DP & measuring decibels - a totally inane question

[tonyvose]

Hello all - my totally inane question that I can't quite answer myself: Is it possible, in using DP, to record a sound and know how loud, in dBs, that the recorded sound actually is? I realize that, in utilizing a mic pre - which you kinda HAVE to do in order to record an 'acoustic' sound with a mic - using a mic pre attenuates and amplifies the sound one is recording.... Can this be accurately done? Can't quite wrap my brain around the method/answer. Thanks for any input. I know it's a weird, possibly dumb question.
Tony

[kevpatt]

Well, decibels is a relative scale. In digital audio, we usually use 0db to represent the maximum amplitude (loudest, full-scale) sound possible a digital system. Then we count down in negative numbers from there. So -6dB is half of the maximum, -12dB is half of that (1/4), -18db is half again (1/16), etc. Total silence is -infinity.

The decibel scale is also used in the real world to describe the absolute amplitude of real-life sounds in the air, as they reach your ears. Is this is what you are looking for? When the scale is used this way, 1dB represents about the quietest sound your ears can detect, and we count up using positive numbers from there. Very loud sounds approach 100dB. 120dB becomes painful.

Really the only thing these two uses have in common is that every 6dB increase represents a doubling of amplitude, and every 6dB decrease represents a halving of amplitude.

In DP, when you see a meter showing -12db, that means that the sound on that channel is at 1/4 of the maximum amplitude for the system. (actually DP uses floating-point math internally, which can handle sounds over 0dB, at least until the final audio output.)

What this doesn't tell you is how loud the sound was in real life before it hit your mic. It might have been 40dB, or 80dB, we don't know. It all depends on how sensitive your mic is, how high your mic preamp was turned up, etc. All we know is that after your mic, and preamp adjustments, and A/D conversion, the resulting signal was about 1/4 of full-scale.

To determine the real-life sound level, you need some other point of reference. Most commonly this is a SPL (sound pressure level) meter, a tool used for measuring real-life sound levels. These are factory calibrated to give you an absolute reading.

If you have a SPL meter, you can use it to adjust ("calibrate") your preamp. Measure a long, constant volume sound with your meter. Let's say it reads 60dB. Now adjust your preamp so that the level meter in DP reads -20dB. Now you've established a reference. Whenever your DP meter shows -20dB, you know the real-life level at the mic was 60dB. If the DP meter shows -30dB, you know the real-life level at the mic was 50dB, etc.

Another way to do this would be to get the specs on your mic, find out how many volts it puts out at a given SPL, and measure the output with a voltmeter. But you need to know how to hook it up correctly, and also measure the ac audio waveform correctly, plus do some math using logarithms. That's why they make SPL meters.

BTW, there is no "inaccuracy" involved in using a mic preamp. You simply adjust the preamp so the recorded levels you get are in a reasonable and desired range. You never want the incoming signal to get up to 0dB, and you probably want it to be at least -40dB on the quiet stuff, as a very rough suggestion.

Now, if you're recording multiple tracks through different preamps, you might care if the preamp levels are the same. You can do this by playing a constant tone simultaneously into both preamps, and adjusting both preamps until the output levels match.

[grouse]

Very informative, kevpatt, thank you!

[Shooshie]

It's a good idea to get some reference recordings and listen to them through your studio monitors, through the same train that plays your recording audio. This is quite simple to do. Just put the CD in your computer, open it, and drag the song into DP. With the Faders set at zero all the way, chances are you won't be able to play it back without blowing a speaker. So, do some reductions with the track fader, but try to keep everything else as you do when you are making music.

It's kind of surprising when you consider all the places at which gain can change. Preamps, CueMix, Tracks Plugins, Sends, Auxes, Master, Output CueMix, external Mixing board, amplifier, and I've probably left out a few. "Plugins" can mean a lot of changes.

The reference track will show you where things "ought" to be. With today's mastered recordings, it is impossible to achieve such loudness without a limiter/expander, but it's best to record at a comfortable level so that "headroom" isn't even a concern, and then let a mastering engineer bring it up to the desired level.

Shooshie

[kevpatt]

That's great advice, Shooshie. A reference track in DP will definitely help put everything in perspective, regardless of your setup.

It's really a shame that the recording industry has pushed "loud" so hard. I guess maybe it started with the ability to over-drive (saturate) tape. Then digital audio forced us into hard limiting because everybody was (by then) so used to "loud". CDs give us 96dB of dynamic range, but most commercial albums have a average(!) sound level of -15 to -10 dB. It's unreal.

Ihmo, in a perfect world, hard limiting, compressing, etc., would never be necessary just to match the expected industry "loudness".

I find it appalling that I need to compress "classical" music to get a -18dB or -15dB rms level, just so people don't think there's something wrong when they pop my cd into their car, because the volume is always turned down to 20% based on all their other cds.

[tonyvose]

Thank you all very much for the informative responses. It all completely makes sense. And, the bottom line for me would be, having a decibel meter / SPL meter would be the most accurate way to go for doing what I want to do. In the meantime, rock on DP7!
Thank you,
Tony

[Phil O]

And, the bottom line for me would be, having a decibel meter / SPL meter would be the most accurate way to go for doing what I want to do.

I've got this one:
http://www.radioshack.com/product/index ... Id=2103667

It may not be the most accurate one on the market, but it does the job for 50 bucks. It has a standard, camera style tripod mounting thread that I put on a cheap mini desk tripod that I got at a camera store. Works great.

Phil

[Pappy725]

There's an app for that.
Mine's on my iPhone. Called, appropriately enough, SPL Meter by Studio Six Digital. And after checking their website, I see they have an interface for the iPhone coming out and several other pro audio apps.

Pappy