# Recording on the computer physics question?

1. Dec 3, 2007

### henry2221

Recording on the computer physics question?
The meters show signal levels in dBFS (decibels below full scale), where a level of 0 dB is the maximum amplitude possible before clipping occurs in a program such ad Adobe Audition.

However my question is why 0 db? When 140 db is the painful amount and etc.? Why can't it be any higher than 0

2. Dec 3, 2007

### fargoth

well decibels are defined as $$log(\frac{I}{I_0})$$
now, if $$I_0=I$$ then this log is 0, $$I_0$$ is the limit in your program..

Last edited: Dec 3, 2007
3. Dec 9, 2007

### JoAuSc

I'm guessing that 0 dB on that program is not the same as 0 dB in the real world. On the program, 0 dB is a somewhat arbitrary amplitude which defines the maximum.

Last edited: Dec 9, 2007
4. Dec 9, 2007

### mgb_phys

dB is a difference between 2 levels.
If only a single dB figure is quoted it is relative to some agreed level, in sound this is usually 0 power ie, no sound and a positive dB number above this is quoted.

It's just as valid to quote a -ve dB figure representing the level relative to the maximum measurable power.

5. Dec 12, 2007

### henry2221

The program shows the dB level going from negative infinite to 0. None of the responses above really tells me much. Can someone please make it clearer.

6. Dec 12, 2007

### mgb_phys

Ok will try again.

dB is the ratio between two values. It's meaningless to talk about the dB of a single level unless you know what you are comparing it to.

For sound levels it is common to set 0dB to the threshold of hearing, then 120dB is about the maximum you can hear before you go deaf.
For recording levels it is common to set 0 dB to the maximum recording level, then all recordable levels are below this and so have negative dB, in logs fractions of a number are negative.

So in a CD with 16bits of range with 0dB being the maximum level, then the lowest level will be 96dB below this, or -96dB.

Since it is a ratio you cannot have a range between 0 and a real number so if you have a maximum recording level of 0dB then an input of 0 the range will be -infinite dB.

This is a simple discussion from the point of view of recording levels.
http://arts.ucsc.edu/EMS/Music/tech_background/TE-06/teces_06.html