxxxxxxxxxxxxxxxxx>]

Thanks to all who responded to my request for information about probe
 microphones
for measuring HRTFs. For the benefit of others wishing to acquire such
microphones, the following is a summary of the responses and whatever additional
information I was able to get.

1) Etymotic Research ER-7-10C probe mic

Etymotic Research may be contacted at (708) 228-0006 (voice) and
(708) 228-6836 (fax).

The Etymotic mic comes with a preamp (ER-7C) for $1500 US, and no assembly is
required by the user. The probe tube is silicone rubber with an OD of .95 mm.
Frequency response equalization beyond 10 KHz is supplied, and there is a
built-in calibrator. It was used in research reported by Wightman and Kistler
at U of Wisconsin (JASA 85(2),858-867,1989) among others, although this
should not necessarily be taken as a recommendation (see comment below,
NOT from the Wisconsin lab).

If the prepayment required by Etymotic Research is not permitted by your
purchasing department, Mimosa Acoustics (Dr. Patricia Jeng) will accept a
purchase order (payment term is net-30). There was a relatively small increase
 in
the price compared to Etymotic's quote. The telephone numbers are (908) 518-0711
(voice) and (908) 789-9575 (fax).

One user of the Etymotic mic felt that it was somewhat noisy although it met
the advertised spec (Noise level: 55 dB SPL equivalent, 20 to 20,000 Hz).

        "We used Etymotic mics (ER-7C, I think) for some in-the-ear-canal
        recording, and I was surprised by how noisy they were.  In fact,
        their noise level is about what the specs said it'd be, but
        that was louder than I expected from glancing at the specs
        in the marketing literature.  You might want to get a demo to
        make sure that (or any other brand) mic is what you want before
        you put in a P.O."


2) Knowles and Sennheiser mics

Some info on the Sennheiser mic is provided in the following comments.
As indicated, the company may be contacted by phone at 0049 5130/600-0.

Knowles Electronics, Inc. can be contacted at

        1151 Maplewood Drive
        Itasca, Illinois  60143
        Phone:  (708)250-5100
        Fax:    (708)250-0575

The company appears to specialize in hearing aid hardware, and it has at least
two products of interest.

a) Knowles can provide a microphone (EM 3046) which the user would need to
 attach to
the appropriate circuitry. The following comments from users are informative.

        "we are working on HRTF and we know two companies building suitable
        microphones, which may be suitable for your purpose:
        One is a German factory: Sennheiser Type KE 4-211; 4,75mm diameter, also
        used by the Danish research group Moeller et al. at Aalborg University.
        They are very easy to handle. Phone only: 0049 5130/600-0.
        The other one is (as you know already) Knowles: Type EM 3046, which are
        very small and hard to handle and solder (you will need some kind of SMD-
        techniques). Both prices are about 50$."

        "Good microphones for this purpose are Sennheiser KE4. The only disadvantage of
        this microphones is, that they are too large for some human ear canals. So they
        cannot be positioned deep enough in the ear canal (appr. 5mm from the entrance)
        I also used Knowles EM 3046 which are much smaller but have less sensitivity.
        These small microphones are fixed in mold of an individual otoplastic.
        The HRTF I measured with both techniques are good enough for psychoacoustical
        experiments about localization of sound sources.
        The microphones cost between $30 and $40. We did not use probe
        microphones because they are much more expensive and it seems that it
        is not necessary to measure very close to the eardrum."

b) Hellstrom and Axelsson (JASA 92(2),907-919,1993) reported using a
Knowles EA 1842 microphone and Martens (Proc 1987 International Computer Music
Conference, 274-281) reported using a Knowles BT-1759 microphone.

c) Mr. Rick Zenardo (Tel: 708-250-5115) at Knowles faxed me info on another of
their products that may be of interest. It is the XL-9073 Microphone Probe Kit
available for about $100 US. The kit includes vinyl (2.21 mm OD) and
 polyethylene
(1.91 mm OD) tubing, individual frequency response curve, resistor, battery
holder, application note, and a technical report. Some assembly is still
required, but it seems less difficult than for their EM 3046 mic (see first
comment above). The mic's sensitivity seems reasonably flat between 300 Hz and 8
KHz.

3) Bruel & Kjaer

The following response describes B&K mics.

        "I know of two in-the-canal probes made by Bruel and Kjaer in
        Denmark.  One is with a thin-tipped hard exponential probe, and the
        other is a hanging tube better fit for use with an earphone in the
        canal, though we haven't used either.  Sorry I do not have the part
        numbers.  Both are to be connected to their calibrated 1/2 inch
        microphone, if I remember correctly, and they do not come cheap.
        Their address in Canada is

                Bruel & Kjaer Canada Ltd.
                90 leacock Road
                Pointe Claire, Quebec H9R 1H1
                514-695-8225."