my profile | my password | search | faq & rules | forum home

» Film-Tech Forum ARCHIVE   » Community   » Film-Yak   » Decibel ranges?

Author Topic: Decibel ranges? Patrick A. Meister Film Handler Posts: 22 From: Switzerland Registered: Aug 2003 posted 04-01-2004 11:24 AM                     Hello! Last days I read something like "-25dB". Well, I just know the sound level of the Dolby Calibration, which is positive! (+85dB) What's up with theese negative Decibel ranges?  |  IP: Logged Darryl Spicer Film God Posts: 3250 From: Lexington, KY, USA Registered: Dec 2000 posted 04-01-2004 01:19 PM                    where are you reading these negatives. If it is on something that is controled by a master volume adjustment like on a stereo it has nothing to do with the actual sound preasure levels.  |  IP: Logged Randy Stankey Film God Posts: 6539 From: Erie, Pennsylvania Registered: Jun 99 posted 04-01-2004 04:46 PM                     The decibel is actually a relative mesurement between two sounds, signals or power sources. The simple formula is: 10 log(Input/Reference) "Input" is the signal, sound or power source being measured. "Reference" is the signal, sound or power source that "Input is compared to. If you are talking about sound pressure levels, the reference would be a source of 1 millibar pressure. If you are talking about power, the reference would be 1 milliwat. That's the simple definition. If you click on THIS PAGE you can get a much better explanation than I could ever give.  |  IP: Logged Aaron Sisemore Flaming Ribs beat Reeses Peanut Butter Cups any day! Posts: 3061 From: Rockwall TX USA Registered: Sep 1999 posted 04-01-2004 04:50 PM                     If you are reading it on a fader control of an SDDS processor or on the display of a home theatre/stereo receiver/amp, then using '0' (zero) as a center reference, the numbers refer to decibels of gain (+) or attenuation (-). In the case of the SDDS, the SPL reading should be 85db per channel with the fader set at '0 db'. You get +10db or -10db in either direction from there. In the case of the home system, usually '0db' reading is with the volume fader in the full up position, with -infinity db in the full down position. -Aaron  |  IP: Logged Patrick A. Meister Film Handler Posts: 22 From: Switzerland Registered: Aug 2003 posted 04-02-2004 09:24 AM                     Thanks to all of you! Now I have to translate this page into german...well, my english isn't the best.  |  IP: Logged Michael Schaffer "Where is the Boardwalk Hotel?" Posts: 4143 From: Boston, MA Registered: Apr 2002 posted 04-02-2004 09:29 AM                        There are about 10 million pages on the internet on which the dB is explained in any language, and you can also go to the library and look it up in just about any dictionary. A dictionary of audio terms would be best of course, you will find something like that most likely in your library too.  |  IP: Logged Patrick A. Meister Film Handler Posts: 22 From: Switzerland Registered: Aug 2003 posted 04-02-2004 09:49 AM                     Good idea! The libary in my school should have something like this...I'll look for it  |  IP: Logged Randy Stankey Film God Posts: 6539 From: Erie, Pennsylvania Registered: Jun 99 posted 04-02-2004 09:53 AM                     Sorry about the language thing. I can often interpret web pages in Spanish or French, although I don't read fluently. I can handle short bursts (1 or 2 sentences) in German. You've probably got a similar situation, only in reverse. It's easy to forget. When I get stuck on a long page like that I often use Systran.com. This page can translate many languages from one to the other. It's good, provided you recognize its limitations: 1) It is a machine translation. 100% accuracy is not guaranteed, especially when it comes to slang, dialects or faulty grammar. It does, however, give you a close enough translation that you can understand the meaning. 2) There are usage limitations. I think you can only translate so many words or pages at a time. (Or during a set time period.) 3) It's a commercial site. They are trying to sell you something: Namely, their Systran language translation software/dictionaries. Always keep such things in the back of your mind when you use sites like this. (Most people do.) I'm sure there are other sites that do language translation. This is just the one that I know best.  |  IP: Logged Patrick A. Meister Film Handler Posts: 22 From: Switzerland Registered: Aug 2003 posted 04-02-2004 11:22 AM                     Well, when I'd read and write more in english (not like in the german forum), I'd be able to understand the language better. I'm learning and learning...  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 04-02-2004 07:19 PM                     The whole decible thing is actually more complicated than has been listed here. For instance...Randy did real well until he gave you the formula. with the 10 multiplier. That works for power measurements but not voltage and most other forms of dB. At its heart, dB is merely a unit of measure that compares one value to another on a log scale (common log, base-10) Normally when someone refers to some level, there is a suffix attached to the dB value...such as dBc which denotes that it is a sound pressure level measured against a "C" weighted scale (mimics the pass band of the range of human hearing from 20Hz to 20KHz). Another popular form is dBa which also is a sound pressure level measurement but using the A-weighted scale that not only accounts for the passband of human hearing but the typical human's frequency response. You will typically find residual noise measurments based on the dBa scale where as overall program levels referenced to the dBc scale. There are tons of other scales and that little suffix will often be a clue as to what it is you are measuring (what the reference is and what the units are). Common ones are dBm, dBv and dBV. More often than not the multiplier is actually 20 such that when one is making a dB calculation it will take the form of 20Log(measured value/reference value). The reason for the difference in multipliers has to do with how logs work and the relationship of voltage, current and power. Some simple rules to remember for working with voltage. Doubling the voltage equals a 6dB increase. Likewise, halving the voltage will result in a 6dB decrease. So if you have a measurement of 300mV...and you raise the level to 600mV...you raised it 6dB. e.g. 20Log(.6/.3)= +6dB. With power there is only a multiplier of 10...if you double the power you only add 3dB. But another nifty rule comes up with this...10 times more power results in a gain of 10dB which sounds about 10 times louder. Normally you don't get to have such a linear feel for sound like that. The next thing to realize for cinema and other sound is log addtion. It is a more complicated subject for the faint of math but will let you understand how adding surround levels such that if you have 85dBc from the left surround and 85dBc from the right surround will yield a combined SPL of 88dBc. So, in the above examples...I've shown how voltages (or currents) compare on the dB scale, how sound pressure levels compare since they have those special suffixes to the dB scale. As others have pointed out...you could have a volume knob that has "0dB" when all of the way up and negative dB numbers as you rotate it counter-clockwise all of the way down to "-infinity." What does it mean when the volume knob (or some call it the attenuator) to say -20dB? Just that you have attenuated the signal, any signal, by 20dB. Its reference is only to maximum and not to some finite number (except maybe when the device clips or otherwise runs out of power rails). If you were to input say a 1KHz tone into your device with the knob at 0dB and note its voltage at the output, then lower the volume knob until it read -10dB...you should be able to predict what the output voltage is using the above formulas. Then, you could take say pink noise and an SPL meter and connect your device up to an amplifier and speaker and do the same experiment measure the pink noise at 0dB...(set your pink noise generator so the level is comfortable)...note what it is...say 75dBc...then lower the volume knob to -10dB and the SPL meter should agree and say 65dBc...presuming your inherent room noise is not contributing a significant amount. Class dismissed. Steve  |  IP: Logged

All times are Central (GMT -6:00)

Printer-friendly view of this topic

Hop To: Select a Forum: Category: Operations -------------------- Digital Cinema Forum Film Handlers' Forum Large Format Forum Ground Level Feature Info, Trailer Attachments & REAL Credit Offsets Category: Community -------------------- Film-Yak Film Handlers' Movie Reviews The Afterlife Bob Maar's Joke-A-Thon Category: Classified -------------------- Equipment and Supplies Wanted/For Sale Help Wanted/Seeking Employment Category: System -------------------- Software Beta Testing Forum

The Film-Tech Forums are designed for various members related to the cinema industry to express their opinions, viewpoints and testimonials on various products, services and events based upon speculation, personal knowledge and factual information through use, therefore all views represented here allow no liability upon the publishers of this web site and the owners of said views assume no liability for any ill will resulting from these postings. The posts made here are for educational as well as entertainment purposes and as such anyone viewing this portion of the website must accept these views as statements of the author of that opinion and agrees to release the authors from any and all liability.