Tell me more about flat frequency response?

A speaker can theoretically have a flat frequency response, but once you put the speaker into an environment, the sound you now hear is the speaker and it's interaction with the environment.  It's this interaction that causes the overall frequency response to no longer be flat by the time it reaches your ears.

As sound travels, any surface the sound reaches causes some degree of reflection.  A tiled room will cause a great deal of reflection of all frequencies.  A room with thick carpet on all the surfaces will cause very little reflection and will tend to absorb high frequencies very well.

When audio is reflected, certain frequencies from the reflections add to the original signal and some subtract, causing an uneven frequency response.  A lot of factors are at play here including the types of surfaces in the room, the size of the room and where you are located compared to the speaker location.  Suffice to say, it all can get pretty complicated.

The goal then is to minimize the uneven frequency response as much as possible.  Short of an anechoic chamber, it's very hard to create an environment with a flat frequency response.  Get it as good as you can, and if your mixes translate well to the real world than you are in good shape.

Yes, though you'll never get it perfect. The small rooms that most of us have are smaller than the actual wavelengths once you start getting down into the lower octaves. So you can get huge peaks and cancellations in the bass as heard in the room, like as much as 40dB between the worst cancellations and peaks. This will wreak havoc with your attempts to mix the bass since you can't hear what's really happening.

At higher frequencies you have issues with reflections off nearby walls coming back at slightly different times, creating subtle cancellations (comb filtering is generally what's going on, a finely spaced set of peaks of cancellations.) So the room is a big issue with getting a nice sound. There's lot of discussion out ther eon the subject to read up on.
Basically you do three things:
 
1. Diffusion, to get things reflecting away from the listener in the mids to higher frequencies, or spreading out the reflections via pseudo-randomly shaped diffusive surfaces.
 
2. Absorption, to suck up bass energy and turn it into heat. Fiberglass insulation is typical for this, such as Corning 703. When they are placed an inch or so from the wall, the bass energy has to get through the insulation, hit the wall, and bounce back through again. This reduces the energy significantly so that it's must lower by the time it gets back to you.
 
You can't use diffusion for low frequences because the waves are too big.
 
3. Listening position placement. There's an optimal position for the mixing position (i.e. where your head will be when mixing), that will start you off at the best you can do before you do the things above. Given a room with X,Y, and Z dimensions, there will be peaks and cancellations are particular places along each axis. There is an optimal position where you are in between as many of them as possible, and it's typically a fairly appropriate spot as well, usually about 38% out from the wall (front to back.) That's your head position, so the desk and speakers will be between you and the wall, and that usually puts the speakers at a reasonable distance from the wall to avoid boominess and such.
 

Room correction systems can only do so much. They can't really do anything about bass cancellations at all, unless I'm missing something.

There are two possible issues. One is that bass from the speakers meets bass coming back from a wall and they combine at a particualr frequency to make it louder. You can use an EQ to bring this down. But the other is when they hit and cancel. You can't fix this with EQ, because they will always cancel out.

so the best thing to have is a flat frequency response from the room?

That is the ideal. However, it is impossible to achieve. Even if you were to manage a flat response for the room (only possible in an anechoic chamber, btw), you still have to contend with the frequency response of your ears, which is not only not flat but changes constantly.


The best you can do, in a practical sense, is try to mitigate the most offensive frequency irregularities through speaker placement, acoustical treatments and optional "room correction" (e.g. ARC). Then monitor at a consistent SPL and don't wear hats while mixing.

Maybe that's why momma told me not to wear my ten gallon hat inside!  She new I was gonna be mixing some day.

@

  In most cases  cases ARC isn't going to be a perfect solution,but then neither are many room treatments. As Bit says,you basically can't get there from here. But you can get close enough to mix productively. In my case ARC took care of a great deal of my room problems and at least made mixing bearable at decent volume levels. A test of my room would reveal a few dips here and there. I offset some of this with headphone mixing and listening to the mix elsewhere in other systems.

  Mixing bass is one of the most frequent problems in the small studio because of the standing waves and the way they interact with the room and your ears. Add to that monitors that can't reproduce the frequencies.

 Like Bit says, your ears adjust to their environment. You have an inner EQ that is constantly trying to overide  the mix as it really is.....sure you still want a go at this??? lol.

sharpdion23

Can I use the IK multipedia ARC plugin without the mic and software to try and get the flatest room possible?
 
In this video he says it the ARC measurement mic, software and plugin works as a combined.

I guess it's because of the budget you mentioned. ARC is a bundle ($199 these days) - you get the whole thing. Basically, the mic is used to measure the room acoustics and tell the software which adjustments must be made, the the plug-in is used in your DAW to make those adjustments live on the mixbus.


In fact, I just checked while replying - you can get the bundle for $169 over at Audio Deluxe.

sharpdion23


@ James. I took a look at the Plugin and I watched some videos about it. In their videos they said it's a combination of three things.
 
1.ARC Mic
2. ARC Software
3. ARC Plugin
 
I'm on a tight budget and I like to work with the things I have inles deemed neccesary.
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@Droddey. What is bass cancellations?
 
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Take a look at this video around 2:30 http://www.youtube.com/watch?v=uSMJ89iOJY8

If it helps you any...ARC=necessity for me. I'd be out of business if I didn't have it. If you can't hear the right stuff coming through your monitors, you can't make the right calls. Be careful what you buy into when you read people talking negatively about ARC. The ones that have a go at it, don't have it and have never used it. It's rare you'll see someone that set it up correctly come on here and bash it as being garbage.
 
I've read about 12 people that were unhappy with it. With each one, I asked them to try my instructions on how to set it up and offered to help them get it sorted. Some tried what I said and were successful, one man replied back telling me he tried everything I said and still failed. The other know-it-alls that ignored me like I wasn't here kept on bashing it.
 
It works for me and several others that are getting good mixes. When I tell you I'd be out of business without it, I'm not lying. I'd never be able to judge the sound I get through my monitors without ARC. I'd be back to mixing through AKG 240 cans...which was a fair fix for me, but still way short of what I get now.
 
-Danny

Hmmm.... I'm getting confused here...ARC, Bass Traps, room size

@Bit. where is his instructions?
 
 
I seem interested in the IK Multimedia ARC from watching the you tube videos about them. I always had a feeling that I may not be mixing or Eqing correctly because even when I like the way it sounds when mixing/EQing and then I listen to it in the vehicle, it sounds different. Though can any mic work with Ik Multimedia ARC software or does it have to be the mic they sell?
 
Is IK Multimedia the only program that does this kind of thing?

Dean's right, ARC and its ilk can do nothing to combat resonant nulls.

Here's what that means: Sound will bounce off walls, the floor and the ceiling, eventually coming back to meet itself. The direct sound and the reflected sound collide, and get added together. If they happen to be in phase, they'll get louder. If they happen to be out of phase, they'll subtract from one another and get quieter. This is what screws up the frequency response of your room.

At certain frequencies, the main signal and the reflected signal will meet completely out of phase with one another, causing them to cancel each other out ("destructive interference"). At that specific spot, and at that specific frequency, there is effectively no sound. That's called a null, a dead spot in the room and the ONLY way to treat it is to provide acoustical absorption so that the reflected signal has less energy to add to the original direct signal.
I don't know if this is related, but sometimes when I am mixing using my monitor speakers, sometimes it's like I can't hear on one side (e.g. my left ear is like deaf) But when I turn my head or move around, I can hear again.
(Note that when nulls are very close together we don't experience them as silence, but rather as comb filtering. That hollow, in-a-pipe sound. That's nasty too, but easier to treat because we're talking higher frequencies. When we talk about nulls we're usually referring to low-frequency problems, where the nulls are far enough apart to create noticeable dead spots.)

If the primary signal and the reflection meet exactly IN phase with one another, then they add together and cause a resonant PEAK. Just the opposite of a null. (It's called "constructive interference".) In that case, you hear way too much of that frequency. ARC can help with that by reducing just that frequency from your speakers' output.

However, ARC cannot "fix" peaks for every position in the room. That's because at a given frequency, there will be peaks AND nulls at different points within the room. What ARC can do is reduce the peaks at one important position - the spot where you sit when you mix. To someone standing behind you, it might still sound like crap!
 
So, "constructive interference" is when the direct sound and the reflecting sound are in phase and make it sound louder and "destructive interference" is when they are out of phase and either become softer or null?
 
ARC only helps with "constructive interference" and not "destructive interference"?
 
Please correct me if I'm wrong.

These are the reasons ARC isn't a complete solution, but rather the finishing touch you apply after addressing problems acoustically as best you can.
 
I mix and EQ in one spot where I sit all the time when I work on projects.

Whatever you decide sharpdion, best of luck. The guys here know what they are talking about moreso than I do and I mean that sincerely....but I have experience with this plug because I own it.... and honest when I tell you....it truly works. Nulls, errors, issues, whatever the case, it truly works for me. I'll leave it at that.

-Danny

This is why mastering rooms typically have more than one subwoofer. At one time I thought that seemed like a needless extravagance, but after reading Dr. Toole's book I understood why they do that. You'd actually benefit greatly from having 3 or 4 subwoofers in a room!

One trick they teach in home theater calibration is that the room response at a given position is reversable. I.e. if you take a sub and put it up on a stand at your head level at the mixing position, you can then go around and measure room response at various places along the floor. When you find the best spot, put the sub there and the response back at the listening position should be basically the same. So you can avoid the huge effort of moving a big sub around and remeasuring many times.

I've not proven to myself that this is true, but it would appear on the surface to be so. If you looked at it as reflection points, using say light, you would have the same reflection points from the head position off of each surface so as to see the sub as you would have looking from the sub to each point where you see a reflection back to the head position.

Presumably, once you found a good position for sub #1, you could take a plot of the frequency response, then start looking on the other side of the room for a position that yeilds as close as possible to an inverse response, right? So where one is weak the other is solid, and vice versa.
 
Though, in general, based on my own experience, I'd argue that you should think hard before using a sub at all in the sort of small rooms most of us have. It creates a lot of complications at the point where the speakers and sub start to crossover. I measured a bazzillion or so different variations back when (with lots of bass traps in place), and that crossover point, using lots of different slopes and frequencies, was messy as measured. I had a box that let me control the xover frequency and slope on both sides, so I could do lots of experiments. Get the xover frequency semi-clean and it would react with the frequencies above or below the xover to make the peaks and dips worse. Make it stop doing that and the xover frequency area would end up usually as a very visible comb filter.
 
Ultimately I sold it and just stuck with the HR824 mkIIs, which had plenty of low end for my needs and I could get a cleaner low end response with just them.