The 15" underhung drivers from TC Sounds can be used in a sealed enclosure where the enclosure volume can be kept reasonable. Sealed systems are less efficient than vented systems, but the simplicity of a sealed system offers the advantage of a lower order system with less group delay. The audibility of group delay distortion at low frequencies is still in question, but the theoretical advantage is that a second order system has a better transient response than a fourth or sixth order system.
The trade-off is the lower efficiency of a sealed system. The design proposed here uses two fifteen inch drivers and equalization in order to reach realistic sound pressure levels, keep the enclosure size down and control cone excursion. Since the typical 15" TC Sounds driver has a practical peak to peak excursion limit of over an inch, it is the ideal driver for sealed enclosure systems.
The following example uses the 5 ohm voicecoil model of the 15" driver. This is a recent motor design with distortion lowering features. I've never come across an underhung motor design this linear over Xmax. Distortion is extremely low as will be shown later.
The crossover and equalization circuit as used in the following LEAP simulations and as-built measurements are part of a CXR-22 equalising ribbon crossover. The low pass for the sub was set at 40 Hz since I'm only interested in the bottom octave for this application.
The first idea was to build a vertical tower with both drivers facing outward. In order to ease the SO objections the box was converted to a horizontal model that can be used as sidetable with "useful" topsurface. External dimensions of the unit are 34" wide, 22.5" deep and 18" high. The corners are 3" radius. The drivers are mounted facing opposite directions on the 22,5" x 18" face. The enclosure is constructed out of 3/4 inch MDF with the driver panels1.5 inch thick. Heavy bracing throughout.
The LEAP simulation below shows three curves. The red curve is the driver in the box without EQ, just a 40 Hz third order low pass filter. This sub has to cover only the bottom octave. Since I plan three subs in my system, I don't require 120 dB @20 Hz out of a single box. The red curve is hardly flat, so some electronical assistance is needed.
The magenta curve is the driver with Linkwitz correction applied, no high pass filter. This will show in the next set of graphs covering the Xmas of the driver. The missing high-pass filter compromises the excursion limits of the driver at 10 Hz as will be seen in the Xmax diagrams.
The green curve has a high pass filter in the system and uses a second order lowpass filter combined with a first order lowpass filter in order to obtain the most interesting response versus Xmax compromise. LEAP predicts 104 dB at 25 Hz with an F3 of 16 Hz. Fc of 50.5Hz @ 20 ohm impedance peak with 100 % fiberglass filling. (Empty box models with an Fc of 58.1 Hz and an impedance peak of 33 ohms.) Almost all of the listening will be done below the system resonance.
Looking at the next curves, it shows that with 2 drivers in a small sealed enclosure there is no way that X-max will be reached before hitting the thermal limit of the drivers. So one never has to worry about bottoming out the drivers as long as the applied power is kept in check.
The green curve is the most desirable, optimum use of Xmax of the driver with protection below 15 Hz. The equalization has also as advantage that from 22 Hz on the Xmax is lower than the non equalized box if one wants the maximum out of the system below 25 Hz. This can only help to control the amount of overall system distortion.
After building the box and measuring and calculating the real volume after bracing etc, it turned out that the internal volume was 5.132 Cft. LEAP predicted an impedance of 33 ohm @58.1 Hz.
The graphs below are 4 impedance measurements with the sealed enclosure under different conditions.
The white trace is the result of an impedance measurements in an "empty" enclousre. With 33 ohm @ 58.05 Hz very close to the predicted LEAP numbers. The impedance may have been a little higher as this first measurement was taken at a limited resolution. The shape of the peak suggests that the reality lies somethere right of the portrayed peak. Lining a sealed enclosure with foam has proven to me being very beneficial. Excellent damping of standing waves.
The yellow trace was measured after lining the enclosure with low density convoluted foam. Average thickness is two inch. The Impedance came in at 35 ohm @54.7 Hz.
The purple trace is the result of a measurement after adding 27 oz of Dacron Hollowfill II. Impedance measured 25.2 ohm @ 52.14 Hz. The last trace was obtained after adding another 27 oz of Dacron Hollowfill II for a total of 54 oz. The impedance meausred 19.25 ohm @ 51.1 Hz.
Adding another 13 oz of Dacron lowered Fs to 50.1 Hz. Following is the list of the applicable Q's in function of filling.
| Fc | Qtc | |
| Empty box | 58.1 | 0.664 |
| 27 Oz DacronII | 52.4 | 0.562 |
| 54 Oz DacronII | 51.1 | 0.526 |
| 67 Oz DacronII | 50.1 | 0.518 |
| After Break In |
Following plot is a near field response as measured with LAUD 3.0 using the script. Both drivers were measured and the responses merged. There is no crossover present in the graph. The little wiggle at 143 Hz and 3dB drop is actually due to transformer coupling of the voicecoil to its surroundings; the polepiece steel, the copper AC-fluxshortring and the topplates.
To be continued.
Copyright (c) 1997-98, by Rudi A. Blondia, ALL RIGHTS RESERVED. Last update June 26, 1998..