共鳴型スピーカーを作ってから、CW運用にはずっとこれを使ってきました。普通のスピーカーと比べて、①音が前に張り出し活発に鳴る、②一定のピッチ(音圧ピーク)に合わせるのが容易、という特長があり、一度これに慣れると手放せなくなります。ただ、１号機も2号機もピッチがやや高目だったので、低いピッチに見合うよう一回り大きな箱で次のバージョンを作ってみました。
タイプ2までは520㏄の小さな箱を使いましたが、今回の箱は容積1300ｃｃ以上。その為、ダクトシステムの共鳴周波数は200Hzオーダー迄下げることができます。スピーカーユニットには直径75mmのものを選び、ダクトの開口面積と深さを振りながらあたり実験を実施。ダクトの深さを50mmにした上でその開口面積を調整し、ヘルムホルツ共鳴周波数245～265Hz(計算値)とした時に聴感的にスムースな特性になりました。音圧のピークは380Hzです。タイプ2と比較すると、共鳴の鋭さに若干鈍さを感じますが、350～390Hzのピッチにて落ち着いてワッチできます。
ダクトの開口面積をもっと絞れば共鳴周波数は下がり、ピーク周波数も更に下がるのですが、聴感特性的に満足できるものが得られませんでした。ピークを300Hz辺りにしたい場合、スピーカーユニットのF0を考えれば口径75mmでは苦しいようです。直径10cm以上のユニットを選ぶべきかもしれません。なお、ヒス低減のため2号機同様直列にLCを入れる場合、Lは大き過ぎない方がFBです。L＝5mH、C=40μF（Q=1.2）辺りが好適でした。
I have been using them for CW operations ever since I made the resonant
type of speakers. Compared to ordinary speakers, they have the following
characteristics: (1) the sound is more forward and lively, and (2) It is
easier to adjust the pitch to a specific frequency (peak frequency). So,
once I got used to this, I couldn't let it go. However, the pitch of both
the first and second version was a little high, so I made the next type
in a box that was one size larger to match the lower pitch.
Up to version 2, I used a small box of 520㏄, but the wooden box I have
chosen this time has a volume of more than 1300 cc. Therefore, the resonant
frequency of the duct system can be reduced down to the order of 200 Hz.
First, a 75 mm diameter speaker unit was selected, and experiments were
carried out while varying the aperture area and depth of the duct. As a
result, audibly smooth characteristics were obtained when the duct depth
was 50 mm and the aperture area was adjusted to a Helmholtz resonance frequency
of 245 to 265 Hz (calculated value). The peak of sound pressure is at 380Hz.
Compared to the version 2, the sharpness of the resonance is a little duller,
but the pitch of 350 to 390 Hz allows for calm listening.
If the aperture area of the duct were narrowed more, the resonance frequency
would be much lower and the frequency of the sound pressure peak would
be even lower, but I could not obtain any satisfactory audible characteristics.
If you want the peak to be around 300 Hz, a 75 mm diameter unit seems too
small considering the F0 of the speaker unit. In that case, a 10cm (or
more) diameter unit might need to be selected. (As with version 2, when
adding LC in series to reduce hiss, L should not be too large; L=5mH, C=40μF
(Q=1.2) was suitable.)
