GRAS 50GI-R User manual
www.gras.dk
LI0084 – Revision 6 June 2014
Instruction Manual
50GI-R Sound Intensity Probe
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LI0084 – Revision 6 June 2014
Revision History
Revision Date Description
1 13 February 2013 First publication
2 19 June 2013 Revision - ordering information for spacers corrected
3 6 June 2014 New part numbers for spacers
Copyright Notice
© 2014 G.R.A.S. Sound & Vibration A/S
http://www.gras.dk
Any technical documentation that is made available by G.R.A.S. is the copyrighted work of
G.R.A.S. and is owned by G.R.A.S.
The content in this document is subject to change without notice. G.R.A.S. 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.
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LI0084 – Revision 6 June 2014
Contents
Introduction and Description ............................................................ 4
Main Components...................................................................................... 5
Handling and Assembling the Probe .................................................. 8
The Microphones ....................................................................................... 8
Probe Design............................................................................................. 8
Physical Strength ....................................................................................... 8
Assembling the Probe ................................................................................ 9
Calibration ................................................................................. 12
Checking the Sensivity and P-I Index using a Pistonphone............................. 12
Checking the Probe’s P-I Index using an Intensity Calibrator........................... 13
Ordering Information.................................................................... 14
Specifications and Warranty .......................................................... 15
Calibration............................................................................................... 15
Warranty................................................................................................. 15
Service and Repairs.................................................................................. 15
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LI0084 – Revision 6 June 2014
Introduction and Description
The G.R.A.S 50GI-R Sound-intensity Probe is a two-microphone sound intensity probe. It has a
pair of G.R.A.S. 40GK phase-matched ½" microphones, two G.R.A.S. 26CB ¼" phase-matched
microphone preamplifiers, solid spacers, remote control handle and connection cable. The probe
is adjustable, durable and fully complies with the following requirements:
• IEC 61043, Electroacoustics - Instruments for the Measurement of Sound Intensity –
Measurements with Pairs of Pressure Sensing Microphones, 1993 for Class 1 Sound-intensity
probes.
Fig. 1. The 50GI–R Sound Intensity Probe
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LI0084 – Revision 6 June 2014
Main Components
The main components of the 50GI-R Sound-intensity Probe are:
• 40GK Sound Intensity Microphone Pair
• Two 26CB Microphone Preamplifiers
• Four solid spacers of various lengths and a spacer cup (see Fig. 6)
• Remote-control handle with (where applicable) up to two push buttons for controlling
measurements
The 50GI-R is delivered in a carrying case similar to the one shown in Fig. 2, complete with
microphones, preamplifiers, standard accessories and a remote-control handle.
Fig. 2. The type of carrying case delivered with the 50GI-R
Microphones
The microphones are high sensitivity, free-field ½" condenser microphones with a uniquely-
designed pressure equalization system that ensures extremely well defined phase characteristics.
The microphones and preamplifiers are mounted on the end of the telescopic arm of the Remote-
control handle. To cover the full frequency range from 30Hz to 10kHz, the 50GI–R is delivered
with four solid interchangeable spacers for spacing the microphones at 12 mm, 25 mm, 50 mm
and 100 mm.
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LI0084 – Revision 6 June 2014
Fig. 3. Pairs of phase-matched ½"microphone cartridges, with and without spacer.
Preamplifiers
The small ¼" diameter and 40mm long microphone preamplifiers are housed in robust, stain-
less steel casings which enable novel probe designs that reduce disturbances to the sound field
otherwise brought about by the effects of shadows and diffraction. Symmetry of design enables
reliable calibrations as described in the proposed standard (ISO/DIS 9614-2) for sound power
measurements using sound-intensity measurements.
26CB ¼" preamplifier
Set
2 x Cable AA0084
Microdot
male
Microdot
male
Fig. 4. 26CB ¼" Preamplifier set supplied with 50GI-R and the 2 microdot cables.
Cables
Serial to USB
Adapter
RS232 BNC BNC 12-pin LEMO
female
12-pin LEMO
male
12-pin LEMO
male
KE0132
AC0024
AA0021
Fig. 5. Cables for control and signal connection to analyzer
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LI0084 – Revision 6 June 2014
Spacers
To cover the wide frequency range (30 Hz - 10 kHz), Type 50GI-R Sound Intensity Probe is deliv-
ered with four solid spacers of various lengths and a spacer cup Fig. 6.
RA0263
100mm spacer
RA0264
50mm spacer
RA0265
25mm spacer
RA0266
12mm spacer
GR0040
Spacer cup
Fig. 6. The various spacers supplied with the 50GI-R.
To select the most appropriate spacer for the actual measurements, refer to Fig. 7.
With ¼" Mics. Type 40BI and 6mm spacer (GU0020)
100 1k 10k
10 100k
12mm (GU0005)
25mm (GU0004)
50mm (GU0003)
100mm (GU0002)
d
Frequency Hz
12 mm (RA0266)
25 mm (RA0265)
50 mm (RA0264)
100 mm (RA0263)
Fig. 7. Frequency ranges covered by the various spacer lengths.
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LI0084 – Revision 6 June 2014
Handling and Assembling the Probe
The Microphones
The 40GK Microphones are a pair of special free-field microphones with extremely well-controlled
phase characteristics. They are delivered as a matched pair each with individual calibration data
as well as data on differences between their phase responses.
These microphones have a unique pressure equalisation system that ensures a well defined
lower-limiting frequency and an extremely low sensitivity to sound pressures at the pressure
equalisation channels. Therefore, they can be calibrated in single-port phase calibrators such as
the G.R.A.S. 51AB Intensity Calibrator.
Probe Design
The design of the Intensity Probe minimizes acoustic reflections and the influence of diffraction.
This has been achieved by removing any reflective components from the sound path at 0° inci-
dence. Since sound waves at 0° incidence are the main contributors to the total sound intensity
level, it is important that disturbances in this direction be minimized.
The thin preamplifier cables (diameter of 2.5 mm) will have no influence on the sound field since
they constitute a highly irregular surface with negligible reflections. The effects of acoustic diffrac-
tion and reflections from the physical parts of the Intensity Probe are below 0.15 dB.
The microphones (40GK) are supplied as sets that include spacers and three preamplifier adapters
(two right-angled and one straight).
The distances between microphones and preamplifiers have been kept to a minimum in order to
avoid problems with stray capacitance and sensitivity to vibration. While amplitude characteristics
are little influenced by vibrations in the conductors carrying the raw signals from the microphones,
the phase characteristics of an Intensity Probe can be critically affected by even very small vibra-
tions. Therefore, the ¼" preamplifiers are mounted in rigid contact with the ½" microphones via
short adapters (right-angled and/or straight). This also eliminates problems with non-matching
capacitances between microphones and preamplifiers that could give rise to phase problems.
Physical Strength
From a physical point of view, an Intensity Probe is robust and easy to assemble and dismantle.
Typically, two points in an Intensity Probe are critical for its physical strength. These points are the
threads on the microphones and preamplifiers as well as the threads on the microphones’ protec-
tion grids. The connections between microphones and preamplifiers are very delicate. Therefore,
the preamplifier threads of the 50GI-R are supported by stainless steel ½" to ¼" adapters.
There is also a protective guard within the ¼" housing of each preamplifier. Furthermore, the
microphones’ protection grids are made of stainless steel that improve their ability to withstand
rough physical treatment – a buckled or damaged protection grid will almost invariably damage a
microphone’s diaphragm beyond repair.
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LI0084 – Revision 6 June 2014
Assembling the Probe
The Intensity Probe can be assembled in either a straight setup or a symmetrical setup:
• The straight setup is for intensity measurements close to surfaces and general source location
measurements.
• The symmetrical configuration, is ideal for sound power measurements, for example according
to the international standard ISO 9614-2 “Acoustics - Determination of sound power levels of
noise sources using sound intensity” where a rotation test is required.
Fig. 8. Straight setup using the 25-mm spacer.
Fig. 9. Symmetrical setup using the 12-mm spacer.
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RA0003
Straight Adapter
RA0001
Right-angled
Adapter
Spacer Cup
GR0040
Spacer
RA0266 40GK matched
pair of micro-
phones
26CB ¼” CCP
Preamplifier
26CB ¼" CCP
Preamplifier
RA0001 Right-
angled Adapter
26CB ¼" CCP
Preamplifier
Symmetrical setup
Straight setup
Fig. 10. Exploded view of probe assembly for both straight and symmetrical setup.
Assemble as follows:
1. Mount one microphones onto one RA0001 Right-angled Adapter and the other micro-
phone onto the other Right-angled Adapter RA0001 (symmetrical configuration) or onto the
RA0003 Straight Adapter (straight configuration).
2. Unscrew the black protection caps from the two CCP Preamplifiers.
3. Mount the microphone-adapter assemblies onto the preamplifiers.
4. Screw the GR0040 Spacer Cup onto the protection grid of one of the microphones.
5. Select the most appropriate spacer for the frequency range you wish to measure. Refer to
“Spacers” on page 7 to select the spacer needed for the actual application.
6. Screw the spacer onto the protection grid of the other microphone.
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