A free-field method to calibrate bone conduction transducers

Abstract

Bone conduction communication systems employ a variety of transducers with different physical and electroacoustic properties, and these transducers may be worn at various skull locations. Testing these systems thus requires a reliable means of transducer calibration that can be implemented across different devices, skull locations, and settings. Unfortunately, existing calibration standards do not meet these criteria. Audiometric bone conduction standards focus on only one device model and on limited skull locations. Furthermore, while mechanical couplers may be used for calibration, the general human validity of their results is suspect. To address the need for more flexible, human-centered calibration methods, the authors investigated a procedure for bone transducer calibration, analogous to free-field methods for calibrating air conduction headphones. Participants listened to1s third-octave noise bands (125-12,500 Hz) alternating between a bone transducer and a loudspeaker and adjusted the bone transducer to match the perceived loudness of the loudspeaker at each test frequency. Participants tested two transducer models and two skull locations. Intra- and inter-subject reliability was high, and the resulting data differed by transducer, by location, and from the mechanical coupler. The described procedure is flexible to transducer model and skull location, requires only basic equipment, and directly yields perceptual data.

The effect of vocal and demographic traits on speech intelligibility over bone conduction

“…Previous studies have shown that sound transmits differently at different parts of the human skull (Snidecor et al, 1959;Gripper et al, 2007;McBride et al, 2008a;McBride et al, 2008b;Osafo-Yeboah et al, 2009;Stanley and Walker, 2009;McBride et al, 2011;Pollard et al, 2013;Tran et al, 2013). Recordings made with bone microphones at different locations are likely to enhance and degrade different parts of the speech spectrum, and this can be relevant for speech intelligibility (Snidecor et al, 1959;Acker-Mills et al, 2005;Acker-Mills et al, 2006;McBride et al, 2011;Tran et al, 2013).…”

“…The playback level of speech was pre-calibrated to perceptually match the loudness of speech playing from a freestanding Infinity Studio Monitor TM 150 loudspeaker, measuring 70 dB SPL at the location of the head with the listener absent (see also Pollard et al, 2013). This level was chosen to make the listening task sufficiently challenging to avoid ceiling effects.…”

Effects of Bone Vibrator Position on Auditory Spatial Perception Tasks

“…To ensure that each listener heard the headphone and BC headset signals as loud as the loudspeaker signals, a loudness matching procedure was used to set the intensity of both devices (Pollard, Tran, & Letowski, 2013). For the headphones, three listeners who did not participate in the study performed this procedure before the experimental sessions began.…”

“…Sound intensity for the loudspeaker trials was preset to approximately 65 dB (A) as measured by a sound level meter located at ear level from the middle of the room where the listener’s head would be centered. To ensure that each listener heard the headphone and BC headset signals as loud as the loudspeaker signals, a loudness matching procedure was used to set the intensity of both devices (Pollard, Tran, & Letowski, 2013). For the headphones, three listeners who did not participate in the study performed this procedure before the experimental sessions began.…”

Design and Characterization of Acoustic Comparison Coupler as a Portable Calibration Media and Mechanical Sound Source for Calibration of Acoustic Measurement Instruments

“…This test is performed using the direct method inside the anechoic chamber and a room that has a background noise around 45 dB separately (Pollard, Tran, Letowski, & Resources, 2013). After that, the result is compared between the two rooms.…”