What sounds would be considered to measure negitive decibles? Links and explenations would be helpful.

Decibels are a dimensionless, logarithmic measure of a ratio. They aren't just used for sound, although that's certainly an important use. The ratio in question in the case of sound is

P/P_0

and hence the decibel measure is the base 10 logarithm of this ratio, where P is the observed pressure from sound and P_0 is some fixed reference pressure. Typically, for sound, P_0 is taken to be on the order of 20 micropascals (2*10^(-5) N/m^2). In order to have a negative decibel measure, you would need P < P_0, or in other words, the observed pressure to be smaller than the 20 micropascal reference. Considering that 20 micropascals of pressure is about as loud as a whisper... from a kilometre away... whatever you're measuring would have to be pretty quiet to measure in negative decibels :)

interesting question.

Even most of the sounds people refer to as silence have a positive decibel value.

side question - Can a decibel value be given to a perceived sound that has no pressure attached to it, i.e., tinnitus? Although you could say that the ringing in your right ear sounds as loud as a 5 db sine wave in your left ear, for example, I still think not.

"even ... silence has a positive decibel value"

well... in that case, lol, we would have vacuum fluctuations be in the negative !

I think i see where this question comes from. I never understood why stereo equipment measures negative decibles for volume... I figure it has something to do with how much it is subtracting from a main signal strength or something, but i really have no freakin idea. Any audiophillies out there?

Decibels is a comparative measure; it really denotes a ratio. When we say that something makes a noise of, say, 50dB, it means that the noise it makes is 50dB above an agreed standard reference. This reference is very low, but in principle a sound could be lower, and thus have a negative dB.

For volume controls, the negative values signify how much the volume control is lowering the output in relation to full output.

And, decibels work at a logarithmic scale, so every time the ratio changes 3 dB, the linear ratio doubles.

Thus, a sound of 53dB is twice the intensity of 50dB.

Hans

A 3db increase doesn't mean twice the sound intensity. It means that it took twice the power to generate the sound. The intensity measured by pressure per square inch would take 6db to double.

wonder what the decibel of a nuclear blast's shock wave would be...