Abstract
Microphones have been extensively studied for many decades and their related theories are well-established. However, the physical presence of the sensor itself limits its practicality in many sound field control applications. Laser Doppler vibrometers (LDVs) are commonly used for the remote measurement of surface vibration that are related to the sound field without the introduction of any such physical intervention. This paper investigates the performance and challenges of using a piece of retro-reflective film directly as an acoustic membrane pick-up with an LDV to sense its vibration to form a remote acoustic sensing apparatus. Due to the special properties of the retro-reflective material, the LDV beam can be projected to the target over a wide range of incident angles. Thus, the location of the LDV relative to the pick-up is not severely restricted. This is favourable in many acoustic sensing and control applications. Theoretical analysis and systematic experiments were conducted on the membrane to characterise its performance. One design has been selected for sensing sound pressure level above 20 dB and within the 200 Hz to 4 kHz frequency range. Two example applications—remote speech signal sensing/recording and an active noise control headrest—are presented to demonstrate the benefits of such a remote acoustic sensing apparatus with the retro-reflective material. Particularly, a significant 22.4 dB noise reduction ranging from 300 Hz to 6 kHz has been achieved using the demonstrated active control system. These results demonstrate the potential for such a solution with several key advantages in many applications over traditional microphones, primarily due to its minimal invasiveness.
Highlights
Published: 3 June 2021The performance of some sound field control applications, such as sound field reproduction and active noise control (ANC), is limited by the physical presence of traditional microphones [1,2]
Conventional microphones use capacitance to represent the displacement of the microphone diaphragm, whereas the remote acoustic sensing apparatus uses the Laser Doppler vibrometers (LDVs) to measure the membrane vibration displacement directly
This paper reports a remote acoustic sensing apparatus that consists of a retroreflective membrane and an LDV at a remote location
Summary
The performance of some sound field control applications, such as sound field reproduction and active noise control (ANC), is limited by the physical presence of traditional microphones [1,2]. In ANC headrest systems, for example, the aim is to reduce the sound at a user’s ears with error microphones installed therein This is not desirable in practice due to their intrusiveness to the user. As the first proposal of such a system [5], it has been one of the main factors limiting the performance To solve this problem, various virtual sensing algorithms have been proposed to use error microphones placed remote from the user to estimate the sound pressure level (SPL) at the user’s ears [6]. Even though the dynamic behaviour of membranes has received significant attentions, this has not yet included retro-reflective material membranes, and the related measured signal quality of LDVs when using them These characteristics are crucial and must be determined before such pick-ups being used in acoustic sensing and control applications.
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