# Kicking off my "Room Measurement" project



## audiobee (Nov 27, 2008)

Hi, everyone.

Being a newbie in this forum, I am contemplating a plan for this "room measurement" project after several days of reading/surfing around:

1. Radio Shack 33-4050 - I assume I'll have more pal around using this than the digital 33-2055 and might prefer analog over digital for a feeling of its range when using MATT or other testing tones.

1.1 BTW, is anyone can tell me where I can find 1/6, 1/12 octave testing tone, be it sine wave, warble or limited bandwidth pink noise - warble or pink noise are preferred to reduce room resonance influences. Also the location for the excel file download in case I need to do it manually.

1.2 I am a bit confused on how to point the mic toward - upward vs. vs. 45% into the center. There are factors to be considered: mic pattern, pressure/free/diffuse field design etc. From what I've read, the ECM8000 is an omni up to 10K+ under a diffuse field, hence good for room measure and better tuned when pointed upward 70%~80%, but not sure of RS)

1.3 I assume the Line Out is direct from its internal built-in pre-amp. Question: is there a consistency, that is, a linear relationship between needle readout & line-out voltage per your calibration measurement?

2. Behringer ECM8000 + M-Audio MobilePre via XLR cable (say, PROCO SIM-10 10')

2.1 This combo seems most popular at Home Theater Shack, so I assume I can cry out for help here if something went wrong (hope not!). The concern is more on the pre-amp quality when using with a condenser mic, esp. at high gain. Several posts have raise noise issue at high gain (beyond 80% knob).

2.2 I guess I have no way to have the ECM8000 calibrated from where I live, so I'll probably just give it a blind date by using the ECM8000 calibration table as posted here. One thing I do not quite understand well: assuming ECM8000 is quite linear 15Hz~20kHz in a diffuse field, do I still need the calibration table when pointing upward; or is it because the calibration table is intended for use when ECM8000 is pointed toward forward center? I got different answers from ETF and Rives... BTW, the RS 4050 manual says pointing at the "source" while some threads here insisted on pointing upward.

2.3 Checking the spec on MobilePre (~$110), it is +/1 0.8dB at 20Hz-20KHz, worse than M-Audio Fast Track Pro (~$150) +/1 0.1dB. I was dancing between these two, but would probably go for MobilePre for its driver stability (lesser functions means easier driver). If I go for MobilePre, I'll need to do a calibration probably using line-out to line-in loop for level matching. Is there any precaution or advices as to your field experiences?

3. Forget me not, the excellent REW. I assume REW will go well with BOTH RS 33-4050 (using its jack line-out feeding into MobilePre RCA line-in) and Behringer ECM8000 (feeding into mic/phantom of MobilePre via XLR cable).

What do I miss? I have Stereophile Test CD2 and ASC Test CD with me which I used in the past using the most sophisticated yet not very reliable device: my ear:surrender:

I have a $300~$500 budget for this whole setup. And if there is any side-benefit, I'd love to record some home sound track, probably using GarageBand comes with iMac...

For reference purpose, I use both Window & Mac in a 802.11n wireless setup.
Equipment wise,
Linderman D680 SACD ->ARC LS26 -> ARC REF110 -> DALI MS5
SONY SE-1 BD -> Proceed AVP -> LS26 -> REF110 -> MS5 with a SONY 47" BRAVIA LCD
Room size, ~13'x20'x9' with a large opening in right-rear side to dining room, placement is along the long side to avoid bass dip.

That's all I got for this newbie introduction.:reading::thud:

Thanks


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## audiobee (Nov 27, 2008)

Added upon my prior post, I guess I am going for safer route by choosing RS 4050 and ECM8000. What has kept bothering me the more I search/read is the choice for a mic preamp + usb vs. firewire connection to my Sony VAIO notebook (no line-in/out). There are choices such as M-Audio MobilePre/Fast Track Pro, E-MU 0404 USB, MBox mini, Edirol UA-25...

For simplicity reason, I am looking at
1. good driver support for Window XP
2. "good" mic preamp (EMU 0404 USB had several good review on this point w/ its massive frontend design, plus using AKM chip which I am familiar with)
3. phantom power for ECM8000
4. preferably USB powered? Not a necessity, I do not go field recording anyway. But from sound quality, an cheap adaptor can do a bad thing.
5. Budget <$200 retail.

Any recommendation from your actual experiences?

Many thanks,:nerd:


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## brucek (Apr 11, 2006)

One of the first things you should do is read and become completely familiar with REW Cabling Basics and the REW HELP files.

It's also good to look through the REW information Index and to know where the Download Page is located for calibration files.



> 1.1 BTW, is anyone can tell me where I can find 1/6, 1/12 octave testing tone, be it sine wave, warble or limited bandwidth pink noise - warble or pink noise are preferred to reduce room resonance influences. Also the location for the excel file download in case I need to do it manually.


All the test tones needed to test a system are included in REW. No external signals are required. 
The excel file downloads are located in the download page, but they are not needed since REW does it all.



> 1.2 I am a bit confused on how to point the mic toward - upward vs. vs. 45% into the center. There are factors to be considered: mic pattern, pressure/free/diffuse field design etc. From what I've read, the ECM8000 is an omni up to 10K+ under a diffuse field, hence good for room measure and better tuned when pointed upward 70%~80%, but not sure of RS)


Don't get too fussed about this. Point a omni-directional ECM8000 vertical (toward ceiling) at the listening position and the Radio Shack at a 45 degree angle toward the front of the theater while measuring at the listening position.



> 1.3 I assume the Line Out is direct from its internal built-in pre-amp. Question: is there a consistency, that is, a linear relationship between needle readout & line-out voltage per your calibration measurement?


The Radio Shack and Galaxy meter are a line-out device. Select the appropriate meter scale for the range you will be using. If it's clipping, change scale.



> 2. Behringer ECM8000 + M-Audio MobilePre via XLR cable (say, PROCO SIM-10 10')


The ECM8000 does require a preamp, so if you also are needing an external USB soundcard, it makes sense to get one with the mic preamp onboard, such as the M-Audio. Cable brand is irrelevant.



> 2.1 This combo seems most popular at Home Theater Shack, so I assume I can cry out for help here if something went wrong (hope not!). The concern is more on the pre-amp quality when using with a condenser mic, esp. at high gain. Several posts have raise noise issue at high gain (beyond 80% knob).


That combo is fine. Often people get a bit overzealous about accuracy of things that are overwhelmed by the changes that occur with a simple movement of the listening position or mic position. Mic positioning and speaker placement and listening position can easily change a signal by several dB. 



> 2.2 I guess I have no way to have the ECM8000 calibrated from where I live, so I'll probably just give it a blind date by using the ECM8000 calibration table as posted here. One thing I do not quite understand well: assuming ECM8000 is quite linear 15Hz~20kHz in a diffuse field, do I still need the calibration table when pointing upward; or is it because the calibration table is intended for use when ECM8000 is pointed toward forward center?


The calibration file we supply is fine for home use. The file renders the ECM flat when used with REW from 10HZ to 20KHz.



> I'll need to do a calibration probably using line-out to line-in loop for level matching. Is there any precaution or advices as to your field experiences?


Yeah, for the external soundcards that have an internal mic preamp, the soundcard calibration routine is performed with the line-out to line-in. This does leave the mic preamp response anomalies out of the loop and will slightly increase the inaccuracy, but the important line stages will be compensated for, so I wouldn't worry about it.



> I assume REW will go well with BOTH RS 33-4050 (using its jack line-out feeding into MobilePre RCA line-in) and Behringer ECM8000 (feeding into mic/phantom of MobilePre via XLR cable).


Yeah, either will work. Personally, I like the Galaxy CM-140 spl meter, which combines a good mic and an SPL meter in one package with a line out. Then you can get an external soundcard without the mic preamp.

brucek


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## audiobee (Nov 27, 2008)

thanks, brucek:clap:

I am now in the modeling, measurement & actual hearing mode. First step is to assemble a set of tools and find some good modeling software.

Has anyone dig into this modeling world, esp for irregular room. I've noticed CARA from Rives Audio.

:nerd:


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## brucek (Apr 11, 2006)

Oops...sorry... I just noticed a mistake in one of my answers. Too big a hurry. I wrote horizontal when I meant vertical. The mic should be pointed at the ceiling while at the listening position. I corrected the answer above...


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## audiobee (Nov 27, 2008)

thanks for the clarification, brucek.

"vertical" for ECM8000 is what I've read in a separate thread posted by you. Yet, I am still confused about RS4050's 45 degree - I guess I've read in the download that it is calibrated with horizontal position. Given Schroeder freq <200Hz in my room, I guess 1KHz will be more than enough for room mode measurement.

Beyond 1KHz, I have yet to understand how that will relate to how we actually hear, except flutter echo that might show some residual energy. Any discussion thread on mid/high room behavior? Hard to find around... 

BTW, is there any thread discussing particularly on room modeling?

've downloaded some excel-based room mode calculator, also used a free software from Ishii Japan using wall reflection method for a retangular room with arbitrary speaker/listener position and wall reflectivity (well it is in Japan, sooooo) which show freq response you can relate to actual measurement. This is quite intuitive and informative. I also tried to download a software called RRC @ RFD Consortium which also uses this wall reflection method, but failed to unzip. I was referred to RRC here in HTShack, but cannot find the post... sort of suck in between.

There is also a CARA package from Rives Audio using finite element for arbitrary room. Anyone has exposure to this software?


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