[MaGG]

I was reading up and was looking particularly at negative decibel readings [ie: -20dB] and realized that 0 dB doesn't actually mean no sound, but means:
Quote:

a sound level where the sound pressure is equal to that of the reference level. This is a small pressure, but not zero

So negative dB ratings therefore mean:
Quote:

It is also possible to have negative sound levels: - 20 dB would mean a sound with pressure 10 times smaller than the reference pressure, ie 2 micropascals.

So, if negative/zero doesn't actually mean no sound, then what's the designation for something that doesn't have any sound, since zero is taken?

Source for quotes: http://www.phys.unsw.edu.au/~jw/dB.html

 

[warpdriver]

No sound would be Minus Infinity
or
-∞

In practice, 0dB means no sound because that's as little as the human ear can detect.

 

[MaGG]

Thanks for the fast response...

Ok, so when looking at a frequency response test [for instance, when checking bass roll-off or the like], what reason would we have to pay attention to the negative dB ratings if we can't even hear at that level?

 

[Tim D]

If you see - db ratings it is most likely a relative measurement. If it is positive it could be an absolute measurement.

0 db absolute means you don't hear it. 0 db relative for 16 bit audio means 96 db in absolute. So of course you can hear frequency deviations in a relative sense because it isn't talking about 0 db absolute.

 

[K2Grey]

Frequency response does not show the actual loudness. It shows the loudness of different frequencies relative to each other. 0 is just a base point, average, level of reference, whatever.

 

[K2Grey]

Frequency response does not show the actual loudness. It shows the loudness of different frequencies relative to each other. 0 is just a base point, average, level of reference, whatever.

 

[BillC]

Quote:

Originally Posted by MaGG Thanks for the fast response... Ok, so when looking at a frequency response test [for instance, when checking bass roll-off or the like], what reason would we have to pay attention to the negative dB ratings if we can't even hear at that level?

You're comparing apples and oranges. dB isn't a "proper" unit of measurement - it just means that you've taken the log of a ratio. As the article you linked to mentions, sound is often measured referenced to the threshold of human hearing - 20 micropascals. Measurements taken in comparison are usually written simply as dB but should more properly include the reference value as dB 20 micropascals. (Note that if you tried to use 0 as the reference value of that ratio the ratio would always be either 0 or infinity - not very useful). So in this case positive dB values indicate that they are audible sounds, negative dB are not.

When you see things like frequency response curves, these are usually based on using one of the curves as a reference - not the 20 micropascal reference from above. In these cases you are looking at changes from the reference - so positive dB values means you're greater, negative dB values means you're less at that point on the curve.

 

[Jam_Master_J]

I think it this case the dB scale is used to measure relative volume. Like for example comparing the relative "hotness" of a recording. A very compressed recording might have an average volume of -2dB, where 0dB represents the maximum possible possible for that format.

 

[Jam_Master_J]

Quote:

Originally Posted by warpdriver No sound would be Minus Infinity or -∞ In practice, 0dB means no sound because that's as little as the human ear can detect.

Actually this guy is more correct I'd say

The dB system we use is just a relative measurement. 0dB is considered the minimum volume detectable to humans, but as you go below 0dB and approach -infinity dB the absolute sound level approaches zero but never quite reaches it.

 

[Ploop]

Why does my Yamaha Home Theatre System use -dB then? for instance,
-99dB is no sound (100dB is mute, which it displays as mute), and the closer to 0 you get, it is increasingly loud (i haven't listened to it above -40 dB).

 

[Illah]

Yeah, as another indicator of the relativity of dB, my Marantz receiver has a volume control that shows it's level of amplification in relation to the reference signal. For example, I typically listen at -20dB, in other words the amplified signal is attenuated by -20dB, while a volume of 0dB would be full amplification, and +6dB would be like over-amping the signal.

This is likely how they measure the headphones. They put in a reference signal represented by 0dB, like my receiver represents 0dB, and then they measure if the headphone reproduces that signal at reference or +/- reference.

--Illah

 

[Jam_Master_J]

It just uses a different reference point.

 

[Illah]

Quote:

Originally Posted by Jam_Master_J It just uses a different reference point.

Exactly - like my receiver. At 0dB it would be the same as yanking out the volume control of your receiver. It's taking the line signal from your DVD/CD/TV and amping it without attenuation. -dB on the volume means it's attenuating the signal to make it quieter. Since the average amp these days is 100W+ per channel if there was no attenuation we'd all be deaf by now!

--Illah

 

[warpdriver]

Quote:

Originally Posted by MaGG Thanks for the fast response... Ok, so when looking at a frequency response test [for instance, when checking bass roll-off or the like], what reason would we have to pay attention to the negative dB ratings if we can't even hear at that level?

The graph you are looking is a relative dB scale.

So the graph may show a range of +10 to -10 dB.

But they are fixing the 0dB level to be, say, 80 dB.

So a -5dB on the graph is actually 75dB

The +/- dB specification on a frequency response is a tolerance. It's sort of like saying these five people are six feet tall, but within a tolerance of 3 inches.

0dB on your receiver is probably the point where the receiver is generating maximum power with a specified input level.