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Author Topic: Setting Reference Levels  (Read 7669 times)

nwboater

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Setting Reference Levels
« on: December 09, 2012, 08:11:12 pm »

With the upcoming 'Loudness' feature I realized that I need to calibrate my audio system to have it working properly. As I understand it MC18 Loudness is based on no correction at 100% or 'O' Db reference level.

Ours is a 'no receiver' system, in other words the soundcard outputs go straight to the power amps. Volume is done only in MC18 using 'Internal' Volume.

I have a Radio Shack SPL meter that I can use for this calibration. (Also have REW and a calibrated mic, but don't think they are needed for setting MC18 reference level.)

I think the first thing that needs to be known is what DB level is reference (for zero Loudness correction) in MC18? Is it 85DB for each channel? If so that would be a very loud level to do adjustments. I think most home receivers use 75 DB for this.

If it is based on 0 DB Volume = 85 db and I want to use test tones at 75 DB do I set the internal volume at -10 DB (the difference of 10 DB from reference and where I want to use the test tones) and then adjust the individual power amps gain so I read 75 DB on the SPL meter? This would be done with music being played and the 'Level' box checked in room correction.

Also there is a setting in Options, Audio for 'Internal Reference Volume Level'. It's default is '100'. Is that percent or could it be DB? If DB, shouldn't it be set at 85 for industry standard?

Any clarification on all this and/or suggestions would be greatly appreciated.

Thanks,
Rod
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hulkss

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Re: Setting Reference Levels
« Reply #1 on: December 09, 2012, 11:32:16 pm »

With the upcoming 'Loudness' feature I realized that I need to calibrate my audio system to have it working properly.
Any clarification on all this and/or suggestions would be greatly appreciated.

Thanks,
Rod

The short story is: 20-20 kHz pink noise recorded at -20 dBFS RMS reads 83 dB SPL, C weighted, slow on all channels except LFE (have your bass management working normally but your subwoofer off and digital volume at 100%). Put the SPL meter at the sweet spot & aimed at the ceiling.

The subwoofer channel must be set relative to the front center speaker to a level 10 dB higher using a real time analyzer (see link). http://www.film-tech.com/warehouse/manuals/TN99051701.pdf

Good stuff to read:
http://www.digido.com/how-to-make-better-recordings-part-2.html

Calibration Files:
http://www.digido.com/media/downloads/category/12-general.html
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mojave

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Re: Setting Reference Levels
« Reply #2 on: December 10, 2012, 07:25:29 am »

The short story is: 20-20 kHz pink noise recorded at -20 dBFS RMS reads 83 dB SPL, C weighted, slow on all channels except LFE
All the calibration discs use bandwidth limited pink noise.

I like using the Disney World of Wonder calibration Blu-ray which uses 83 dB for all channels and then has you increase the LFE by 3 dB when finished. 

Rod, you could just arbitrarily put in a reference level and not really worry about it. Just increase your volume to the maximum you will listen to and set that as reference. Now when you listen below that level, the loudness feature will kick in.
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mojave

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Re: Setting Reference Levels
« Reply #3 on: December 10, 2012, 09:45:18 am »

Good stuff to read:
http://www.digido.com/how-to-make-better-recordings-part-2.html

Calibration Files:
http://www.digido.com/media/downloads/category/12-general.html
Richard Casey, the producer of the WOW disc, has referenced the Digido article and stated that is in line with how the calibration disc is produced and why the disc uses 83 dB for Reference Level.

The left and right limited pinknoise calibration files on the second link are identical to the WOW disc. One can use the Order Channels DSP to route them through the other channels. Unfortunately, the link doesn't include the LFE channel calibration signal. This signal is from 10-80Hz with the higher frequencies being played louder. This is so bass deficient subwoofers will still calibrate fairly decently.
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nwboater

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Re: Setting Reference Levels
« Reply #4 on: December 10, 2012, 09:49:06 am »

Hi Hulkss & mojave,

Thanks for all the info and links. Enough reading to need an Excedrin or two! Seriously though some very good stuff there.

I didn't make it clear in my original post that I was planning to use the 'Level' tones in MC Room Correction. I'd be interested in knowing why it's better to use the linked test tones, or the World of Wonder Blu-Ray.

Also Hulkss when you say "slow on all channels except LFE" did you mean 'show' on all channels? And what do you mean by this?


Mojave - If I just set an arbitrary level for Reference I don't believe that Loudness will work properly. That's what got me into thinking I should calibrate the system. For example if I set Reference at a lower level than it should be then Loudness would not be doing adjustments until I turned listening volume down below that point. I would not have enough Loudness correction in that example. Conversely if I set Reference at a level higher than it should be then Loudness will over correct.

Are you referring to the setting in Options, Audio for 'Internal Reference Volume Level'?  It's default is '100'. Is that percent or could it be DB? If DB, shouldn't it be set at 85 (or 83?) for industry standard?"

Thanks,
Rod
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Vocalpoint

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Re: Setting Reference Levels
« Reply #5 on: December 10, 2012, 10:34:27 am »

Rod, you could just arbitrarily put in a reference level and not really worry about it. Just increase your volume to the maximum you will listen to and set that as reference. Now when you listen below that level, the loudness feature will kick in.

+1. This is what I would do.

Attempting to calibrate to industry (studio grade) specs - is probably not ideal for the home environment. And really - if you have an accurate SPL meter - "calibration" literally comes down to marking a tic on the console where the volume control for the main monitors should be (this assumes that all other gear in the entire chain has been properly gain staged).

But be aware - using the industry standard (20-20 kHz pink noise recorded at -20 dBFS RMS reading 83 dB SPL) is a lot louder than you think. Probably way too loud for the home environment.

If it were me - I would just find a comfy volume level (I still can't believe you are letting MC control this - one slip of that volume slider could end your power amps or speakers :) and let that be your guide.

VP

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Juniorjbl

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Re: Setting Reference Levels
« Reply #6 on: December 10, 2012, 10:35:30 am »

"C" weighted and "slow" response time are settings on an SPL meter.

Subscribed and watching this thread.
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nwboater

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Re: Setting Reference Levels
« Reply #7 on: December 10, 2012, 10:47:11 am »

+1. This is what I would do.

Attempting to calibrate to industry (studio grade) specs - is probably not ideal for the home environment. And really - if you have an accurate SPL meter - "calibration" literally comes down to marking a tic on the console where the volume control for the main monitors should be (this assumes that all other gear in the entire chain has been properly gain staged).

But be aware - using the industry standard (20-20 kHz pink noise recorded at -20 dBFS RMS reading 83 dB SPL) is a lot louder than you think. Probably way too loud for the home environment.

If it were me - I would just find a comfy volume level (I still can't believe you are letting MC control this - one slip of that volume slider could end your power amps or speakers :) and let that be your guide.

VP



Thanks VP.

Why do you say that calibrating to industry specs is "Not ideal for the home environment"?

Yes I've been concerned about the very high volume if calibrating at 83 db. I think that's why receivers have you do it at 75, and internally adjust so Reference is at proper levels.

You bring up another point of concern and that's toasting my speakers from an accidental volume change error. But in a multi channel active crossover system with sound card straight to power amps how would you do it externally? That would be one massive ganged set of pots, or another electronics box that could also fail and cause a problem.

Cheers,
Rod
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nwboater

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Re: Setting Reference Levels
« Reply #8 on: December 10, 2012, 10:48:50 am »

"C" weighted and "slow" response time are settings on an SPL meter.

Subscribed and watching this thread.

Thanks. I see those settings on my meter. I had them set properly but had forgotten about them.

Rod
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Vocalpoint

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Re: Setting Reference Levels
« Reply #9 on: December 10, 2012, 11:11:26 am »

Why do you say that calibrating to industry specs is "Not ideal for the home environment"?

Strictly from a volume perspective. The industry spec was created for mixing multi-track audio for film etc where certain standards (like playback gear in theaters etc) comes into play. Sitting thru a feature film picture at home with your reference level set to 83SPL for two hours will get annoying. Unless maybe you have a true "theater" at home with 50 or 60 feet between you and the speakers :)

But in a multi channel active crossover system with sound card straight to power amps how would you do it externally? That would be one massive ganged set of pots, or another electronics box that could also fail and cause a problem.

If it were me - I would immediately get a pre-amp in front of all those power-amps with an old school volume control. Not only would this allow you to "set" (and control) your pre-determined volume setting (like 8 on the knob or whatever) to indicate what 75db (or whatever) is per your SPL meter - but it would ensure that MC cannot influence the output (software volume can disrupt "bit-perfectness") or heaven forbid - trash any gear due to an unplanned (and sudden) wide volume swing.

Simply put - while I love MC - I would never hardwire any gear to any software volume control.

VP
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Matt

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Re: Setting Reference Levels
« Reply #10 on: December 10, 2012, 11:16:47 am »

If it were me - I would immediately get a pre-amp in front of all those power-amps with an old school volume control. No only would this allow you to "set" your pre-determined volume setting (like 8 on the knob or whatever) to indicate what 75db (or whatever) is per your SPL meter - but it would ensure that MC cannot influence (or trash) any gear due to a sudden wide volume swing.

Simply put - while I love MC - I would never trust thousands of dollars worth of gear to any software volume control.

A lot of us (myself included) use the program this way.  It's safe.

If you don't want to do it, fine, but please don't spread FUD.
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mojave

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Re: Setting Reference Levels
« Reply #11 on: December 10, 2012, 11:18:49 am »

I didn't make it clear in my original post that I was planning to use the 'Level' tones in MC Room Correction. I'd be interested in knowing why it's better to use the linked test tones, or the World of Wonder Blu-Ray.
The "Level" tones in JRiver are full band pink noise and at a slightly lower dBFS level than the WOW tones. I think they probably calibrate to the same reference level since you have more energy across the frequency band at a lower energy level. I'll have to check and see at home sometime. They are probably close enough to use. I think one of the reasons for using limited band pink noise is to eliminate the greater variations in speakers in the upper frequency ranges. This can greatly impact the overall perceived loudness.

Quote
Mojave - If I just set an arbitrary level for Reference I don't believe that Loudness will work properly. That's what got me into thinking I should calibrate the system. For example if I set Reference at a lower level than it should be then Loudness would not be doing adjustments until I turned listening volume down below that point. I would not have enough Loudness correction in that example. Conversely if I set Reference at a level higher than it should be then Loudness will over correct.

Are you referring to the setting in Options, Audio for 'Internal Reference Volume Level'?  It's default is '100'. Is that percent or could it be DB? If DB, shouldn't it be set at 85 (or 83?) for industry standard?"
I say that arbitrary works because music seems to be mixed anywhere within a 20 dB window. In other words, reference level is sort of a moving target depending on how hot they have mixed it. With movies, the audio is supposed to be mastered to similar levels, but I still find that I listen to various movies within a 10 dB window. Some movies have less low frequency content and they turn up the levels in the upper bass or elsewhere.

Quote
Are you referring to the setting in Options, Audio for 'Internal Reference Volume Level'?  It's default is '100'. Is that percent or could it be DB? If DB, shouldn't it be set at 85 (or 83?) for industry standard?"
Yes, and it is in percent. You might find that 70% is 83 dB so you set the Internal Reference Volume level to 70%. If you don't want your system to play louder than Reference, then set Maximum Volume to 70%, too. Note that if you change some of your EQ in Parametric EQ, you might need to re-calibrate the reference level.

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Vocalpoint

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Re: Setting Reference Levels
« Reply #12 on: December 10, 2012, 11:24:37 am »

If you don't want to do it, fine, but please don't spread FUD.

Matt,

FUD? Since there is no way for you to ever guarantee that MC can't suddenly go from 0-100 in an unplanned burst - with big powerful amps connected - you can't say it's 100% safe either. A random mouseclick from my 8 year son could easily send the volume rocketing.

Jeez - the guy just asked how I would do it.

VP

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nwboater

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Re: Setting Reference Levels
« Reply #13 on: December 10, 2012, 11:28:30 am »

Vocalpoint - Thanks very much for your further clarifications.

Matt - It's absolutely amazing the things I learn here. I now know what FUD means, after looking it up!

mojave - Thanks for all your explanations. It all really helps.


Looks like I need to spend a little more time deciding just how I want to do the calibration, but at least now I have a lot more good information.

Cheers,
Rod
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Vocalpoint

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Re: Setting Reference Levels
« Reply #14 on: December 10, 2012, 11:30:29 am »

Vocalpoint - Thanks very much for your further clarifications.

No prob - let us know how it goes!

VP
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Matt

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Re: Setting Reference Levels
« Reply #15 on: December 10, 2012, 11:31:29 am »

FUD? Since there is no way for you to ever guarantee that MC can't suddenly go from 0-100 in an unplanned burst - with big powerful amps connected - you can't say it's 100% safe either. A random mouseclick from my 8 year son could easily send the volume rocketing.

Volume Protection doesn't allow the volume to go up too quickly.
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nwboater

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Re: Setting Reference Levels
« Reply #16 on: December 10, 2012, 11:33:31 am »

Matt,

FUD? Since there is no way for you to ever guarantee that MC can't suddenly go from 0-100 in an unplanned burst - with big powerful amps connected - you can't say it's 100% safe either. A random mouseclick from my 8 year son could easily send the volume rocketing.

Jeez - the guy just asked how I would do it.

VP



VP - I appreciate the warnings. As I said this has been a concern of mine. But it is really unpractical to have a preamp in front of all the power amps in a 5.1 active eq system.

Also I have 'Volume Protection' enabled in MC and that at least helps avoid a big accidental volume change.

Rod
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Vocalpoint

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Re: Setting Reference Levels
« Reply #17 on: December 10, 2012, 11:48:05 am »

VP - I appreciate the warnings. As I said this has been a concern of mine. But it is really unpractical to have a preamp in front of all the power amps in a 5.1 active eq system.

Well - I only have a single one (my Pio Elite) and it works great allowing all my poweramps to do their thing without issue.

Also I have 'Volume Protection' enabled in MC and that at least helps avoid a big accidental volume change.

Yes - I totally forgot about it (Sorry Matt :) - but then again - I always do this anyway:

"Use 'Internal Volume' and leave the System Volume at 100%. This is advantageous because you get a 64bit volume and also because it allows Media Center to have as much signal headroom as possible, which is especially important if you do processing (upsampling, room correction, bass management, etc.) have MC at 100%"

For the very reasons it lists. And then just use the Elite to handle all "volume" duties (for all inputs) as required.

VP
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mojave

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Re: Setting Reference Levels
« Reply #18 on: December 10, 2012, 12:00:02 pm »

Volume Protection in combination with the Maximum Volume setting make JRiver foolproof. With the new ZoneSwitch feature, one can have all movies and music with a genre of kids go to a zone with the maximum volume set at a lower level so the kids can't even turn it up all the way.

With most systems, turning up the volume is going to hurt your ears more than anything. I have 83 dB efficient speakers with a single 5 1/4" driver, 89 dB speakers with a single 6 1/2" driver, and 90 dB speakers with eight 6 1/2" drivers per speaker. My amps put out over 400 watts per channel at 8 ohm and a little under 800 watts at 4 ohm. I've been to maximum volume with all three speakers with no problems.

I recently had some 101 dB 4 ohm speakers in for a demo. I had some guys over for prime rib and listening and we turned them up to max, too. ;D It was fun getting 130 dB peaks (with subs too) at the listening position.
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nwboater

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Re: Setting Reference Levels
« Reply #19 on: December 10, 2012, 12:02:45 pm »

Well - I only have a single one (my Pio Elite) and it works great allowing all my poweramps to do their thing without issue.

Yes - I totally forgot about it (Sorry Matt :) - but then again - I always do this anyway:

"Use 'Internal Volume' and leave the System Volume at 100%. This is advantageous because you get a 64bit volume and also because it allows Media Center to have as much signal headroom as possible, which is especially important if you do processing (upsampling, room correction, bass management, etc.) have MC at 100%"

For the very reasons it lists. And then just use the Elite to handle all "volume" duties (for all inputs) as required.

VP

Guess I should have explained earlier that what I have been referring to as Active EQ is really Active Crossovers. With L, R & C at 2 or 3 channels each most preamps can't handle the total number of channels.

I believe that with 64 bit processing 'headroom' is not an issue. Perhaps Mojave or Matt could address that, but I know I've read good explanations here before.

Rod
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mojave

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Re: Setting Reference Levels
« Reply #20 on: December 10, 2012, 12:31:51 pm »

Using internal volume with the receiver set to its highest level is best unless you hear some hiss. This would be the same as an amp without gain controls such as most consumer amps. If the highest level on a receiver is +18 (Pioneer Elite is either +16 or +18, I can't remember), then 0 on a receiver is technically -18 dB. Receivers set 0 way down from max to allow for variation in efficiency in subs/surrounds/mains, EQ, bass boosting, and combined bass due to bass management. They also push all channels but the LFE down another 10 dB to compensate for the LFE being mixed -10dB relative to the other channels. With an HTPC and letting it decode, you do the +10 dB with your sub amp's gain controls and have more headroom available in the HTPC for volume, EQ, etc. All this attenuation by the receiver is done at a lower quality that the volume attenuation in JRiver.

I just received a two channel balanced tube preamp (battery powered) in the mail today to try with my system. I've been using a computer my whole life for media playback and this will be the first time trying a preamp in the signal chain. I don't understand how adding something to the signal chain will improve it, but I'm willing to try it out. A member of another forum sent it to me to try.


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nwboater

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Re: Setting Reference Levels
« Reply #21 on: December 10, 2012, 12:39:49 pm »

Using internal volume with the receiver set to its highest level is best unless you hear some hiss. This would be the same as an amp without gain controls such as most consumer amps. If the highest level on a receiver is +18 (Pioneer Elite is either +16 or +18, I can't remember), then 0 on a receiver is technically -18 dB. Receivers set 0 way down from max to allow for variation in efficiency in subs/surrounds/mains, EQ, bass boosting, and combined bass due to bass management. They also push all channels but the LFE down another 10 dB to compensate for the LFE being mixed -10dB relative to the other channels. With an HTPC and letting it decode, you do the +10 dB with your sub amp's gain controls and have more headroom available in the HTPC for volume, EQ, etc. All this attenuation by the receiver is done at a lower quality that the volume attenuation in JRiver.

I just received a two channel balanced tube preamp (battery powered) in the mail today to try with my system. I've been using a computer my whole life for media playback and this will be the first time trying a preamp in the signal chain. I don't understand how adding something to the signal chain will improve it, but I'm willing to try it out. A member of another forum sent it to me to try.


Thanks.

And have fun with the new toy. Are you going to use it in a separate 2 channel system or with your mains? OT, but a lot of people like tubes partially because of some added harmonic distortion (Can't remember if it's even or odd order) that can be pleasing. Hope I'm not starting another war with this one!

Cheers,
Rod

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mojave

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Re: Setting Reference Levels
« Reply #22 on: December 10, 2012, 01:01:20 pm »

This is with my mains which are GR-Research LS-6 speakers.
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Juniorjbl

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Re: Setting Reference Levels
« Reply #23 on: December 10, 2012, 01:16:44 pm »

Guess I should have explained earlier that what I have been referring to as Active EQ is really Active Crossovers.
Rod

This is also one of the reasons I was requesting (in the other thread) to move settings from the PEQ to their own named page/tab. Also to include the differences between Bessel, Butterworth and Linkwitz Riley slopes and orders.

BTW being able to have a 1/3rd octave eq per channel for house eq would be great as well. Just the audio geek inside wishing, wishing, wishing.........
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hulkss

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Re: Setting Reference Levels
« Reply #24 on: December 10, 2012, 08:48:31 pm »

+1. This is what I would do.
But be aware - using the industry standard (20-20 kHz pink noise recorded at -20 dBFS RMS reading 83 dB SPL) is a lot louder than you think. Probably way too loud for the home environment.
VP

This setting limits the maximum SPL to 103 dB from a single speaker. Loud enough for most, but definitely not "way too loud".
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hulkss

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Re: Setting Reference Levels
« Reply #25 on: December 10, 2012, 10:14:56 pm »

My amps put out over 400 watts per channel at 8 ohm and a little under 800 watts at 4 ohm.....I recently had some 101 dB 4 ohm speakers in for a demo. I had some guys over for prime rib and listening and we turned them up to max, too. ;D It was fun getting 130 dB peaks (with subs too) at the listening position.

130 dB is painful for most humans.
Checking physics:
A 101 dB efficient speaker is defined as 101 dB SPL at 1 meter distance with a 2.83 volt signal.
At a listening distance of 4 meters (about 13 feet) the level would be 101 + 20 × log(1 ÷ 4) = 89 dB
Now turning up the voltage to reach 130 dB (41 dB gain) would require 2.83 x 10^(41/20) = 318 volts
Calculating the power required (318 volts into a 4 ohm load) (318^2)/4 = 25,281 watts  :-X


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mojave

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Re: Setting Reference Levels
« Reply #26 on: December 11, 2012, 11:31:04 am »

The peaks were recorded with an Omnimic about 10 ft from the speakers. Also, actual volume loss in a small room are much less than the theoretical. We were playing back the gunfight in Open Range and the gunshots are what caused the peaks so we weren't listening to the regular content that loud. We only had the volume up all the way for a few minutes. JTR Speakers are rated at 101 dB at 2 volts. The signals in the midbass where the peaks were recorded were probably coherent so you can get up to 9-12 dB of volume increase from both mains and subwoofers. I was using a 40 Hz crossover with a 12 dB/octave slope so the gunshots where right where both IB subs and mains were producing output. Total power available for subs and mains was 6000 watts with possibly double available for peaks.

If you look at the Reference Capable Speakers Spreadsheet you can see that the JTR Triple 12HT is also rated at 101 dB with 2 watts. It is shown to be able to produce 124.8 dB peaks at 12 ft with 3200 watts. The methodology/accuracy of the spreadsheet seems to have been fairly well vetted at AVSForum.

To recalculate:
101 dB at 1 M = 91.32 dB at mic position (worst case scenario)

This is with 1 watt. To calculate the dB increase between watts you use dB = 10 x Log (watts1/watts2). If I have 1600 peak watts for a speaker then 10 Log(1/1600)=32.04 dB change. The voltage required can be much less than calculated because peak output doesn't require the input of more voltage due to the amp's transformers.

91.32 + 32.04 = 123.36 dB per speaker

The subs are a little trickier. The subs are 87 dB at 1w (87.3 @ 2.83v). With 8 drivers this equates to 96 dB at 1 w. However, for each doubling wired in parallel you are halving the impedance which gives another 3 dB (per Bill Fitzmaurice). Since there are 4 pairs you should gain another 9 dB by halving the impedance into each pair which would make them have an efficiency of 105 dB.

At 10 ft the subs would be 95.32 dB with 1 watt. If they gain the same as the mains, then they could output 95.32 + 32.04 = 127.36 with going from 1 watt to 1600. At peak power they could increase dB by 39.44 dB which would put them at 134.76 dB. Interestingly, this matches almost exactly what WinISD predicts as maximum output without exceeding driver excursion limits.

If you start at 91.32 dB and get just 6 dB from coherent signals to the subs and mains you need 32 dB of gain which is certainly possible.

If you want to double check with continuous volts, my subwoofer and mains amps are capable of a combined 250 volts continuous draw. 250 volts at 4 ohms = 15,625 watts. I can't do this continuously right now since I haven't added the additional circuit that I plan to add. However, peak watts shouldn't be an issue.
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Matt

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Re: Setting Reference Levels
« Reply #27 on: December 11, 2012, 12:01:13 pm »

15,625 watts

That's 21 horsepower ?!?

I think the entire service into my house is about 1/3 of that.
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hulkss

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Re: Setting Reference Levels
« Reply #28 on: December 11, 2012, 09:12:34 pm »

OK, I'll try my method again with your clarifications:
The linked spreadsheet says 101 dB sensitivity normalized for 8 ohms, so.....
101 dB SPL at 1 meter distance with a 2.83 volt signal
At a listening distance of 3.05 meters (about 10 feet) the level would be 101 + 20 × log(1 ÷ 3.05) = 91.3 dB
Now turning up the voltage to reach 123.4 dB (32.1 dB gain) would require 2.83 x 10^(32.1/20) = 114 volts
Calculating the power required (114 volts into an 8 ohm load) (114^2)/8 = 1624 watts

So both mains at 1600 watts peak = +6 dB with a coherent output from each main and you get to 130 dB  ;D

Just a couple comments:
I was using a 40 Hz crossover with a 12 dB/octave slope so the gunshots where right where both IB subs and mains were producing output.
A typical 2nd order L-R crossover is -6db at the crossover point to provide flat output when the mains and subs are summed. Most of the increase in output would come from the R+L channels.

With 8 drivers this equates to 96 dB at 1 w
I think that should be at 8 watts (1 watt for each of 8 drivers).

However, for each doubling wired in parallel you are halving the impedance which gives another 3 dB (per Bill Fitzmaurice). Since there are 4 pairs you should gain another 9 dB by halving the impedance into each pair which would make them have an efficiency of 105 dB.
Halving the impedance adds 3 dB of voltage sensitivity not power efficiency.
The small room you mentioned will provide substantial room gain to the subwoofers.
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