Measurement Techniques

Near Field Measurement Technique

Any sound field is described by two complementary acoustic properties, the scalar value 'sound pressure' and the vector value 'particle velocity'. In the acoustic near field, acoustic particle velocity is the dominant acoustic property. The Microflown is the only sensor which can physically measure the acoustic particle velocity directly. Two principles can be described on why measuring in the near field with particle velocity is beneficial. The sound pressure level and particle velocity level are of similar magnitude in the free field. If the sound wave reflects off a rigid surface, the sound pressure doubles and the particle velocity reduces to zero. Therefore at the surface the particle velocity component of the background noise and reflections are almost immeasurable. Secondly, close to a vibrating (sound emitting) surface, the first layer of air is incompressible and moves with the vibrating surface. The sound pressure level is reduced as compared to the particle velocity level. Therefore close to the surface the sound pressure of the emitted sound field is almost immeasurable. In addition to these specific near field principles, there is a difference in directivity between sound pressure and particle velocity. The polar pattern of a sound pressure measuring microphone is omni-directional. The Microflown, measuring particle velocity, is directional with a figure of eight polar pattern.  This all results in a 40dB less susceptibility of particle velocity compared sound pressure. The above described principles lead to direct benefits in the application field.  There is no need to create anechoic conditions and thus measurements can be undertaken in real conditions, for example in a manufacturing environment or in closed cavities such as a car interior. Directly linked to the measurement results there are additional advantages from using a particle velocity based method in comparison to sound pressure based techniques. Such as more broad banded results, a higher spatial resolution, a higher dynamic range, more accurate measurements of the sound field and significantly shorter measurement time.