ABSTRACT
Ubiquitous remote monitoring will be crucial in the medical and healthcare fi eld in the future. Since a mobile phone has the basic functions for providing ubiquitous service, we investigated its use as a remote terminal. No other devices are required; simply replacing the mobile phone’s audio-microphone with a low-frequency microphone provides the functions required for the terminal. This chapter describes how we designed a low-frequency microphone as the key device, how it is used to detect bio-signals and how it is applied to ubiquitous medical and healthcare monitoring. Microphones are widely used in everyday life, especially in mobile phones. A
microphone is an acoustic sensor as well as a pressure sensor, with a very high gain of about 10 mV/Pa and an SN ratio of about 68 dB in the frequency range of 20 Hz to 10 kHz. It is very sensitive and accurate in the audio-frequency range; few pressure sensors have such high gain and accuracy. We considered these superior characteristics as a pressure sensor and expanded the low frequency range from 20 Hz to 0.1 Hz so that the microphone worked in the frequency range of 0.1 Hz to 10 kHz. We applied it to a security sensor to monitor the symptoms of fi re, earthquake and break-ins [1]. The low-frequency microphone was easily created just by closing the back chamber and reinforcing the pressure-sensitive fi lm, but it was too large for installation in a mobile phone. When the microphone was placed in a thin air cushion under a bed mattress, it was able to detect bio-signals from the subject’s heartbeat, respiration, body movement and snoring. The micro-vibration generated from these vital functions acted on the pressure inside the air cushion even through the thick bed mattress. We proposed a pressure-based pneumatic bed sensing system using the large microphone [2]. Furthermore, as these bio-signals, especially those pertaining to changes in the frequency of heartbeat and body movement, are closely related to sleep depth, we presented frequency-based sleep depth indices for estimating sleep stages [3]–[5]. The bed sensing system, which is a candidate ubiquitous health monitoring system with a function for sleep monitoring, was network-connected to 45 subjects for 18 months to examine its validity [6]. Unfortunately, the system was not practical due to the high cost of the bed sensing terminal device and specially designed network, and the lack of portability, thus severely limiting the market for the system. This paper describes a novel solution to the problem: a newly designed compact low-frequency microphone for installation on mobile phones.
