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Why are Decibles negative?


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3 replies to this topic

#1 of 4 ToddAC

ToddAC

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Posted April 18 2003 - 03:02 AM

This is probably a stupid question, but would someone enlighten me?

ToddAC

#2 of 4 Bob McElfresh

Bob McElfresh

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Posted April 18 2003 - 03:38 AM

Each manufacturer can be different, but in general:

Your receiver is designed to produce POWER, not volume. But people want a rough idea of the volume. So the people who designed your receiver did this:

- They designed the receiver to produce some maximum watts per channel. They hooked up a "reference" 8 ohm speaker and measured the volume from 1 ft in front at this power.

- As they reduced the power with the volume control, the sound dropped off so they made the front-panel display reflect how much volume BELOW MAX they got.

Some receivers show "0.00 db" for their MAX power output. Others pick a reference volume like 75 db and make the display reflect rough volume below this REFERENCE volume.

In all cases, the display does not really tell you what volume you are getting at your listening position. This depends on your speakers, angle, and distance to the speakers.

But the display is a rough indication so it's usefull to see.

Hope this helps.

#3 of 4 Vince Maskeeper

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Posted April 19 2003 - 03:33 AM

The db scale is actually a measurement of difference between two values. It's sort of like weight- you can't measure the difference in weight between two objects, unless you have 2 objects- measuring the weight difference between My TV and My TV doesn't work too well.

If you dont have 2 values specifcally to measure- you need to declare a 2nd value as a reference value against which everything will be measured, in order to measure in decibels.

As such, a reference value is created- and the intensitey you wish to measure is weighed against that reference.

When the two intensities are equal- the ref value and the tested value- the math works out to be, essentially, 0db. If your tested value ends up lower than the ref, then it would be a negative number. So your DB scale is measuring your current power in relation to a given ref point.

So, if you choose the "maximum" intensity (or the max you'd want it to perform at) as your ref value, which is the standard in many cases, then the DB scale becomes sort of a measure of potential. Most intensity you seek to measure will be either be equal to or less than that maximum, thus the negative scale.

Hope that helps.

See also:
http://www.phys.unsw....au/~jw/dB.html
http://www.sfu.ca/so...ok/Decibel.html
http://www.jimprice....prosound/db.htm

-V
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#4 of 4 Allan Jayne

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Posted April 21 2003 - 12:34 AM

When measuring the quality of audio circuits, there is usually a reference level, referred to as zero dB (decibels). Good quality means a uniform, or flat, response and poorer quality means degradation of the response at certain frequencies. Degradation is expressed as negative decibels, accentuation (also undesirable except for tone controls) is expressed as positive decibels.

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