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PostPosted: Mon Jan 09, 2012 9:29 am 
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Hi,

Firstly, thanks for a wonderful site. I've already learned a lot and am looking forward to doing a lot more reading here. :)

Apologies if I've overlooked the answer, but I'm trying to track down information on how to test a room, both before, during and after applying treatments. I'm especially interested in getting a reasonably accurate sound from my monitors for mixing.

I know that my room (a home hobby attic style arrangement) is giving me unreliable information. A quick glance at the current layout is enough to show that it has obvious faults that need fixing. I downloaded an LFSineTones test file, and sure enough the volume and character of the sound appears to change as it works through the lower frequencies. Even my newbie ears, as yet uncalibrated by experience, can hear very noticeable effects.

But there's obviously a big range of possible difference between say a bass guitar and a banjo, and I don't want to rush ahead and try and solve an obvious problem without properly evaluating what additional and overlapping problems that I also have, and which I might actually make worse. Is it practical, or useful, for me to make up other test files that go across a fuller range, maybe with a synth or using instrument samples of some kind? Perhaps there other similar test files already around that I could use, or are there better ways than that anyway?

Any assistance or direction towards answers would be great.

Thanks,

Chris
(aka Baffled of Perth Hills....)


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PostPosted: Mon Jan 09, 2012 11:31 am 
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Hi Chris, and Welcome! :)

The best way I know of to test a room is with REW (Room EQ Wizard), which you can download for free from the Home Theater Shack website. (I think you have to be a member there to download, but membership is free too.)

You also need a good measurement mic, and a good full-range speaker to do the tests. You'll need to play around a bit to figure out how it works, and how to get valid results from it. You should position the mic at the listening position, aimed forwards and angled upwards slightly, at around 30° to 45°. Then you need to very carefully measure the exact location of that mic, in order for future comparisons of tests to be valid. Each time you want to make a new measurement, you have to get the mic back in the exact same position, accurate to within a few mm, so measure very, very carefully. (One method is to attach a plum bob to a fixed point on the ceiling, and mark the exact length of the string, so each time you want to get the mic back to the correct location, hang the plumb bob from the same point, at the same length, and position the tip of the mic at the point of the plum-bob.)

REW can do a whole bunch of tests, and can display the results in various ways. The problem then becomes interpreting those results correctly! :)

You can also post photos / diagrams / dimensions of your current room here on your thread, so we can help figure out what might be wrong, and how to fix it. What seems to be wrong, to your ears? Have you treated it at all yet?


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PostPosted: Mon Jan 09, 2012 12:41 pm 
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Hi Stuart,

Thank you so much for the welcome and for such a quick and informative reply. :D

I have to go out for a couple of hours now but I'll download the tool you suggested, and also post some more information when I get back.

Cheers,

Chris


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PostPosted: Mon Jan 09, 2012 2:15 pm 
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Chris - I've moved you post to the acoustics forum where it's more appropriate.

A simple test you can do is to sit in front of your speakers and cup your hands behind your ears, it eliminates the rear of the room reflections so you can get a better idea of what is coming from the speakers and what effect the room is having.

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PostPosted: Mon Jan 09, 2012 2:51 pm 
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Thank you John. I was a bit tentative about posting straight into the acoustic forum with my very first post, but it's good to be here now.

Thanks for the ear cupping tip. I'll try that. Your words elsewhere about open air work were interesting too. I've got quite a decent sized open verandah space that might be usable for doing some kind of approximately open air test too. Not directly relevant to the room, but I might learn a bit more about the monitors and the test file(s).

And thanks for such a great site too. Before I press any more buttons, there's an orange one on the site software, just above me that I need to press to feed a wee bit more kero into the site engine.


Chris


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PostPosted: Mon Jan 09, 2012 4:22 pm 
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Ok, I have downloaded the REW test program. Whilst I'm reading the instructions I'll post a couple of pictures.

I haven't attempted to do any room treatment as it's just been used as a general practice space with no real thought to recording or mixing before.

I've read quite a few DIY projects at other sites and have been struck by the lack of detailed objective information about the condition of the room before and after the work. Mostly, the builder seems to have some general ideas about what various panels and traps will do, from a broad perspective, but they never seem to talk about any before and after testing. Of course they all agree that the room was perhaps 'boomy' or whatever, before and that it's definitely 'better' now... but that's all a bit vague. :?

So I'm keen to start by getting some baseline figures so that, firstly, I know what problem(s) I'm actually trying to solve and how they overlap, but also to get a bit more knowledge about what the pitfalls are. I assume that the 'perfect' room is a logical and technical impossibility as it might get used for tasks that require different characteristics. Indeed, I'm pretty much a blank slate, a solid block of ignorance...

My equipment is basic hobby stuff but (I hope) not complete junk. It should be good enough to at least get some practice with the testing routine while I learn what needs to be upgraded. It's also purely for fun and interest, not as a commercial venture.

Currently, pretty much everything is wrong, but I see that as actually a bonus. If the improvements were only going to be subtle my inexperienced ears might not pick them up. This way I should be able to tune both the room and my ears as I go. The first task would seem to be to shift the whole circus around so that the monitors point down the room and get out of the corner. The desk is not as close to the wall as it looks, but it all needs to move off that wall altogether.

Dimensions: 4.7metres wide, 8.3meters long, walls 1.5m, ceiling ridge 2.4m. Carpet on the floor.
Image

My hobby is songwriting, so full band recording isn't an aim. Just a comfortable space for a one man band that isn't too badly skewed for mixing .
Image

Monitors, showing the sophisticated and absorbent isolation stands...
Image

I have Pro Tools 10 as the main DAW and have since added an extra monitor alongside the iMac. I also have a reasonable mic stand, a Rode M3 mic and a cheapo Ashton dynamic mic. The interface is an MBox 3 Pro (or the Eleven Rack in the pic).

Thanks for looking. Not looking for specific treatment suggestions at this stage, but all enlightenment is welcomed especially about testing and understanding results. It can only get better...

Chris

PS The pics are hosted at my own web space, but I could shift them if necessary. Haven't found the site instructions for upload method and required sizes yet.


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PostPosted: Tue Jan 10, 2012 7:25 am 
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... just following along with your thread [... always trying to learn :)].

If I may ...

To go along with the REW software, there are recommendations for the BEHRINGER ECM8000
cal mic. It is one of the more 'affordable' cal mics.

During my build, I DID try to do a 'before and after' snapshots with REW, as a way to document the progression of layout and treatment. REW is handy, in that it allows you to keep adding new 'shoots' all within the same file. Makes it easier to compare.

My 1st issue with REW is trying to interpret WHAT the data/graphs are showing :? True, I was rather eye-ball deep in trying to build the room .... learning on the job :shock:

Another issue ... I've recently added new powered monitors to the equation. For the past few weeks I've been evaluating them. Tonite I hope to re-shoot the room with these monitors.

Anyway ... look forward to following along. As you are aware, this is quite a unique site. The MODS, and several of the experienced 'visitors' are so generous with their knowledge, experience AND their patients ... for those of us trying to learn, understand.


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PostPosted: Tue Jan 10, 2012 10:34 am 
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RJHollins wrote:
.

To go along with the REW software, there are recommendations for the BEHRINGER ECM8000
cal mic. It is one of the more 'affordable' cal mics.



Hi,

Thanks very much for joining in. :) I see that the mic you mention is available reasonably locally, so I might slip down to the city and get one on Saturday.

I just found your thread about building a control room in the basement - most interesting.

I've done a couple of preliminary recordings with my current mics where my head would be, and the results are suitably alaraming! :wink: I think I'll spend the next few days trying to get at least basic grasp of testing before I start lumping the furniture around.

What I've done so far is to just playing testing tones through the monitors and recording back into Pro Tools to see what the signals look like. Here's one from a Low Frequency Sine Tone file. Apparently it's a "chromatic scale of sinewave tones spanning 24Hz-262Hz":

Image

It sounds pretty much like it looks - big perceived drops in volume at some spots and louder in others.

Long way to go yet... :)

Chris


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PostPosted: Tue Jan 10, 2012 11:05 am 
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Without knowing more about the specific tones, it's hard to say what you are looking at there. Also, simple graphs like that of single tones only tell you about the frequency domain, but your real problems in the room are in the time domain as well: In other words, how the room response changes over time, both during the tones and AFTER the tones stop playing. You can best see that on waterfall plots, impulse response plots, ETC curves, and things like that.

By the way, I love your speakers decoupling "pads"! :) VERY original. Never quite seen anything like that before.... :lol: Not sure how effective they'd be at decoupling, but they win the prize for being the most "creative" decoupling devices so far this year! :)

---

OK, on to a more serious note: Before thinking about treatment for your room, I'd suggest that you should think about layout. Priority #1 should be to get perfect symmetry around your mix position, and in very close second place, Priority #2 is getting the geometry correct. Practically on the heels of both of those, Priority #3 is getting your first reflections points treated correctly. Tripping along right behind is Priority #4: bass traps. Large ones, and lots of them. Then, a little distance further back, comes Priority #5: the rest of the room treatment.

So you should start by re-arranging your room such that your desk, chair and speakers are facing that far window, and centered about the axis of the room, right under the ceiling peak. Then set up the speakers and listening position in the correct geometry, then add treatment at all first reflection points (thick absorption), then install large bass traps (maybe superchunks) in the corners.

If you take accurate measurements at each stage, with your mic in the exact same position each time, then you'll be able to see the small steps in progress as you add each item. That will help to figure out the next steps in treatment.

- Stuart -

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PostPosted: Tue Jan 10, 2012 11:58 am 
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Soundman2020 wrote:
Without knowing more about the specific tones, it's hard to say what you are looking at there. Also, simple graphs like that of single tones only tell you about the frequency domain, but your real problems in the room are in the time domain as well: In other words, how the room response changes over time, both during the tones and AFTER the tones stop playing. You can best see that on waterfall plots, impulse response plots, ETC curves, and things like that.


The tone file was downloaded from Mike Senior's site and is one of the resource files for his book Mixing Secrets For The Small Studio ( Download page) . It seems like a very well regarded book. A similar file was included with an article in Sound on Sound magazine.

I'm playing the sound out into the room, so shouldn't the mic be picking up the sound developments over time too? I mean if there's reverbs, etc they should all be picked up. The tricky part is presumably teasing out what each point on the graph actually consists of. As you said above, the interpretation is the ticklish part. :)

My thinking was that my main focus is accurate mixing and I'm not so concerned about the room characteristics for recording instruments with a mic ( I mainly play directly in). So if I record what my ears would hear, by placing the mic in that spot then the mic should give me a reasonable idea of how much the signal diverts from the sort of even sounds I'd get if the room was dead true (or through headphones. That file sounds dead level through cans). My hope at this stage is to get the room as accurate as possible with good layout and some appropriate treatment and then compensate for any remaining bad areas by knowing the points which I'll need to adjust for.

Quote:
By the way, I love your speakers decoupling "pads"! :) VERY original. Never quite seen anything like that before.... :lol: Not sure how effective they'd be at decoupling, but they win the prize for being the most "creative" decoupling devices so far this year! :)


They seem to work pretty well. I've got the packaging sorted and hope to be selling them soon at a supermarket near you... I'll put you down for a set if you like. :wink:

Image


Quote:

So you should start by re-arranging your room such that your desk, chair and speakers are facing that far window, and centered about the axis of the room, right under the ceiling peak.


Major re-arranging to get everything facing down the room instead of across it is definitely the priority. I'm keen to get on with it, but I'm going to spend a few days learning more about testing first, especially as I can guarantee some obviously bad results in the existing position.

I'm interested in trying out John's 'open air' version too. If I set it up in the garden there will be no walls for the sounds to bounce off, so I'll be interested to see what testing there looks like.

Thanks for your input. I've learned a lot here in the last day or so, but the old brain can only take it in so fast! :roll: We'll get there...

Cheers,

Chris


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PostPosted: Tue Jan 10, 2012 1:13 pm 
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This is the same LFSineTone file recorded with only a single speaker turned on. The mic was placed about 5" from the cone and a thick folded doona put over it all. The idea was to try and remove the room as much as possible. but of course I can't remove the effect of the speakers themselves or the mic. Judging which is which isn't easy... It's much less erratic than the first shot though, which is what you'd expect.

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PostPosted: Tue Jan 10, 2012 2:05 pm 
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Quote:
I'm playing the sound out into the room, so shouldn't the mic be picking up the sound developments over time too?
Yes, but you are not seeing them in those highly simplified waveform graphs! They really aren't much use at all, except as a first approach to finding out if you have uneven response.

The problem isn't reverb (although that's an issue too, but isn't really valid for small rooms anyway, since small rooms cannot actually have a reverberant field). The problem is room modes. Modes are standing waves, a form of resonance, which build up rapidly and "store energy" (not strictly true, but its a good way to think of it). After the tone STOPS, the speaker cone stops moving, but the energy carries on resonating for a while, dying out slowly over a period of time. You CANNOT see that in your simple waveform graphs, and even if you could, you would not be able to analyze it from a waveform. It needs to be transformed into the time domain, and the frequency domain, since tones can excite modes that are not at their own frequency, but are nearby. You'd never see that on a waveform. Rather, you need graphs such as ETC (energy time curves), IR (impulse response), waterfall plots, and things like that, which you find in proper acoustic testing software, such as REW. For example, you'd never be able to see a reflection in a waveform plot, but they stick out like sore thumbs in IR and ETC graphs. You'd never be able to see the overall RT-60 across the entire spectrum (or rather, the modal equivalent in a small room) from a waveform, but it's glaringly obvious in waterfall plots, and somewhat obvious from an IR graph (or an ETC). And so on.

Sorry, but that's a very simplistic, basically meaningless test that you are trying to run: It tells you that you have a problem with the room, but it cannot tell you what that problem is: Is it the speakers? Flutter echo? A reflection? Comb filtering? A mode? Reverberation? Some other form of resonance? What is the amplitude of the problem? What is the Q of the problem? Is it related to other problems? Etc. As you just discovered, you cannot even determine if you are seeing a speaker problem or a room problem from such a test! It tells you very little that is useful at all, other than "You seem to have a problem".

Quote:
I mean if there's reverbs, etc they should all be picked up.
Small rooms cannot possibly have a reverberant field, or at least not a statistically valid one. In order to have reverb, you need a free path of at least six or seven times the longest wavelength of interest. That's impossible in small rooms, for low frequencies: their dimensions are just nowhere near big enough to support a reverberant field: All you have in small rooms is direct field, reflections, and modal behavior (plus some other stuff that isn't really worth bothering about).

Quote:
The tricky part is presumably teasing out what each point on the graph actually consists of. As you said above, the interpretation is the ticklish part.
Exactly! but you have to be looking at the right type of graph! :)

Quote:
My thinking was that my main focus is accurate mixing and I'm not so concerned about the room characteristics for recording instruments with a mic ( I mainly play directly in).
Well, it's hard not to be blunt about this, so I guess I have to be! :) Your thinking is not correct. Your goal in mixing is to hear exactly what was recorded, accurately, without any coloration from the room, the speakers, or the sound system. Therefore, the room must be neutral, acoustically. It must not do ANYTHING to the sound, except reproduce it with extreme accuracy. So the room must have flat frequency response, and flat RT-60 response, and evenly spread modal response. The RT-60 must be LESS than that of the associated live room, or you'd never hear reverb tails from the natural sound of the live room itself. It must be symmetrical and perfectly balanced, in order to have an accurate sound stage, clear stereo image, and perfect phantom center. There must be no first order reflections getting back to the mix position within 15 ms of the direct signal, and 15 dB down from it. Etc. There are a whole slew of characteristics that a good control has to meet, and they are all related to the room not doing anything except tell you the truth. I've seen a few people go into a great room for the first time, listen to the session for a bit, and declare that the room sounds terrible because there's no warmth to it. That's the whole point! The truth sounds ugly, sometimes, but that's what an engineer needs to hear: the truth: He has to hear exactly what is in the music, and nothing of the room. If he can make the mix sound good in such a room, then it will automatically "translate" well, and sound great elsewhere. But if the room is coloring the sound, or not telling the truth, then the engineer will subconsciously compensate for that, and even though the mix sounds good in the room, it won't sound good elsewhere: it wont translate, because the room "lied" to the engineer.

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My hope at this stage is to get the room as accurate as possible with good layout and some appropriate treatment and then compensate for any remaining bad areas by knowing the points which I'll need to adjust for.
Well, some people can do that, but I never have been able to. If I try to mix in a room that doesn't tell the truth, then my mixes don't translate well, since I subconsciously "corrected" the room defects in the mix, so it sounds bad in places that don't have those defects, or that have different defects. I find it really hard to keep in mind all of the deficiencies of a room, and apply those to the mix while also trying to mix! Most people's auditive "memory" only lasts a few seconds anyway. That's why it is so hard to compare mixes without doing direct A/B switching on the fly: your brain just can't remember what the "old" version sounded like for more than a few seconds. So mixing in such a poor room is not easy (for me at least), and requires constant reference to a known track, in order to refresh my memory of what is wrong with the room, and apply it to the mix. Tiring, boring, non-creative. Not the way I want to mix. But if you can handle that, then fine.

Quote:
I'm keen to get on with it, but I'm going to spend a few days learning more about testing first, especially as I can guarantee some obviously bad results in the existing position.
To be really honest, I suggest that you don't waste your time learning about testing, but rather learning about small room acoustics and studio design. The testing won't tell you how to design and build your room, and you won't really need to test much until you get the basic room into shape, with the basic treatment in place. THEN you test, to see what still needs to be done.

Sorry to be so blunt and to-the-point, but that's kind of the way we are around here: tell it like it is, no sugar coating. It tends to get your attention better! :)

Quote:
I'm interested in trying out John's 'open air' version too. If I set it up in the garden there will be no walls for the sounds to bounce off, so I'll be interested to see what testing there looks like.
An "open air" studio in the middle of a an empty field is great! Until it rains... :) And isolating such a room is pretty hard to handle as well... :)

Quote:
This is the same LFSineTone file recorded with only a single speaker turned on.
That's the way you should ALWAYS run acoustic tests! If you test with more than one speaker on, then you have no idea if you are testing the room, the speakers, or the interference patterns created between them. Test one speaker at a time, calibrate one speaker at at time, then do the final stereo test with your ears.

Quote:
The mic was placed about 5" from the cone
Which cone? :) 5" from the woofer means it is way off-axis for the tweeter, and therefore it is not a valid test. Correct testing distance for speakers is one meter (roughly 39 1/2"). Closer than that, and the fields from the various cones and ports have not yet fully merged.

Quote:
The idea was to try and remove the room as much as possible.
The only way to do that is, well, to remove the room! One "old" way of testing speakers was to bury them in sand, out in the open, facing upwards, with the ground being exactly level with the front face of the speaker: Hang a proper omnidirectional mic with perfectly flat response exactly one meter above the speaker, exactly on the acoustic axis, and measure. That is an real infinite baffle measurement. More modern measurements are usually done in anechoic chambers.

Quote:
but of course I can't remove the effect of the speakers themselves or the mic.
That's why you need a proper measurement mic. Like Brian said, The ECM 8000 is inexpensive, and not too bad.

What mic and speaker are you using for these tests right now? To me, it looks like a lot of what you are seeing on your waveform is speaker-related, especially the bass roll-off.

- Stuart -

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PostPosted: Tue Jan 10, 2012 3:13 pm 
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Hi again,

Thank you so much for taking the time and trouble to post all that Stuart. :)

I'll digest as much of it as I can, and also work through the REW instructions. They seem to have a pretty good forum for it over at Home Theatre Shack, so I'll spend some time there and return here when I'm more up to speed on it.

Cheers,

Chris


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PostPosted: Tue Jan 10, 2012 8:55 pm 
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Soundman2020 wrote:
Sorry to be so blunt and to-the-point, but that's kind of the way we are around here: tell it like it is, no sugar coating. It tends to get your attention better! :)

- Stuart -


Blunt is good. :mrgreen:

Oddly enough, over the years I've found that often the best way to get detailed help is to actually jump in and get something wrong. It often elicits a much more wide ranging response if you do or say something that looks obviously in error. I don't do it deliberately, but I never mind looking uninformed or even mildly foolish, because it frequently it seems to give people the opportunity, or permission or something, to fire off with both barrels. Then you get the chance to gather a lot of information quickly. :)

I went out last week to help put a bushfire out and the guy with me was far more experienced than I am, and he sees me as a newbie at it (with justification). So while I drove the fire truck he fired off a steady stream of instructions on how to drive, where to go, etc. and then on the fire-ground he kept up a barrage of instructions on every conceivable aspect of operating the gear and fighting the fire. Not everything he said was necessary, but it was still a great learning experience. I'd rather get too much blunt info than not enough any day. And we got the fire out, which was the main thing.

OK, back to the studying...

Cheers,

Chris


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PostPosted: Sat Jun 02, 2012 5:00 am 
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Sorry to jump on the topic but I'll be doing similar calculations in a few months and I wanted to ask: You mention a 'full-range' speaker for the testing via REW; is there any particular 'cal speaker' you'd recommend for the tests (in the same way Behringer ECM8000 as the cal mic)?

Thanks,
Paul


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