# First up graphs, advice appreciated



## dodittydada (Jan 25, 2009)

First measurement. Room has some bass traps and touches of other treatment. Thoughts? Im just trying to learn to interpret these things. Obviously a big problem between about 60Hz and 260Hz.

Two channel studio monitoring, no sub. Only gone up to 3K due to RS meter. Only the left channel is measured here for the moment


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## Anthony (Oct 5, 2006)

*Re: first up graphs, advice appreciated*

What are your speakers and sub? Are you planning on using an EQ for the sub?

Room effects and a gradual roll-off of the speakers can contribute to the low output in the 100 to 200Hz region. Then comes the conundrum. If you want SPL at the low end, you have to jack up the sub, but then that peak at 60Hz will be noticeable and boomy.

If you do choose to EQ, you can tame that peak and raise the level of the sub for good results.

I have a similar problem in my setup: my mains roll off a tad above 80 Hz leading to low output in the high-bass region.


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## brucek (Apr 11, 2006)

*Re: first up graphs, advice appreciated*

Well, since this a studio with two channel monitoring, with no sub, then your effect on the monitors is limited to positioning of the speakers and treatment. Usually the listening position in front of the mixer board is fairly well fixed, so I suppose you have to start playing with the monitors to get the smoothest response, or get them near-field enough to help remove the rooms influence. Hopefully you're able to add treatment behind and overhead.

You'll want to change the scale of your waterfall to 15Hz to 100Hz (max 200Hz) and change to LOG mode so you can see the waterfall the same way that you hear (logarithmically) and to help to expand the lower octaves where you want to see more of what's happening.

For your response plot use horizontal scale of 20Hz-3000Hz.

EQ is usually a no-no for mixing, so that's out, but you should be able to get it a lot better than it is now...

brucek


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## Anthony (Oct 5, 2006)

*Re: first up graphs, advice appreciated*

Sorry, missed that about the sub and studio (no coffee this morning).

Bruce's suggestions are good:
Nearfield -- use the IR windowing feature in REW to eliminate reflections from the measurement
Treatment -- help smooth out the response, especially in the low end.


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## dodittydada (Jan 25, 2009)

*Re: first up graphs, advice appreciated*

thanks guys, Ill experiment and see what I can come up with.


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## dodittydada (Jan 25, 2009)

*Re: first up graphs, advice appreciated*

ANthony,

What exactly is IR windowing. Im looking as we speak but havent found it yet... Ok I have found it, just investigating what it does and how I should set it


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## Wayne A. Pflughaupt (Apr 13, 2006)

*Re: first up graphs, advice appreciated*


Never used it so I can't tell you how it's done, but "what it is" is a super-quick, snap-shot measurement that shuts off the signal before it has a chance to start echoing around the room. The result is a measurement akin to anechoic.

Regards,
Wayne


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## brucek (Apr 11, 2006)

*Re: first up graphs, advice appreciated*



> Nearfield -- use the IR windowing feature in REW to eliminate reflections from the measurement





> What exactly is IR windowing.





> shuts off the signal before it has a chance to start echoing around the room


But, in this case, you don't want to alter the IR gating, since the gating puts a limit on the lowest frequency and the resolution of the response. For example, if you were trying to limit reflections from surfaces 1 meter away to eliminate comb filtering when high frequency testing, you might use a 6msec gate (d=(time*speed)/2), but this would limit the lowest frequency of usable response information to 167Hz (1/gate time). Not much use in this case where we're measuring 20Hz-3KHz. The default windows are much better for this low to mid frequency testing.

brucek


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## Anthony (Oct 5, 2006)

*Re: first up graphs, advice appreciated*

That is true, windowing lowers your effective frequency, but you can allow second and third reflections to keep the window longer and still see the lower octaves. You start introducing some comb filtering, but you get the extended response.

When I test speakers in my basement, I have to do this since it's not that big of a room and there's no way to make it truly anechoic (it's also my theater, and bar, and game room). Just another lever to play with for testing. Usually, though, I'm only interested down to 60 or 80 Hz.

What are the dimensions of the studio? That might give some insight as to the peak/valleys. Construction? (concrete, drywall, etc).


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## dodittydada (Jan 25, 2009)

*Re: first up graphs, advice appreciated*

Ok, small improvement, but not in the area Im looking for! Two speakers here, the left only was lost when the comp. crashed.

I cant get anywhere near enough more trapping I dont think to help the low bass. So Im now thinking other trap types for solutions. Helmholtz got a wide enough Q in some form?

Purpose built $$$ studio's aside, I.E your typical make shift room for mxing, how close to flat can one be realistically trying to achieve.? Within 10db be a nice job? I need to know where the finish line should be.


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## brucek (Apr 11, 2006)

> how close to flat can one be realistically trying to achieve.?


The dip at 200Hz is quite bad. What's the problem? 

Does a single speaker measured near-field in the middle of the room enjoy a flat response? Lets be sure that the room is the culprit.

brucek


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## dodittydada (Jan 25, 2009)

I am sure the speakers are fine.


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## dodittydada (Jan 25, 2009)

hmm, Im real suss on my chart. I mean, Im trapped to the * in here. Two front corner traps are a foot thick each, the ones behind the speakers 8 inches, rear traps are 6 inches thick....they are all 6ft tall and nearly reach to the ceiling. yet I still have a massive dip at 200hz in a rectangular room nearly 20ft deep??? I didnt expect flat of course, but this dosent seem right. I guess Im especially suss seeing its a dip and not a peak. Im sitting at the 38% point towards the front, I should be out of the way of any major modes.I just dont know what to do next besides start looking at slot resonators.


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## thewire (Jun 28, 2007)

dodittydada said:


> hmm, Im real suss on my chart. I mean, Im trapped to the * in here. Two front corner traps are a foot thick each, the ones behind the speakers 8 inches, rear traps are 6 inches thick....they are all 6ft tall and nearly reach to the ceiling. yet I still have a massive dip at 200hz in a rectangular room nearly 20ft deep??? I didnt expect flat of course, but this dosent seem right. I guess Im especially suss seeing its a dip and not a peak. Im sitting at the 38% point towards the front, I should be out of the way of any major modes.I just dont know what to do next besides start looking at slot resonators.


Have you tried some trapping on the ceiling between the LP and the mains before?


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## Anthony (Oct 5, 2006)

What are your exact room dimensions and relative speaker placement from wall/side/ceiling?

200 Hz 1/2 wavelength is about 5.5 ft. If you have an 11' dimension in the room and the speakers are right at this point, they'd be at a null.

Is there anything in the room that could be an accidental helmholtz or panel resonator? Sliding glass door, open doorway to another room. Dozens of empty water jugs in the corner?  Originally my riser was acting as a panel resonator and sucking out a lot of bass until I added more insulation and doubled up on the plywood.

but that response is odd.


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## JohnM (Apr 11, 2006)

Try measuring forwards and backwards of your current position, 38% is not gospel, it is only a reasonable starting point to begin measuring to find the actual optimum for the room. Try moving the speakers also, as far as your space can accommodate. Easiest to use the RTA and the Pink PN generator when moving around, so you get immediate feedback of the effects.


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## dodittydada (Jan 25, 2009)

Anthony said:


> What are your exact room dimensions and relative speaker placement from wall/side/ceiling?
> 
> 200 Hz 1/2 wavelength is about 5.5 ft. If you have an 11' dimension in the room and the speakers are right at this point, they'd be at a null.
> 
> ...



hmm, how about a fridge? Got one of those centre of the back wall. Could it possibly be sucking bass? with a closed door probably not. Width of the room MAY be around 11ft, not sure. Ill go measure it.


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## JimP (May 18, 2006)

Have you calibrated your sound card and do you have any graphs of what the room was doing before adding any acoutic panels and bass traps.

I think another worthwild question is are we really sure that your amplifier speaker combination is not causing these dips?


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## dodittydada (Jan 25, 2009)

JimP said:


> Have you calibrated your sound card and do you have any graphs of what the room was doing before adding any acoutic panels and bass traps.
> 
> I think another worthwild question is are we really sure that your amplifier speaker combination is not causing these dips?



unofrtunately, no testing before treatment. What possible things amp/speaker wise could cause a dip like this? At least then I know where to start investigating. thanks.


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## JimP (May 18, 2006)

I'm just saying that before you make yourself crazy fixing the room, be sure that you don't have something else going on with your electronics/sub/speakers. Unfortunately, if near field testing doesn't give you a flat response curve, then dragging it all outside to test it would be about the only other way to check it. How many feet of snow do you have?


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## dodittydada (Jan 25, 2009)

Ok, with much hassle, been moving speakers around. Hows she looking now in your experience?


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## Anthony (Oct 5, 2006)

It definitely improved your 200 Hz response. Still got that nasty resonance at 55 Hz or so. 1/4 Wave for that is about 5', full wave is 20'. You mentioned a 20' deep room. It looks like you may need some absorption to break up that standing wave.

To me, that resonance would sound more objectionable than the lower bass output.

But that being said, 8dB is pretty low for the bass. I would say, the speaker designer might have counted on some room gain to boost the bottom end, but if these are nearfield monitors, that wouldn't have been the case.


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