Adding a third sub to cancel "Deep Mid-Bass Null"

[Richx200]

Hello;

I have a problem with deep mid-bass cancellation null. (see attached) I tried moving/pointing the 2 subs around the room along with moving the mic position; sometimes I would get a little change, but the sub was in an impossible position like in front of a door. I posted this problem before and got very good responses but none helped.

I read that adding a 3rd woofer (doesn't have to match existing) could smooth out the Null.

I have 2 SB 4000 and thinking about adding a SVS 3000 Micro or SVS SB 2000.

Will a third sub cancel the Null ?

What speaker would better ?

Do I have to match the frequency response of the SB 4000 ?

 

[kemmler3D]

Hello;

I have a problem with deep mid-bass cancellation null. (see attached) I tried moving/pointing the 2 subs around the room along with moving the mic position; sometimes I would get a little change, but the sub was in an impossible position like in front of a door. I posted this problem before and got very good responses but none helped.

I read that adding a 3rd woofer (doesn't have to match existing) could smooth out the Null.

I have 2 SB 4000 and thinking about adding a SVS 3000 Micro or SVS SB 2000.

Will a third sub cancel the Null ?

What speaker would better ?

Do I have to match the frequency response of the SB 4000 ?

Looks like you are trying to fill in 35 and 50hz.

It might be easier to diagnose with less smoothing on the graph.

Otherwise, a third sub can help in theory, but only if you get it in the right spot, you might also need to adjust phase and such.

IME some EQ can really help with this kind of thing. Although it's often said "you can't EQ a null", in practice, if you have more than one source for that frequency, and you also have a lot of headroom, you kinda can, sometimes.

In my room the subs are around 9dB above the mains at a relatively low gain setting. I don't listen very loud in general. I have plenty of nulls but in practice they are filled in by the 9dB headroom and it ends up flat after EQ.

 

[FrantzM]

Have you tried MSO? Multi Subwoofers Optimizer? It will require a DSP box, the most popular option is the miniDSP2x4 HD. The results will surprise you.
There are other options. I am only familiar with what I suggested and extremely satisfied.

If you come to add a 3rd or more subs. MSO will optimize the output of your subs. Not the easiest software to master but well worth it and it’s free, although a donation is strongly suggested. It’s that good.


Peace

 

[ppataki]

Just simply use the Room Sim component of REW
You will easily see if adding a third sub would make any difference or not (and where)

 

[Richx200]

Looks like you are trying to fill in 35 and 50hz.

It might be easier to diagnose with less smoothing on the graph.

Otherwise, a third sub can help in theory, but only if you get it in the right spot, you might also need to adjust phase and such.

IME some EQ can really help with this kind of thing. Although it's often said "you can't EQ a null", in practice, if you have more than one source for that frequency, and you also have a lot of headroom, you kinda can, sometimes.

In my room the subs are around 9dB above the mains at a relatively low gain setting. I don't listen very loud in general. I have plenty of nulls but in practice they are filled in by the 9dB headroom and it ends up flat after EQ.

I have 10db of correction from the SVS DSP and I have a 20db problem. The more dbs I add from Anthem, (SPL) the sloppier the bass gets. I tried moving the subs all around the room and in the places it gets a little better are not available; doors and patio. I read that adding a sub would help, even if it is a smaller one.

Furthermore, I will try ppataki's suggestion and see what happens.

 

[DVDdoug]

The more dbs I add from Anthem, (SPL) the sloppier the bass gets. I

Maybe the filter isn't narrow enough. But it might just be "too much". +10dB requires 10 times the power and +20dB requires 100 times the power! That's why they say "you can't EQ-out a null'. You end-up clipping the amplifier and/or pushing the speaker into distortion. And since the null only occurs at one place (or is the worst in on place) in the room, you can end-up with a bump in other locations. And bumps are usually more annoying than dips.

 

[Keith_W]

Just simply use the Room Sim component of REW
You will easily see if adding a third sub would make any difference or not (and where)

Paul stole my favourite reply!

Anyway the OP might like to read about Geddes 3 sub method here (from Geddes himself) or here. Again, this can be simulated in your room with REW's room sim.

FYI it is possible to get a good result with 2 subwoofers + DSP. But this requires a bit of luck. I am lucky because I get to place my subs where they need to go - you may not be able to (doors, furniture, etc. might get in the way). Before you go and buy another subwoofer, see if you can get a good result with your existing subwoofers.

 

[ppataki]

Paul stole my favourite reply!

It is Peter, but no worries

 

[alex-z]

SVS 3000 Micro has dramatically less peak output than even an SVS SB-2000, you need something more capable if you want to add SPL at 35Hz. Frequency response does not need to match perfectly but you want enough dynamic range to keep up with those SB-4000.

What kind of seating do you have? A good place for a third sub is often underneath the listening position, or an end-table next to it. A DIY build is really good for these situations where a traditional big box sub will not fit.

 

[Richx200]

SVS 3000 Micro has dramatically less peak output than even an SVS SB-2000, you need something more capable if you want to add SPL at 35Hz. Frequency response does not need to match perfectly but you want enough dynamic range to keep up with those SB-4000.

What kind of seating do you have? A good place for a third sub is often underneath the listening position, or an end-table next to it. A DIY build is really good for these situations where a traditional big box sub will not fit.

If I can't resolve the null by any other means and I have to get another sub since the SB4000 has a full-featured amp at 1200 watt continuous and a peak of (I think) 4000 watts, what would I need to cover the SB4000. My listening average's about 80dbs my DR, may reach 100–105dbs at extreme; 20–25 dbs?

 

[Richx200]

Paul stole my favourite reply!

Anyway the OP might like to read about Geddes 3 sub method here (from Geddes himself) or here. Again, this can be simulated in your room with REW's room sim.

FYI it is possible to get a good result with 2 subwoofers + DSP. But this requires a bit of luck. I am lucky because I get to place my subs where they need to go - you may not be able to (doors, furniture, etc. might get in the way). Before you go and buy another subwoofer, see if you can get a good result with your existing subwoofers.

Good stuff ... Thank you

 

[win]

More subs gives you more tools to play with. Most people also have no problem with subs at half gain, there is usually a ton of headroom to play with because they are so much louder than passively amplified mains. It's just a matter of getting some dsp and using Multi-Sub Optimizer. With three subs, you should be able to fix that.

 

[Richx200]

More subs gives you more tools to play with. Most people also have no problem with subs at half gain, there is usually a ton of headroom to play with because they are so much louder than passively amplified mains. It's just a matter of getting some dsp and using Multi-Sub Optimizer. With three subs, you should be able to fix that.

It's my understanding that a "Null" cannot be corrected with "EQ". So far I've tried ARC Genesis, SVS's sub EQ, and moving the speakers around for hours. I checked REW room sim, and it shows that the only place that corrects the "Null" is if I place the 3rd sub in the center of the room. I'm not sure if the REW room sim Is correct; I might have screwed up the settings. At this point I don't and 3rd sub or more DSP is going to solve my problem.

 

[Juhazi]

That null at 35Hz (low bass) could be from vertical mode, since moving subs doesn't help. First just change mic height to verify. Second sub on a shelf might help. In my HT vertical null is at higher freq though.

 

[Keith_W]

It's my understanding that a "Null" cannot be corrected with "EQ". So far I've tried ARC Genesis, SVS's sub EQ, and moving the speakers around for hours. I checked REW room sim, and it shows that the only place that corrects the "Null" is if I place the 3rd sub in the center of the room. I'm not sure if the REW room sim Is correct; I might have screwed up the settings. At this point I don't and 3rd sub or more DSP is going to solve my problem.

How correct the REW room sim is depends on how close your room looks like a rectangle. If you have a non-rectangular room, try the Amroc room mode calculator or Audieum.

Some nulls can be corrected by EQ, but you have to be very clever with DSP to do it. The first thing you can try is to give it a little boost to see if it can be filled. Chances are, you will be able to fill it a little bit. You can even try a bigger boost but be aware that it will sound worse everywhere else in the room.

Otherwise, a high Q reverse all pass filter may be able to tackle it. Remember: a null is due to the interaction between the direct sound from the sub and its reflection. The reflection has to travel a longer path, so it is time delayed with respect to the direct sound. Therefore, applying a minimum phase boost will not correct the null because the energy is injected at the beginning of the impulse, i.e. at the wrong point in time. The energy needs to be injected before the impulse happens, which means you need either a linear phase or mixed phase FIR filter.

 

[DonH56]

A true room-mode null is due to signal cancellation from the sound interacting with room boundaries. The sound bounces off a boundary and recombines with the direct sound but out of phase so the sound cancels. Since it is due to physical things, frequency and distance/placement, EQ will not in general fix it. If A-B=0, then increasing the signal level 10x yields 10A-10B=0 -- same result, but wasting a lot more power. Sound treatment can help, but at low bass frequencies is generally prohibitive due to the large size and volume of treatment required. The wavelength at 100 Hz is about 11 feet, so even quarter-wave treatment is pretty thick/deep. The usual solutions are to move the listening position or add subwoofers appropriately placed to counter the room modes.

Room modes article: https://www.audiosciencereview.com/forum/index.php?threads/room-modes.25433/

Todd Welti has a good article on subwoofer placement, and room simulators help, but unless you have a sealed rectangular room are likely to need to move the sub around to optimize the response. (There is a program that handles non-rectangular rooms but I do not have the link handy, and it tells you the frequency of the nulls, not necessarily what to do about them.)

https://www.harman.com/documents/multsubs_0.pdf

You may be able to find a solution with two subs depending upon where you can place the subs. MSO is another good thing you could try: https://www.andyc.diy-audio-engineering.org/mso/html/

HTH - Don

 

[scotchrocks]

I used to have a bad null at some seating positions around 70-90Hz with one sub (SVS PC12-Plus) that drove me crazy. I eventually added 2 more subs similar to yours (SVS PC-4000) and used Multi-Sub Optimizer with a MiniDSP 2x4 to get the three subs working relatively well together. It definitely took patience, trial and error, and testing the limitations of your room/furniture setup, but eventually I ended up with the two PC-4000s in the front and the PC12-Plus behind the seating position. This gave me pretty even bass response given the room restrictions. I'd love to add more subs one day, but it's good enough for now.

To help with the subwoofer positioning, I used the REW modal simulator to preliminarily test at a high level how some potential sub placements might affect the null: https://www.roomeqwizard.com/help/help_en-GB/html/modalsim.html This simulator works best in rectangular rooms.

One thing to note is that MSO v2 just came out in May 2024 with some major new features. I've only used v1 in my setup, but will probably take some time one of these days to re-run things with v2.

Another option I've been considering is a new receiver with Dirac Live Bass Control, which can also help tune multiple subs. But, MSO has been good enough for my needs so far.

 

[raindance]

It is Peter, but no worries

Is this the definition of "robbing Peter to pay Paul" ???

 

[Richx200]

How correct the REW room sim is depends on how close your room looks like a rectangle. If you have a non-rectangular room, try the Amroc room mode calculator or Audieum.

Some nulls can be corrected by EQ, but you have to be very clever with DSP to do it. The first thing you can try is to give it a little boost to see if it can be filled. Chances are, you will be able to fill it a little bit. You can even try a bigger boost but be aware that it will sound worse everywhere else in the room.

Otherwise, a high Q reverse all pass filter may be able to tackle it. Remember: a null is due to the interaction between the direct sound from the sub and its reflection. The reflection has to travel a longer path, so it is time delayed with respect to the direct sound. Therefore, applying a minimum phase boost will not correct the null because the energy is injected at the beginning of the impulse, i.e. at the wrong point in time. The energy needs to be injected before the impulse happens, which means you need either a linear phase or mixed phase FIR filter.

Here Is a screenshot of REW Room Sim with room dimensions.

 

[ppataki]

Here Is a screenshot of REW Room Sim with room dimensions.

Try playing around inversing the subs
All permutations
It can (and does usually) make a huge difference

Edit: and definitely tick align subs individually