A particle velocity sensor to measure the sound from a structure in the presence of background noise

The performance (or quality) of a product is often checked by measuring the radiated sound (noise) from the vibrating structure. Often this test has to be done in an environment with background noise, which makes the measurement difficult. When using a (pressure) microphone the background noise can be such that it dominates the radiated sound from the vibrating structure. However, when using a particle velocity sensor, the Microflown [1,2], near the vibrating structure, the background noise has almost no influence (it is almost cancelled) and the sound from the structure is measured with a good S/N ratio. The experimental results are explained in terms of the different boundary conditions at the surface of the vibrating structure for the pressure and the particle velocity

A source path contribution analysis on tire noise using particle velocity sensors

Road tire noise is an important topic of research where acoustic particle velocity based testing techniques can be expected to bring new insights. Modal analysis can be carried out using non contact particle velocity sensors, and PU sound probes can be used to measure the radiated sound without a need to use anechoic testing conditions. A further breakdown of the overall sound pressure levels measured in to its various sources can be made by applying a source path contribution analysis, using the PU probes to measure velocity, sound intensity and, for determining the reciprocal transfer path, the sound pressure. The concept of using this type of transfer path analysis will be outlined and illustrated by a tire noise case

An intuitive handheld acoustic noise source finder

ABSTRACT - An apparatus has been developed to find acoustic sound sources in the near field of a radiating object operating in a noisy environment. It is based on two orthogonally placed particle velocity probes (two Microflowns[1], [2]). The complete signal processing is done in real time with battery powered analogue circuitry, resulting in a very small and handheld measurement device. One Microflown is used to display the sound level and to listen to the source whilst rejecting the background noise and another Microflown is used to create a stereo\ud indication in which direction the device must be moved to pinpoint the noise source

Optimisation of a two-wire thermal sensor for flow and sound measurements

The Microflown is an acoustic sensor measuring particle velocity instead of pressure, which is usually measured by conventional microphones. In this paper an analytical model is presented to describe the physical processes that govern the behaviour of the sensor and determine its sensitivity. The Microflown consists of two heaters that act simultaneously as sensors. Forced convection by an acoustic wave leads to a small perturbation of this temperature profile, resulting in a temperature difference between the two sensors. This temperature difference, to which the sensitivity is proportional, is calculated with perturbation theory. Consequently the frequency dependent behaviour of the sensitivity is analysed; it is found that there are two important corner frequencies, the first related to the time constant velocity of heat diffusion between the sensors, the second related to the heat capacity of the heaters. The developed model is verified by experiments. Previously a very good model has been given for the performance of the Microflown in a channel, i.e. with both heaters between fixed walls walls in the positive and negative z-direction. Here, a model is presented that describes the situation of the present used sensors: without walls under and above them. Model predictions are compared to experimental result

The microflown

This thesis deals with a novel acoustic sensor, the microflown. This micromachined sensor is capable of measuring the particle velocity directly. When both particle velocity and acoustic pressure variations are known an acoustic phenomena can be fully described. The microflown is a microphone which consists of two temperature sensors and a heater. The particle velocity alters the temperature distribution and thus the temperature of both the sensors. The temperature difference of both sensors is proportional with the particle velocity