Advances in broadcasting have significantly increased the demand for sophisticated microphones and loudspeakers. At STRL, we are conducting research on flexible loudspeakers for multichannel audio and other applications, on narrow directivity microphones for sophisticated program production, and on compact and reliable high-performance silicon microphones.

Flexible loudspeakers

We are conducting research on lightweight flexible loudspeakers for applications such as the 22.2 multichannel sound system of Super Hi-Vision.

In FY2009, we analyzed the operating principles of loudspeakers made of electroactive elastomer (a polymer with rubber-like elasticity). The theoretical analysis of deformation of a thin elastomer membrane revealed that the change in planar directions is much greater than in the thickness direction. We prototyped a semi-cylindrical loudspeaker that works by making changes in the surface directions and a flat loudspeaker that works by making changes in the thickness direction and compared their characteristics. The results showed that the speakers have different reproduced sound pressure levels (Figure 1). We theoretically derived that the ratio of the fundamental frequency response to second harmonic distortion should be proportional to the ratio of audio signal amplitude to DC bias voltage, and we confirmed this result experimentally. We also designed and prototyped a driver and made significant improvements to the frequency response over 2 kHz.

Figure 1. Comparison of acoustic pressure from semi-cylindrical and flat speakers

Narrow-directivity microphones

We are conducting research on a narrow-directivity microphone with excellent rear response suppression for recording in noisy environments and for multichannel recording by combining an acoustic tube with a second-order acoustic gradient element. By FY2008, we had prototyped a 30-cm-long standard microphone and a 15-cm-long short model, and we were able to suppress the low-frequency rear response by 20 dB. We used these microphones in various programs and in recording tests for 22.2 multi-channel sound.

In FY2009, we prototyped a 55-cm-long model (Figure 2) with an elon-gated acoustic tube. This mi-crophone sup-presses the side response by up to 20 dB for frequencies below 1 kHz (standard model: about 10 dB) and has narrow directivity across almost all frequencies. We also analyzed the operating principles of the acoustic tube in order to estimate the directivity of the microphone very precisely.

Figure 2. Prototype microphone using long acoustic tube for rear-cancellation

Silicon microphones

We are developing an ultra-compact, high-performance silicon condenser microphone composed of a silicon diaphragm facing a back-plate. In FY2009, we began to develop the core technologies for a stored-charge (electret-condenser) silicon microphone.

Our silicon microphones have excellent durability, heat-tolerance, and good acoustic properties, but they require a bias voltage of 48 V between the diaphragm and the back plate, which limits their portability.

To solve this problem, we are developing a stored-charge silicon microphone that does not require a bias voltage. A potential difference between the diaphragm and the back-plate is created by accumulating a charge in a dielectric layer formed on the back plate with oxide and nitride films (Figure 3).

We had to develop a means of accumulating a charge in this dielectric layer efficiently. To do so, we apply a positive potential, relative to the diaphragm, to the back plate and in that state, expose it to soft X-rays. This ionizes the air between the diaphragm and the back plate and generates negative ions which go on to transfer charges into the dielectric layer (Figure 4). We prototyped a structure similar to the microphone and found that charges can be efficiently accumulated in the dielectric layer by passing soft X-rays through the diaphragm. Currently, we can generate a potential difference of about 25 V using an accumulated charge, but we know that this can be increased by, for example, increasing the X-ray exposure time. This development raises the prospects for development of a stored-charge silicon microphone.

Figure 3. Stored-charge-type silicon microphone

Figure 4. Soft X-ray charge-accumulation technology