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Discussion Starter · #1 ·
I've been doing some reading on THX, curiosity as none of my stuff is THX certified. I understand about using (0) on your reciever as a median point. And that the pink noise level is 85 db. But then I'm confused. After you set all your levels to 85 db...where do you get the 105 db that is reference level? Is that what a THX movie plays at with your reciever set to (0)?

BTW- I was watching Master and Commander at the chapter 4 battle scene and I did some checking and my db meter was showing a peak of 95 db. I cannot imagine how LOUD 105 db would be in a 14x20 room like mine. At the level I was at I was noting all the spots where my house rattles!
 

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When you calibrate the noise to 85 dB, that noise is 20 dB below the 105 dB (you don't need to calibrate at max settings).


Your volume control could be at any single value to reach that reference. It's -17 dB on my system. When you watch a movie at reference level, you set the volume at that level again and you will hit peaks of 105 dB.


I find dialog often too loud at reference level, so often watch 7 to 10 dB below reference,
 

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Here is a quote from a THX document proposing game reference levels that speaks to Dolby/DTS levels-


3.6. Playback Reference Level

In the consumer electronics industry, there is an

established reference level standard based around the

motion picture level of 85dBc. For home systems,

though, there is not an array of surround speakers. This

means that all the main loudspeakers (Left, Center,

Right, Surround Left, Surround Right for a 5.1 system)

are all set to:

85dBc (7 )

This reference level can be achieved readily using either

test noise embedded in products, or a commercially

available calibration disc, along with an inexpensive

sound pressure level meter,


The full document is at http://thx.com/library/pdf/GamesRefLevels.pdf


So for THX the 0dB or -20dB setting on the gain control should be 85 dB at the listening position. Your gain control should allow you to go to some positive dB setting (+15 on mine) to play at higher SPL. I did not see anything on the THX site suggesting a 105dB reference level.


In my 13.5 x 23 room 105 dB is hearing protector country. That battle in the fog is a great way to scare/impress people with your setup
, and it is a pretty good movie, taken from two very good books.
 

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Discussion Starter · #4 ·

Quote:
Originally Posted by trekguy /forum/post/0


and it is a pretty good movie, taken from two very good books.

I assume the title to one book is the obvious, what is the other? Sometimes I read too, but it's "funner" to watch!


So, when all noise levels are set to the 85db mark, when viewing a movie the highest high will be 105 db (THX) and the lowest low would be ..0 db? Providing you have the volume control set to whatever point you set the 85 db levels to.


Since the THX level is determined at the labs theater, their reference is, like you said..ow. Is there a formula that would lower the reference level db in ratio to the decrease in sq. ft. of my room to their theater? I'm just curious, I put the knob where it sounds great to me...and when my lady is around or the parents visit obviously it's turned down a good deal more!


If it's unreasonable to listen to movies at the THX certified level (at home), is it safe to say that this is more for commercial (size) theaters? If that's so, why are there THX recievers? You don't listen at that volume at home and a large theater is going to have more than a couple thousand dollar reciever powering it.


Pleeze exkuse my speling, the spell check won't work.
 

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Let's start over.



The pink noise used for "calibration" is recorded at -20db. Remember with digital media 0db is full strengh.


Using the THX model every speaker should play at 85db when given a -20db "reference" signal, at the main listening position.


Now time for some math. If -20db is played at 85db, what is 0db played at? 105db.


The LIFE channel tops out at 115db. The decoder adds 10db for added effect.


In a theater environment the SEPAL cannot exceed 120db, so when you play a signal on all channels at full strenght when "calibrated" the SPL stays just below this number. This is why THX want the AVR VOLUME indicator to be at 0db, so the theater operators know not to turn it up anymore. Kind of a K.I.S.S. thing.



This is a very high level for the home environment. Most "normal" people prefer a 75db level. My wife likes 65db.
 

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Discussion Starter · #6 ·

Quote:
Originally Posted by soho54 /forum/post/0


Remember with digital media 0db is full strengh.

I did not know that. Ok now I understand the 0 db/volume knob relationship. I never really got why other friends recievers always show a negative db on the display even though we're rockin.


I know, I'm slow, but, is the THX ceritification on home epuipment really necessary? I see that the reciever is capable of producing those levels, and you would want speakers that won't catch fire trying to do so, so I see the need for "matched" epuipment, but if you don't play that loud....I mean you say "normal" for 75 db and I probably sit around 85 db (I've been told I'm not normal, but for other reasons) but the difference between even 85 and 105 is HUGE. I did read up about the non-linear(?) relationship of the ratio between increasing db numbers and noise levels.
 

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Quote:
Originally Posted by tonyfan70 /forum/post/0


I did not know that. Ok now I understand the 0 db/volume knob relationship. I never really got why other friends recievers always show a negative db on the display even though we're rockin.


I know, I'm slow, but, is the THX ceritification on home epuipment really necessary? I see that the reciever is capable of producing those levels, and you would want speakers that won't catch fire trying to do so, so I see the need for "matched" epuipment, but if you don't play that loud....I mean you say "normal" for 75 db and I probably sit around 85 db (I've been told I'm not normal, but for other reasons) but the difference between even 85 and 105 is HUGE. I did read up about the non-linear(?) relationship of the ratio between increasing db numbers and noise levels.


Yep, every 6db increase in SPL represents a doubling in sound pressure output.
 

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OK, now for part two.

Quote:
I know, I'm slow, but, is the THX ceritification on home epuipment really necessary?

No, but some people like to use it. It is also nice for comparisons sake, if someone says they were listening to so-and-so at -15db you can relate. (-15db is my sweet spot, btw)

Quote:
I mean you say "normal" for 75 db and I probably sit around 85 db

Now are you talking calibration or peak SPL?


If you calibrate to 85db at 0db on the AVR you get peaks of +105db

If you turn the AVR down to -10db (same as calibration at 75db) you get +95db peaks

With the AVR at -20db(or cal. 65db) you get +85db peaks when things blow up.


What you are really doing is setting the max SPL. It is just really hard to be in a room trying to calibrate at +100db levels, so you use a partial signal (-20db) to save your ears.
 

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Quote:
Originally Posted by crackyflipside /forum/post/0


Yep, every 6db increase in SPL represents a doubling in sound pressure output.


That's 3dB, isn't it? And, 10dB to sound twice as loud...
 

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Doubling the voltage to a speaker gives you +6db.

Doubling the watts to a speaker gives you +3db.

Boosting a signal by 6 dB doubles the amplitude.

Humans on average require a 10db increase of SPL to hear a subjective doubling of the loudness.
 

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Quote:
Originally Posted by soho54 /forum/post/0


Doubling the voltage to a speaker gives you +6db.

Doubling the watts to a speaker gives you +3db.

Boosting a signal by 6 dB doubles the amplitude.

Humans on average require a 10db increase of SPL to hear a subjective doubling of the loudness.

Fine and dandy, but does +6dB or +3dB mean "doubling sound pressure output"? I think that it's +3dB, but could be wrong...
 

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Discussion Starter · #13 ·

Quote:
Originally Posted by soho54 /forum/post/0


OK, now for part two.


No, but some people like to use it. It is also nice for comparisons sake, if someone says they were listening to so-and-so at -15db you can relate. (-15db is my sweet spot, btw)


Now are you talking calibration or peak SPL?


If you calibrate to 85db at 0db on the AVR you get peaks of +105db

If you turn the AVR down to -10db (same as calibration at 75db) you get +95db peaks

With the AVR at -20db(or cal. 65db) you get +85db peaks when things blow up.


What you are really doing is setting the max SPL. It is just really hard to be in a room trying to calibrate at +100db levels, so you use a partial signal (-20db) to save your ears.

Excellent, excellent responses. Now I understand where the 85 db level comes from..simply to save yourself pain and to have a "standard". BTW, the 95db's that I mentioned was peaks in chapter 4.


Now I see the point in having THX certified AVR's with the apples to apples idea. Since watts per channels power specs seem to be ... varied
between manufacturers, does the THX kinda level that field? I mean is 100w/ch on a THX Pioneer close in power to a 100w/channel THX Denon (for ex.)
 

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Quote:
but does +6dB or +3dB mean "doubling sound pressure output"?

I guess I failed to make my point before.


In the following, dB is power and Db is SPL


If you double the power behind a sound you get a change of 3dB in the power level, which is a change of 6Db in the sound pressure level, and a change of 6Db is a doubling of SPL.


Going from .5 volts to 1 volt raises SPL by 6Db


Going from 100watts to 200w raises SPL by 3Db


I'm trying to show that there is two funky decibel levels we could be talking about, and why everyone mixes them up.
 

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Quote:
Originally Posted by tonyfan70 /forum/post/0


I've been doing some reading on THX, curiosity as none of my stuff is THX certified. I understand about using (0) on your reciever as a median point. And that the pink noise level is 85 db. But then I'm confused. After you set all your levels to 85 db...where do you get the 105 db that is reference level? Is that what a THX movie plays at with your reciever set to (0)?

BTW- I was watching Master and Commander at the chapter 4 battle scene and I did some checking and my db meter was showing a peak of 95 db. I cannot imagine how LOUD 105 db would be in a 14x20 room like mine. At the level I was at I was noting all the spots where my house rattles!

With properly encoded Dolby Digital source material and a correctly calibrated THX preamp or receiver, 0dB on the volume knob means that dialog will be reproduced at 74dB SPL. The electronics don't matter, the speakers don't matter, and the room doesn't matter.


With the default encoder settings the dialog level is at -27dBFS; the FS meaning full-scale digital where 0dBFS is the loudest possible signal. 74 dB SPL for the dialog @ -27dBFS means 0dBFS = 101dB SPL possible peak output from the main channels. The LFE channel gets a 10dB boost allowing for 111dB peaks. This isn't too loud provided that the system has low distortion and the room is not too reverberant. A lot of speakers don't have low distortion, and the average living room is somewhat live.


Without any dialog normalization, calibrating at 75 dB SPL using consumer test signals recorded at -30dBFS produces maximum main channel peaks of 105dB SPL @ 0dBFS.


Using 85 SPL with professional test signals recorded at -20dBFS produces maximum peaks of 105dB SPL @ 0dBFS.


That doesn't make 75, 85, or 105dB reference level.
 

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Drew, man chill.



I thick everyone here is grown up enough to decide what is to loud for their own comfort.


Also you are reading a little to much into dialnorm.


Here is a nice site for everyone : THX-Certification Skip on down to the THX at Home section.
 

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Quote:
Originally Posted by whoaru99 /forum/post/0


Fine and dandy, but does +6dB or +3dB mean "doubling sound pressure output"? I think that it's +3dB, but could be wrong...

Let's write out how the sound pressure level and sound power change with various dB increases. Further, let's be fussy, and show each answer to three decimal places (much more precision than is needed for the kinds of measurements we are talking about).


+3 dB means the sound pressure level increases by a factor of 1.413, and the sound power increases by a factor of 1.995. People say this is "doubling the sound power", because 1.995 is close enough to 2.000 to not worry about the difference.


+6 dB means the sound pressure level increases by a factor of 1.995, and the sound power increases by a factor of 3.981. People say this is "doubling the sound pressure level", or "increasing the sound power by a factor of four"; again, the numbers are close enough. (Therefore, +6 dB is the answer whoareu99 was looking for.)


+10 dB means the sound pressure level increases by a factor of 3.162, and the sound power increases by a factor of 10. In this case, the "factor of 10" is exact.


+20 dB means the sound pressure level increases by a factor of 10, and the sound power increases by a factor of 100. In this case, the "factor of 10" is exact, and the "factor of 100" is exact also.


Although some people don't like using math formulas, the following formulas will give all the preceding results. For an increase (or decrease) of N dB (whether N=+3, +6, +10, +20, -3, -20, or some other number):

new sound pressure / old sound pressure = 10^(N/20)

new sound power / old sound power = 10^(N/10)


There is one kind of deciBel (dB) (sorry, no alternate unit Db allowed in these parts
). How the dB change affects the sound pressure level, and sound power, is given by the formulas.
 

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When I said "two funky decibel levels we could be talking about,," I was talking about sound pressure levels and sound power levels.



I didn't mean there were two decibel scales.


I try to keep everything at an 8th grade level.



Nice write-up though.
 

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Thanks for pointing out the difference between sound pressure and sound power - that's what got me.
 
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