It is always best policy to have driver have a pass band an octave higher than the crossover point for a fourth order filter and two octaves for a second order filter and four octaves for a first order filter.
Does anyone know if the Velodyne DD series satisfy these criteria? I have a DD-15 and added a DD-12 to fill in a broad dip I had in the 60 to 80 Hz region. Like the original poster, I am interested in the high end of the frequency response for listening to music.
I recently replaced my surrounds with two NHT Classic 3's, which are acoustic suspension type. I chose them because they have a 2" aluminum dome mid-range driver like the 2" drivers in the Energy Veritas 2.3i's I have in front. And they do in fact blend very well, excepting in the mid-bass region where the Veritas have two 6.5" woofers that are ported and the NHT has one 6.5" woofer that's not. So in order to achieve a better balance in the mid-bass region I am thinking of having the XO for the NHT's at 150 Hz and 6 db/oct for the DD-12 and 12 db/oct for the DD-15. The crossovers themselves will be custom made, the reason being is that I want the XO that's right for the speakers and not have to rely on the sub or the receiver or the player to do it with one size fits all.
My question then is, do either of these driver's (which feature "high gain servo" control) meet TLS Guy's criteria (low distortion levels 4 octaves for the DD-12 and 2 octaves for the DD-15 above a XO at 150 Hz)? Or would it be better to use a higher slope and/or lower XO frequency and not try to use the XO to match the mid bass response of the NHT's to the Veritas?
I'm sure there have been measurements done on the D-12 and DD-15 but I am having trouble finding them and I'm doubtful that any of them looked at distortion in the 200 Hz to 300 Hz and higher region. Most reviews, even if they do look at distortion, usually focus on the lowest frequencies in the driver's range, not the highest.
My ears tell me the above XO settings will work okay but I'm interested in others opinions and experience with similar drivers. I do not seem to be bothered by the directivity issue that has been mentioned as justification for low XO frequencies and high slopes. It seems to me that one's perception of the overall quality of the sound that's most important. I think distortion is likely more important than direction but it's not so easy to detect just by listening alone.
Edit: I should probably explain that when I refer to XO I actually mean low-pass filter. I have all speakers high-pass filtered at 80 Hz, 12 dB/oct. Therefore, in the case of the NHT's, I am not relying primarily on the subwoofer response in the octave below the 150 Hz XO frequency. In this range, both the speakers and the subwoofers make large contributions to the sound.