EPL Acoustic System Assembly Manual
The EPL Acoustic System is designed for use in small animals such as mice and guinea pigs. It
has two miniature dynamic earphones, whose outputs mix, and a miniature electret microphone
coupled to a small probe tube to measure sound pressure at the tip of the nosepiece (Fig. 1).
This system produces high output (>90 dB SPL) from ~100 Hz to ~60 kHz (Fig. 2). The
microphone is used for calibrating the earphone outputs when the system is in the ear canal and
is suitable for measuring otoacoustic emissions (OAEs) such as distortion product (DP)OAEs.
The acoustic system is under continued development at EPL. To receive updates about
improvements, register your use of the system at EPL (ijs@epl.meei.harvard.edu). See the EPL
Cochlear Function Test Suite Users’ Manual for a description of the calibration procedures for
the EPL Acoustic System.
Figure 1: Diagram of the EPL Acoustic System
Figure 2: A typical output curve
1. Coupler Body and Speakers
Quantity
Item
Manufacturer / Description
Part # / Size
1
Coupler Body
see below
per STEP file
1
Support Rod
Aluminum
1/4" dia x 6" L
1
Probe Tube
Thin wall hypodermic tubing
0.050” OD x 0.625” L
1
Nosepiece
Thin wall hypodermic tubing
0.120” OD x 0.270” L
2
Speakers
CUI Miniature Dynamic earphones
CDMG15008-03
1
Speaker Connector
Molex
22-23-2041
2
Damping Foam
Open cell foam
1/2" dia x 3/8"
2
Screw Cap
Scintillation vial cap, polypropylene
22-400
1. Have the coupler body made via stereolithography from the EPL STEP file that contains all
dimensions in Product Manufacturing Information format. For a copy of the STEP file, email:
ijs@epl.meei.harvard.edu. Our supplier, Prototek, can build this in high-density optically
cured resin.
2. To finish off the coupler body, ream the probe tube hole (0.050”), nosepiece hole (0.120”)
and microphone pocket (0.161”). Cement a 1/4" support rod through hole in coupler body
with epoxy.
3. For the probe tube, cut stainless steel tubing to size, debur, and cement in place with epoxy.
4.
5.
6.
7.
8.
The probe tube end should be flush with bottom of microphone pocket. The nosepiece
should protrude 0.150" from coupler for use in mice. For use in guinea pigs, add 0.150” to
the nosepiece and probe tube and arrange so that they remain coaxial and end flush.
Epoxy the 4-pin Molex speaker connector to the 4-hole flat on the coupler body (Fig. 3C).
Connect each speaker to two pins of the
4-pin Molex via a twisted pair of wires (2"
30AWG). Solder the speaker wires
according to the polarity shown in Figure
3A. Solder the wires to the 4-pin Molex
as shown in Figure 3C. Mount the
speakers in the coupler body pockets,
threading wires through the pre-molded
slot in the coupler body (Fig. 3B).
Cut damping foam to size, place over
each speaker, and then twist the screw
caps into place on the coupler body.
To connect the 4-pin Molex to the dual
channel equalizer (See #2 below) wire
two coaxial cables to two BNC plugs at
one end, and to a Molex 22-01-3047
shell and four Molex 08-50-0114 crimp
terminals at the other end: see
Connecting BNC cables to Molex shell
for details. Solder connections so that the
center wire of the BNC cable goes to the
+terminal of the speaker.
[OPTIONAL] Paint the coupler body with
CuPro-Cote conductive copper-bearing
paint
to
provide
electromagnetic
shielding for reduction of stimulus pickup
by recording electrodes. Paint should
overlap the support rod to allow
grounding via wire. Leave an unpainted
Figure 3: Wiring speakers into the coupler body
area near nosepiece to avoid current
path to the animal.
2. Dual Channel Equalizer
Quantity
Item
Detail / Size
4
Connectors
BNC
2
Inductor
100µH
2
Capacitor
2µF
2
Resistor
22!
1
box to fit
Bud CU-3000A
In a sealed system the miniature dynamic earphones generate large pressures at low
frequencies, and the TDT SA-1 two-channel amplifier provides sufficient power to damage the
transducers (see EPL PXI System Overview). To reduce the low and midrange response, we
use an equalizer circuit that is a high-pass LC filter with a 12 dB/octave slope and a cutoff at
10kHz (Fig. 4).
Figure 4: Dual Channel Equalizer. A: circuit diagram B: photo of assembled device
3. Probe Tube Microphone
Quantity
Item
Manufacturer / Description
Part # / Size
1
Probe Tube Microphone
Knowles Electret Condenser
FG-23329-P07
1
Probe Tube Microphone Sheath
Tygon® tubing
3/32 x 5/32
1
Connector
CUI miniDIN-5M
5 pin male
Figure 5: Photo of the probe tube microphone alone (left) and another microphone
positioned within the protective-tubing assembly (right)
1. We use a small Knowles electret condenser microphone FG-23329-P07 at the end of the
probe tube to measure ear-canal sound pressure (Fig. 5). The FG-23329-P07 comes with
1m of shielded 3-conductor wire preattached. [If unavailable, solder shielded cable (Cooner
NMUF 3/32-6550 S, #32 stranded conductors) to a FG-23329-C05 microphone.]
2. If using the FG-23329-P07, remove the shrink wrap that covers the microphone.
3. Slip 1" length of 3/32 x 5/32 plastic tubing over cable and microphone body; secure cable to
tubing at the end opposite the microphone with electronics-grade adhesive.
4. Terminate the other end of the cable with a miniDIN-5M connector.
4. Probe Tube Microphone Bias Box
The FG-series microphones can take a maximum supply of 1.6V, so we use a 1.5V alkaline cell
to provide isolated, low-noise power. A bias box uses a battery to supply operating power and
removes DC offset from the microphone signal before going to the preamplifier. Hand wire as
per schematic (Fig. 6) or use a printed circuit board.
Figure 6: Microphone Bias Box. A: circuit diagram B: photo of assembled device
5. Microphone Amplifier
For a microphone amplifier, we use an ER10C amplifier system from Etymotic Research. A
complete ER10C system also includes an acoustic probe with two sound sources and a probe
tube microphone. However, this acoustic probe is not useful above ~16 kHz. When purchased
in this complete configuration, the ER10C system has an internal equalizer tuned for best
response with the attached sources and microphone.
Alternatively, the amplifier can be ordered as a stand-alone device (for less than half the price);
however, this device will not have an equalizer or a signal path from input to output. Use in our
system requires construction of a dummy equalizer; we connect pins 1, 2 and 3; no connection
to pin 4, which is circuit ground. See their schematic or ask Etymotic for information.
Input to the ER10C is via a BNC to miniDIN-7M cable, shield to pin 1, signal to pin 3.
For those with experience in electronics assembly, an alternative to the bias-box plus ER10C is
the custom EPL FG-series amplifier (Fig. 7). The input amplifier is LME49710NA (needs a
power source) for exceptionally low input noise. Output amplifier is LF356 (needs a power
source) for cable drive capability.
Figure 7: Circuit diagram of the EPL FG-series custom microphone amplifier with built-in bias box
6. The calibration coupler
The EPL Acoustic System must be
calibrated to know the relation between
SPL at the tip of the nosepiece and
voltage from the probe tube microphone.
For a description of the calibration
software, see the EPL Cochlear Function
Test Suite Users’ Manual.
The hardware and setup for calibration is
illustrated in Figure 8: a reference
microphone (Larson-Davis "” condenser
microphone) is placed a reproducible
Figure 8: The setup for a “microphone + probe tube”
distance from the tip of the acoustic
calibration
system by means of a “calibration
coupler” and a “reference microphone
shield”. The shield can be made by milling off the end of the removable microphone windscreen.
The calibration coupler is machined from brass or transparent plastic with a flange that prevents
the system nosepiece from touching the diaphragm of the reference microphone.
7. Acknowledgement
This is a joint effort of the Eaton-Peabody Laboratory Engineering Core. Development
supported by NIDCD Research Core Center Grant P30DC005209.