was mucking around a bit today, and perhaps this data will be of use to some.
First of all, the sub I quickly threw together was the thor of seigfreids, I was hoping to get flat to 20 in room but fell a little short. Like Otto, my mains 'only' go flat to 30, and I was curious about the last little bit below. The following graph shows the raw, and then eq'd response to about as low as I could get it.
raw sub, plus flat to 20.jpg
(hope I can make sense of my poor notations!!!!)
Then, taking my needs as an example, I aimed for a roll off from 30 hz. First up I used the geq of the deq 2496 to cut the bands at 40, 50 etc etc by the maximum amount, namely -15 db.
sub to 20 plus -15 above 40.jpg
As we can see, and as can be expected, the signal drops by (only) 15 db. In fact, whilst the sweep is in progress I can still hear it past the cutoff point we wanted.
And in any case I don't think using the geq in this manner yields a satisfactory low pass filter, unless -15 db is acceptable.
Next I looked up Bruces recipe for a low pass filter. Bit of a pain really, had to enter all the figures into the filters panel, then convert them to the form used by the deq! By the way, the maximum cut allowed in the deq peq section is -15, so it is not an exact match of Bruces but nonetheless the best we can get.
flat to 20 plus bruce low pass.jpg
The graph using Bruces recipe is the darker one, and strangely enough, the result is worse than by just using the geq function.
So, it seems that Waynes take on the 'shelf' filter is correct, the frequencies in question are simply attenuated by 15 db all the way up, which is not the same as a low pass function.
OF COURSE, it is perfectly possible that I did it all wrong so let's bear that in mind!!! ha ha ha.
Otto, seeing these results I feel for what you want to do the DCX 2496 may be more suitable???, it will after all give a genuine low pass function with peq as well.
Give me a little while and I'll test that as well, I have one here I need to nut out and this is as good a project as any to learn on eh??
I know you were looking for a second hand unit Otto, but when new the two are comparable in price??
sorry, looking at the graphs I first aimed for a cutoff of 40 hz, it was only after seeing Bruces formula that I realised I should have gone for thirty (to compare). Forgot to save the second set of measurements, can redo it if needed. So the cutoffs don't exactly match, but hope there is enough here to tell the story.