SEAS CA18RNX 2-way floorstander
Copyright 2010 Troels Gravesen

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Download driver specs: SEAS CA18RNX and SEAS 27TDC.

Having recently finished the Peerless NOMEX 164 speaker, the front panel routings would nicely fit with these SEAS drivers, thus another 2-way system offering an excellent cost-performance ratio.

There are good reasons for the popularity of 2-ways. Most people want a slim cabinet and placing a single e.g. 6" driver on a 20 cm baffle calls for serious baffle step loss, that is: The response from the driver will fade 6 dB/octave downwards from around 580 Hz (11,600/20 = 580 Hz). So, we have to add a rather large coil to the single driver to compensate for this loss and get a flat response in all of the midrange. Adding a second woofer run through a larger coil can fill this gap, thus we achieve an overall flatter response and we get twice the membrane area in the bass section (so much needed) and we get roughly twice the power handling. Last but not least we can increase overall system sensitivity by a factor of two (+ 3 dB).

Those familiar with these pages may immediately ask what the price is for this, because there's usually a price to pay for anything we do with loudspeakers. Hmm... You have to pay for an additional two woofers and you have to make your cabinet bigger, i.e. twice the volume of a single driver cab, but you can maintain baffle width and only increase cabinet depth so your wife won't get another heart attack - at least when she sees the speakers from the front. Finally you get half the impedance of a single driver set-up, the latter not a problem for any modern amplifier.

These SEAS CA18RNX drivers hold the classical coated paper cones, but here with a 39 mm voice coil former allowing greater power handling compared to CA18RLY's standard 1" voice coil. Overall frequency response is better for the CA18RLY, but despite a somewhat whobbly midrange they sound smooth, detailed and throw a punchy bass. Smoothing the midrange further took at lot of components and the sound? Not better, thus this simplistic crossover only smoothing some peaks in the 8-10 kHz range.
17 liter/driver makes F3 = 47 Hz and you may increase cabinet volume to e.g. 42 liters and have F3 = 43 Hz. No problem, the CA18RNX can easily handle 21 liter. F3 = 40 Hz will require 24 liter/driver, a total of 48 liters. To do so, increase cabinet depth and maintain height and width + placement of drivers on front panel. Also maintain a port tuning of 42 Hz.

Overall system sensitivity is around 89-90 dB/2.8 volts and impedance stay above 4 ohms. No problem for the majority of amplifiers.

QUESTIONS having the answer NO:
Can I use another tweeter? Can I change the front panel layout and driver placement? Do you have more detailed drawings? This should eliminate the bulk of mails.
Can I use it up-right? Oops...a yes. Yes, in my set-up I had it tilted some 3-4 deg, but vertical dispersion is fairly good, so take your pick.

The Crossover



Crossover layout

Final layout a little different from drawing.


The drawings here are from the CNO-25, as they share the same dimansions.


Cabinet damping

Add 8 mm felt to all internal panels except front.
Add 30 mm acoustilux according to drawing. 4 layers at bottom to reduce standing waves between bottom and top.



Left: SPL of drivers driven directly without crossover. Right: Individual and summed response of drivers (blue = without lower bass).

Left: Inverting tweeter polarity. Point of crossover = 2700 Hz. Right: System impedance, minimum = 4 ohms.

Test cabinet set-up.