I used to think that if I had had a chance to develop a loudspeaker, I would try to make a system that would reproduce main frequency range of music and voices with one loudspeaker driver.
○Although the main frequency range may change depending on a source, the lower range is 100–200Hz and the higher range is 5–8kHz.
○When you use two to three drivers for the main range, the sound quality changes around the crossover frequency so that the reproduced sound tends to be incoherent.
At the beginning of 1960’s in my NHK Technology Laboratory days, I had a chance to develop a monitor loudspeaker for broadcasting to be utilized to check the sound quality of broadcasting programs. According to the required specifications, the maximum sound pressure level was 114dB or more and the reproducing band was 50-15000Hz. Thus, it was very hard to satisfy the specifications with one driver. At least a combination of two drivers for low and high frequency ranges had to be prepared.
○Famous and prestigious loudspeakers in those days were “Goodman,” “Jensen,” “TANNOY” and “RCA,” but there were few suitable ones from the broadcasting monitoring point of view. Only RCA’S LC1 motivated me to study.
○LC1 was a two-way, co-axial loudspeaker whose crossover frequency was 2,000Hz. It was very expensive, as I remember; with the price twice as much as our monthly salary. We bought one as a reference material and studied on it. It was very informative, but at the same time, we got a lot of fights to try to make a better one by ourselves.
＜To realize a two-way composite loudspeaker:＞
Basic specs were:
・A two-way loudspeaker with aφ30cm woofer and aφ5cm tweeter
・Both drivers should have the same output sound pressure level (measured on the front axis at 1m from it, with an input power of 1 watt) and the same maximum input power.
・A crossover frequency, where woofer’s directional characteristics turn to be worse, should be chosen to be under 1.5kHz.
Basic policies were:
・Synthesis output sound pressure of both drivers should be flat.
・Interference by scattered sounds from two driver’s diaphragms should be minimized. Attenuation should be implemented only with a mechanical vibration system, without any electrical or acoustical assistance.
To obtain these goals:
A woofer should have no peak around a higher resonant frequency, and the response should be decreased proportionally to 1/ f2 beneath the frequency.
・There was no way but to choose the diaphragm shape to be a flat
・A special corrugation should be developed to have a piston motion within the utilized frequency range dips in the response.
・Cone materials having proper damping factor should be developed to prevent peaks and dips in the attenuation band.
The tweeter response should be controlled by both resonant frequency and Q (quality factor) of the vibration system.
・The electrical circuit should be consisted with one capacitor only to prevent an excess input in the lower frequency of the tweeter.
Based on the above design concepts, I made a prototype as shown in the picture, with whose performance we were almost satisfied with.
Through making the prototype, I strongly realized that the key point of loudspeaker development was the diaphragm.
・I ordered cone paper to Toyo-Cone through Mitsubishi Electric Corp.
・I found that there was a lot of know-how in the choice of cone materials,beating, pressing and forming for paper cone
manufacturing. I also felt impatience with the fact that I could not participate in the work nor touch the know-how.
・Anyway, it was apparent that the important factor contributing to sound quality was the diaphragm. Accordingly, the lifeline of a loudspeaker depended on the result of cone paper production. So I thought if we were to face the loudspeaker production seriously, we should make the cone paper by ourselves. I made a proposal to prepare facilities for cone paper development in our laboratory, but it was rejected because of the reason that NHK should not go so far as to enter that area.
・Since a diaphragm parameter changes by two to three percent according to atmospheric conditions such as temperature and humidity, we should work on the production considering that point seriously.
・I asked Mitsubishi Electric Corp. to manufacture the product. They started by designing dies for the driver and finalized the loudspeaker system named 2S-305 in two years.
That 2S-305 was found to comply with the Broadcasters Technical Standard (BTS). Until quite recently, it has been used not only as a broadcasting monitor, but also as a loudspeaker for studio recording.
2S-305, Monitor Loudspeaker for Broadcasting