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The impulse response of an LF system is usually simulated (for the purpose of comparing it to other LF systems) with the assumption that the driver has no lowpass roll-off higher up. In such case the impulse response of a sealed box depends only on Fc and Qtc. Higher Fc systems will have the impulse response ending quicker, but this is not associated with the quality of the response, which is better described by the shape (number of overshoots and their heights) depending on Qtc only. As long as you are able to keep the Qtc within 0.5 - 0.58 range, the impulse response is going to be perfect. If you want your low Qtc system to have the f6 at around 40Hz however, you will need a driver with Fs/Qts around 80Hz.
Comparing a single 10" driver to 2 7"ers of the same Xmax at the same low frequency and overall SPL you are likely to get more distortion from the 2 7"ers because of more suspension surface while total radiatng surface is roughly the same in both cases. Another factor may be the linearity of the spider, which at a given excursion is going to be better in the case of bigger woofer, because of larger difference between inner and outer diameter of the spider.
Of course statisticaly at a given Qtc one 10" will have lower Fc while two 7" drivers will be more power efficient.
Some people may preceive higher Fc closed boxes using small drivers as "faster" but IMHO this is due to less masking of the higher frequencies (responsible for "speed") by the low bass, and because of higher distortion of small drivers compared to bigger ones.
PS. To calculate the time domain impulse response of the closed box based on Fc and Qtc, you need to be able to calculate the complex response in the frequency domain first (with constant frequency step), insert mirror-image conjugate values above the Nyquist frequecy and then calculate the inverse discrete Fourier transform. I do not know any ready made software to calculate this from just the Fc and Qtc values provided by the user.
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1st February 2007, 03:22 PM
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