Audio and ultrasonic frequency response measurements using regularized inverse filtering

Skyrocketing growth in digital voice communication technology prompts inclusion of ultrasonic microphones in modern smartphones. Inaudible ultrasonic waves provide safe and effective way for ambient intelligence in indoor environments. It is important to measure microphone response over combined audio and ultrasonic range. Conservative measurements based on frequency sweep present signal equalization challenge. Most ultrasonic tweeters produce narrowband ultrasonic tones, while audio speakers resonate below 1 kHz. We present a simple methodology based on emission of specially constructed noise stimulus that overcomes this limitation. The Ole Wolf 25 x16 mm 1 W 8 Ω speaker connected to B&K power amplifier delivered the stimulus. High frequency B&K type 2670 reference microphone was used to record responses at 192 kHz. Experiments were performed in two stages. First, the blue noise was emitted, its response was recorded, and new stimulus with flat spectra between 15–25 kHz was derived using regularized inverse filtering technique. Second, constructed stimulus was emitted and recorded by reference microphone and device under test. The frequency response was derived and found in good agreement with conventional stepped sweep measurements.