Transducers Part 3

Paper Sessions

Co-chairs:
Alexander Voishvillo, JBL/Harman Professional Solutions - Northridge, CA, USA
Finn T. Agerkvist, Technical University of Denmark - Kgs. Lyngby, Denmark

P12-1 A Stepped Acoustic Transmission Line Model of Interference Tubes for Microphones—Francesco Bigoni, Aalborg University - Copenhagen, Denmark; Finn T. Agerkvist, Technical University of Denmark - Kgs. Lyngby, Denmark; Eddy Bøgh Brixen, EBB-consult - Smørum, Denmark; DPA Microphones - Allerød, Denmark
This paper presents an extension of the standing-wave model of interference tubes for microphones by Ono et al. The original model accounts for three acoustic parameters: tube length, tube radius, and constant acoustic conductance per unit length. Our extension allows a varying conductance per unit length along the side wall. The assumptions behind the extended model and its ability to predict the frequency response of interference tubes are validated through simulations and by fitting the model parameters to frequency response measurements of a tube with varying conductance per unit length, using two different mountings. Results suggest that a tube with varying conductance per unit length is most effective at attenuating the off-axis sound if the conductance per unit length is decreased towards the tail end of the tube. 

P12-2 Improving Audio Performance of Microphones Using a Novel Approach to Generating 48 Volt Phantom Powering—Joost Kist, Phantom Sound B.V. - Amsterdam, Netherlands; TritonAudio. - Alkmaar, Netherlands.; Dan Foley, Audio Precision - Beaverton, OR, USA
The introduction of the 48-volt phantom powering circuit in 1966 led to IEC 61938:1996. A key aspect of this powering circuit are the 6.81 k? precision resistors that are in parallel to the emitter-follower of the microphone preamplifier. These resistors act as a load on the emitter-follower that causes added distortion. A new approach is presented whereby, in series of these 6K8 resistors, an electronic circuit is placed that acts as a high input-impedance current source, which does not load the emitter-follower. By making this change, THD is decreased by 10 dB while also slightly improving the gain. Measurement results are presented comparing audio performance of a conventional 48-volt phantom power circuit and this new circuit along with circuit details. 

P12-3 Challenges and Best Practices for Microphone End-of-Line Testing—Gregor Schmidle, NTi Audio AG - Schaan, Liechtenstein; Mark Beach, Beach Dynamics - Cincinnati, OH, USA; Brian MacMillan, NTi Audio Inc. - Tigard, OR, USA
Due to the increasing use of microphones in many applications such as automotive or artificial intelligence, the demand for fast and reliable microphone test processes is growing. This paper covers various aspects of the design of an end-of-line microphone test system. A prevailing challenge is to properly control the sound source, as loudspeakers have a tendency to vary their performance due to many influences. The acoustic environment for the test must provide reproducible conditions and is ideally anechoic. Noise from outside must be damped across the measurement bandwidth, so that it doesn’t affect the results. Different testing requirements for various types of microphones are shown. Different methods for defining limit criteria are discussed. 

P12-4 Shotgun Microphone with High Directivity by Extra-Long Acoustic Tube and Digital Noise Reduction—Yo Sasaki, NHK Science & Technology Research Laboratories - Tokyo, Japan; Kazuho Ono, NHK Science & Technology Research Laboratories - Setagaya-ku, Tokyo, Japan
A prototype of a shotgun microphone having higher directivity than a conventional microphone has been developed to capture target sounds clearly. The shotgun microphone has a structure in which an acoustic tube is attached on a directional microphone capsule. The directivity pattern is formed by adjusting an acoustic resistor attached to orifices along the length of the tube. The prototype we developed has a 1-m long acoustic tube designed on the basis of a numerical calculation. It also includes additional microphone capsules and a digital signal processing circuit that reduce undesired acoustical signals arriving from directions other than the front. Measurements show that the developed shotgun microphone prototype achieves even higher directivity than conventional shotgun microphones. 

P12-5 High Power Density for Class-D Audio Power Amplifiers Equipped with eGaNFETs—Andreas Stybe Petersen, Technical University of Denmark - Kgs. Lyngby, Denmark; Niels Elkjær Iversen, Technical University of Denmark - Kogens Lyngby, Denmark; Michael A. E. Andersen, Technical University of Denmark - Kgs. Lyngby, Denmark
This paper presents how to optimize the power density of Class-D audio power amplifiers. The main task is to ensure that the ratio between the ripple current and the continuous output current is larger than one. When this is satisfied soft switching conditions are facilitated. Optimizing the amplifier power stage for soft switching while playing audio result in a more evenly distribution of the power dissipation between switching devices and filter inductors. Measured results on 150 Wrms test amplifiers equipped with eGaNFETs shows that the power density can reach 14.3 W/cm3, with THD+N levels as low as 0.03%. Moreover safe operating temperatures below 100°C when playing music with peaking powers of 200 W is achieved. Compared to state-of-the art, the power density of the amplifier module is improved with a factor 2–3. 

P12-6 Estimation of the Headphone "Openness" Based on Measurements of Pressure Division Ratio, Headphone Selection Criterion, and Acoustic Impedance—Roman Schlieper, Leibniz Universität Hannover - Hannover, Germany; Song Li, Leibniz Universität Hannover - Hannover, Germany; Stephan Preihs, Leibniz Universität Hannover - Hannover, Germany; Jürgen Peissig, Leibniz Universität Hannover - Hannover, Germany
The study presented here investigates and compares three different methods regarding their suitability for determining the relative openness of circumaural and supraaural headphone types, namely: (1) the Pressure Division Ratio (PDR), (2) the Headphone Selection Criterion (HPC), and the Acoustic Impedance Curve (AIC). Measurements were conducted by using a custom built acoustic impedance measuring tube and an artificial dummy head (KEMAR 45BC-12). The results show that the openness of headphones can be determined best by their low-frequency acoustic impedance curves. Estimations using PDR and HPC show large measurement variations especially in the low frequency range where the perceptual occlusion effect dominates. We introduce the Occlusion Index (OI) that characterizes well the acoustical “openness” and possibly can be used as a reliable indicator for the perceived headphone occlusion.