Yet Another N00b - Graphs

[sparky5]

Hi All,

I have been into audio for a long time, but just starting to learn about room acoustics and specifically how to measure responses.

I am trying to set up a home studio mixing room (on the cheap). I did a measurement of the room using the Radio Shack SPL with calibration files, along with my Behringer B2030A monitor speakers (cheap, I know, but not bad).

Please see attached graphs and provide any comments. I have no idea what I am looking at here really so any help is very much appreciated. I am guessing I need to treat the room to flatten things out so the impulse tracks more in line with the dotted line on the graph?

cheers

[MikeD]

Looks like your target level is too low. If you notice most of the response is above the dotted line. I think there's a way to automatically have REW give you the proper target but others are more experienced to tell you how than I am.

[sparky5]

Thanks for the fast response.

I am not sure I completley understand but would love to learn more.

I found an option to do with target responses in REW and enabled it. See below.

Is this the sort of line you were talking about?

I did the measuring at 80db. Should I do the measurement at a much higher level?

Thanks

[Wayne A. Pflughaupt]

I did the measuring at 80db. Should I do the measurement at a much higher level?

Your measurement level is fine. If you want to adjust the Target Level that appears on the screen, click on the “Target Settings” icon on the left side of the screen. There you’ll find a field to enter the desired Target Level. You probably want it at about 80 dB.

Please see attached graphs and provide any comments. I have no idea what I am looking at here really so any help is very much appreciated. I am guessing I need to treat the room to flatten things out so the impulse tracks more in line with the dotted line on the graph?

Well for starters, the Radio Shack meter is only accurate up to about 3 kHz, so you put much stock in anything you’re seeing above that point. If you want accurate measurements up to 20 kHz, it will require additional equipment – a calibrated mic and a pre-amp with phantom power.

Treatments – bass traps could probably smooth out response from ~100 Hz up to maybe ~400 Hz (I’m not sure how high they will affect). But since you’re on a budget I doubt they will be an option for you. I know that speaker location options are usually extremely limited in a home studio, but if you have some options you might experiment with that. Your best bet will probably to get a decent quality parametric equalizer to smooth out some of the peaks and dips.

Regards,
Wayne

[sparky5]

Thank you very much for the comments - they are very helpful.

I am able to swing the cash for some bass traps, diffusers e.t.c so I may give that a go.

To be honest, more important for me at the moment is learning the theory of acoustics from a purely intellectual point of view, and I think REW could help me do that.

So, to attenuate some of the bass frequancy hills in the graph I take it the appropriate amount of bass trapping (in the right locations) might bring this back towards the smooth dotted line.

How about the nulls? Those dips below the line? For the very low bass frequencies I think I would need a sub - my monitors will just not produce that kind of volume down to that level as the speakers are too small.

As for the higher frequency nulls (lets assume that the Radio Shack meter is accurate, I want to understand this more in theory than in practice right now) is relocating the speakers the only option to remove the nulls here? I got to thinking about professional studios - surely they would apply some sort of treatment, not move their nice monitors to weird locations to get the right response.

Perhaps it is the original design of a professional studio room that limits the nulls in the first place, and then they need only add trapping and diffusing to eliminate the peaks?

Any information on these ideas, and how to understand the graphs in depth would be very much appreciated. Are there some really comprehensive guides or examples around. I really, really want to get good at this stuff. I have a few acoustics textbooks I am going to study as well to understand things more.

Thanks so much.

[Wayne A. Pflughaupt]

How about the nulls? Those dips below the line?

My understanding is that traps will help with both peaks and dips in the frequency range that they function.

For the very low bass frequencies I think I would need a sub - my monitors will just not produce that kind of volume down to that level as the speakers are too small.

Correct.

As for the higher frequency nulls (lets assume that the Radio Shack meter is accurate, I want to understand this more in theory than in practice right now) is relocating the speakers the only option to remove the nulls here? I got to thinking about professional studios - surely they would apply some sort of treatment, not move their nice monitors to weird locations to get the right response.

Speaker relocation probably won't affect the upper frequencies much, but it never hurts to experiment. You can use REW's RTA feature to see the response changes in real time as you move the speaker.

Regards,
Wayne

[sparky5]

One other question (at this point anyway! ) - does anyone have any recommendations for free (or cheap) software graphic equalisers for use in tuning my room? I have found a few like the 31 band from karmafx dot net but they are VST plug-ins.

Are there any that sit at the operating system level (Windows) and will work on all audio from all applications? For example, I am not sure how a software equaliser could be used with Room EQ in trying to judge the impact of any EQ changes.

Thanks!!

[glaufman]

Wayne A. Pflughaupt wrote:

[FONT="Comic Sans MS"]
My understanding is that traps will help with both peaks and dips in the frequency range that they function.

Yes, that's correct. Assuming the peaks and dips are due to acoustics. Corner traps are especially useful for room modes.

[Jarrod]

sparky5 wrote:

One other question (at this point anyway! ) - does anyone have any recommendations for free (or cheap) software graphic equalisers for use in tuning my room? I have found a few like the 31 band from karmafx dot net but they are VST plug-ins.

Are there any that sit at the operating system level (Windows) and will work on all audio from all applications? For example, I am not sure how a software equaliser could be used with Room EQ in trying to judge the impact of any EQ changes.

What sound card are you using? My SoundBlaster X-fi has a 10-band EQ function in its DSP and a nice software interface. The EQ gets applied to all sound since it is implemented in the soundcard's DSP itself. The entry-level versions of this card can be had for ~$45. Probably not that useful for anything but crude EQ, though, since the bands are fixed.

[Weasel9992]

sparky5 wrote:

How about the nulls? Those dips below the line? For the very low bass frequencies I think I would need a sub - my monitors will just not produce that kind of volume down to that level as the speakers are too small.

I can see the way your speakers roll off at 60Hz or so, but the big valleys above that are modes...looks like you've got something at 50Hz, 100Hz and 200Hz, which it what I'd expect to see if they're related to the same dimension or dimensions. Bass trapping will push down the peaks and bring up the valleys.

Simply adding a sub will actually make things worse at 100 Hz and 200Hz. Think of it this way: have you ever seen a wave tank? It's a rectangular box with plexiglass sides with an oscillating arm on one end. If you start the oscillator you'll produce standing waves that are a function of the size and shape of the tank. Those standing waves will never change position as long as those variables remain the same. If you pour more energy into the oscillations you'll produce higher peaks and lower valleys though.

Your room is the same. The more energy you pour into it the worse the peaks and valleys will get (without treatment). Yes, you could make some headway with a single modal frequency if you place the sub carefully, but the gain probably won't be offset by the problems you'll cause.

I'm not saying that no one should use a sub. I am saying that subs make room treatment even more important though.

Frank

[MikeD]

I think your graph still doesn't have the correct target level. Pretty much everything is above the 75db target. If you move the target level to 80db (no re-measurement necessary, just use the target level button on the left side of the screen) then your graph will look more normal.

[glaufman]

Weasel9992 wrote:

I can see the way your speakers roll off at 60Hz or so, but the big valleys above that are modes...looks like you've got something at 50Hz, 100Hz and 200Hz, which it what I'd expect to see if they're related to the same dimension or dimensions. Bass trapping will push down the peaks and bring up the valleys.

If it starts at 50, and that affects 100 and 200, it should also affect 150.

Simply adding a sub will actually make things worse at 100 Hz and 200Hz. Think of it this way: have you ever seen a wave tank? It's a rectangular box with plexiglass sides with an oscillating arm on one end. If you start the oscillator you'll produce standing waves that are a function of the size and shape of the tank. Those standing waves will never change position as long as those variables remain the same. If you pour more energy into the oscillations you'll produce higher peaks and lower valleys though.

Your room is the same. The more energy you pour into it the worse the peaks and valleys will get (without treatment). Yes, you could make some headway with a single modal frequency if you place the sub carefully, but the gain probably won't be offset by the problems you'll cause.

One benefit of adding a sub, besides getting lower extension, and as you alluded to, is it can be placed (depending on interior design considerations) where it can do the best job of providing a smooth response, which usually is NOT where the main speakers are optimally placed for higher freuqencies. With 60Hz extension on the mains, you might get away with an 80Hz crossover to the sub, in which case it won't be putting out much energy at at 200Hz, so it won't hurt that situation. Similarly, because it can be optimally placed, it could help at both 50 and 100Hz.

But... if smoothness is your primary goal (as opposed to adding LF extension) bass trapping is probably the way to go first and foremost here.

If I were you, I would probably confirm what we think we're seeing: move the mic around to take scans in a few different locations, and superimpose the scans (don't average them). Peaks and valleys that show up on every single scan are most definitely modal. Anything that comes and goes is more likely another phenomenon... bass trapping might still help those, but perhaps different kinds of bass traps...

[sparky5]

Thanks so much for all the comments guys. Feel like I am starting to learn a thing or two.

Since my first post I purchased a Behringer ECM8000.

So, the first graph is from my listening position:



And the waterfall:



On your recommendation, I have taken measurements using different mic placements.

This is from the back left of the room:



This is from the back right of the room:



This is from the front left of the room:



This is from the front right of the room:



And I bought some acoustic batts. They are Sonobatt Silencer +. Their specs are as follows:

For 100mm Nominal Batt thickness, sound absorbtion coefficients from data sheet (not too shabby me thinks) -

125hz - 0.54
250hz - 1.11
500 - 1.23
1000 - 1.12
2000 - 1.02
NRC - 1.10

I am awaiting 8 Bass Trap bags from Ready Acoustics (should be here in a week or so) and will be filling all four corners of the room at 100mm from floor to ceiling. But just for fun now, I grabed 16 batts and shoved 8 in each rear corner, and then put some pillows up at side reflection points to see what would happen. Here is the result:



And the waterfall:



Now I know that this is just a crude placement of some batts, but I can't see much has changed. Am a bit worried now that the bass trapping when properly applied will not do much. Although it does look like the ringing at around 200hz has improved (I am guessing this is one of the room modes? What are the other modes of this room?)

How important is it to treat the early reflection points (compared to bass trapping for example). Given the current small changes with some crude bass trapping, am reluctant to spend any more money if it is not worth it.

This is a small room by the way (typical sized, more or less rectangular study).

Thanks for any more help!

VidarAus

[sparky5]

I threw my room dimensions into modecalc and got the following modal frequencies:



Looks like the dips in the graph would correspond to these frequencies, which are the modes right? And proper bass trap placement (in all the corners) should help this?

[glaufman]

Not sure, but sounds like one thing limiting the effectiveness of what you've done so far could be you've heavily treated 2 corners, while leaving another 10 untreated... by this I mean, the corners in question are 4 x floor-wall corners, 4x wall-wall, corners, and 4x ceiling-wall corners... each mode could be effected by up to 8 of these... thereby you could have improved performance in an area that isn't contributing heavily to the major issues...
Which before plots do these after plots correspond to... the listening position? In the before plots, it seems the front of the room gives smoother response...

Also, when you say you grabbed 8 of the batts for each corner, how did you arrange the 8... did you double up to increase the thickness? You can see by the coeffs that absorption at 100mm drops off below 250Hz... so as is you can't expect them to do much at 60Hz... when you double up on the, I'm not sure if they have ot be bonded in some way, but I wouldn't think so...

[sparky5]

glaufman wrote:

Not sure, but sounds like one thing limiting the effectiveness of what you've done so far could be you've heavily treated 2 corners, while leaving another 10 untreated... by this I mean, the corners in question are 4 x floor-wall corners, 4x wall-wall, corners, and 4x ceiling-wall corners... each mode could be effected by up to 8 of these... thereby you could have improved performance in an area that isn't contributing heavily to the major issues...
Which before plots do these after plots correspond to... the listening position? In the before plots, it seems the front of the room gives smoother response...

Also, when you say you grabbed 8 of the batts for each corner, how did you arrange the 8... did you double up to increase the thickness? You can see by the coeffs that absorption at 100mm drops off below 250Hz... so as is you can't expect them to do much at 60Hz... when you double up on the, I'm not sure if they have ot be bonded in some way, but I wouldn't think so...

Thanks for the input!

Yeah, the batts were just thrown in the corners in their original packaging. I am awaiting some ready acoustics bag traps to set them up properly, but was just trying to see the difference in the meantime with what I had.

I guess (as I have the batts and am awaiting the bags) I will just build the traps, hang them in the corners, and test again and post the results.