Microflown, a New Particle Velocity Sensor, The

Sound and Vibration, Feb 2005 by de Bree, Hans-Elias

The Microflown is the world's first dedicated MEMS-based acoustical particle velocity sensor. Microflowns offer an alternative to laser vibrometers, pressure transducers and mini accelerometers. The working principle and some recently developed applications will be discussed.

Working Principle. The Microflown is an acoustic sensor directly measuring particle velocity instead of sound pressure, which is usually measured by conventional microphones. This transducer is not a hot wire anemometer in the traditional sense. As shown in Figure 1, the micromachined sensor is based on two heated extremely thin wires, not one as in the classical anemometer.

A particle velocity signal in the perpendicular direction of the wires changes the temperature distribution instantaneously, because the upstream wire is cooled more by the airflow than the downstream wire. The resulting resistance difference provides a broadband linear signal (0 Hz up to at least 20 kHz) with a figure of eight directionality that is proportional to the particle velocity up to sound levels of 135 dB. The lower (noise) level is in the order of -10 dB (i.e., 20 × 10^sup -9^ m/s) in a 1 Hz bandwidth at 1 kHz.

The sensor itself can withstand temperatures up to 500° C. Due to the operation principle (measurement of the temperature difference between the two wires), the sensitivity of the sensor is not affected by temperature variations.

At low frequencies, the sensitivity of the particle velocity transducer increases at a rate of 6 dB per octave. This behavior is believed to be related to the thermal boundary layer on the wires. Between 100 Hz and 1 kHz the frequency response is relatively flat. Between 1 kHz and 10 kHz there is a roll off of 6 dB per octave, caused by diffusion effects related to the distance between the two wires. Above 10 kHz the sensitivity decreases an additional 6 dB per octave because of the thermal heat capacity of the wires.

A flat frequency and phase response signal can be obtained by either using the option of the Microflown signal conditioner that comes with the probes, by inserting calibration values in the analyzer software or with a post-processing routine.1

The Microflown Probes. There are several product lines in terms of sensor configuration and packaging. In Figure 2, the PU probe is shown where the Microflown transducer is combined with a small electret condenser microphone. The velocity transducer is mounted on a small, solid cylinder, and the condenser microphone is mounted inside another, hollow cylinder. The geometry of this arrangement increases the sensitivity of the velocity transducer by about 10 dB.

A miniature series has been developed in addition to the 1/2 in. series. Both 1D as 3D sensors are available as shown in Figure 3. The 3D sensor on the left, called the USP (Ultimate Sound Probe) is capable of measuring broadband sound pressure (20 Hz-20 kHz) and the 3 orthogonal particle velocity components of a sound field in one spot. The PU-scanning probe at right is used mainly for close proximity measurements.

Calibration. The Microflown can be calibrated in several ways. The Microflown can be fixed to a shaker and its output compared to the vibration velocity or the Microflown can be placed close to a vibrating surface in the very near field and its output compared to the surface velocity. It also possible to utilize a sound pressure microphone as a reference. The Microflown can then be calibrated in a standing wave tube, in an anechoic room or with a nearfield calibrator.

Noncontact Measurement of Structural Vibrations. In the very nearfiold, defined by some theoretical constraints that are usually met, the measured acoustical particle velocity deviates from the structural velocity to be measured by a maximum of 1.5 dB.4 Therefore, the Microflown offers an alternative to a laser vibrometer or an accelerometer. The scanning probe is small, handheld and simple to use. Arrays of velocity probes are capable of measuring nonstationary sources.

Extended Sound Intensity. Compared to a traditional sound intensity probe (based on two pressure microphones) a Microflown-based sound intensity probe is small, easy to use and covers a wider frequency range without the use of spacers. 3D sound intensity can be measured with only 4 channels. The most important difference is that (in contrast to a traditional intensity probe) the measurement error of a Microflown-based sound intensity probe is not dependent on the pressure intensity index.5,6 Therefore, the Microflown-based intensity probe can be used in acoustic environments where traditional probes fail such as car interiors and aircraft cabins. Keyhole measurements become possible.

Rapid In Situ Reflection Coefficient Determination. With a PU probe the sound pressure and particle velocity can be measured very close to a damping material and therefore the acoustic surface impedance can bo determined. With this measurement technique the absorption coefficient of acoustic samples can be determined in situ (without the use of a Kundt's tube). Broadband measurements (100 Hz-20 kHz) at both normal and oblique angles of incidence can be made. The total setup is small (less than 60 cm) and lightweight (less than 1 kg). A calibration and measurement can be performed within 60 seconds.8