Hi all,
I'm wondering if I can get some advice.
I have 2 subs. DIY BMS18" in sealed boxes.
places in front corners of room on the outside of the left and right speakers. (typical setup)
I have used various room correction software e.g. Dirac version 1, YPAO, Manual EQ etc
my issue is that, even though my subs are time aligned and level matched to each other and I verify with REW. when I measure the FR of one sub, then the other sub, and then combined, most of the frequencies sum greater when combined like they should.
but there is a specific frequency that doesn't. 40 to 50hz huge null.. this doesn't happen in the individual sub responses. once combined the magnitude at this frequency is lower than that of a single sub. As if they aren't aligned.. but they are. for most frequencies. Just not at 40 to 50Hz.
if I reverse the phase on one sub they sum beautifully at 40 to 50hz, only to ruin the rest of the frequencies as this is obviously incorrect.
How do I fix this? I have read that all pass filters work to change the phase but keep the magnitude the same. But others tell me its not a phase issue but a room issue?. have I not just confirmed that the phase is the issue? I just need to know why this is happening and the correct approach to fixing it.
thanks.
I'm wondering if I can get some advice.
I have 2 subs. DIY BMS18" in sealed boxes.
places in front corners of room on the outside of the left and right speakers. (typical setup)
I have used various room correction software e.g. Dirac version 1, YPAO, Manual EQ etc
my issue is that, even though my subs are time aligned and level matched to each other and I verify with REW. when I measure the FR of one sub, then the other sub, and then combined, most of the frequencies sum greater when combined like they should.
but there is a specific frequency that doesn't. 40 to 50hz huge null.. this doesn't happen in the individual sub responses. once combined the magnitude at this frequency is lower than that of a single sub. As if they aren't aligned.. but they are. for most frequencies. Just not at 40 to 50Hz.
if I reverse the phase on one sub they sum beautifully at 40 to 50hz, only to ruin the rest of the frequencies as this is obviously incorrect.
How do I fix this? I have read that all pass filters work to change the phase but keep the magnitude the same. But others tell me its not a phase issue but a room issue?. have I not just confirmed that the phase is the issue? I just need to know why this is happening and the correct approach to fixing it.
thanks.
You need to provide the dimensions of the room, plus a sketch of sub placement. Distance from walls matters. Room surfaces matter. Brick, glass, cinder block, masonry are particularly prone to make standing waves. Clutter breaks up standing waves. Ceiling height, shape, floor & ceiling coverings matter. Placement of measurement microphone matters. Rough surfaces and acoustic treatments break up standing waves.
This is a phase problem caused by a room problem, it happens. Assuming the subwoofers are identical.
You may compare two individual phase responces in listening position.
Are your subs stereo?..
Also you may applay FIR phase correction)).
Actually this is the root of the problem. Many home systems use a crossover to sum the two channels below the crossover frequency, and install ONE sub speaker. Your room size is likely at or above the wavelength of 50 hz. The ear cannot locate bass anyway in a room of that size. It requires a 3rd amp channel to run one sub, but having a null at the wavelength or half wavelength of the room size is inevitable with two subs in a reflective (hard wall) room. Here is the blurb on a Rane 23S "The Rane AC 23S active crossover can be configured as stereo 2 or 3-way, or mono 4 or 5-way. It employs 4th-order Linkwitz-Riley filter alignments to minimize phase difficulties in the critical crossover region"
I own a used Nady C23SW, and it has a cutover frequency for the summed sub channel adjustable from 250 to 50 hz. It also has a switchable roll off below 30 hz to avoid subsonics from a turntable (LP) picking up footsteps ripping the wire or suspension of your 18" driver. I lost a 10" woofer that way, likely, with a 35 w/ch amp. Dynakit ST70.
BTW if you need a 3rd channel mono amp, a Peavey M-3000 is affordable used. Or a MMA-875T (75 w) or MMA-8150 (150 w). You can replace the 30 year old filter caps and clean the heat sink yourself. The MMA models require a 4 ohms sub, the 8 ohm output is transformer coupled and rolls off below 50 hz. The M-3000 is 130 w 8 ohms, 210 w 4 ohms. M-3000 minimum frequency 10 hz. None of these have a fan.
Last edited:
Thanks for the replies. Really helps. I'll do some more research and maybe give some more info soon
Subs are single output from AVR..mono, to inuke 6000dsp
Individual eq applied to both subs and delays.
My hunch is what you have is interaction between the subs and the room. Once you drop below about 300hz all the reflections in the room start to unmerge and space out according to the room dimensions. You wind up with a three dimensional pressure map of peaks and nulls that varies with frequency. If you try and eq a null, nothing will happen. You can throw infinite energy at it, nothing will still happen because the null is a location where two waves of the same frequency are 180 deg out of phase and cancel perfectly. You need to move something physically. Experiment with the sub placement, asymmetry might even help.
As you have DSP you can notch one sub and boost the other for that range. Not ideal since those are the frequencies where you have to move more air, but saves rearranging the room
Brian
Brian
Notching one and boosting the other is exactly what I have done ..issue is the subs are too big to try and find another location.. unless I put one in the opposite corner.
However, for some reason when I do that the one at the rear drops off very quickly below 30hz where at the front it's good to 10hz.. but it does get rid of the 40 to 50 Hz issue.
Seems if I had access to all pass filters / Dirac bass control processor it would sort this issue to some degree.
But at least I know now
However, for some reason when I do that the one at the rear drops off very quickly below 30hz where at the front it's good to 10hz.. but it does get rid of the 40 to 50 Hz issue.
Seems if I had access to all pass filters / Dirac bass control processor it would sort this issue to some degree.
But at least I know now
You need a third and possibly a fourth sub in order to manage the modal behaviour of the sound field in your room. That is the summary of above post. There is practically no way around it, active room correction systems or really huge (100's of cubic feet) acoustic measures aside.