Volume 6, Number 4, December, 1998 - Noise News International

(a)
M 4
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Fig. 3. Three dimensional sound intensity probe (H. Suzuki et al., lASl 1995).
(b)
crophones, the time delay between the first and the
second microphone enables the sound intensity to be
computed and the direction of the intensity vector to
be discerned. A notable design innovation.i' illustrated
in Fig. 3, is the implementation of a
three-dimensional intensity probe (Ono Sokki, Japan)
that consists of four microphones that are arranged
at the apexes of a regular tetrahedron. A
four- channel FFT analyser is used.
Gain Compensation WithVoltages to
ExtendInstrument Dynamic Range
Designers of acoustical measuring instruments usually
[lave to compromise between the uncertainty of
measurement and the dynamic range of the instrument.
As the dynamic range increases, the uncertainry
of the quantity measured increases. A
precision measuring amplifier' that is not limited
by the above restrictions has been designed and
tested; its characteristics are shown in Fig. 4. As the
input signal increases, the instrument inserts
attenuators in the input circuits. With each attenuator
(say 10 dB) insertion, the output from a logarithmic
converter decreases by a fixed value (for
example, 0.1 volts). However, a voltage of 0.1 volts
can he added to the output of the converter to compensate
for the loss due to attenuation. As more
attenuators are activated, more voltages are added
by a summing amplifier. The added voltages are de-
~'T-lin,--__o .....---9--1
I
I
I I
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Atlenuators
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..-"
v; ....-...
v: ...-.0'-. ....---
Fig. 4. Gain compensation with voltages to extend dynamic range (G. Wong,
lASA 1979).
rived from precision voltage
trimmers such that the voltages
exactly compensate for the uncertainty
(smaller than 0.001 dB)
of each of the attenuators. The
prototype instrument has a
l20-dB dynamic range with an
uncertainty of less than 0.1 dB.
Miniature Noise
Measuring Device
A relatively small noise dose
badge is shown in Fig. 5, (Cirrus Research, United
Kingdom). It has a diameter of 47 mm and measures
the sound exposure of the wearer over the
A-weighted sound level range from 80 to l30-dB.
The microphone frequency range is from 31 Hz to 8
kHz. After the measuring session, the dose badge information
is transmitted via an infrared link to a hand
held reader; and the data can then be printed or stored
in a computer for analysis.
Miniature SoundLevelMeters
Designers of miniature instruments have two challenges:
1) miniaturization and 2) satisfactionof the requirements
of international sound level meter
standards. These challenges were met twenty years
ago with analog tecbnology'' Recently, a wide dynamic
range miniature sound level meter has been developed
by Etymotic Research in the United States,
and is shown in Fig. 6. The device, based on digital
technology, has a dynamic range (with A-weighting)
from 30 to 140 dB with manual and auto range controls.
Extensive type testing has shown that the device
can satisfy the requirements of international standards
over the required operating temperature range. The
shirt pocket miniature sound level meter has provision
for an RS232 data link for computer data logging.
NoiseIsolationCapfor Detection of the
Presence of Electromagnetic (EM) Field
The presence of an electromagnetic
field produced by power
transformers, cell phones, radio
phones, etc., may affect acousti-
V s
cal measuring instruments such
as sound level meters, personal
sound exposure meters and other
sensitive accessories. International
standards are being drafted
to enforce electromagnetic compatibility
requirements and test
procedures for sound level meters.
A simple test for the effects
of EM is to install a noise isolation
cap made of non-metallic
1998 December NoiselNews International 211