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A high sensitivity - high SPL design

by Curt Campbell & Wayne Wendel

Uluwatu: the coolest surfing Beach in Bali

When I designed the Stentorians, I made sure there would be plenty of drivers available for the near future. I was assured there were several thousand on hand.
That sounded like a good supply to me, but the supply seemed to run out sooner than I had hoped. Doing the math, several thousand drivers, used a dozen at a time, equates to only a couple of hundred pairs of Stens, give or take. When Adam at Madisound suggested I try something similar with the SB Acoustics SB13PFC25-08 woofers, and the SB29RDNC-C000-4 tweeter, it didn’t take too much investigating to get me on the bandwagon.


Main / Design Goals

Similar in concept to the Stentorians, but the tinker in me refused to be satisfied reinventing the wheel again. Since I was using more expensive drivers, it seemed logical that the design should also exhibit better performance as well.
Fortunately, I had some ideas in both driver layout and crossover that turned out favorably to enhance the performance of the design. The SB13’s favored a vented alignment, and you will notice I broke with tradition and front mounted the port. Based on the room position preference of many Sten users this speaker is designed for near wall placement.
–Yes, that is a 4” flared port. The air velocities at high SPL’s demanded this diameter.
While the max SPL model’s somewhere around 118 dB, there is another advantage to using six mid-woofers per line: The combined surface area of the six drivers nearly equals that of a typical 15” woofer. This design exhibits a surprising amount of tactile presence. 


Major specifications

A 2.5-way vented design with 6 SB Acoustics SB13PFC25-08 woofers in a series parallel configuration graced by the quite capable SB29RDNC-C000-4 ring radiator. The 94 dB tweeter sensitivity came in handy…

Nominal Impedance

6 ohms - The impedance curve stays well above 5 ohms for the most part. Where it dips below 5 the impedance phase is nearly resistive, so it will be an easy load for any competent amplifier.
Dimensions: 8.5” wide, 47” tall, 15” deep
Basic sensitivity: Conservatively rated at 92 dB / 2.83v / 1 meter
Max SPL: (Modeled at the woofer published Xmax) 118 dB / 1 meter 

Driver Selection

The woofer: SB Acoustics SB13PFC25-08.
SB really demonstrated its aptitude here. While the motor structure is quite simple, it appears obvious that every facet of the driver design and materials was engineered and optimized as if it was to be their flagship driver. I hope SB keeps this driver around for a long time.

The Tweeter:  SB Acoustics SB29RDNC-C000-4
The 94 dB sensitivity provided by this ring radiator design was necessary to keep up the system sensitivity of the woofers. Its response, power handling capabilities, low distortion plots, and of course its small mounting diameter made it an easy choice to pair with the SB13’s. 


Placement suggestions:
In my listening room I have them orthogonal with the wall and about 10 feet apart. They are sitting about 4" from the front wall and on 8" stands. These love to hug the wall but respond favorably when moved out into the room as well.

The Bottom Line:
OK, I know some of you are saying "All that drivel about balance is fine, but how do they ROCK?" To answer that very question, I recently spent many enjoy
able hours going through my old vinyl from the heady days of rock, and I'm here to tell you that they can kick some serious booty.
These ain't no sissy audiophile speaker.

They like it LOUD.

The Uluwatus are copyright 2014 by Curt Campbell &Wayne Wendel
Free for non-commercial use.
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