G R A S 40SA Probe Microphones Instruction Manual
- June 12, 2024
- G R A S
Table of Contents
40SA Probe Microphones
Instruction Manual Instruction Manual
40SA/40SC Probe Microphones
LI0123 – March 2022
www.GRASacoustics.com
40SA Probe Microphones
Revision History
Any feedback or questions about this document are welcome at
gras@grasacoustics.com.
| Date| Description
---|---|---
1| July-07| First release
2| September-16| Addition about cleaning; Calibration coupler: new P/N: RA0326
3| October-19| Page 12: max body temperature changed to 70 °C
4| March-22| Page 9: Updated text regarding calibration charts
Warning: do not use the Probe Microphone for measurements on the human
body. It is neither intended nor approved for measurements on the human body
and the internal high voltage may be harmful.
Note: unless individually specified, the term “Probe Microphone”
throughout this document refers to both types, i.e. Type 40SA and Type 40SC.
Copyright Notice
© 2016-2022 GRAS Sound & Vibration A/S
http://www.grasacoustics.com
Any technical documentation that is made available by GRAS is the copyrighted
work of GRAS and is owned by GRAS.
The content in this document is subject to change without notice. GRAS Sound &
Vibration A/S is not liable or responsible for any errors or inaccuracies that
may appear in this document.
Trademarks
Product names mentioned in this document may be trademarks or registered
trademarks of their respective companies and are hereby acknowledged.
Introduction and Description
The GRAS Probe Microphones Types 40SA and 40SC (Fig. 1.1) are small, compact
units for sound-pressure measurements in small enclosures, in harsh
environments and in very close proximity to sound sources. The high acoustic
input impedance at the tip of their probes minimizes their influence on the
acoustic field, while the stainless-steel tubes used in manufacturing these
probes can withstand high tip temperatures, up to 800 ºC with the 160 mm long
probe GR0261.
1.1 Differences between Type 40SA and Type 40SC
Both types shown in Fig. 1.1 have similar sizes, performances and
applications. They differ only in the type of internal microphone and
preamplifier used and in the way they are powered.
1.1.1 Probe Microphone Type 40SA
The Type 40SA uses an externally polarized internal microphone with a
preamplifier that requires a constant-voltage power supply, e.g., GRAS Power
Modules Types 12AA, 12AD or 12AK. It has a 3 m cable terminating in a 7-pin
male LEMO plug 1B, which plugs directly into these power modules.
1.1.2 CCP Probe Microphone Type 40SC
The Type 40SC uses a prepolarized internal microphone with a CCP 1
preamplifier that requires a constant-current power supply, e.g., the GRAS CCP
Supply Type 12AL, or any other CCP compatible power supply. It has an
integrated BNC output for, via a coaxial cable, drawing power from, and
delivering the signal to, the constant-current power supply. Fig. 1.1 The two
GRAS Probe Microphones covered by this document
Left; CCP Type 40SC
Right; Type 40SA
1.2 Description
The Probe Microphone is a small robust unit with a smooth frequency response
from 2Hz to 20kHz (see Fig.1.2). It can be used with both stainless-steel
probes and flexible probes of various lengths. With stainless-steel probes,
measurements can be made with temperatures as high as 800°C at the tip of a
160mm probe (GR0261). Fig. 1.2 Typical frequency response of the Probe
Microphone with a 40 mm probe
The small diameter of the probe ensures a very high acoustic impedance, which
minimizes its disturbance to the sound field. For closed-coupler measurements,
the Probe Microphone uses internal pressure equalization to balance out static
pressure differences.
The Probe Microphone consist of a probe, a ¼″ condenser microphone, a
preamplifier and an impedance matching tube (see Fig. 1.3). The sound-pressure
fluctuations at the tip of the probe are transmitted through the probe to the
condenser microphone within the body of the Probe Microphone. The condenser
microphone senses these pressure fluctuations on its diaphragm, and the
preamplifier converts the resulting high-impedance output signal from the
microphone to a low impedance signal suitable for connection to a standard
signal analyzer or other measurement equipment. Fig. 1.3 The principle of the
Probe Microphone
The sound-pressure fluctuations transmitted through the probe to the
microphone continue to an impedance-matching tube. This tube is approximately
1 m long with the same inside diameter as the probe itself. This long tube
reduces reflections and ensures that the Probe Microphone has a smooth
frequency response. The impedance-matching tube is coiled around the
preamplifier within the housing of the Probe Microphone.
The impedance-matching tube terminates in a small opening within the housing
thus ensuring that the static pressure within the Probe Microphone is the same
as that at the tip of the probe. Consequently, the static pressure within the
rear-vented condenser microphone will also be identical to the static pressure
at the tip of the probe.
The microphone preamplifier used is based on a standard GRAS preamplifier. In
the case of the Type 40SA, similar to Preamplifier Type 26AC with an output
cable terminating in a 7-pin LEMO plug for direct connection to any standard
preamplifier power supply, e.g., GRAS Power Modules Types 12AA, 12AD or 12AK.
In the case of the Type 40SC, similar to Preamplifier Type 26CB with an
integrated BNC output socket for connection to a CCP supply, e.g., the GRAS
CCP Supply Type 12AL.
The Probe Microphone is ideal for measurements in small enclosures or for
measurements in the very near field of a sound source. Fig 1.4 shows an
example of a setup for measuring sound pressures inside an exhaust system. The
tip of the probe is inserted into the exhaust pipe through a small hole (ø 1.3
mm) in the wall of the exhaust pipe. Fig. 1.4 Setup for measuring sound
pressures within an exhaust pipe
Operation
2.1 Carrying Case and Accessories
The Probe Microphone is supplied with a kit of various accessories and tools
for calibrating a probe and for adapting a probe to particular measurement
requirements. It is delivered in a carrying case (see Fig. 2.1), which, apart
from the Probe Microphone itself, includes five stainless-steel probes of
various lengths. There is also a flexible probe and extra flexible tubing for
making probes of various lengths and a needle for cleaning the probe.
The lengths of the stainless-steel probes can be shortened for specific
applications using the file and pliers supplied (see Fig. 2.2).
The kit includes calibration adapters for both frequency-response calibrations
and levelresponse calibrations. The two level-response calibration adapters
fit into the coupler of a pistonphone or calibrator intended for ½″
microphones. They permit precise calibration with both stainless-steel probes
and flexible probes. Since the outside diameters of the stainlesssteel probes
and flexible probes are not the same, two different calibration adapters are
provided. There is also a calibration adapter for frequency-esponse
calibration. This is intended for use with two ½″ microphones used
respectively as emitter and receiver (these two microphones are not included
in the kit).
The aluminum disc doubles as both heat sink and tool for loosening and
tightening the probes attached to the Probe Microphone. A tube of silicon
grease for O-rings is also included for sealing minor leaks. Fig. 2.1 The
Probe Microphone kit in its carrying case. Shown here the Type 40SA.
A needle for cleaning (not shown here, see Fig 2.2) is also part of the
delivery. Fig. 2.2 Tools and accessories of the Probe Microphone
2.2 Connection to External Power Supply
2.2.1 Probe Microphone Type 40SA
The Type 40SA contains a standard microphone preamplifier similar to the GRAS
Preamplifier Type 26AC. This requires a constant-voltage power supply of 28 V
to 120 V and a polarization voltage of 200 V, e.g., from a GRAS Power Module
Type 12AK or similar. The wiring diagram of the male 7-pin LEMO 1B plug is
shown on Fig. 2.3.
2.2.2 CCP Probe Microphone Type 40SC
The Type 40SC contains a standard microphone preamplifier similar to the GRAS
Preamplifier Type 26CB. This requires a constant-current power supply, e.g.,
from a GRAS CCP Supply Type 12AL or other CCP-compatible power supply. The
Type 40SC has an
integrated BNC coaxial output socket for this purpose. Fig. 2.3 7-pin LEMO
female socket 1B (external view)
2.3 Changing the Probe
There is a set of four stainless steel probes of various lengths. To change
one of these, use Heat Sink GR0267 as a tool to unscrew the probe from the
housing of the Probe Microphone, see Fig. 2.4. Apply a thin smear of silicon
grease to the end of the new probe but be careful not to clog the probe hole
with the grease. Mount the new probe on the housing and tighten it with the
tool. Change flexible probes in the same way. Fig. 2.4 Unscrew the probe using
the Heat Sink GR0267 as a tool
2.3.1 Probe Length and Frequency Response
The frequency response of the Probe Microphone depends on the length and
diameter of the probe fitted. Because of sound-pressure losses in thin tubes,
a probe will, depending on its length, reduce the level of sound, particularly
at higher frequencies. Fig. 2.5 shows an example of the frequency response of
a Probe Microphone for various lengths of stainless-steel probes. The values
are also given in Table 2.1. For lengths other than those given in Table 2.1,
interpolate between the given lengths. For the individual responses of a
specific probe microphone, use the data from the Calibration Chart for that
probe microphone. Fig. 2.5 Frequency response for various lengths of
stainless-steel probes
| Freq. (H z) | 20 mm | 40 mm | 80 mm | 160mm | Tolerance (± dB) |
|---|---|---|---|---|---|
| 0.25 | -6.25 | -6.25 | -6.25 | -6.25 | N/A |
| 0.31 | -1.34 | -1.34 | -1.34 | -1.34 | N/A |
| 0.4 | 2.56 | 2.56 | 2.56 | 2.56 | N/A |
| 0.5 | 3.30 | 3.30 | 3.30 | 3.30 | N/A |
| 0.63 | 3.04 | 3.04 | 3.04 | 3.04 | N/A |
| 0.8 | 2.28 | 2.28 | 2.28 | 2.28 | N/A |
| 1 | 1.56 | 1.56 | 1.56 | 1.56 | N/A |
| 1.25 | 1.52 | 1.52 | 1.52 | 1.52 | N/A |
| 1.6 | 1.43 | 1.43 | 1.43 | 1.43 | N/A |
| 2 | 1.38 | 1.38 | 1.38 | 1.38 | 1.5 |
| 2.5 | 1.34 | 1.34 | 1.34 | 1.34 | 1.5 |
| 3.15 | 1.29 | 1.29 | 1.29 | 1.29 | 1.5 |
| 4 | 1.25 | 1.25 | 1.25 | 1.25 | 1.5 |
| 5 | 1.16 | 1.16 | 1.16 | 1.16 | 1.5 |
| 6.3 | 0.98 | 0.98 | 0.98 | 0.98 | 1.5 |
| 8 | 0.89 | 0.89 | 0.89 | 0.89 | 1.5 |
| 10 | 0.80 | 0.80 | 0.80 | 0.80 | 1.5 |
| 12.5 | 0.71 | 0.71 | 0.71 | 0.71 | 1.5 |
| 16 | 0.67 | 0.67 | 0.67 | 0.67 | 1.5 |
| 20 | 0.63 | 0.63 | 0.63 | 0.63 | 0.5 |
| 25 | 0.58 | 0.58 | 0.58 | 0.58 | 0.5 |
| 31.5 | 0.49 | 0.49 | 0.49 | 0.49 | 0.5 |
| 40 | 0.40 | 0.40 | 0.40 | 0.40 | 0.5 |
| 50 | 0.31 | 0.31 | 0.31 | 0.31 | 0.5 |
| 63 | 0.22 | 0.22 | 0.22 | 0.22 | 0.5 |
| 80 | 0.18 | 0.18 | 0.18 | 0.18 | 0.5 |
| 100 | 0.13 | 0.13 | 0.13 | 0.13 | 0.5 |
| 125 | 0.09 | 0.09 | 0.09 | 0.09 | 0.5 |
| 160 | 0.04 | 0.04 | 0.04 | 0.04 | 0.5 |
| 200 | 0.04 | 0.04 | 0.04 | 0.04 | 0.5 |
| 250 | 0.00 | 0.00 | 0.00 | 0.00 | Ref. |
| 315 | -0.04 | -0.06 | -0.09 | -0.14 | 0.5 |
| 400 | -0.03 | -0.08 | -0.16 | -0.32 | 0.5 |
| 500 | -0.02 | -0.12 | -0.31 | -0.68 | 0.5 |
| 630 | -0.06 | -0.19 | -0.46 | -1.01 | 0.5 |
| 800 | -0.04 | -0.23 | -0.61 | -1.37 | 0.5 |
| 1000 | -0.08 | -0.31 | -0.77 | -1.69 | 0.5 |
| 1250 | -0.11 | -0.38 | -0.93 | -2.02 | 0.8 |
| 1600 | -0.14 | -0.46 | -1.09 | -2.34 | 0.8 |
| 2000 | -0.36 | -0.71 | -1.43 | -2.86 | 0.8 |
| 2500 | -0.65 | -1.08 | -1.94 | -3.65 | 1 |
| 3150 | -0.95 | -1.44 | -2.41 | -4.37 | 1 |
| 4000 | -1.20 | -1.76 | -2.88 | -5.12 | 1 |
| 5000 | -1.57 | -2.24 | -3.57 | -6.24 | 1.5 |
| 6300 | -2.01 | -2.77 | -4.29 | -7.32 | 1.5 |
| 8000 | -2.72 | -3.63 | -5.43 | -9.05 | 1.5 |
| 10000 | -3.93 | -4.98 | -8.01 | -11.27 | 1.5 |
| 12500 | -6.20 | -7.60 | -8.98 | -13.79 | 3 |
| 16000 | -6.88 | -8.48 | -11.70 | -18.13 | 3 |
| 20000 | -7.40 | -9.91 | -13.13 | -19.55 | 3 |
| 25000 | -10.41 | -14.40 | -15.16 | -21.49 | N/A |
Table 2.1 Frequency response of the Probe Microphone (referred to 250Hz) for
various probe lengths
2.4 Cutting Probes down to other Lengths
2.4.1 Stainless-steel Probes
A stainless-steel probe can be cut down to a required length using the pliers
and the file. Place the probe into the lower cut-out of the pliers’ jaws close
to where the tube is to be cut (see Fig- 2.6). Score a groove around the
circumference of the tube using the file and gently break off the tube. Fig.
2.6 Use the pliers and the file to cut the probe down to the required length.
2.4.2 Flexible Probes
Use a pair of scissors to cut a flexible (Teflon) probe down to a required
length. The nipple attached to a flexible probe can be removed or fitted as
follows. To remove it from the flexible probe, place the flexible probe in the
upper cut-out of the pliers’ jaws (see Fig 2.7) and unscrew the nipple by
turning it anticlockwise. To fit a flexible probe to the nipple, place the
flexible probe in the upper cut-out of the pliers’ jaws, and screw the nipple
onto the flexible probe by turning it clockwise Fig. 2.7 Use the pliers to
hold the flexible probe when removing or fitting the nipple
2.5 High-temperature Shielding
For measurements at very high temperatures, use only stainless-steel probes.
The temperature of the main body of the Probe Microphone should not exceed
70°C. To protect the body from heat conducted along the probe, mount the Heat
Sink GR0267 on the probe between the hot end of the probe and the main body of
the Probe Microphone as shown in Fig 2.8. Fig. 2.8 Showing the Heat Sink
GR0267 mounted on a stainless-steel probe
2.6 Cleaning
If the probe needs to be cleaned, dismantle it and use the needle shown in
Fig. 2.2.
With the probe removed from the microphone assembly, gently guide the needle
through the probe tube.
Calibration
The following explains how to calibrate both the level and frequency responses
of the Probe Microphone.
3.1 Level Calibration
To calibrate the Probe Microphone with a pistonphone such as the GRAS Type
42AA, first ensure that the pistonphone is fitted with a coupler for
calibrating ½″ microphones (in the case of the Type 42AA, this is standard).
The Probe Microphone comes with two ½″ calibration adapters, i.e.:
- GR0265 for stainless-steel probes
- GR0266 for flexible probes
Fit the appropriate adapter to the pistonphone’s coupler. Insert the tip of
the probe into the calibration adapter 2 (see Fig 3.1) and turn on the
pistonphone. For a Pistonphone Type 42AA, the
sound pressure at the probe tip will be 114 dB re. 20 µPa. Since the load-
volume correction for the probe is 0 dB, the only corrections necessary will
be for the barometric pressure. Fig. 3.1 Level calibration using Pistonphone
Type 42AA
3.2 Frequency-response Calibration
Frequency-response calibration can be performed either as a free-field
calibration using a suitable sound source and ¼″ microphone in an anechoic
chamber or as a pressure-response calibration using the Calibration Coupler
RA0236 included with the Probe Microphone.
3.2.1 Calibration Coupler RA0326
To use this coupler, an emitting microphone and a receiving microphone are
required. These are not part of the standard Probe Microphone.
The emitting microphone is inserted in one end of the coupler and the
receiving microphone in the other end (see Fig 3.2).
The probe tip is inserted in the appropriate hole in the side of the coupler.
There are two of these on opposite sides of the coupler, one for stainless-
steel probes and one for flexible probes. The hole not in use should be closed
off with an insert pin.
2 The probe tip must not make contact with any surface inside the calibration
adapter since this will block the entrance to the probe. Fig. 3.2 Set up for
pressure-frequency response calibration
Apply the test signal to the transmitting microphone and measure the responses
of both the receiving microphone Probe Microphone. The response of the Probe
Microphone can then be seen as relative to the response of the receiving
microphone. Since the sound pressure within the coupler will vary across the
diameter of the coupler, make sure that the tip of the probe is positioned at
the diametral centre of the coupler 3.
Service and Repair
Repairs should be carried out only by qualified personal. The Probe Microphone should not be dismantled with power on because of high-voltage circuits.
Specifications
Nominal sensitivity at 250Hz: 3mV/Pa (individually calibrated)
Dynamic range : 40dB to 166dB (re. 20µPa)
Frequency response (relative to tabular values) :
2Hz – 20Hz:……………………………… ±1.5dB
20Hz – 250Hz:……………………………… ±0.5dB
250Hz – 1kHz: …………………………………±0.5dB
1kHz – 2kHz:……………………………….. ±0.8dB
2kHz – 5kHz:………………………………………. ±1dB
5kHz – 10kHz: ……………………………….±1.5dB
10kHz – 20kHz:……………………………….. ±3dB
Electrical Output Impedance :…………. <50Ω
Power supply:
Type 40SA:
Single 120V (2.5mA) to 28V (0.7mA)
Dual ±60V (2.5mA) to ±14V (0.7mA)
Type 40SC:
2mA to 20mV (typically 4mA)
Temperature Range:
Operating: -25ºC to 70ºC
Probe temp. (with heat sink): max. 800ºC
Pressure-equalization time constant:
Internal to tip static pressure: typically 0.1s
Dimensions:
Length (housing): …………………………..83.8mm
Diameter: ……………………………………12.7mm
Weight:…………………………………………… 40g
Probe tube outside diameter: ………………1.25mm
Probe tube inside diameter:………… 1mm
Cable length: …………………………………3m
Cable diameter: ……………………..2.5mm
Accessories included :
Pistonphone adaptor for 1.25 mm:……………… GR0265
Pistonphone adaptor for 1.6 mm: ……………….GR0266
Heatsink and tool: ……………………….GR0267
Calibration coupler:……………………………….. RA0326
1.3mm pin for calibration coupler: ……………GR0263
1.6mm pin for calibration coupler:………………. GR0264
Pair of pliers: …………………………………………………YY0004
File: ………………………………………………..YY0005
Silicone grease: ………………………………..MI0016
Teflon tubing (L: 1m, Ø: 1.6mm): ………….EK0018
20mm Probe tube:……………………………….. GR0258
40mm Probe tube:………………………………… GR0259
80mm Probe tube: ……………………………GR0260
160mm Probe tube: …………………………….GR0261
Flexible Probe tube: …………………………..GR0401
Needle (for cleaning) ……………………………SK5546
Accessories available:
Pistonphone:…………. Type 42AA
Type 40SA:
Power modules: Types 12AA, 12AD or 12AK
Extension cable 3m: AA0008
Extension cable 10m: AA0009
Type 40SC:
CCP Supply: Type 12AL
Power modules: Types 12AA, 12AD or 12AK (use with CCP Input Adapter AG0002)
Manufactured to conform with:
**** CE marking directive: 93/68/EEC
WEEE directive: 2002/96/EC
RoHS directive: 2002/95/EC
GRAS Sound & Vibration continually strives to improve the quality of our
products for our customers;
therefore, the specifications and accessories are subject to change.
References
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