Tell me about dB's

MrNightly

Ok.

Trying to explain to my brother in law a couple days ago, why my AVR was running at -20dBs during a movie. He's an engineer and I explained that I calibrated my system to 80 decibels at 0dbs on the AVR with my SPL blah blah blah...

He said he understood about the calibration, and then said, "why don't you just calibrate 0dbs to be 0 volume?" I didn't have an answer. It dawned on me, that there must be a reason for this "-" instead of the positive.

Who invented this idea anyways? Is there a logical explanation when the engineer asks why in the world are you in the negatives???

When I really sit down and think about it, I don't understand it either.

F1nut

On AVR's, zero represents "Reference Level", so -dB is under the RL and + is over. All in all, pretty effing stupid if you ask me, but then I'm not into HT.

McLoki

Reference level is 85 db dialog with 105db peaks from each channel. Your sub (due to all other speakers set to small and just cuz it can) gets average levels of 95db and peaks of 115db.

The test tones on Video essentials are calibrated to be 30db under reference (that is why you set them to 75db) The test tones on Avaya are 20db under reference (that is why you calibrate them to 85db.)

Movies themselves can run over or under reference. Usually they are close (+ or - 3 to 5 db) but it can make a big difference. That is why if you have not heard a movie before and listen close to reference level, you may want to turn it down a bit for your first viewing.

The point of it is so you can compare with someone else. Since reference level is measured from your listening position, the same movie in two different rooms with 2 different systems should be the same volume if they have both been calibrated to reference level and are both playing with the volume control at the same level.

BTW - actually playing a movie at reference level (volume at 0) takes some pretty serious equipment especially a monster sub.

Michael

BTW - the words volume and reference level are what you want to search on to get more links like this one...

unc2701

Orginally, it was a recording/ preamp thing. It used to be if you hooked your line-level source straight to the power amp it would be at the same volume as if you we through the pre at 0db. No boost, nothing taken away.

So that's the origin of the negative thing. These days it's basically meaningless, unless you calibrate your system to some set level.

McLoki

Here is a cut and paste from this article on THX certification....

A few words on "Reference Level"

Before we start talking about the pieces and parts, we first need to take a quick refresher on the concept of "Reference Level", as you are going to be hearing that term quite a bit in the coming paragraphs. Simply stated, Reference Level is a standard, known, predictable and reproducible playback volume level. When movie sound tracks are crafted, they are done so on systems which are locked at this level. The sound artist does not play around with a big volume knob when doing his/her work. If the sound artist wants something to be loud, they make that sound loud within the sound track. When they want something to be soft, they make that element soft within the sound track. Movie theaters set their playback level by the exact same rules, so when the movie is shown, you hear EXACTLY what the sound artist heard when they were making the piece. Loud, soft, in-between, it's all there, and no one touches the master volume knob over the course of a two-hour movie.

To achieve THX certification, components must play at this reference level without breaking, distorting, buzzing, rattling or any other distracting effects.

Reference level is by any definition, objective or subjective, quite loud. It basically mirrors the dynamic range of the studio system, which in the case of all movie sound tracks, is 105 dB. Any single channel of the system is calibrated to play 0 dB FSD (the loudest sound the sound track can contain) at 105 dB (115 dB for the LFE channel). While that is really, REALLY loud, its important to remember that there is 105 dB of dynamic range and the artist can put a sound at any level they want. So while a system's volume may be set to reference level, dialogue within the sound track can, and most often is, at a normal, natural level. Reference level, with the dynamic range available, permits a movie to have that normal, natural dialogue, and then suddenly a spectacular, loud car chase without anyone touching the volume control. Every element in the sound track comes out as it should.

Now, having said all that, watching a movie at reference level in a home theater is almost never done. It can be extremely loud to begin with, but the close spaces typical of home theaters make it perceptibly even more so. Reference level is still very important in home theater though for several reasons. Because it is the absolute loudest a sound track should ever be played, its fairly intuitive that its a good idea to have a system that can competently go that loud. It gives you a sort of "safe maximum" volume level, even though you may never push it that high. Even more important though is knowing what volume you are at RELATIVE to reference level because if we go too low, we literally lose the quietest sounds since they are pushed below the audible threshold, surrounds lose their presence, the perceived spectral distribution of the track is altered, and dialogue intelligibility suffers.

Michael

EricBurg

Most all newer surround recievers have a - to + range scale for volume, so setting your levels to 0 makes the most sense and makes it easy to remember/locate. However, this is just basically an arbitrary setting and you really could pick just about any number you wanted. Some of the older surround recievers didn't use the - scale and actually started with 0 and went to +40 (my old onkyo comes to mind). On those recievers you just would find a setting about 3/4 total volume to set your levels, where you would get no audible distortion. Its only important to set up all your levels for balanced playback at your primary seating positions, because you won't ever listen to any tracks at the same levels, since they vary so widely.

Eric

MrNightly

EricBurg wrote:

Most all newer surround recievers have a - to + range scale for volume, so setting your levels to 0 makes the most sense and makes it easy to remember/locate. However, this is just basically an arbitrary setting and you really could pick just about any number you wanted. Some of the older surround recievers didn't use the - scale and actually started with 0 and went to +40 (my old onkyo comes to mind). On those recievers you just would find a setting about 3/4 total volume to set your levels, where you would get no audible distortion. Its only important to set up all your levels for balanced playback at your primary seating positions, because you won't ever listen to any tracks at the same levels, since they vary so widely.

Eric

Ya, that makes sense. I think I just had never really questioned why everything was a negative. After reading that THX article, it explained it a little better.

Just trying to get my understand down, because it is a little strange that you run a movie in the negatives!!! Everything is calibrated and all is well