Discovery-CENTER

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ScanSpeak Discovery CENTER
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DRIVERS CROSSOVER CABINET MEASUREMENTS SPEAKER-KIT CROSSOVER LAYOUT

A ScanSpeak center speaker has been on the to-do list for a long time, so here it is. Initially I was thinking of Illuminator drivers, but my recent experience the Discovery-861, convinced me of using the Discovery drivers as they offer exceptional performance. I wouldn't hesitate to use this center speaker with the Ekta-mkII, The Ekta Grande, Ekta-25 or any middle sized floor-stander or the 3-way classics, being ScanSpeak or other brands.
Some may think my use of crossover components' quality is extravagant, but these drivers deliver when fed an undistorted signal. Worth every cent. Eventually there will be a low-cost version with STANDARD-Z caps for the midrange and Superior-Z for tweeter.

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Basics:
4-driver speaker
Dimensions: 65 x 38 x 25 cm, WxDxH.
System sensitivity: 89 dB/2.8V/1 meter.
Impedance: 4-8 Ohms.
Point of crossover: 600 and 2600 Hz.
Power requirement: 20+ watts/channel.
Power handling: 100 watts. Please also read: http://www.troelsgravesen.dk/power-handling.htm, and remember any burned driver is a misused driver.

Useful links (please read before writing!):
http://www.troelsgravesen.dk/tips.htm
http://www.troelsgravesen.dk/crossovers.htm
http://www.troelsgravesen.dk/LCR-RC.htm
http://www.troelsgravesen.dk/choices.htm

DRIVERS
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Download specs here: D2608/913000 12W/4524G00 18W/8434G00

CROSSOVER
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Pay notice to the two 18W/8434G00 drivers being connected in series. The 18W is quite efficient and even with the 8 Ohms version being use here, we have to use the 12W in 4 Ohms to match efficiency.

CABINET
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Cabinets were made from 20 mm Baltic birch and bracing/mid-cab/port from 15 mm BB. Front panel I made from 25+10 mm black MDF (HDF). I suggest 19-25 mm.
The chamfering of the front panel is optional, but I think it looks cool..

Workshop images

Rear panel showing the port and midrange slot. Coordinates for holes in bracing/mid-cab measured from top and rear.Placement not critical.
I made the rear panel from several pieces as can be seen, makes it easier to make the port and slot.

Glue the upper panel in midrange cabinet before cabinet assembly, so you can slide in the structure.

Port in place.

Sanded and ready for lacquer.

Damping of left and right compartment.
The rear panel free to carry crossovers and be cored by acoustilux.

Mid and tweeter cab damping. Right: Start of routing for drivers.

Faceting the front panel is optional. I think it looks great.
Doing it on a table saw is easy, but can be done by hand: http://www.troelsgravesen.dk/tips.htm#Faceting
Sawblade angle set på 23 deg. Guide set to 6 deg. (for the vertical faceting) and 7 deg for the horizontal faceting.
Length of vertical faceting: 250 mm. Length of horizontal faceting: 200 mm.
Both to a depth of 25 mm - if panel thickness allows.

I made front panels 35 mm, excessive, but looks nice. I suggest 19-22 mm like the rest of the cabinet. Remember to chamfer the midrange cabinet hole, read here: http://www.troelsgravesen.dk/chamfer.htm. Very important, in particular if you use more than 19 mm. Do like this:

Picture taken from another construction. Mark the midrange cabinet panels and chamfer to some 6-7 mm from the edge of the driver rebate.

The midrange cabinet filled with sheeps wool, 75 grams.
Behind 18W drivers, add one folded sheet of acoustilux, 46 x 50 cm to cover crossover and the side panel.

MEASUREMENTS
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A few comments on MEASUREMENTS before you start interpreting the readings below.
First of all, if we think measurements will tell us how a speaker sounds, we're wrong. The perception of sound is way too subjective to be reflected in any measurements we can perform. A loudspeaker system is meant to give us a satisfying idea of an acoustic event and for some people a pair of 5 USD ear-plugs are enough, others spend 200 kUSD on a truly full-range pair of speakers - and the latter may not be happier than the former.
Measurements may give us an idea of tonal balance of a system, i.e. too much or too little energy in certain areas, although dispersion characteristics play a vital role here. A two-way 7+1 and a three-way 7+4+1 may display similar horizontal dispersion, yet sound very different. Measurements may tell us about bass extension if far-field measurements are merged with near-field measurements. In addition to this, ports may contribute to bass extension. Most of we diy'ers do not have access to an anechoic room for full-range measurements from 20-20000 Hz.
What cannot be seen is what kind of bass performance we get in a given room. Bass performance is highly dependent on in-room placement of your speaker and the same speaker can be boomy in one place and lean in another. Actual SPL level at 1 meter distance and 2.8V input is useful for en estimate of system sensitivity and combined with the impedance profile may give an idea of how powerful an amplifier is needed to drive the speaker to adequate levels.
What measurements do not tell is the very sound of the speaker unless displaying serious linear distortion. The level of transparency, the ability to resolve micro-details, the "speed" of the bass, etc., cannot be derived from these data. Distortion measurements rarely tell much unless seriously bad, and most modern drivers display low distortion within their specified operating range.
Many people put way too much into these graphs and my comments here are only meant as warning against over-interpretation. There are more to good sound than what can be extracted from a few graphs. Every graph needs interpretation in terms of what it means sonically and how it impacts our choice of mating drivers, cabinet and crossover design.
What measurements certainly do not tell is the sonic signature of the speaker, because speaker cones made from polypropylene, aluminum, Kevlar, paper, glass fiber, carbon fiber, magnesium, ceramics or even diamonds all have their way of adding spices to the stew. Nor do measurements tell what impact the quality of the crossover components add to the sound, from state of the art components to the cheapest of coils and caps, they all measure the same if values are correct, yet sound very different.

Frequency response measured at 0.5 meter, normalised for 1m/2.8V.