Typical instrumentation used to measure NVH include
microphones, accelerometers and force gauges, or load cells. Many NVH facilities
will have semi-anechoic chambers, and rolling road dynamometers. Typically
signals are recorded direct to hard disk via an Analog-to-digital converter. In
the past magnetic or DAT tape recorders were used. The integrity of the signal
chain is very important, typically each of the instruments used are fully
calibrated in a lab once per year, and any given setup is calibrated as a whole
once per day.
Analysis of sound and acoustics plays a role in such
engineering tasks as product design, production test, machine performance, and
process control. For instance, product design can require modification of sound
level or noise for compliance with standards from ANSI, IEC, and ISO. The work
might also involve design fine-tuning to meet market expectations. Here,
examples include tweaking an automobile door latching mechanism to impress a
consumer with a satisfying click or modifying an exhaust manifold to change the
tone of an engine's rumble. Aircraft designers are also using acoustic
instrumentation to reduce the noise generated on takeoff and landing.
Acoustical
measurements and instrumentation range from a handheld sound level meter to a
1000-microphone phased array. Most of the acoustical measurement and
instrumentation systems can be broken down into three components:1) Sensors
2) Data Acquisition
3) Analysis
Ø Sensors
The most common sensor used for acoustic measurement is
the microphone. Measurement-grade microphones are different than typical
recording-studio microphones because they can provide a detailed calibration
for their response and sensitivity. Other sensors include hydrophones for
measuring sound in water or accelerometers for measuring vibrations causing
sound. The three main groups of microphones are pressure, free-field, and random-incidence,
each with their own correction factors for different applications. G.R.A.S.
Sound & Vibration has a selection guide detailing the difference between
microphones. Other well known microphone suppliers include PCB Piezotronics and
Brüel & Kjær.
Ø Data Acquisition
Data acquisition hardware for acoustic measurements
typically utilizes 24-bit analog-to-digital converters (ADCs), anti-aliasing
filters, and other signal conditioning. This signal conditioning may include
amplification, filtering, sensor excitation, and input configuration. Another
consideration is the frequency range of the instrumentation. It should be large
enough to cover the frequency range of signal interest, taking into account the
range of the sensor. To prevent aliasing, many devices come with antialiasing
filters, which cut the maximum frequency range of the device to a little less
than one-half the maximum sampling rate, as prescribed by the Nyquist sampling
theorem. Dynamic range is a common way to compare performance from one instrument
to another. Dynamic range is a measure of how small you can measure a signal
relative to the maximum input signal the device can measure. Expressed in
decibels, the dynamic range is 20 log (Vmax/Vmin). For example, a device with
an input range of ±10 V and a dynamic range of 110 dB will be able to measure a
signal as small as 10 µV. Thus, the input range and the specified dynamic range
are important for determining the needs of your instrumentation system. Some
well known vendors include Prosig, OROS, Brüel & Kjær, National
Instruments, and LMS.
