Octave Analysis and SLM Analysis_1375381
Octave Analysis and SLM Analysis_1375381
Octave Analysis and SLM Analysis_1375381
Octave Analysis and SLM Analysis_1375381
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Octave Analysis and SLM Analysis

Overview

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. 

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