# I have a BAD room null...



## bayn

Hey all, 
I have a terrible null in my living room when my equipment is oriented one way (the one way that works with my projector of course) and I had a simple question.

My simple question is, can treating a room get rid of (or move) a null?

If it matters, the null is around 60hz, and actually its quite large. Its easy enough to move around and hear the sound come back or when I orient the equipment another way (that doesn't work with my projector) full sound comes back. Its so bad I don't need to measure anything to notice the dead zone.


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## robbo266317

What size is the room and what options do you have to locate the speakers?


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## bayn

Hey Robbo'
The room is roughly 17x13 and one side is 100% open to my kitchen. The null is found when sitting across the short end of the room (the end that gives me a huge wall to mount a screen on) and there are not many options for speaker locations as they are floor standing models.

On the other hand if I sit using the long 17' length of the room, the null is moved and away from the seating area but that wall has 2 windows and would require a drop-down screen, which I'd like to avoid hence the question if I can just move or eliminate the null.

Either way, as I mentioned I have floor standing speakers for the front and back so while I have some wiggle room for placement, it is not huge and doesn't seem to eliminate the null which of course is right over the seating.


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## Zing

bayn said:


> My simple question is, can treating a room get rid of (or move) a null?


It absolutely can!

I had a horrible null around 110Hz in my room. TrueRTA, a 6" bass trap and a whole lot of patience during a trial-and-error process yielded the most unlikely location for that bass trap. Thankfully, it also yielded perfect results.


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## bayn

Great news Zing!
I was unsure if treatments could fix the issue or if they could affect but not resolve it.

Any way of finding out if treatments will work before buying? I intend on buying a full treatment setup eventually but this room is not really conducive for them so I was holding off and black treatments in a white room stand out . I think all I can really put into place is maybe 2 sets of corner traps (stacked, in the front 2 corners) since I don't have any other corners; so is there any way to figure out if those bass traps will affect the null before buying?

Not that I can't store the traps if they don't affect this but I'd rather just wait until I get a room before buying all matching traps if this room is a loss or I need more traps to get rid of the null.


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## Zing

You'll have no way of knowing, with any degree of certainty, where a trap needs to be placed in order to fix the null in your room. A corner is a good place to start but it's not the be-all-end-all location. That was my point about the unlikely location for my bass trap - it _*wasn't*_ in a corner - but rather in the center of my front wall near the floor (essentially behind and below my center speaker).

Honestly, some sort of analyzing software would benefit you the most. Then, and only then, will you be able to know _precisely_ what plagues your room's response and what the best corrective action will be. It could be something as simple (and as free) as speaker and/or sub positioning. You'd be surprised what just a few inches closer or further from a boundary will do to the response of your room.


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## Wayne A. Pflughaupt

bayn said:


> If it matters, the null is around 60hz, and actually its quite large.
> ne.


What exactly do you mean by "large?" A true null - one that can't be readily equalized - is typically narrow, but deep. Those will typically only respond to bass traps or relocating the speaker. 

If its broad and spans, between say, 40 and 80 Hz, problems like that can usually be dealt with with an equalizer (assuming we're talking about a subwoofer, of course).

Regards,
Wayne


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## bayn

Well I do have REW, and have used it but I couldn't read much out of it. Best course of action may be to take more measurements and then just post them for some assistance in the REW forum first.

Second I mentioned it was "large" by the spectrum it seems to cover. 
It affects both my subs and main speakers and as I mentioned you can easily tell the impact/sound of both sets of speakers are affected. 

For my sub's I have dual MFW-15's from AV123 and they are quite capable, though when standing in the null you can barely hear or feel them but silverware in the kitchen can be heard rattling around. The speakers are a set of Rocket 850's with other rockets and the same situation happens where gain needs to be increased just to get back up to around a 75db calibration but impact and sound is still nothing compared to other positions. (What I'm describing then IS a null right?)

If I run a sweep, are there tell-tale things to look for that will direct me to speaker location or EQing vs. buying treatments? How will I be able to isolate the graph to so I can "see" what I'm hearing and point to the problem and perhaps a fix? Again I don't mind buying treatments its just the room is horrible and I'm not sure going to the "next step" in it would be worth it right now.


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## Wayne A. Pflughaupt

> Well I do have REW, and have used it but I couldn't read much out of it. Best course of action may be to take more measurements and then just post them for some assistance in the REW forum first.


We’re here to help. We don’t bite!

REW is definitely your first course of action. Otherwise you’re shooting blind as to where in the frequency spectrum the problem is, how wide/narrow it is, and/or exactly where in the room it’s located. The problem you mentioned standing in the null, for instance – bad as it sounds, it’s really only a problem if it’s evident at your listening position. REW can help us determine those things, and it’d help guys like Zing with acoustics treatment experience determine if it’s something treatment could resolve.

But we can’t do much of anything for you without some graphs! That’s the first place to start. :T

Regards,
Wayne


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## Zing

First of all, a null looks like this:










Look at the yellow trace. Do you see the three lowest points (approximately 35Hz, 61Hz and 115Hz)? Those are nulls at those frequencies, a sudden and severe drop in SPL.

Now that I know you're using dual subs, I'm guessing they're the biggest culprit. A null is typically caused by competing signals of the same frequency and thereby cancel each other out. *Increasing the gain will only make it worse!* I'd almost guarantee you could significantly improve, if not completely correct, your issue by correctly adjusting the phase of your subs.


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## bayn

I'll check and pull out my REW stuff to try and run a few sweeps, see what I get. Now that I see what i'm looking for I can try to isolate it.

I'm curious. Assuming that I see one or more dips, they could not only be affecting each other (bass wise) but also affecting the main's too? (If the freq/null is high enough?) 

I'm going to run some default sweeps per the instructions on the REW forums (I have also a mic so I should be able to run the full thing) and post up asap. Any other info you can throw my way? As in "Try a sweep this way and that way" so I can get as much info for you guys as possible?

=== edit ===


I posted some of my REW graphs a while ago when I first got this setup and it looks like that IS a null between 70 and 100hz.
I'm still going to try and get some new graphs but that is the exact setup that is having the problem. Any thoughts with the graphs? Is there a good way to check/set phase using REW or any other methods?


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## robbo266317

You will need to set the "default"settings to 15-200 Hz 
there's an icon top right to do this.
Resubmit when done.

That is a major hole. Have you tried simply reversing the phase of the subs?


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## Zing

Your null is only about a half octave wide. That's almost EQ territory but not quite. You should look at some other options first and I'd start with your crossover setting. You could experiment with a setting of 70-60Hz (your 850s won't have a problem with that) to see if that helps and by how much. You could also try 90-100Hz (your MFW-15s won't have trouble with that). Be sure to take a measurment after each change. I'd guess the lower setting will yield the smoother response.

I'd also suggest moving your subs (even just one of them). If you can, pull them out a bit from the walls. Frankly, since you have two, you're not lacking in output capability. As such, they really don't need to be in or near a corner. Corners offer the lowest extension and the highest output but seldom offer the smoothest response. Co-locating would be something worth considering too.


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## bayn

Robbo, 
Yeah, i'll need to set the graph correctly but you are correct in setting the phasing of the subs as I have not tried that yet.
Should the first option be to try just 1 sub by itself to see if the dip is gone or not as large? (I'm guessing this may immediately show if phase plays a large part in it.)

Zing - I've never eq'ed anything, would my emotiva mmc-1 pre be able to do that or would I need another device to properly eq the sound? I don't believe I've ever seen settings for adjusting eq'ing anywhere in it but as a newbie I may not know what to look for.

Thanks for the help guys, I now have some stuff to go on to see if I can eliminate the dip. Of course being a Monday, it seems now that the soonest I can look at this will be Tuesday evening or Wednesday but I'll play with these tips and try to have graphs with results by tomorrow or the day after.
Thanks for the help!


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## robbo266317

It's a good idea to measure each seperately and see if it is an interaction between them or the room.
And as Zing says you can move the subs and see if it helps.


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## fredk

It looks like you have a number of options for tackling that null. Do your subs have variable phase adjustment or is it just a switch? I have seen it suggested that you can also use the distance setting on your receiver to make slight phase adjustments.

How did you originally determine the location of your subs? One aproach I have seen is to find the best location for one sub, and then try different locations with the sub to smooth the response.


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## bayn

Good tips Fredk, 
the location was pretty much in keeping them with the front's and they do have a phase adjustment knob on them so I can tackle that. I had some work stuff come up so hopefully this weekend I can tackle some of this quickly.


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## bayn

Ok, so I took new measurements and these are with 1 sub disconnected (each side) and with both on. 
Also the phase has been adjusted on each and measurements taken but not shown here but so far I could not really adjust either of these null's. :hissyfit:

When looking at the pictures the first null occurs at 71-74db and the larger one happens at 103 at the lowest point. Currently the crossover is set to be 80 and the subs are located about 34" apart. They sit between the two front mains so it looks like " LF sub C sub RF ". 

It doesn't look like either sub specifically is the problem if the graph does not change with either on/off does it? 

Both subs on, with phase set differently:









Right sub disconnected:









Left sub disconnected:









Finally, this is a sweep with both subs off, I dont know if it will help seeing how they react on their own but you guys know way more than I do about this stuff :T


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## Zing

Excellent work there, Mr. Bayn! :yay2:

It's _my_ opinion (and you know what they say about opinions) that your null problems lie solely with speaker/sub locations and their interaction with the room. At this point, there are only two more things you can do.

The first is to experiment with location. Move a speaker out further away from a boundary (rear and/or side wall) and run the sweep again. Then move it closer to a boundary and run the sweep. If you haven't found any improvement, then it's time to experiment with acoustic treatment. Fret not. Though you'll encounter a slight expense, it will yield the results you seek.

The least expensive option will be the DIY route. The most common approach is burlap from a fabric store and some Owens Corning 703 from a home improvement store. You want three 2" thick panels to make a 6" thick trap. Personally, I went with Rockboard 60 (rigid mineral wool) because of its higher NRC rating at lower frequncies and a ReadyTraps bag. But there's a plethora of options available to you if your go the pre-made route.


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## Wayne A. Pflughaupt

Neither one looks bad, really. Equalization might even help the one between 70-80 Hz, although Zings advice to experiment with placement is good (assuming that's an option). It appears that your bass management isn’t working well on the mains, though...

Regards,
Wayne


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## Zing

Wayne A. Pflughaupt said:


> It appears that your bass management isn’t working well on the mains, though...


Silly question - you're not using a crossover on one or both of the subs in addition to the one in your processor, are you Bayn?


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## bayn

As far as I know, no.
Both are set as high as possible on the subs, then using the xo on the pre, it is now set to 80.
I will try to play with the crossover and see if it changes anything with the graph and I may also try to move one of the subs. Looks like i'll just have to get my hands dirty and then hit the graph button again!


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## Zing

bayn said:


> As far as I know, no.
> Both are set as high as possible on the subs,


Are they defeatable? Can you simply bypass them altogether?


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## bayn

Wayne, what do you mean by "your bass management isn’t working well on the mains"? Does that mean the cross over isnt set right?

In looking at the part of the graph, just past 100, shouldnt they be "picking up" the sound faster? Does that mean the cross over is set too low on the subs and too high on the speakers causing that dip? I'm just trying to make sense of what i'm seeing.

Zing, no as far as I know I can not bypass them using the LFE port on the subs, but I thought using the crossover on the pre would limit what is sent to each, no?


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## Zing

bayn said:


> Zing, no as far as I know I can not bypass them using the LFE port on the subs, but I thought using the crossover on the pre would limit what is sent to each, no?


If you're using the LFE inputs on the subs, it's very likely that the subs' crossover controls are indeed being bypassed.

And yes, setting your XO in your pro limits what's directed to the subs. But remember - a crossover is NOT a brick wall. Just because you have your pro set to 80, your subs may start playing a 100Hz signal, just not very loud. Equally, as the signal decreases past 80Hz, your mains are still playing a 60Hz signal, it's just that their output decreases as the fequency decreases.


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## bayn

So then why is that dip there at the crossover area?
It certainly looks like the sub quits early and the mains start late. Or is something else going on?
How is that corrected?


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## fredk

I agree with Wayne. Those sweeps don't look bad at all. You have two distinct dips, but they are not that deep at all.

Is your room height 8'? a 71Hz null at ear height (~4') would match with a room hight mode. I see that you have the same dip at 71Hz in your mains.


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## Wayne A. Pflughaupt

bayn said:


> Wayne, what do you mean by "your bass management isn’t working well on the mains"? Does that mean the cross over isnt set right?


The mains should show a fairly sharp roll-out below the crossover point; 24 dB/octave is typical these days. Your crossover is set at 80 Hz, but notice at 40 Hz (one octave below 80 Hz) response has not dropped at all. 

Of course other factors can come into play, such as the speaker's acoustic response in the room. Graphs comparing the mains' operating full range vs. with the bass management engaged would be the best way to tell for sure, but judging only from the graph you posted, the bass management for the mains doesn't look good.

Regards,
Wayne


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## paulspencer

That response is probably more offensive to your eyes than ears! 

What you are getting suggests that the room has a decent amount of bass damping already. What kind of construction does the room have? Drywall light timber framed construction tends to act as a very large bass trap, and in rooms like this, the improvement might not be what you expect. 

Looking at how left and right subs have an almost identical response, it suggests you might in fact improve slightly with some experimentation. Ideally you would place one of the subs in the listening position, then place your mic where you might want to put the sub. Now you can measure quickly in all kids of sub locations, but with moving the mic instead of the subs. This might give you some hints as to what you can try, and then start moving the subs and measuring again. If you can arrange your subs so that you can get one sub with a peak to match the dip of the other, you'll get an improvement.

Just out of curiosity, I merged all the charts together:










As you can see, above 65 Hz, all of the speakers have the same peaks and dips. 

The mains have a 4th order roll off below ~120 Hz but it stops at 70 Hz. This makes me wonder if this is actually the response of the mains without any high pass. Are mains set to large?


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## bayn

Hey Paul, 
The room is just a regular drywall room, it is my living room so it goes up to 8ft then gradually slopes to 9ft. Nothing special about the room except that it is open (completely) on 1 side to the kitchen and flows out to the rest of the house.

Unfortunately I don't understand much about sound orders and their rolloffs (I've read on it, but it never clicks for me  ) But I think I see what you guys are referring too. It looks like my mains are trying to produce sound below the cut off and that should not be so.

The mains are set to small for just to clarify but I'm guessing that because they produce sound they can be interfering with the sub's, is that correct? I'm also unsure what you mean by "without any high pass" but if I can graph anything for you to help answer that then let me know.

Sorry guys, I’m trying to learn as I go


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## paulspencer

bayn,

A 4th order high pass will filter out lower frequencies and let higher frequencies pass through - not eliminate them, but add a slope. 4th order means 4 x 6 db ie 24 db/octave. So if set to 120 Hz, then it filters so that the response is -6db at 120 Hz, then it rolls off continuously below that point at the given slope. At 60 Hz, the filter is -30db (24 + 6). Of course, in an acoustic measurement you also get it summed with the acoustic response.


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