If you're reading-up on the Skytec CW-1 Speaker because you're thinking of building one yourself then you need-to-know that the Skytec speaker that you see in the pictures is the outer 'cosmetic' housing. The Skytec speaker consists of a tube-within-a-tube ... the somewhat 'fat' outer PVC pipe housing and a narrower (also PVC) pipe inside.
When you look at the pictures that show the front of the Skytec speaker that smaller 'pipe' sticking out the front (in the middle of the dark plastic 'surround') is the inner portion that provides the acoustic resonance. The inner pipe is built with the same (roughly) 30 degree 'bend' upper section just like the outer pipe; the inner pipe is concentric within the outer pipe but I don't believe that has any effect on performance.
The outer ('fat') PVC pipe has a 3 1/2 inch outer diameter (OD) and the inner (resonant) pipe has a 1 7/8 OD (inside diameter is about 1 3/4 inches). The actual electrical speaker is sitting down at the base of the inner 'pipe' (pointing straight up).
(as has been mentioned by another reviewer) The Skytec speaker is tuned by adjusting the length of the inner pipe - the portion that sticks out the front (in the midst of the dark 'surround') is a sliding piece that fits (fairly tightly) inside the portion that is just protruding out from the dark plastic 'surround'.
In my situation I have been experimenting with different audio frequencies for CW reception, specifically that are slightly lower than the 700-750 Hz that used to be considered the 'standard'. I wanted to see if I liked a 600 Hz note (tone) and to get the Skytec speaker resonant at that lower frequency I made a replacement (longer than the original) slider-piece out of a length of toilet-paper tube (I slit the tube along-its-length, cut out a narrow sliver, and then taped the tube back together so that its OD fits tightly inside the Skytec speaker's inner 'pipe' that is sticking out of the dark 'surround'). With that (slightly) longer 'slider-tube' I can now tune the Skytec speaker to 600 Hz.
Here is some good reading material for acoustically resonant CW speakers:
An Electro-Acoustic CW Filter
QST Apr 1983
A Resonant Speaker For CW QST (Hints & Kinks) Dec 1987
More On Resonant Speakers QST (Hints & Kinks) Jan 1989
Additionally, here is a good article that discusses how to build a robust, wooden cased,
speaker.
A High Quality Speaker System
for the Ham Shack
OST Oct 2004
It does not have an acoustically resonant speaker but when I read the article I kept thinking that it would make an excellent 'surround' for an acoustically-resonant design.
- Paul, WB5AGF