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Noise sources that are of interest to us at present are those stemming from vibration, electromagnetic and acoustic sources. The tools we have developed to capture noise data are based on a versatile laptop computer equipped with a DAQ card (6062E National Instruments). This card provides E Series high-performance to PCMCIA (PC Card) computers with up to 500 kSamples/s, 12-bit resolution, on 16 single-ended (8 differential) analog inputs. The DAQCard-6062E has analog and digital triggering, two 12-bit analog outputs, two 20-bit 24 MHz counter/timers, and eight digital I/O lines.

This card is controlled by an easy-to-use versatile software package written in Visual Basic. Inputs to the software include data sample rate, number of samples, gain, channel number and output file management capabilities. The data is analyzed using Origin 6.1 scientific graphing software.

Accelerometer

To measure noise from vibration sources we employ a Model 731A Seismic Accelerometer coupled with the Model P31 Power Unit/Amplifier from Wilcoxon Research. This is an ultra-low noise system with capability to measure to the sub-micro g level using a high sensitivity piezoelectric sensor. A piezoelectric sensing element creates a current when it is deformed so it is very sensitive to vibration.

Magnetometer

To measure noise from electromagnetic sources we use a Mag-03MC100 three axis magnetic field sensor coupled with the Mag-03PSU power supply unit from Bartington Instruments. The Mag-03MC100 is capable of measuring a magnetic field in a range of and provides three analog outputs of 0 to 10V. For our unit this gives a scale of .1 with the relationship between the magnetic field and the analog outputs being extremely linear. The Mag-03MC100 by default ships in a dc response mode where the analog outputs reflect the total magnetic field from each axis including the static background magnetic field. In addition, the Mag-03PSU provides a 4.5 kHz low pass filter to discard HF noise components and a 0.1 Hz dc isolation high pass filter. The series capacitor of the high pass filter may be short circuited to provide a dc response. When the high pass filter is include, the static component of the magnetic field is rejected and the alternating component of the magnetic field is output. This is what we are interested in so we normally run the Mag-03 with the high pass filter for ac response.

Microphone

A dynamic microphone would be preferable to an electret condenser microphone because a dynamic microphone is able to produce and output voltage without the requirement of an external power supply. The problem is that cheap dynamic microphones do not have a very high sensitivity and for the applications we are looking at, namely recording ambient room noise, a cheap dynamic microphone does not even reach the threshold at which it stops producing its own noise. There are dynamic microphones that we are looking into that are in the expensive-but-not-outrageously-expensive category. Ideally we’d like a microphone with a sensitivity range from less than 20 Hz to 20 kHz but microphones that have a very low frequency range are inherently expensive. The other important factor in choosing a microphone is whether to get a directional or omni-directional type. Obviously, if we are doing ambient noise test, omni-directional would be the preferred type.