Intensity Probe - Larson Davis Model 2250
The intensity probe is a standard piece of equipment in many industrial
and academic acoustical facilities. Microphones alone measure
sound pressure fluctuations. But with two precision microphones
whose phase responses are well-matched, spaced a precisely known
distance apart, the particle velocity at the probe location can be
calculated from the difference (gradient) in pressure. Then, with
both pressure and velocity, sound intensity can be calculated from
their product.
Intensity, the rate of energy flow through a unit area, can tell us
about radiation properties of, say, a sheet metal panel that's
vibrating, a guitar's top plate, or the tweeter, woofer, and port of a
loudspeaker. Intensity can also be summed or integrated over area
to tell us the total sound energy emitted from an object per unit
time. This quantity, Sound Power, is the basis of many
regulations (especially in Europe and Japan) governing machinery and
noisy equipment such as lawn mowers, earth movers and tractor, and even
home vacuums.
Microflown Demo Kit from
This kit contains
three Microflown sensors, a new way to measure particle velocity.
The Microflown consists of two very small wires (on the level of MEMS
devices). By sending current through the wires, their resistance
can be measured. Of course, resistance depends on temperature,
and air flow across the wires cools them. In effect, the
Microflown is similar to a hot-wire anemometer. However, it is
novel due to its small size, and the use of two wires for differential
measurement of the temperature effects (more at www.microflown.com).
Once the particle velocity is known, it can be combined with pressure
measurements for a variety of applications which require sound
intensity and/or acoustic impedance. The
sensors included
in the kit all have small precision microphones co-located with the
Microflown sensor. The signals are routed through a conditioning
unit, and can then be fed to a dynamic signal analyzer or into LabVIEW
for recording and processing.
The apparatus is here through a partnership between Microflown and
Kettering University. Daniel Ludwigsen's winning proposal to
their University Contest will use the Microflown sensor to measure the
input impedance of brass instruments. The input impedance is an
important measure of the characteristics of an instrument, making a
significant contribution to proper intonation and playability.