G.R.A.S. Sound & Vibration
Sound-intensity Probe Type 50AI - Page 13
4.
Handling and Assembling the Probe
4.1
The Microphones
The Microphones Type 40AI (Fig. 1.3) 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 which 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. Intensity Calibrator Type 51AB.
4.2
Probe Design
The design of the Sound-intensity Probe minimises acoustic reflections and the influence of
diffraction. This has been achieved by removing any physical, reflective components from the
sound path at 0° incidence. 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 minimised.
The thin, 2 mm diameter preamplifier cables will have no influence on the sound field since they
constitute a highly irregular surface with negligible reflections. The effects of acoustic diffraction
and reflection from the physical parts of the Sound-intensity Probe are below 0.15 dB.
Microphones are supplied as pairs (Type 40AI) or as sets (Type 40AK) which 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 any stray capacitance and sensitivity to vibration. While amplitude
characteristics are little influenced, the phase characteristics of a Sound-intensity Probe can
be critically affected by even very small vibrations in the conductors carrying the raw signals
from the microphones. 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, which could give rise
to phase problems.
4.3
Physical Strength
From a physical point of view, a Sound-intensity Probe should be robust and easy to assemble
and dismantle. Typically, there are two points in a Sound-intensity Probe which can be identi-
fied as critical for physical strength and are the most likely to suffer damage and are the most
difficult to repair. These points are the threads on microphones and preamplifiers as well as on
the microphones’ protection grids. The connections between microphones and preamplifiers are
very delicate and carry both microphone signals and microphone polarisation voltages. There-
fore, the preamplifier threads of the Type 50AI are supported by stainless steel ½″ to ¼″ adapt-
ers. There is also a protective guard within the ¼″ housing of each preamplifier. In addition, the
microphones’ protection grids are made of stainless steel to withstand rough physical treatment
since a buckled or damaged protection grid will almost invariably damage a microphone’s dia-
phragm beyond repair.
4.4
Assembling the Probe
The Sound-intensity Probe can be assembled in either a straight configuration or a symmetri-
cal configuration The straight configuration (Fig. 4.1), is for intensity measurements close to
surfaces and general source location measurements. The symmetrical configuration (Fig. 4.2),
is ideal for sound power measurements, for example according to international standard ISO
9614-2 “Acoustics - Determination of sound power levels of noise sources using sound inten-
sity” where a rotation test is required.
The probe on the Type 50AI consists of the parts shown in Fig. 4.4. Assemble as follows.
