The following example uses the 5 ohm voicecoil model of the 12" driver. The box has a volume of 3 Cft and is tuned to 20 Hz. The equalization circuit is part of the new CXR 32 crossover. This is a dual 3 way equalising crossover. More detail on this unit can be found in another section on my website.
The LEAP simulation below shows three curves. The red curve is the driver in a sealed box without EQ, just a 70 Hz fourth order low pass filter applied. With EQ to flatten the curve, this 12" driver in a sealed enclosure would get about 96-97 dB @ 20 Hz.
The magenta curve is the 12" driver in a 3 Cft vented enclosure tuned to 20 Hz. There is no high pass filter in the circuit nor is there any electronical assistance. This setup is obviously more efficient in the low end. Over 100 dB at 20 Hz is possible. But notice the potential excursion problem below 20 Hz as there is no protection for the driver.
The yellow curve is the 12" driver in a 3 Cft vented enclosure tuned to 20 Hz. There is a second order highpass filter in the system with boost. SPL at 30 Hz exceeds 106dB. This is without any room gain etc. F3 is 19 Hz, F10 is 16 Hz. The combination of the 4th order driver alignment with the second order highpass make this a 6th order alignment. The advantage is obvious. Protection of the driver below port tuning while being able to adjust the Q of the filter in order to boost the Xmax at the tuned port frequency.
The green curve is the 12" driver in a 3 Cft vented enclosure tuned to 20 Hz. There is a fourth order highpass filter in the system with boost. SPL at 30 Hz reaches 106 dB. F3 is 19 Hz, F10 is 16 Hz. There is no need for a 4th order HP as there is not really any additional gain compared to a 2nd order HP filter.
The curves below belong to the same drivers/set-up. It shows Xmax in function of frequency. This shows well how a boosted highpass filter can protect the driver as well as boost the output substantially while flattening the curve. This function comes however with an increased group delay.
With respect to group delay, all those alignments exhibit similar (within 1.5 milliseconds) group delay at the Fc of approximately 45 Hz. It is below Fc that group delay goes "out of control" for the higher order alignments. The group delay scale is at the righthand side of the above figure. The group delay curves are located somewhat at the left hand side in dotted lines. As expected, the sealed alignment has lowest group delay as a second order system.
To be continued with measurements on the built unit.
Copyright (c) 1997-98, by Rudi A. Blondia, ALL RIGHTS RESERVED. Last update June 22, 1998..