Decibels??? Two scales. What's the go? - Home Theater Forum and Systems - HomeTheaterShack.com

Old 02-29-12, 06:48 AM Thread Starter
Shackster

Join Date: Dec 2011
Location: Yarrowitch, NSW, Aus.
Posts: 7
Decibels??? Two scales. What's the go?

Hi all.
Not sure where to post this question on the forum but seeing as my use of REW has raised it I'll try here. Web was still confusing. Hoping someone might have a layman's explanation.

This is probably elementary stuff for most of you on this forum but how do decibels on my SPL meter relate to the readout on my amp? I realize that distance from speakers make a difference to the meter, but with the amp on 0dB it is still quite loud and for normal listening is in the negative.

Also noticed that the REW graphs read in the negative dB's. What is the relationship?

Thanks and cheers,

LesH.
Harbspan is offline

Old 02-29-12, 02:07 PM
REW Author

John

Join Date: Apr 2006
Location: UK
Posts: 6,307
Re: Decibels??? Two scales. What's the go?

dB values are always relative to some reference. Positive dB figures mean bigger than the reference, negative dB values mean smaller than the reference. 0dB means the same as the reference.

REW's 'dB FS' graphs (e.g. the impulse graph) are dB relative to Full Scale, where full scale is the largest level that can be represented by the digital data. As nothing can be larger than the largest level the values are either zero or negative, -6dB FS, for example, is a signal that is half the largest level.

In amplifiers the dB figure represents a gain. That might be relative to unity gain, where 0dB would mean the output signal is the same size as the input, or some other reference, such as the gain required to reproduce a signal at theater reference level if the system has been correctly calibrated.

In an SPL meter the scale is relative to 20 micro-pascals, which is the nominal threshold of human hearing at 1kHz. 75dB is a level that is about 5,600 times louder than the hearing threshold.
JohnM is offline
Old 03-01-12, 05:52 AM Thread Starter
Shackster

Join Date: Dec 2011
Location: Yarrowitch, NSW, Aus.
Posts: 7
Re: Decibels??? Two scales. What's the go?

Thanks John, Makes more sense now that I know what it relates to. Nice explanation too. I'm happy now!!!

LesH.
Harbspan is offline

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