What is "reference level"? - Page 4 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #31 of 81 Old 11-19-10, 08:14 AM
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Re: What is "reference level"?

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A9X wrote: View Post
Dynamic amplifier power is nothing more than marketing rubbish.

10kW, really? For an 88dB speaker on it's own at 4m with 800W you get THX ref level at the LP of 105dB. As I presume it is 2 or 4 pi space and not a room where the reverberant sound will also add to the apparent level, and the recommendation from Toole (IIRC, not in front of me ATM) is 3dB not 6dB loss per doubling of distance, then we're back to 400W. High, but not unrealistic.

Now get some sensible speakers with decent efficiency and volume displacement and reference at the LP is easy.

Dialogue is supposed to be mastered at the 75-85dB level and with peak loudness at 105dB, a 20dB+ crest factor is not unrealistic. Just make sure you have enough speaker and amp to do it.

Calculator used is from here. [first google hit]
No, actually not 10 kW. According to your example, you need 20 kW. The problem comes down to what we mean by "Watts".

The first mistake in all the calculators is that there is no such thing as "Watts RMS" in these calculations. The simplest, arithmetic form of the equation is Volts, RMS X Amps, RMS = Watts, average. So now we know that we are taking an average power.

The second problem is the speaker sensitivity measurement. As I understand it, the sensitivity measurement is taken driving the loudspeaker with a sine wave. Beginning with that, and working through the calculator with your example, we get 105 dB SPL with 800 W average. But that would be a scaled version of the speaker sensitivity measurement at 105 dB instead of 88 dB. That is not home theater. My question in all this is how to relate this to home theater, which I presume is employing movie soundtracks, and not sine waves.

I believe this all comes down to the "dynamic headroom" which is allowing for the difference between average and peak power in the program content. In the speaker sensitivity test, using sine waves, the peak power in the speaker is 2X the average power. This happens because the sine wave applied to the speaker has a peak voltage that is the square root of 2 higher than the RMS value of the waveform. Since power is proportional to the square of the applied voltage, at the peak of the waveform the instantaneous power is twice the average power. This, BTW, is the genesis of "peak power ratings", which are not rubbish. As we say, figures don't lie, but liars figure. Peak power is often presented to be the same thing as average, which it isn't, and that part is the marketing rubbish. But I digress.

As we all know, 2X the power is 3 dB. So your amplifier needs 3 dB of "dynamic headroom", just to reproduce a sine wave. However, this headroom is already included in the amplifier's power spec, so we don't add it in twice. So what dynamic headroom should we be allowing for when we wish to reproduce movie soundtracks?

Some years ago, this problem was faced by broadcasters who had the same problem trying to broadcast program material without over modulating. A noise signal was developed, based on work done characterizing radio program material. This noise signal became known as USASI noise and a standard containing the definition of this noise source is given here: http://www.nrscstandards.org/SG/NRSC-1-A.pdf

Page 17 of this standard shows this noise spectrum. Note that it rolls off at 6 dB/octave on both sides of the peak at about 200 Hz. A study of the peak-average ratio, or crest factor of USASI noise reveals that it is 10 dB. This means that to reproduce 88 dB of USASI noise requires 10 dB of dynamic headroom, or 10X amplifier power to reproduce the random peaks of the signal. In other words, you can run the 88 dB speaker sensitivity test with a 1 W amplifier, but if you used USASI noise you would need a 10 W amplifier to produce 88 dB on your SPL meter.

Going through the SPL calculator with your example, and listening at 105 dB, you need 8 kW of amplifier power, or if you use the -3 dB/double distance rule for a reverberant room instead of the -6 dB rule for free-field conditions, it requires 2 kW. (You quadrupled the distance to 4 m so you saved 6 dB of amp power.)

Now, in your example, you were quoting a 20 dB crest factor. I only subscribe to 10 dB, which I have justified above. However, the broadcast industry's program material model may be too compressed for the movie industry, and 20 dB may be closer to the truth. If that were true, then add another 10X to amplifier, and get 20 kW required power.

Last edited by gsmollin; 11-19-10 at 08:26 AM.
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post #32 of 81 Old 01-01-11, 11:44 AM
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Re: What is "reference level"?

Since nobody has really address this question, I will. Reference level is not 75db or 85db. It is actually 0 digital reference, or the loudest digital signal before system overload. So reference levels for the cinema would be 105db for each main channel, and 115db for the LFE based on 0 reference. This would be continuous output, and nobody can really stand this kind of output continuously.

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post #33 of 81 Old 01-01-11, 06:53 PM
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Re: What is "reference level"?

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Sir Terrence wrote: View Post
Since nobody has really address this question, I will. Reference level is not 75db or 85db. It is actually 0 digital reference, or the loudest digital signal before system overload. So reference levels for the cinema would be 105db for each main channel, and 115db for the LFE based on 0 reference. This would be continuous output, and nobody can really stand this kind of output continuously.
Does this mean it's impossible for a signal to ever peak above these?
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post #34 of 81 Old 01-01-11, 10:27 PM
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Re: What is "reference level"?

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Does this mean it's impossible for a signal to ever peak above these?
Yes the signal can peak above 0db, however it is pure distortion at that point. It is truly not something you want to hear.

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post #35 of 81 Old 01-10-11, 06:54 PM
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Re: What is "reference level"?

your question to me is very broad.... you speak of what is reference, do you mean reference level speakers? reference level processors? reference level amplifiers? maybe your looking for mid-fi equipment which is still very expensive but not in the 100k level? mid-fi is generally 18-28k+ for a theater system approximately.. first if you say that this is the last system you want to buy you need to define a budget, before you go any further.

just my .02

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post #36 of 81 Old 01-10-11, 09:38 PM
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Re: What is "reference level"?

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THOMASWMAZ wrote: View Post
your question to me is very broad.... you speak of what is reference, do you mean reference level speakers? reference level processors? reference level amplifiers? maybe your looking for mid-fi equipment which is still very expensive but not in the 100k level? mid-fi is generally 18-28k+ for a theater system approximately.. first if you say that this is the last system you want to buy you need to define a budget, before you go any further.

just my .02
I think you're totally lost my friend. This thread is discussing the strict definition of "reference level" sound pressure levels IE the sort defiined by Dolby or THX or whomever.
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post #37 of 81 Old 01-10-11, 11:14 PM
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Re: What is "reference level"?

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Emuc64 wrote: View Post
I just recently bought my first house. With the low interest rates, I'm paying $225 more per month for my single family detached house.
Congrats on your first home. The hard work pays off. And now the sweet joy of ownership - playing music to your "reference level."

I can't even rent a room for $225 around here. So envious!

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post #38 of 81 Old 12-28-11, 10:13 PM
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Re: What is "reference level"?

I have little problem in reaching reference levels as I have Klipsch RF-82s for mains and my 5 speakers are driven by a new 80lb Outlaw 7500 power amp and my subwoofer is the 160lb SVS PB13 Ultra (with Sledge amp). Watching War of the Worlds proved to be profound. I metered the system at 116.5 db with both the power amp and sub driving that sound pressure level effortlessly. For your interest, the scene where the pods blow up the bridge behind Ray's (Cruise) house is the loudest in the movie.

SPSs at 90db is more then good for me for 2 hours while 82db-85db is a comfortable continously LOUD listening level.

Thanks for everybody's analysis. You guys definitely know a lot more than me.

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post #39 of 81 Old 02-21-12, 04:16 AM
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Re: What is "reference level"?

Had a good summer storm here today, lots of thunder and lightning so I put TRON on and cranked it up to eleven and threw away the knob.
That movie is AWESOME at reference levels. I judge my reference level the same way as we did it when I worked at the local twin cinema, the dialogue should be clearly audible and let a couple of Kw of amps and LOTS of subs take care of the rest.
A soundtrack with good dynamic range like TRON is a great indicator.
Trouble is you can only play it during the day, the sound would carry blocks away during the night!!!
I use two active 15" vented subs via FBD plus twin 12" sealed just to augment the centre channel.
-Plus a bunch of amps from the receiver pre-outs.
Add a 6meter screen and Bob's your uncle!!
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post #40 of 81 Old 02-21-12, 09:47 AM
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Re: What is "reference level"?

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waricle wrote: View Post
Had a good summer storm here today, lots of thunder and lightning so I put TRON on and cranked it up to eleven and threw away the knob.
That movie is AWESOME at reference levels. I judge my reference level the same way as we did it when I worked at the local twin cinema, the dialogue should be clearly audible and let a couple of Kw of amps and LOTS of subs take care of the rest.
A soundtrack with good dynamic range like TRON is a great indicator.
Trouble is you can only play it during the day, the sound would carry blocks away during the night!!!
I use two active 15" vented subs via FBD plus twin 12" sealed just to augment the centre channel.
-Plus a bunch of amps from the receiver pre-outs.
Add a 6meter screen and Bob's your uncle!!

This is arguably the best explanation of FUN in one's system that I ever read. That is movie watching.

My two favourite lines are...

Cranking it up to eleven and throwing away the knob.
Add a 6 meter screen and Bob's your uncle.


That's the way it's done and bollocks to the neighbours.

"There is one who comes after me who's sandals I'm unworthy to unloose." John the Baptist
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