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High Fidelity

Sentinel

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A. Design
B. Listen
C. Specs
D. Crossover

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High Fidelity

Sentinel Floor Mount Loudspeaker

Description of this page

The Sentinel Floor Mount Loudspeaker is a large three way acoustic suspension loudspeaker,
built as a first design attempt to investigate time delay and crossover design.



Speakers Side & Front


Speaker Top Section


A. Designing and Building the Speaker

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

B. Listening to 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.


C. Specifications of the Speaker

Specifications : Three Way Time Equalised Acoustic Suspension Floor Stander
Bass Enclosure : 100L Braced and Lined
Mid/Treble Enclosure : 4L Time Delayed and Lined
Frequency Response : 65hz-18,000hz at -3dB
Efficiency : 89dB at 1W at 1 metre
Impedence : 6.7ohms typical with full impedence equalisation of drivers
Power Handling : 30W rms
Crossover, Bass to Midrange : 533hz 2nd Order Butterworth Network
Crossover, Midrange to Treble : 3,000hz 2nd Order Butterworth Network
Impedence Correction : Woofer - Inductance, Midrange and Tweeter - Inductance and Resonant Impedences
Bass Driver : Etone 618 300mm plastic/paper cone
Rola 12UAX-8 300mm doped paper cone
Midrange Driver : Fosters C100K03 100mm paper cone
Tweeter Driver : Fosters D025N15 25mm silk dome

D. The Crossover Network



Circuit for the Crossover Network