Thermal noise amplification at low frequencies in thermal flow sensors

In recent years thermal flow sensors have been created which can measure the direction of sound. These thermal flow sensors have cutoff frequencies up to several kilohertz. This paper discusses the amplification of the thermal noise at low frequencies in such thermal flow sensors. Two different noise models for thermal noise at low frequencies are discussed and compared with measurements. To measure the thermal noise a low frequency amplifier was designed and the noise sources from within the amplifier were identified. With this amplifier, noise measurements were taken in both an oven and a vacuum chamber. The models for noise amplification match the measurements taken. However, noise from external devices and 1/f noise did influence the results in such an extend that more measurements need to be taken to make the results credible.