Acoustics measurements are performed for a variety of
reasons, including: product design, production testing, machine
performance, and process control. Crystal Instruments’ Spider
series has capable acoustic measurement facilities including real-
time octave, 1/3 octave filters, and sound level meter functions.
Crystal Instruments provides an easy to use yet powerful toolbox for
acquiring and viewing acoustic signals. Digital octave band filters
and raw time data recording can be performed simultaneously for
a detailed investigation of noise problems.
The Spider series meets the requirements for measurements from
4 input channels going up to 512 channels!
Onboard IEPE (ICP®) transducer power capability allows for
direct connection to pre-polarized microphones when used with an
ICP microphone preamplifier. Traditional condenser microphones
are also easily accommodated by connecting the direct voltage
signal from the microphone power supply into an input channel.
White and pink noise signals can be produced using the waveform
generator. This feature is very useful when performing absorption
measurements using a speaker.
Real-time Octave Analysis
The acoustic data acquisition software option for Spider
hardware includes real-time octave filters, sound level meters,
and microphone calibration functions. These three operations
allow users to perform many acoustic measurement operations.
The octave analysis option applies a bank of real-time filters with
1/1, 1/3rd, 1/6th, or 1/12th octave resolution. The input time stream
is split into fractional frequency-band signals (octave bands) which
can be saved. Frequency weighting can be applied to the octave
bands to simulate human hearing, and time weighting can be
applied to adjust sensitivity to short duration events. The resulting
octave spectra can be saved periodically and displayed on a
waterfall plot to observe how the spectrum changes in time. The
RMS time history can also be saved as a time trace of a given
octave band.
The 1/1 and 1/3 octave analysis is implemented using a real-time
band-pass filtering with decimation technique. The data stream is
processed continuously, and fed into a bank of decimation filters.
Band-pass filters are then applied to the output of each stage
of the decimation filters. This provides extremely accurate filter
shapes that comply with worldwide acoustic standards: ANSI std.
S1.11:2004, Order 3 Type 1-D and IEC 61260-1995.
Acoustic Measurement: Sound Level Meter
The Sound Level Meter (SLM) is a related application in the
acoustic data acquisition software. This module is also referred to
as an Overall Level Meter. The SLM applies a frequency weighting
filter to the input signal and time weighting to the filter’s output.
Various acoustic measurements are then extracted from both the
input and output signals of this frequency weighting filter.
All of the features that you would expect from an acoustic
measurement device are present…and then some! A, B, C, and
linear weighting functions; fast, slow, impulse, and peak detectors;
and user selectable high and low-pass filtering. The tremendous
dynamic range that all Crystal Instruments products offer take the
worry out of setting voltage ranges precisely to avoid under-range
or overload conditions.