4 ms·

That text mentions that dB is relative, but misses how the values are defined. A simple definition could be: - If we are talking about dB in context of amplitu

That text mentions that dB is relative, but misses how the values are defined. A simple definition could be: - If we are talking about dB in context of amplitude 6dB more means twice as much amplitude, 6dB less (or -6dB) means half the amplitude. - If we are talking about dB in context of power 3dB more means twice as much power, 3dB less (or -3dB) means half the power. dB is often also used to measure attenuation instead of gain. Here a positive dB value would actually mean less output compared to reference value. And: For dB values often a suffix is used, to denote to what the value is relative to. E.g. dBV is relative to 1V, dbm is relative to 1 milliwatt.

lutusp 10y ago
> - If we are talking about dB in context of amplitude 6dB more means twice as much amplitude, 6dB less (or -6dB) means half the amplitude. > - If we are talking about dB in context of power 3dB more means twice as much power, 3dB less (or -3dB) means half the power. Yes, and there's a reason for this relationship -- the two decibel scales (amplitude and power) are often interchangeable, because (in many contexts including sound waves in air and electrical waves in conductors) power varies as the square of amplitude. So the same decibel ratio can be used to express both amplitude and power. https://en.wikipedia.org/wiki/Decibel https://en.wikipedia.org/wiki/Decibel
pitaj 10y ago
What you said is kinda confusing so I'm gonna clarify 3dB = √2 which is the amplitude ratio which is equivalent to doubling the power, since power is square proportional to amplitude 6dB = 2, so a 6dB increase in power would be doubling the power, and a 6dB increase in amplitude would be 4x the power a negative dB is just the reciprocal of the radio, so -6dB is equal to 1/2
- ramshorns 10y ago
  > 6dB = 2, so a 6dB increase in power would be doubling the power Pretty sure this part isn't right. Decibels are about power ratios, not amplitude ratios. 10 dB is a power ratio of 10, so 20 dB is an amplitude ratio of 10. 6 dB is a power ratio of 10^(6/10) ≈ 3.981 or an amplitude ratio of 10^(3/10) ≈ 1.995.), while 3 dB is a power ratio of 1.995 or an amplitude ratio of 1.413. (It's probably sometimes defined so that 3 dB is a power ratio of exactly 2, making 30 dB 1024 instead of 1000.)
  - pitaj 10y ago
    Yes I was wrong about the 6dB. That's a quadrupling of the power, not a doubling. It's twice the amplitude. Unfortunately I can't edit my comment.
jeffwass 10y ago
I'm reminded of my old boss's words when I worked as an EE at MIT Lincoln Lab. Whenever someone (self included) distinguished between the meaning of decibel on a signal's amplitude or power, he'd deadpan : "A decibel is a decibel is a decibel". Eg, a filter that attenuates a certain frequency by 24 dB is well defined. Regardless of whether you're interested in the resulting voltage or power of the output signal. Of course for absolute terms a reference like dBm is used.

lutusp 10y ago

> - If we are talking about dB in context of amplitude 6dB more means twice as much amplitude, 6dB less (or -6dB) means half the amplitude. > - If we are talking about dB in context of power 3dB more means twice as much power, 3dB less (or -3dB) means half the power. Yes, and there's a reason for this relationship -- the two decibel scales (amplitude and power) are often interchangeable, because (in many contexts including sound waves in air and electrical waves in conductors) power varies as the square of amplitude. So the same decibel ratio can be used to express both amplitude and power. https://en.wikipedia.org/wiki/Decibel https://en.wikipedia.org/wiki/Decibel

pitaj 10y ago

What you said is kinda confusing so I'm gonna clarify 3dB = √2 which is the amplitude ratio which is equivalent to doubling the power, since power is square proportional to amplitude 6dB = 2, so a 6dB increase in power would be doubling the power, and a 6dB increase in amplitude would be 4x the power a negative dB is just the reciprocal of the radio, so -6dB is equal to 1/2

ramshorns 10y ago
> 6dB = 2, so a 6dB increase in power would be doubling the power Pretty sure this part isn't right. Decibels are about power ratios, not amplitude ratios. 10 dB is a power ratio of 10, so 20 dB is an amplitude ratio of 10. 6 dB is a power ratio of 10^(6/10) ≈ 3.981 or an amplitude ratio of 10^(3/10) ≈ 1.995.), while 3 dB is a power ratio of 1.995 or an amplitude ratio of 1.413. (It's probably sometimes defined so that 3 dB is a power ratio of exactly 2, making 30 dB 1024 instead of 1000.)
- pitaj 10y ago
  Yes I was wrong about the 6dB. That's a quadrupling of the power, not a doubling. It's twice the amplitude. Unfortunately I can't edit my comment.

jeffwass 10y ago

I'm reminded of my old boss's words when I worked as an EE at MIT Lincoln Lab. Whenever someone (self included) distinguished between the meaning of decibel on a signal's amplitude or power, he'd deadpan : "A decibel is a decibel is a decibel". Eg, a filter that attenuates a certain frequency by 24 dB is well defined. Regardless of whether you're interested in the resulting voltage or power of the output signal. Of course for absolute terms a reference like dBm is used.