Sentinel Floor Mount Loudspeaker
Description of this page
The Sentinel Floor Mount Loudspeaker is a large three way acoustic suspension loudspeaker,
The Sentinel Loudspeaker was my first foray into loudspeaker design and building. It was created in 1979 and built on the rear steps of a flat I was living in behind the Cremorne Fire Station. The name derives from the interstellar artefact in the Arthur.C. Clarke novella 'The Sentinel', which became the basis of the film 2001 - A Space Odyssey.
I wanted to experiment with time delay correction and refraction, hence the separate mid-treble enclosure on the top. The delay was established by feeding a square wave into the speaker and listening for the change in tone as the drivers were moved relative to each other. The speaker displayed this effect beautifully when listening to the attack on transients such as cymbals.
This design was inspired by the Bowers & Wilkins 801 matrix speaker. My system is a 100 litre acoustic suspension bass enclosure system mounting a cleverly designed mid/treble enclosure at ear height. The crossover is the limiting factor to performance. I used 12dB/octave networks, which do not have a flat phase response. A 6dB/octave network would have created the correct response, but would be inadequate to control the frequency limits of the used drivers. The next step would be to design a 24dB/octave network which keeps the drivers in phase.
I would now consider the time delay between the bass and midrange driver to be of no great import. The greatest impact on transient reponse is the time delay between the midrange and treble drivers plus, more importantly, control of cone breakup on the bass and midrange drivers by doping. Further build improvements of the cabinet are recessing the drivers to be flush with the surface of the baffle. When these speakers were built I had no access to a depth controlled circle cutting router.
After fifteen years use the cone suspension on the Etone woofers rotted, and I replaced them with a pair of MSP 12UAX8 drivers, to no great detriment in the sound. The speakers no longer exist, having been broken up two years ago due to crumbling enclosures and lack of room in the lounge room. They cost about $400 to build. All I now have are the crossovers, the 12UAX8's and the two mid/treble components now fitted into the compact bookshelf speaker.
Set of plans for the speaker
The setup created a tight vertical listening angle, and was designed for correct ear height when seated on a typical chair. Unfortunately my lounge is slightly lower and you had to sit with a straight back to hear the best response. The overall tone of the speaker was muddy due to the unmodified midrange. The bass response was adequately controlled due to adequate bracing and insulwool lining. The treble was unique in producing accurate attack on symbols and drums. When the system was decommissioned, the top mid/treble enclosure was recycled into a compact bookshelf speaker with admirable performance. This was due to experiments in cone doping producing significantly improved results.
Circuit for the Crossover Network