Notching Out Modes

Max Warasila

Max Warasila

Graduate
Feb 20, 2013
1,217
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Richmond, VA
I've been experimenting lately with a lot of different ways to approach tuning systems. By no means do I know what I'm doing yet, but I've had been able to get at least decent results so far.

One of the particular things I've been doing has been searching for and gently removing room modes that are particularly offensive. I know I wrote "notching" above, but I prefer using a high Q parametric to bring it down just enough to prevent it from going nuts in the space. I'm curious, though, if there is a particular range that is worth doing more than other parts of the frequency spectrum (Ivan, if you respond - I know it depends on the room size :)), or if there are certain cutoff points which you don't bother with. So far, I've found that most modes over 1k are hopeless to try to find and avoid, and that below about 120Hz, removing them messes with too much of the frequency content down low, meaning I have to turn everything else around it up to compensate, if only a few dB.

Thoughts? I'm open to criticism of the process too, I was told about it a while back by someone who I worked with briefly on a one-off, so I'm not too attached to it.
 
Re: Notching Out Modes

The only real tool for the adjustment and removal of room modes is the D9 EQ. For tuning around room response, I've found the best tools to be loudspeaker placement, directivity, and aim.

How are you determining the presence and magnitude of room modes in the first place?
 
Re: Notching Out Modes

Max Warasila said:
I've been experimenting lately with a lot of different ways to approach tuning systems. By no means do I know what I'm doing yet, but I've had been able to get at least decent results so far.

One of the particular things I've been doing has been searching for and gently removing room modes that are particularly offensive. I know I wrote "notching" above, but I prefer using a high Q parametric to bring it down just enough to prevent it from going nuts in the space. I'm curious, though, if there is a particular range that is worth doing more than other parts of the frequency spectrum (Ivan, if you respond - I know it depends on the room size :)), or if there are certain cutoff points which you don't bother with. So far, I've found that most modes over 1k are hopeless to try to find and avoid, and that below about 120Hz, removing them messes with too much of the frequency content down low, meaning I have to turn everything else around it up to compensate, if only a few dB.

Thoughts? I'm open to criticism of the process too, I was told about it a while back by someone who I worked with briefly on a one-off, so I'm not too attached to it.
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But what you "think" you are doing is not what you are "really doing".

With room modes, there are points of Buildup AND suckout-due to the freq of interest and the dimensions of the room.

So when you "cut a freq" of buildup at one point, you will ALSO be reducing the level in the areas of "suckout".

Now you may not care about those areas-but there are people in those areas.

You will not find "room modes" at 1KHz. You will find reflections, but the same thing applies. If you reduce the level going to the mains (via eq), then the lower level areas will also go do by the same amount.

Now if you are talking RT60 times, then eq can help by reducing the amount that the room is "energized" by the sound system.

But RT60 is different than room modes. Yes they are both caused by reflections, but the end result can be very different.

Of course all of this only matters if you are concerned with the overall audience area and not a single point (like many FOH guys).

Of course the real answer is "it depends" on a lot of particulars.
 
Re: Notching Out Modes

Rob Timmerman said:
The only real tool for the adjustment and removal of room modes is the D9 EQ. For tuning around room response, I've found the best tools to be loudspeaker placement, directivity, and aim.

How are you determining the presence and magnitude of room modes in the first place?
Click to expand...

Ideally, that's what I'd do, but for when I'm stuck with boxes and angles that aren't optimal, it's nice to have some techniques.

Right now, usually all I can do is use my ears and some quick mental math, as I'm still fighting with my measurement tools. I'll go through and calculate what waves are likely to be modes, then do a quick sweep or two (whether with pink noise, a narrow parametric and my ears or through an FFT) and see what actually stands out enough to be worth getting rid of. If I'm doing it the poor man's way with my ears I'll bring it down to where I can't hear it and then usually bring it back 3-6dB to make sure I didn't go too crazy with it. If I'm using the tools... well I'll be honest I'm still sifting through the best tool to look at to find them, but I'll usually just look for deviations over 6dB at the calculated points and correct as such.

I've only messed with this a few times so far.
 
Re: Notching Out Modes

Max Warasila said:
Right now, usually all I can do is use my ears and some quick mental math, as I'm still fighting with my measurement tools. I'll go through and calculate what waves are likely to be modes, then do a quick sweep or two (whether with pink noise, a narrow parametric and my ears or through an FFT) and see what actually stands out enough to be worth getting rid of. .
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Here is a FUN and VERY DISTURBING test to do the next time you have a little bit of time.

Put a 50Hz sine wave into the system. It doesn't need to be loud-just a comfortable level.

Now walk around the room and notice the peaks and dips. THESE are the room modes. Make a note of where they are.

Now change the freq (not to anything within a 1/4 or multiple of the freq. So say 60 or 65Hz. Now walk around the room again.

Notice how the peaks and dips are in different places now.

It should bring a new case of questions and confusion.
 
Re: Notching Out Modes

Ivan Beaver said:
Here is a FUN and VERY DISTURBING test to do the next time you have a little bit of time.

Put a 50Hz sine wave into the system. It doesn't need to be loud-just a comfortable level.

Now walk around the room and notice the peaks and dips. THESE are the room modes. Make a note of where they are.

Now change the freq (not to anything within a 1/4 or multiple of the freq. So say 60 or 65Hz. Now walk around the room again.

Notice how the peaks and dips are in different places now.

It should bring a new case of questions and confusion.
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Not really. I accepted the hell that is interference a long time ago. Obviously I'm using different terminology than you are about "modes" - mine is probably the wrong one.

Unless you're are guiding me towards an entirely different topic here. In that case I must confess that I'm hopeless when it comes to reading minds.
 
Re: Notching Out Modes

If a room node is driving me or the BE nuts at mix position, I'll gladly take it out of the system eq. However during tuning, I'm much more concerned with the average response. As we know, what shows up in one position may be completely cancelled in another. You can use this to your advantage, as nobody will miss something they never had. In other words, if you cut 160 at FOH you will probably make those around you happy (if the punters even notice), and those that didn't have it in the first place will be none the wiser.

Of course, if that doesn't work just turn up the kick drum...I think Evan is on to something there.
 
Re: Notching Out Modes

Ivan - what is the definition of room mode?
And for that matter, lobes or nodes?

Is it just a hot spot or cancelation at a particular frequency?
 
Re: Notching Out Modes

Glenn Adams said:
Ivan - what is the definition of room mode?
And for that matter, lobes or nodes?

Is it just a hot spot or cancelation at a particular frequency?
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Room modes are based on the dimensions of the room-with the smallest (usually the height) dimension being the dominating mode.

Since different freq have different sizes, when the lengths of the waves (or multiples) equal the dimensions then there will be "standing waves", or points at which the reflected waves either sum together (causing boosts) or cancel (causing nulls).

As should be obvious-as the room gets bigger, the room modes move down in freq.

There is no "hard line" when the room mode part of the room start to die away and the diffusion zone starts to take over.

As with many other things audio-there is a sizable "grey area".

Yes the room modes will cause hot spots at some places at one freq and cancellations at other places with the same freq-as I described earlier in my experiment. These hot spots and nulls will move around the room at different freq.

I HIGHLY suggest actually doing it-not just assuming I am correct.

It is often quite eye/ear opening.
 
Re: Notching Out Modes

I am interested in the test you outlined. I get what you are saying.
I know you are correct. I've heard it but could never combat it. What you do for it in one area throws it off someplace else.

I do think the test has a slight problem.
1 - because you are using only 1 sustained fixed frequency at a time, although it would find the points quicker.
and 2 - music does move around a bit, even the bass guitar. Making the setting up and nailing it down even harder. There may only be one 5 second part of one song that may make a 4 sq ft patch suck real bad once in the whole show.


But more important, once you find it at one place for 50hz, and another for 65hz, what can you do about it?
 
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Re: Notching Out Modes

Well, Glen, that's kinda where Evan's strategy comes in.

The fourth thing I was ever told was that my job was damage control. "It isn't perfect and you better accept that it never will be, no matter how hard you try."
 
Re: Notching Out Modes

Glenn Adams said:
I am interested in the test you outlined. I get what you are saying.
I know you are correct. I've heard it but could never combat it. What you do for it in one area throws it off someplace else.

I do think the test has a slight problem.
1 - because you are using only 1 sustained fixed frequency at a time, although it would find the points quicker.
and 2 - music does move around a bit, even the bass guitar. Making the setting up and nailing it down even harder. There may only be one 5 second part of one song that may make a 4 sq ft patch suck real bad once in the whole show.


But more important, once you find it at one place for 50hz, and another for 65hz, what can you do about it?
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You MISSED the whole idea of the test.

It is not for actual use or trying to figure out modes in a room.

it is an EDUCATION test. Yeah things that many people don't care about-but things that really help raise your level of understanding.

It so not for "finding solution", but rather to simply show what can happen and make you aware of it.

Let's say that you kick drum (with proper mic of course ;) ) is tuned to 60Hz. And the FOH position is in a 60Hz null in a room. If the FOH guy doesn't leave that position (very often) then they will never be able to get enough kick in the mix-yet most of the rest of the room has to much kick (is that even possible;))

It is simply part of the "overall understanding" of what is going on-rather than a "quick answer".

It is not about "finding solutions" , but rather an academic exercise to further ones learning.

The more you know and understand-the better you are about to realize what you can and (often more importantly) cannot fix.
 
Re: Notching Out Modes

Ivan Beaver said:
You MISSED the whole idea of the test.
h
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>>> Not really, just wondering if any of the more experienced members had figured something out. Like I said, I get it.


"Let's say that you kick drum (with proper mic of course ;) ) is tuned to 60Hz. And the FOH position is in a 60Hz null in a room. If the FOH guy doesn't leave that position (very often) then they will never be able to get enough kick in the mix-yet most of the rest of the room has to much kick (is that even possible;))"

>>> Only when it interferes with you heart beat is it enough.
 
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