Transducers Trends - Part 3 of 3 by Dave Bogema

There were several papers dealing with the relatively new Microflown sensor at SAE. The Microflown sensor measures particle velocity directly in a sound field, and is sometimes combined in the same unit with a microphone to measure sound pressure. Three applications were presented at SAE, including one to measure acoustic properties of materials, and another in which the Microflown sensors were used in an acoustic holography array.

Because the sensors measure particle velocity directly, and sound pressure can be measured simultaneously at the same point, the sound field can be much more easily characterized and measured than with traditional microphone-only techniques. The Microflown sensor can be used in several different ways. In characterizing sound fields, one traditional approach is to use a microphone pair to measure sound intensity. Drawbacks to this method include a limited frequency range and a large probe size. The Microflown can be used to measure sound intensity because it measures sound pressure and particle velocity simultaneously (sound intensity is the product of sound pressure and particle velocity). With the Microflown, a larger frequency range can be measured at one time, and the probe size is very small compared to traditional sound intensity probes.

Another interesting property of the Microflown is that the particle velocity sensor indicates the direction of sound propagation and this can be listened to in real time with a pair of headphones. This is a very handy tool to use when locating the source of a noise, since the Microflown probe is extremely directional, whereas a traditional microphone is omni-directional, responding only to sound pressure. Also, particle velocity measured with a traditional sound intensity probe is calculated mathematically, and it is not possible to actually listen to the directionality of the sound in real time with this type of probe.

The Microflown has also been employed in non-contact vibration measurements. When the surface of a structure vibrates, it moves the air particles near it. The Microflown, positioned close to the surface of an object, measures the particle velocity of the air. When very close to an object, this can be assumed to be very similar to the velocity of the surface. Thus, the surface velocity of the object can be measured with the Microflown, without adding mass or damping like traditional contact-sensors (accelerometers, ect.) would. In this case, the Microflown represents an alternative to the laser vibrometer.

At MTS, we have had the opportunity to evaluate some of the microflown sensors. My first impression is that there are a lot of possibilities for the application of these sensors in making noise and vibration measurements faster and easier. In the future, we will probably be seeing more and more applications of the Microflown as various applications are explored and developed.