Why are stereos measured in negative decibels?
January 9, 2004 2:39 PM   Subscribe

Why is the volume of my stereo receiver measured in negative decibals? That is, 0 decibals is loudest (absolutely speaker-blowing deafening), -50 is a good loud setting, and -70 is good for quiet late-night music. My previous stereo was similar, as are most of the others I've seen in stores and friends' homes. WHY!?!?!
posted by The Michael The to Technology (13 answers total) 4 users marked this as a favorite
Check the top result for a good explanation. Basically, each decibal is a ten-fold increase in sound. If no sound was labelled as zero, ten times that would still be zero. So the loudest volume is used as a reference instead.
posted by hyperizer at 3:11 PM on January 9, 2004

Sorry, that's each "bel" is a ten-fold increase. And I mispelled "decibel."
posted by hyperizer at 3:14 PM on January 9, 2004

Okay, I buy the technical explanation, but the user experience is still crap - no other device in the world asks me to increase a negative number (which looks like decreasing a positive number if you miss the "-") toward zero to increase the output. I've always wondered why my receiver did this, and couldn't figure out if it was defective or I was being dense. Thanks for this question (and answer!) - though I buy the answer, it doesn't make me any happier about the experience.
posted by kokogiak at 4:00 PM on January 9, 2004

This is an amazing question. I never thought to ask. When I finally dropped a little money on a real receiver, I just sorta assumed I was the one who wasn't getting it. Anything that weighs that much must know what it's doing.
posted by yerfatma at 4:07 PM on January 9, 2004

Puzzle me this, then: my Denon AVR system goes from -60 to +18, and is normally played at about -24 (were I not in a condo, I'd probably play at -16ish).

If 0 is the loudest, then my +18 is, what, overdrive?
posted by five fresh fish at 4:51 PM on January 9, 2004

fff, your Denon system "goes to 11" ;)

Really, the numbers seem so arbitrary, they could be letters or anything that denotes a graduating scale. Why the non-standard numbering. FFF has got a winner there, so non-standard, it must be cool/technically superior, right?

Sorry to rant, not being helpful - stopping now.
posted by kokogiak at 5:21 PM on January 9, 2004

The number is decibels, yes. It is negative because with the knob you are controlling how much the signal is attenuated. Properly speaking, you are not turning your stereo up when you are turning it up -- you are throttling it less. At the zero point, you are not throttling it at all. At any positive value, you are, yes, increasing the signal above the reference input signal strength to the amplifier circuit.

The number has a very specific and useful meaning, it's just that it requires you know (a) how much your amplifier amplifies, and (b) something about how that works. Not an ideal user experience, but I suspect it's one of those things that started out as a pro and "prosumer" feature that has now migrated into the consumer-grade equipment to make it look fancier.
posted by majick at 5:56 PM on January 9, 2004

Strictly speaking, a decibel is a unit of measurement of a ratio between two signals: S db = 10 log (P1/P2), where P1 and P2 are power measurements of a signal. A decibel measurement is therefore always relative to some arbitrary point. In sound, 0 db is most often defined as the threshold of human hearing (which is variable from person to person, of course). That's why you will see references to jet engines at 150 db, rock music at 110 db, etc. Sound is also often defined as power per surface area -- something that your stereo knob obviously isn't calibrated for since it doesn't take into account the rest of your system (speakers, distance from the speakers to you, etc.).

Your stereo knob is probably ("probably" because there's no telling what the manufacturer had in mind) indicating the electrical power delivered relative to an arbitrary level. The negative numbers are part of the math: if the highest level your amp can output is considered 1 (i.e., whole), then every setting lower than that is a fraction. Since the log of 1 is zero and the logs of fractions between 1 and 0 are all negative, that gives you a zero at the top of the scale and negative numbers below that. In the case of fff's Denon, the arbitrary level chosen for comparison is set such that the knob indicates power levels above and below that arbitrary level, hence the positive and negative db readings.
posted by joaquim at 7:09 PM on January 9, 2004

I suspect that for Denon, the 0 represents the point at which the system meets its distortion specs. Keep pushing higher, and you're treading unknown territory.
posted by five fresh fish at 8:28 PM on January 9, 2004

,,,and beware - there be monsters paddling about, in those waters, which have been known to swallow entire ocean going vessels!
posted by troutfishing at 9:42 PM on January 9, 2004

majick's got the best answer so far.

Basically, you are looking at a relative dB measurement. There's no way that the amp can actually KNOW what the absolute dB level is at your ears since it doesn't know what sort of speakers it is driving, or how far away YOU are. Therefore, it expresses the relative attenuation of the signal it is getting from whatever the source device is.
At "zero dB" the full strength signal is passing through the amp.

It is indeed descended from pro equipment, where it makes a lot more sense to use a scale like this, since (for example) on a mixer you're balancing one signal against another. It also helps to know how much signal you are attenuating to maintain quality, and I suppose lots of other reasons. (Caution: I am an amateur recording enthusiast, not any sort of expert.)

On your basic receiver's volume knob, it's pretty much useless. Just mark the loudest spot you ever want to hear it at and ignore the marketing....
posted by dragstroke at 9:48 PM on January 9, 2004

And known to blow out your speaker coils by driving them with straight DC when the amp chops the peaks.
posted by five fresh fish at 10:51 PM on January 9, 2004

That happens more often with car stereo equipment.
posted by Keyser Soze at 3:10 PM on January 10, 2004

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