SPL Meter Aiming & Reading - AVS Forum
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Old 02-22-2012, 07:24 PM - Thread Starter
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A couple of questions on balancing the home theater sound with a Radio Shack SPL meter.

1. Which direction should the microphone point at?
I have read various answers for this and after trying all of them the results are different so I think it is important to establish the proper way of pointing it. I have read that pointing it at each individual speaker from the listening position allows you to get a true reading for each speaker individually. Its a bit hard since you have to repoint your meter for each speaker but if this is the most accurate way then why not.

I have also read that you aim the SPL meter from the listening position directly to the ceiling, that way you get a more balance sound from each speaker and it does not require re-aiming the SPL meter.

I have also read that one should point the SPL meter from the listening position directly to Movie Screen since this is they way you will be listening to your theater therefore it is the way it should be calibrated. Sounds reasonable but is it the correct way, in fact which is the correct way?

2. Various reports show the the Radio Shack SPL meter is not so accurate on the low frequency range and have posted correction values such as this;
10hz........+20db
12.5hz.....+16.5db
16hz........+11.5db
20hz........+7.5db
25hz........+5db
31.5hz.....+3db
40hz........+2.5db
50hz........+1.5db
63hz........+1.5db
80hz........+1.5db
100hz......+2db
125hz......+.5db
160hz......-.5db
200hz......-.5db
250hz......+.5db
315hz......-.5db
400hz......0db
500hz......-.5db
630hz......0db
800hz......0db
1k...........0db
1.25k......0db
1.6k........-.5db
2k...........-1.5db
2.5k........-1.5db
3.15k......-1.5db
4k...........-2db
5k...........-2db
6.3k........-2db
8k...........-2db
10k.........-1db
12.5k......+.5db
16k.........0db
20k.........+1db

So if I measure my LFE at 20Hz and it reads 80 DB's on the SPL meter does that mean it is actually 87.5 DB's in which case if 80 DB is the target I need to lower the volume by 7.5 DB's down to 72.5?

3. What crossover frequency should be used?
I read that most settings recommend setting your crossover to 80 hz, I think that is also the THX standard as well. However when reading manuals and set up guides for sub woofers they seem to think that 60 hz (assuming your mains do well there)is ideal for good bass.
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Old 02-22-2012, 07:45 PM
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Quote:
Originally Posted by OzzieP View Post

1. Which direction should the microphone point at?
I have read various answers for this and after trying all of them the results are different so I think it is important to establish the proper way of pointing it. I have read that pointing it at each individual speaker from the listening position allows you to get a true reading for each speaker individually. Its a bit hard since you have to repoint your meter for each speaker but if this is the most accurate way then why not.

Definitely don't do it this way.

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Originally Posted by OzzieP View Post

I have also read that you aim the SPL meter from the listening position directly to the ceiling, that way you get a more balance sound from each speaker and it does not require re-aiming the SPL meter.

Not this way either.

Quote:
Originally Posted by OzzieP View Post

I have also read that one should point the SPL meter from the listening position directly to Movie Screen since this is they way you will be listening to your theater therefore it is the way it should be calibrated. Sounds reasonable but is it the correct way, in fact which is the correct way?

Don't do it this way, either.


Aim the meter at the point between your front speakers and above your center speaker where the wall behind your front speakers meets the ceiling.


Quote:
Originally Posted by OzzieP View Post

So if I measure my LFE at 20Hz and it reads 80 DB's on the SPL meter does that mean it is actually 87.5 DB's in which case if 80 DB is the target I need to lower the volume by 7.5 DB's down to 72.5?

Provided that correction table's values are correct, yes. But why do you have a "target" for 20Hz?

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Old 02-22-2012, 07:52 PM
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An SPL meter is best used to set the individual channel levels. I would mount it on a tripod or mic stand at the main listener's seat and aim it straight up.
For frequency response checks I would use a calibration mic.
The Radio Shack SPL meter correction charts are about worthless. These mics have been manufactured for decades, probably in different factories with many different suppliers for all the internal parts. From batch to batch the frequency response is all over the map.
I'll let others answer your other questions.

Kevin
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Old 02-22-2012, 09:53 PM - Thread Starter
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sivadselim

Thanks for the Reply, You suggest;
Aim the meter at the point between your front speakers
This would be my center speaker.

and above your center speaker where the wall behind your front speakers meets the ceiling.
O.K. then above my center speaker aiming to the point where the ceiling meets the wall. I have a pretty high ceiling and if I put this angle from the listening position it will point approximately 60 degrees up.

Is there a logical or acoustic reason for aiming it this way?
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Old 02-22-2012, 09:58 PM - Thread Starter
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Speedskater

I understand the Radio Shack is probably the poor mans version of a calibration microphone which last time I checked they the cheap one of those cost about 15 times the price of the radio shack version plus you still need a device to display the results of this microphone. I do have to agree with you that those SPL correction charts are probably useless for the reason you stated, but I suspect it does reflect where the flaws are in the Radio Shack SPL (Low Frequencies).

Do you have a calibration microphone you recommend?
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Old 02-23-2012, 12:11 AM
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Quote:
Originally Posted by OzzieP View Post

........it will point approximately 60 degrees up.

OK

Quote:
Originally Posted by OzzieP View Post

Is there a logical or acoustic reason for aiming it this way?

Because it looks cool as fonk.

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Old 02-23-2012, 01:53 AM
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Quote:
Originally Posted by OzzieP View Post


Do you have a calibration microphone you recommend?

I bought mine, Behringer ECM8000, from Herb Singleton, founder of Cross-Spectrum
http://www.cross-spectrum.com/

Very reasonable prices for a calibrated mic.

I use that for all acoustic measurements, and the RS SPL meter for pink noise dbl level setting only.
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Old 02-23-2012, 05:15 AM
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Quote:
Originally Posted by OzzieP View Post

1. Which direction should the microphone point at?

http://www.gearslutz.com/board/studi...c-ceiling.html
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Old 02-23-2012, 05:58 AM
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Quote:
Originally Posted by OzzieP View Post

Speedskater

Do you have a calibration microphone you recommend?

I second the recommendation of the Behringer ECM8000. I've probably bought a dozen over the years (people keep borrowing them and not returning them!). Cheap, effective and reasonably reliable. For under $60 if it gets lost, damaged or dies, so what?

You also need something like a USB computer audio interface with XLR mic inputs and phantom power. There are many good competitive devices, with the M-Audio Mobile Pre probably being one of the more highly regarded and popular.

Finally, you need some good measurement software. Room Equalization Wizard is good, well supported freeware. I believe that there is a conference related to its use here on AVS.
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Old 02-23-2012, 06:28 AM
 
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Another recommendation for REW. Just use a good quality microphone with the program.
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Old 02-23-2012, 09:17 AM - Thread Starter
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Found the ECM8000 for $54, and the REW software, still trying to find what to interface the microphone to the laptop with.

I think this would work:
http://www.m-audio.com/products/en_us/FastTrack.html
$149
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Old 02-23-2012, 11:32 AM
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Quote:
Originally Posted by OzzieP View Post

Found the ECM8000 for $54, and the REW software, still trying to find what to interface the microphone to the laptop with.

I think this would work:
http://www.m-audio.com/products/en_us/FastTrack.html
$149

ART USB DUAL-PRE
http://www.bhphotovideo.com/c/produc..._Dual_Pre.html
$69
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Old 02-23-2012, 11:46 AM
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Quote:
Originally Posted by Speedskater View Post

An SPL meter is best used to set the individual channel levels. I would mount it on a tripod or mic stand at the main listener's seat and aim it straight up.
For frequency response checks I would use a calibration mic.
The Radio Shack SPL meter correction charts are about worthless. These mics have been manufactured for decades, probably in different factories with many different suppliers for all the internal parts. From batch to batch the frequency response is all over the map.

Exactly. This may help too:

Comparison of Ten Measuring Microphones

--Ethan

RealTraps - The acoustic treatment experts
Ethan's Audio Expert book

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Old 02-23-2012, 12:42 PM - Thread Starter
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Quote:
Originally Posted by localhost127 View Post

ART USB DUAL-PRE
http://www.bhphotovideo.com/c/produc..._Dual_Pre.html
$69

That Dual Pre look like will do the trick and good price, but it has more
knobs than I expected. I have no clue what phantom on or off is for, and would one need to use the gain if your just taking spl readings? I am hoping it comes with instructions. I have taking readings with a radio shack SPL and a test CD with various frequencies but have never really use gain or phantom.
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Old 02-23-2012, 01:16 PM
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Quote:
Originally Posted by OzzieP View Post

That Dual Pre look like will do the trick and good price, but it has more knobs than I expected.

I have no clue what phantom on or off is for,

Set to "on" to make sure the mic gets the power that it needs to operate. It's good switch to turn on and just leave on, because if the mic doesn't need the power, it won't take it.

Quote:


and would one need to use the gain if your just taking spl readings?

Use the gain control to obtain the desired vertical position of the graph in whatever analysis software that you are using.


Quote:


I am hoping it comes with instructions.

Your best source of instructions will come with the software, and advice you can obtain from this forum.

Here is a relevant thread in *this* forum:

"Room EQ Wizard (free measurement and parametric EQ setup software)"


I have taking readings with a radio shack SPL and a test CD with various frequencies but have never really use gain or phantom.[/quote]
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Old 02-23-2012, 02:32 PM - Thread Starter
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Thanks, now wait for it to arrive and hook it all up, hopefully mac compatible or else I will have to use my old windows laptop.

So the consensus is to aim the mic toward the ceiling and floor area above the center channel then.
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Old 02-23-2012, 04:48 PM
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REW will run on a Mac. I think for most home measurements where you put the mic matters more than which way you aim it. Of course I'm prepared to be wrong.

What are you trying to measure?

If you are just looking at room modes, or trying to integrate a sub with mains, you can likely get useful results using REW and your radioshack SPL meter (nothing to buy assuming you have cables to connect your mac to your receiver, and to the RS meter.) Sure they may not be the most accurate, but they can still be useful. And it will let you get used to the software while you wait for your other new toys to arrive

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Old 02-23-2012, 06:05 PM
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When Dolby sweeps a theater they use four microphones in each corner of the seating area all aimed up at the ceiling.

I have tried both aiming at the screen and aiming at the ceiling at home and prefer the aim to ceiling results.

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Old 02-23-2012, 06:22 PM
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Quote:
Originally Posted by Glimmie View Post

When Dolby sweeps a theater they use four microphones in each corner of the seating area all aimed up at the ceiling.

I have tried both aiming at the screen and aiming at the ceiling at home and prefer the aim to ceiling results.

you "prefer", as in they give you a better "looking" (more "pleasing") response on the graph, or a more accurate response of the actual acoustical behavior of the space.

you should consult with the mic mfg'r on their recommended orientation. eg, is it free-field (direct) mic, or diffuse-field ?
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Old 02-23-2012, 07:00 PM
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is it free-field (direct) mic, or diffuse-field ?

Can you explain to me the difference between the two? And how they should be best used? This is not a test, I'm just looking to learn without wasting hours on google. The terminology seems to be all over the place. I'm sure this terminology means something to somebody familiar with microphones, but not much to me. If you you know the difference, great - please share. If not, no problem, I'll look it up later if I think it matters. Thanks.

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Old 02-23-2012, 07:51 PM
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Quote:
Originally Posted by dknightd View Post

Can you explain to me the difference between the two? And how they should be best used? This is not a test, I'm just looking to learn without wasting hours on google. The terminology seems to be all over the place. I'm sure this terminology means something to somebody familiar with microphones, but not much to me. If you you know the difference, great - please share. If not, no problem, I'll look it up later if I think it matters. Thanks.

free-field is relevant for non-environment or anechoic rooms - eg, direct signal is what is being measured. diffuse-field for a non-anechoic room with reflections.

http://www.globalspec.com/learnmore/...nt_microphones

"Free field microphones are used to measure a plane wave that is propagating freely in one defined direction. Diffuse field measurement microphones, also known as random incidence microphones, measure sound waves from all directions."

http://www.acousticvibration.com/mea...icrophones.htm

check with the manufacture of the mic as for recommended orientations re: diffuse-field mics (eg, 70* on-axis). free-field should be pointed at the source.

this is an inexpensive and popular mic for room measurements:
http://www.daytonaudio.com/index.php...icrophone.html
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Old 02-23-2012, 08:26 PM - Thread Starter
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That would explain all of the different ways that these measurements are being taken. I know the Audyssey Microphone point straight up when they take their measurement. I would not be surprised then that tons of people that have been trying to balance there sound have microphones pointed in all sorts of direction and getting incorrect results.

That Dayton microphone also looks like a nice one, I assume that is a Diffused Field Microphone, so in that case if your trying to balance your sounds which direction should it be oriented?
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Old 02-23-2012, 08:37 PM
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Quote:
Originally Posted by localhost127 View Post

you "prefer", as in they give you a better "looking" (more "pleasing") response on the graph, or a more accurate response of the actual acoustical behavior of the space.

you should consult with the mic mfg'r on their recommended orientation. eg, is it free-field (direct) mic, or diffuse-field ?

I prefer the sound results of the ceiling aim.

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Old 02-24-2012, 01:55 PM
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btw, I have a 2009 27" i7 iMac and REW on the Mac OsX platform works - no need to run VMWare, just like WinDoz there are some quirks here and there, support on HTS page may have tips for you.
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Old 02-24-2012, 02:42 PM
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Quote:
Originally Posted by OzzieP View Post

That would explain all of the different ways that these measurements are being taken. I know the Audyssey Microphone point straight up when they take their measurement. I would not be surprised then that tons of people that have been trying to balance there sound have microphones pointed in all sorts of direction and getting incorrect results.

That Dayton microphone also looks like a nice one, I assume that is a Diffused Field Microphone, so in that case if your trying to balance your sounds which direction should it be oriented?

Here's a good read regarding mic's...Herb from Cross Spectrum sells these with cal files (various ordering options). He suggests 10-20 degrees off vertical (where vertical is capsule pointed at ceiling). I bought the Dayton EMM-6 with "Basic Plus".
http://www.hometheatershack.com/foru...rvice-usa.html
http://www.cross-spectrum.com/measur...ed_dayton.html
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Old 02-24-2012, 02:47 PM
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Quote:
Originally Posted by localhost127 View Post

you "prefer", as in they give you a better "looking" (more "pleasing") response on the graph, or a more accurate response of the actual acoustical behavior of the space.

you should consult with the mic mfg'r on their recommended orientation. eg, is it free-field (direct) mic, or diffuse-field ?

FWIW< here's Ethan Winer's short take from his measurement mic test linked above

"Although omnidirectional microphones supposedly receive sound equally from all directions, when measuring rooms and loudspeakers the convention is to "aim" the microphone upward. No omni microphone has exactly the same frequency response from all directions, though microphones with tiny diaphragms and slim bodies are often more uniform than larger models. So when balancing loudspeaker volume levels on a surround system, pointing the microphone toward the ceiling favors all of the loudspeakers equally."

I'd add that if you want to capture the room effects that Audyssey is attempting to correct for, straight up seems to make the best sense overall - - equally favoring at least 360 degrees on the vertical plane of the mic.
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Old 02-24-2012, 05:01 PM
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Speedskater
I understand the Radio Shack is probably the poor mans version of a calibration microphone which last time I checked they the cheap one of those cost about 15 times the price of the radio shack version plus you still need a device to display the results of this microphone. I do have to agree with you that those SPL correction charts are probably useless for the reason you stated, but I suspect it does reflect where the flaws are in the Radio Shack SPL (Low Frequencies).

Do you have a calibration microphone you recommend?

The Radio Shack meter is a surprisingly good SPL meter (at least mine is) but not so good as a cal. mic.

I have two older ECM8000 mics and an even older Mity-Mic. But other good options are available now.

Kevin
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