Use Delay to cancel room mode @ 25hz???

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
I have multiple Subwoofers and I was wondering if I could use a delay to cancel a room mode @ 25hz? I am still trying to understand about phase, polarity etc, but I am still coming to grips with all the technical terms.

So Could someone help me understand this and how much delay would I need to achieve this?

And forgive my ignorance, I am doing my best to learn what I can.

Cheers Dave
 
The room is not that big. Approx 3500cf, but it is solid concrete. This is the waterfall and you can see the decay @ 25 hz is really bad. I was led to believe that changing the phase would cancel out a certain frequency? Or is delay and phase different altogether?

134089


Cheers Dave
 
The room is not that big. Approx 3500cf, but it is solid concrete. This is the waterfall and you can see the decay @ 25 hz is really bad. I was led to believe that changing the phase would cancel out a certain frequency? Or is delay and phase different altogether?
Dave,

Delay and phase are two different things.

A 25 Hz cycle is twice as long as 50 Hz, it's decay will show up as twice as long on a waterfall even without figuring room reverb.

Using two LF sources, one delayed, you could cancel certain frequencies in certain places, while increasing their amplitude in others. Not a good idea.
 
Thanks for the info. So how do you change the phase of a speaker to help cancel out a problem frequency?
If the "problem frequency" is at the crossover point, delaying the upper or lower band (usually upper) may be required to get a smooth phase response through the crossover region.

Generally parametric or graphic EQ are used to reduce or boost amplitude at problem frequencies within the pass band of the speaker.
The correct application of EQ also smooths the phase response as the frequency response is flattened.

EQ won't correct room problems, but will make them more tolerable.
 
You use delay to reduce the differences in position between sub and main to help the crossover range integration. Bass traps, as in real ones, not a block of foam in the corner, can do wonders. Moving them around can help. If your peak is that low, I would try and reduce the sub output by playing with the alignment a bit. Make sure you don't have it in the corner. Center between the mains is easiest to integrate. Then try some eq. My personal taste has them roll off in that range. Just no music down there. killing a peak is pretty easy to do with eq. Trying to boost dips is where subs get into trouble.
 
This is odd.

First, response is flat down to 10 Hz. Second, i don't see any other resonances. Is your room completely cubic, 3 m * 3 m * 3m ?

That was after EQ. The room is heavily trapped (Designed by a proffesional) , but the 25hz area is the only remaining problem with the room.

This is the after EQ in room response after appling the house curve and 80hz LP filter applied. Subs and amps still have a bit left in them.

134088
 
Last edited:
You could try a double bass array. Typically you would use more drivers spread across the front and back walls to create a plane wave (google double bass array for examples), but at 25hz 4 subs might be enough. You would put two in the front of the room, two in the back. Have them roughly 1/4 of the room width in from the side walls. Drive the front ones normally with a mono signal. Drive the rear ones with the same signal as the fronts but inverted in polarity and delayed by the propagation time from the front subs to the back subs (if your room is 3m long, about 3/344=0.00872 seconds). The idea is that as the wave from the front subs arrives at the back of the room, the inverted polarity signal is produced and cancels the signal from the front subs so it is no longer heard. You will probably have to re-eq after doing this to get it to sound right.
 
You could try a double bass array. Typically you would use more drivers spread across the front and back walls to create a plane wave (google double bass array for examples), but at 25hz 4 subs might be enough. You would put two in the front of the room, two in the back. Have them roughly 1/4 of the room width in from the side walls. Drive the front ones normally with a mono signal. Drive the rear ones with the same signal as the fronts but inverted in polarity and delayed by the propagation time from the front subs to the back subs (if your room is 3m long, about 3/344=0.00872 seconds). The idea is that as the wave from the front subs arrives at the back of the room, the inverted polarity signal is produced and cancels the signal from the front subs so it is no longer heard. You will probably have to re-eq after doing this to get it to sound right.

Hi John,
Thanks for the info, I have seen that kind of approach before, not sure it is doing in my room. I have 4 subs in an IB at the front, but I will also have 3 IB subs in the back right hand side of the room. I can't get them in the same position as the fronts though. That last graph is with only the 4 subs at the front playing.
 
Member
Joined 2008
Paid Member
Sorry to be late to the interesting discussion.

First, if I had only your 25-Hz "problem" I'd be laughing. Purely a visual/graph thing that only bothers wannebee engineers, I'd guess. Not something any human could hear on any ordinary source. There are few systems that wouldn't sound better with a bump-up at 25 Hz, esp. a bigger boost through the low bass is likely needed to sound right (hint: flat bass sounds lousy).

Except for absolutely heroic architectural treatments, not much can be done to directly grapple with problems like this... a little trim here, a little boost there... many good ideas have been mentioned but the effect in most rooms I've tried is meager.

But I'm with Geddes - who is really repeating the clear chapter in Toole's book. In short, you want something an engineer-at-heart would hate: heterogeneity. Mix and match three or more sub-woofer housings, locations, tuning, dipoles, and so on.

One thing I'd like to ask this forum is whether the heterogeneity technique works better with a single mixed-bass signal or some hodge-podge of the stereo signal found in the source?

Ben
 
Last edited:
Seems like a dream response curve to me.

I deal with canceling room modes by using main speakers with multiple BR ports pointing in different directions to cancel modes at specific points in the room. The result is that there is still some gain from the phase variations caused by height/length/width/materials of the room, but it is evenly distributed, no very apparent phase issues, from 33-87hz near flat with no peaks. As opposed to very peaky and clearly audible phase issues from 30-42hz before.

Your frequency response from 0 to 50hz seems very good. I envy you.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.