Confusing notation for decibels

[GigaCluster]

I've always been taught (and it's repeatedly confirmed on decibel documents I find online) that decibels are units of sound intensity and are measured from zero -- absence of sound -- to something like 140.

However, stereo equipment such as amplifiers measure decibels with negative values, with loudness approaching zero. Also, I was recently reading some document that was trying to explain this situation, throwing the word "gain" around.

Could anyone shed some light on this?

 

[WinkOsmosis]

The equipment reduces volume I assume.

 

[Barnaby W. Füi]

I believe those guages are showing relative numbers, not absolute numbers. 0 is when the signal will begin to clip.

 

[GigaCluster]

If so, then when the amplifier shows zero, it means it's not modifying the intensity of sound? So if it begins clipping at zero, does this mean that the original track on the CD clips???

 

[ebecker]

Check this link

 

[GigaCluster]

Thanks for the site; it's quite in-depth. I learned that certain devices (amplifiers) provide a positive gain while others (analog cables) provide a negative gain through power loss. However, that does not answer my question: why does an amplifier get loudest when it approaches zero? Let's say that when the amplifier is at zero and the sound is at its loudest, the sound intensity as measured with another device is 150 dB. Does that mean that the amplifier is not modifying the intensity at that setting and that the original intensity as processed by the CD player is 150 dB? In other words, why doesn't the amplifier AMPLIFY the sound wave by adding gain to it, instead of approaching zero loss?

I hope my question makes sense.

 

[Savij]

The CD/DVD is recorded at a certain "reference" volume that is usually quite loud. For most listening you want to reduce the volume so that your neighbors/roomates/parents/cats don't come by and kick your ass. So your receiver cuts your volume and displays the number of decibles that it cut the volume by.

 

[ebecker]

A negative db just means the output power is less than the input power. 0db = input and output ratio are equal. It has to do with the input impedance and output impedance

http://otto.cmr.fsu.edu/~elec4mus/topics/decibel.html

The GAIN of an AMPLIFIER commonly is expressed in decibels, and this can lead to further confusion: an amplifier with 3 dB of gain literally means it doubles the power in the signal, and an amplifier with 60 dB gain would be expected to increase the power one million times. This would be true if the IMPEDANCE of the input and output of the amplifier were the same, but this is seldom the case in practice. For instance, a power amplifier typically has a high INPUT IMPEDANCE and a very low OUTPUT IMPEDANCE for driving a LOUDSPEAKER. The true power gain of such an amplifier is very high because the input signal has essentially no LOAD and thus supplies almost no CURRENT or power. This power gain could be correctly expressed in decibels if the actual input and output powers were taken into consideration. This, however, is rarely done; it works out correctly only if the two impedances are the same, as is sometimes the case in amplifiers used in broadcasting and some sound systems. The impedance of such units is usually 600 OHMS.

When amplifier gains are expressed in decibels, what is almost always meant is VOLTAGE GAIN, and as we have seen, a simple voltage ratio cannot be expressed in decibels by the definition of the DECIBEL. The formula used to calculate amplifier gain is

dB (gain) = 20 log [V(output)/V(input)]

and again, it would be correct only if the impedances at the input and output were the same. It is small wonder that decibels cause so much confusion!

 

[Savij]

if your take your lowest volume (-75 on mine) setting. At that volume it's taking 75 decibles away from the volume the CD was recorded at. When I turn the volume up to -45, it's now subtracting only 45db from the CD's volume. At 0db you should be hearing the audio at the volume it was recorded at.

 

[beer]

Primarily, it's designed as a calibration point. With receivers, you calibrate it to 'reference volume' at a reading of 0 dB on the reciever. The 0 dB on a receiver usually corresponds to a reading of 85 dB or so @ about 10 feet from the speakers. (listening point). Hence, zero is a reference point, much like 0 Volts is considered a 'reference point'. It's just out of engineering convention.

 

[GigaCluster]

I am reviving this topic because it still does not make sense. From what everyone is saying, without an amplifier, a CD would sound loudest, since all an amplifier does is reduce the sound. So, a CD player with speakers plugged into it would sound louder than a CD player with an amplifier, at any other than HIGHEST (0 dB) volume. And thus, it would be impossible for the amplifier to actually increase the volume since there is positive decibel setting.

That doesn't make sense.

Am I misunderstanding something?

 

[Bulldozer]

Try looking at it this way. Say you have an amplifier rated at 100w. If there was no volume control the amplifier would amplify all signals with 100w of power. The volume knob reduces the amount of amplication that takes place. It may do this by attenuating the input signal.

 

[GigaCluster]

Breakthrough, Bulldozer. Thank you for that. I just hope that's right.

Why, though, WHY, would such a confusing notation be picked? Why not show the dumb consumer like me how WELL the amplifier is performing, not how much it is impeded?

Again, thanks.

 

[Zenmervolt]

Originally posted by: GigaCluster
Breakthrough, Bulldozer. Thank you for that. I just hope that's right.

Why, though, WHY, would such a confusing notation be picked? Why not show the dumb consumer like me how WELL the amplifier is performing, not how much it is impeded?

Again, thanks.

Because professionals wanted it this way and it's easier to have one standard instead of two.

ZV