Convert measurement to cal? - Page 2 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #11 of 19 Old 03-13-09, 03:56 PM Thread Starter
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Fran
 
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Re: Convert measurement to cal?

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Tonmeister wrote: View Post
Hello Fran, hello brucek,

there is always one bad thing in equalizing a soundsystem without tuning the room: the first sound wave would be EQed to the frequency response matching all reflections in the room. So first aim has to be to correct the room with hardware material (diffusers, resonance plates, Helmholtz resonators and other stuff). When then room reflections and reverb time are smooth, you may EQ the electronic equipment at a second step.

Fran, ist that what you want to do?

Yours

Olaf
Yes, Olaf, I don't intend to EQ my system. My only interest is measuring and improving the response of my room.

I have 13 broadband absorber panels installed now, and just made 9 more. This will allow me to install panels over the listening position (which is also my recording position) and cover additional corners. I'm hoping for another increment of improvement.

Fran

E ho`okani pila kakou ma Kaleponi

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post #12 of 19 Old 03-13-09, 04:33 PM
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Re: Convert measurement to cal?

Okay Fran,

first let me ask you: what ist the longest diagonal in your room? This value is for the deepest frequency your room would be able to reproduce correct. It hast to be, that minimum one half-cycle hast to fit in there.

Frequencies below this value never would be reproduced correct in that room in case of acoustic limitations.

Second it is a good idea, to destroy reflections as good as possible using diffusers and non-parallell walls.

Third aim is to level reverberation time to a good value. You will find clues on the www, especially at the website of the AUDIO ENGINEERING SOCIETY [AES], the German "Institut fuer Rundfunktechnik" [IRT] or at the "Verband Deutscher Tonmeister" [VDT]. Look for MULTICHANNEL SURROUND SOUND SYSTEMS AND OPERATIONS, Reference Room (AES TC-MBAT Infomationsdokument: Mehrkanal-Surround-Systeme und Anwendungen)

Sincerely
Olaf G.
post #13 of 19 Old 03-13-09, 07:27 PM Thread Starter
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Fran
 
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Re: Convert measurement to cal?

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Tonmeister wrote: View Post
Okay Fran,

first let me ask you: what ist the longest diagonal in your room? This value is for the deepest frequency your room would be able to reproduce correct. It hast to be, that minimum one half-cycle hast to fit in there.

Frequencies below this value never would be reproduced correct in that room in case of acoustic limitations.

Second it is a good idea, to destroy reflections as good as possible using diffusers and non-parallell walls.

Third aim is to level reverberation time to a good value. You will find clues on the www, especially at the website of the AUDIO ENGINEERING SOCIETY [AES], the German "Institut fuer Rundfunktechnik" [IRT] or at the "Verband Deutscher Tonmeister" [VDT]. Look for MULTICHANNEL SURROUND SOUND SYSTEMS AND OPERATIONS, Reference Room (AES TC-MBAT Infomationsdokument: Mehrkanal-Surround-Systeme und Anwendungen)

Sincerely
Olaf G.
Olaf, thanks so much for taking time to share your knowledge.

Here's a model of my room:



The corner to corner length (excluding the hallway) is approximately 24 feet (7.3 meters or so). I'm really lucky to have such a generous sized room for my home studio. I suppose my room should be OK down to 24 hz or so, yes? I'm only interested in recording acoustic guitar, so I really only need to be able to capture and playback down to 80 hz or so, depending on how my guitar is tuned.

Unfortunately, my wife frowns on my making the walls non-parallel. I'd _really_ like to knock down some walls and raise the ceilings but she's not at all interested in spending our money that way. So far I'm using broad band absorbers (4" thickness of Owens Corning OC703 compressed fiberglass) to treat reflections and attempt to reduce low frequency cancellations and reinforcements.

In the past when I've tried to access information at the AES it's only available for members (or at least that's what I remember) and my German language skills are utterly nonexistent. I have some suggestions for BCC papers but haven't delved into them yet.

Thanks again for your advice.

Fran

E ho`okani pila kakou ma Kaleponi

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post #14 of 19 Old 03-13-09, 09:54 PM
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Mike Bentz
 
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Re: Convert measurement to cal?

In concept, making a cal file out of your speaker measurement is a great way to isolate the frequency response effects of room treatment in your room.....

However, the problem is you're only measuring on one-axis and your speaker is spraying sound in many different directions. The sound you measure at the listening position is the culmination of all that sound bouncing around the room and landing at the listening position. The tonal balance of the sound to the sides of the speaker isn't going to have the same frequency response as the sound measured on-axis. Because of this, the frequency content of the reflections are different.

However, the thinking is still flawed because the purpose of acoustical treatment is to deal with the time-arrival / frequency balance of all the later arriving reflections. As the reflections arrive later and later, our ears perceive that reflection as shifting from a tonal imbalance to a distinct reflection. The significance is that a late enough arriving reflection may induce a big peak/dip in the frequency response, but it won't be perceived as a tonal imbalance. Because of this, adding EQ to make it measure flat actually creates a new tonal imbalance and doesn't address the audible impact of the later arriving reflection.

So all that said, I would highly recommend diving into the ETC and exploring the audible correlations of that plot.

-Mike Bentz
~It's all about compromise~


"It's territorial with the soundboard. So you're mixing and some dude comes by spewing opinions and trying to turn knobs. It's akin to going up to an artist and painting over his unfinished masterpiece. You just want to shove your paint brush up his nose and throw the soundboard out the window!"
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post #15 of 19 Old 03-14-09, 08:41 AM
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Re: Convert measurement to cal?

Quote:
The significance is that a late enough arriving reflection may induce a big peak/dip in the frequency response, but it won't be perceived as a tonal imbalance. Because of this, adding EQ to make it measure flat actually creates a new tonal imbalance and doesn't address the audible impact of the later arriving reflection.
I would think that if we are EQ'ing at our listening / measurement position, and the gating of the impulse response is set to the point where the signal enters the noise, that all reflections (including late arrivals) would be taken care of?

brucek
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post #16 of 19 Old 03-16-09, 06:46 AM
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Re: Convert measurement to cal?

Hello again,

planning a good studio for reliable (sound engineering) listening is something, that deals with many complex processes. Nevertheless your overall transmission from source to loudspeaker should be as flat as possible, to ensure, that the tonal balance of the first arriving (not reflected) sound is correct. All electronic equalizing is a sellout, that makes the original sound bad to get the reflected soundmix better. So it is always the first aim to tune your room, not EQing electronically. REW is a good tool helping withal, but use it in the right way:

First take a look at the WATERFALL tab - you see low frequency resonances of your room that might be antagonized by a combination of inclined resonance plates (some said 7 or more, no you don'n need to break down your walls! ;-)) and Helmholtz resonators.

Second zoom deep into your IMPULSE tab - from 2.5ms to about 15ms you will see first reflections from floor, walls and ceilling. Try to scatter them with diffusers and again inclined resonance plates. Incoming wave angle is equal to leaving wave angle, use a mirror, to locate the best place for the diffusers! It has to be, that REW will show no discrete peaks between 0.5ms and 15ms!

Third make a RT60 measurement (Topt, one third octave bands). It shold be 0.25*(V / V0 ) with V=volume of your room and V0=100m (cubic meters). For example: a Room of 7 meters length, 5 meters widh and 3 meters height should be RT60 at 0,2625s from 200 Hz to 4 kHz and +/-0.05s tolerance.

If you have done those three things, you may have a look at the tab MEASURED and if your over all frequency response is not within 4dB tolerance (+/-2dB) from 40 Hz...16 kHz, then go to FILTER ADJUST and EQ your setup... but not until having done step one, two and three!

Yours


Olaf G. Guenther
post #17 of 19 Old 03-17-09, 12:32 AM
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Mike Bentz
 
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Re: Convert measurement to cal?

Quote:
brucek wrote: View Post
I would think that if we are EQ'ing at our listening / measurement position, and the gating of the impulse response is set to the point where the signal enters the noise, that all reflections (including late arrivals) would be taken care of?

brucek
Just as a very simple example....say you have a single reflection that arrives 30 seconds late. If you make your window 45s long, you're going to see dips and peaks in the frequency response. Do you think an EQ is going to get rid of the 30 second delay sounding like an echo?

-Mike Bentz
~It's all about compromise~


"It's territorial with the soundboard. So you're mixing and some dude comes by spewing opinions and trying to turn knobs. It's akin to going up to an artist and painting over his unfinished masterpiece. You just want to shove your paint brush up his nose and throw the soundboard out the window!"
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post #18 of 19 Old 03-17-09, 08:55 AM
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Re: Convert measurement to cal?

Quote:
Do you think an EQ is going to get rid of the 30 second delay sounding like an echo?
But his room isn't large enough to produce a reflection of 30 seconds. Sound travels ~1100 feet/second, so his room would produce reflections inside the ears gate time of ~50msecs and should not be perceived as two discrete sounds. Is that not correct?

So, all the reflections will be gated by REW and produce a response graph and waterfall that can easily be adjusted with treatment and completed with EQ if needed.

brucek
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post #19 of 19 Old 03-17-09, 10:00 PM
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Mike Bentz
 
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Re: Convert measurement to cal?

The Haas Window is non-linear with frequency. Also, the smearing effect still continues well inside the Haas Window. EQ doesn't fix the smearing (even though the smearing isn't heard as two distinct sounds).

The shape of the ETC has a huge effect on what is perceived as well. Specular reflections reduce intelligibility and ruin imaging...same with sparsely distributed reflections. You want the reflections to decay in amplitude over time too....don't want swelling or flat spots, single spikes, etc...

Long story short, EQ'ing a room with a poor ETC to measure a flat frequency response is not going to sound as good as a room that has properly treated the reflections....even though the frequency response of the latter could be quite similar to the EQ'd system.

In a studio setting, you want to try real hard to achieve an ITD that is as long as possible as this will dramatically improve the referencing of the mix. And then usually, you want the Haas Kicker (the first reflection) to be very apparent so that it and all the reflections following are perceived as separate from the direct sound.

-Mike Bentz
~It's all about compromise~


"It's territorial with the soundboard. So you're mixing and some dude comes by spewing opinions and trying to turn knobs. It's akin to going up to an artist and painting over his unfinished masterpiece. You just want to shove your paint brush up his nose and throw the soundboard out the window!"
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