Why so negative?

October 20, 2010 9:30 PM

Why does the sound system show negative db?

My roommate's sound system has, on the front, a measure of the decibels that it is producing at the time, whenever you change the volume. I tend to watch tv at -50db, my roommate at -42db (he works in clubs and so has harmed his hearing), while videogames tend to be played at -65db (don't know why they're louder, but they are).

I know how the logarithmic scale of decibels work, and I understand the negatives are in comparison to something. I'm not about to turn the thing up to 0db, for fear of blowing out the speakers. But what I had learned was that for sound 0db was the quietest sound a normal human could hear. Since these sounds aren't impossible to hear, what's the baseline here? And furthermore, what determined that baseline?

posted by Hactar to technology (12 answers total)

Basically, 0dB is when the receiver/amp/whatever is putting out its full power, whatever that may be. The negative values indicate how much attenuation is being applied to the signal.

It's similar in pro mixers and outboard gear, where a setting of 0db on the fader or knob means the signal is passing through unmolested, pushing the faders up amplifies the sound, pulling it down will attenuate it.
posted by jjb at 9:47 PM on October 20, 2010

I'm not about to turn the thing up to 0db, for fear of blowing out the speakers.

That's pretty much it.

Decibels don't measure sound per se, but intensity or power, of which sound is merely one example.

In this case, it's measuring the electrical signal passing through the line. 0db is the acceptable limit of the given piece of equipment. Positive measurements (usually indicated in red) reflect how far beyond that acceptable limit you're pushing the signal.

In recording, for example, 0db is the point at which the signal will begin to distort (analog) or clip (digital). Those are bad. (Unless you want tape saturation, in which case the former is good. The latter is always, always, always bad, however.)
posted by Sys Rq at 9:51 PM on October 20, 2010

Along with what jjb said, which I think is the main explanation, you should keep in mind that there's more to decibels than just measuring sound. A decibel scale can be used to measure any ratio of "in" versus "out" on a logarithmic scale. For example, 10 log(Pout/Pin) as a representation of attenuation/amplification of Power P in decibels. This is why 0dB is not the "quietest sound a normal human could hear" in this context - as jjb is alluding to, the decibel scale being used isn't actually measuring sound decibels like you are thinking of.
posted by Diplodocus at 9:52 PM on October 20, 2010

Here's a good dB primer. That's a full scale representation, so 0dB is the maximum level the system can achieve before clipping. Any output is therefor less than or equal to the maximum output, so it's less than or equal to 0db.

As to why video games need a lower volume setting it could be the video game system has a higher line output level or the games themselves are "mastered" to a higher sound level (or compressed in such a way that they seem to have a higher sound level). Incidentally, the louder video games might make the system clip before you reached 0dB on the volume, since it would be driving it harder.
posted by 6550 at 9:55 PM on October 20, 2010

Decibels are a relative unit. The scale you're used to defines 0dB to be a certain pressure, roughly equal to "the quietest sound a normal human can hear". Similarly, your amp defines 0dB to be the loudest signal it can emit.
posted by Serf at 9:56 PM on October 20, 2010

You're talking about two different scales. The one you're thinking of is dB(SPL), or the audible sound pressure level. There is no way that the sound system can measure this accurately, because it depends on: the number and type of speakers hooked up, how far the listener is from the speaker, the type of source program audio, the level of background noise in the room, etc. You need a 'loudness meter' to measure dB(SPL) because it takes into account all of those things. The only thing the sound system can measure is how much it's amplifying the input signal, a mere out:in ratio, and for that it's using the logarithmic decibel scale because it more closely approximates our hearing which is also logarithmic.
posted by Rhomboid at 10:10 PM on October 20, 2010

A "dB" is not an absolute unit of measurement, like inches or meters or degrees centigrade. It's always relative to something else. Think of it like a percentage -- "dB" acts a lot like "%", in that they are always relative to some other measurement in absolute units.

To make things more clear, never use "dB" by itself, always use "dB (relative to X absolute value)".

In this case, as jjb says, the volume indicator on the sound system is calibrated so "full output of the amplifier" is indicated as "0 db". This really is "0 dB (relative to full output of this amplifier)".

If you were to measure the output of the amp, it would be some absolute value in volts (an absolute measurement of electrical potential), like 1 volt. You could convert that value to a standard measure in decibels relative to a standard voltage, like dBV or dBu or dBv, by doing some simple math.

When the sound system is hooked up to some speakers, they emit sound, and you can measure the "volume" of that sound in absolute units, as pressure. Again, if you pick a reference level to be "0 dB", you can measure the output in dB relative to that -- and there are standards for doing so, based on what humans can typically hear.

The important thing is that the "0 dB (voltage relative to the amp)" is not the same as "0 dB (sound pressure relative to human hearing threshold)". They are separate, although related, values. You can't simply take one value and find the other -- you need to know how the speakers take the amp signal and produce sound, in order to convert from one to the other. (And since the amp doesn't know what speakers are attached to it, it cannot possibly show you a calibrated sound pressure level.)

Think of it like a water valve with a hose attached. When you turn the valve fully open, it's at 0 dB (relative to full output of the valve). Then you measure the pressure of the water coming out of the hose -- that might be 10 PSI, or 47 dB (relative to some baseline PSI). The conversion from one to the other depends on the exact characteristics of the hose.
posted by xil at 10:17 PM on October 20, 2010 [2 favorites]

And just to be clear, because the dB is a unitless measure of a ratio, if you want to express an absolute value you have to specify what the reference value is. The wikipedia page gives some examples of common scales: dB(SPL), dBw, dBi, dBV, etc.
posted by Rhomboid at 10:21 PM on October 20, 2010

Previously. Though I think the answers here are a bit better than I received back then.
posted by The Michael The at 4:23 AM on October 21, 2010

Your next question is going to be, if a set my amplifier to 0 dB (relative to the maximum the amplifier can produce) and I measure the loudness when sitting on my coach, it measures 120dB (relative to the lightest sound the typical human ear can ear.) So why the amp isn't designed to display 120dB in the first place?

There are two reason:

[1] The loudness you hear depends on how far you are sitting from your speakers. In the kitchen, all you can hear is a murmur because it's so far from the living room. The amp can't display how many dB you're hearing because it doesn't know you're in the kitchen.

[2] The loudness you hear depends on which speakers you are using. For the same amount of voltage coming from the wire, some speaker will sound much louder. It's called the efficiency of the speaker. You can think of it roughly like the mpg of your car. Some speaker convert the electric energy into sound energy very efficiently, other, not so much. Typically, larger speakers are more efficient. But the amp doesn't know how efficient your speakers are.
posted by gmarceau at 10:27 AM on October 21, 2010

From what I understood by reading about this after noticing the same thing on my new receiver, the system is generally set to show dB ratings relative to the loudest sound the human ear can listen to over a sustained period without risking hearing damage. So no, don't turn it above 0. My son grabbed the remote one day and mashed the volume button down, and it got loud. This scared him, so he held the button tighter, which made things even worse before we got it away from him - the volume went above 0, and it was absolutely deafening. The speakers could definitely handle it, but we sure as heck couldn't.

We usually have it between -45 (radio) to -20 (movies), with TV generally falling in around -30 or so. That's plenty loud enough for us. But I suppose it's nice to know that we can push the system well past 11 if we ever have the need to.
posted by caution live frogs at 11:04 AM on October 21, 2010

Good explanations. Just to make that part 100% clear: "db" is not a unit of measurement at all.
posted by oxit at 12:23 PM on October 21, 2010