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The scientific “unit” we call the decibel

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Re: The scientific “unit” we call the decibel

#391

Post by davrosthedalek »

lifthrasiir wrote: post #40

The only thing you should know is that any use of bel and thus decibel should ideally have the reference level suffixed (usually in parentheses or subscript), not implied. The absolute sound pressure level is dB(SPL). The human-perceived loudness level is dB(A) and similar. The RMS voltage expressed in power is dB(u) (formerly dB(v), not same as capital dB(V)). And so on. And then each different instance of dB unit i…

That is of course not true. dB without reference is perfectly fine to use for gain and attenuation. dBm or any of the variants would be flat out wrong.

Re: The scientific “unit” we call the decibel

#392

Post by MyOutfitIsVague »

jayd16 wrote: post #351
klodolph wrote: post #234

Earlier quoted context omitted.

Sure, but the author is wrong. “How old is your son?” “He’s 3.” Clear in context. People write things like dB SPL (A-weighted) in spec sheets because spec sheets benefit from being unambiguous. Most of the time it’s really clear, like you’re talking about insertion loss or amplifier gain and there’s only one reasonable way to interpret it.

When you say 35 it would be strange for it to be anything but years. 3 days, weeks, months or years are ironically all common units when someone is "3".

Not where I live. You'd never specify how old somebody is with a bare number and have it mean anything other than years in the US. "My kid is 3" is always 3 years. So is "How old is your Tammy?" "Three". That only ever means years. Every other unit is always explicit. In my decades as a parent and being around other parents of kids and newborns, I've never experienced an exception to this.

Re: The scientific “unit” we call the decibel

#393

Post by davrosthedalek »

marcosdumay wrote: post #383
davrosthedalek wrote: post #332

Earlier quoted context omitted.

There is a legitimate use of dB without a reference point. An attenuator attenuates by -20dB, not by -20dBm.

This is right and all... But this usage still leads to confusion about what you are measuring your filter by. There are filters we measure on power, there are filters we measure on signal amplitude, and "signal amplitude" can be ambiguous on some contexts too. There should be a way to specify this one better.

Well, dB is fully specified in that regard. It's always power. You can calculate the voltage gain from it under certain assumptions, and under normal assumptions you get that factor 2. But a -20dB attenuator will always reduce the power by a factor of 100.

Re: The scientific “unit” we call the decibel

#394

Post by camtarn »

margalabargala wrote: post #367
camtarn wrote: post #156

Earlier quoted context omitted.

...as long as the container has straight sides. The area of the opening needs to be the same as the area of the container all the way down, otherwise the two areas don't cancel out.

Sure you can do that, and now you're measuring something different. The volume of an exotic container is measured in different units than the depth of rain falling from the sky evenly.

I have no idea what you mean here. Exotic containers? I'm talking about something like a measuring cylinder or a straight-sided glass or mug.

This StackOverflow answer probably does the subject more justice than I can: https://earthscience.stackexchange.com/questions/14587/what-...

but I'll try and explain it succinctly:

What you're measuring is the volume of rain falling per unit area in a given time (usually 24 hours). If you're collecting rain in a vessel, you divide the volume collected by the area of the vessel's mouth to get the volume collected per unit area. And in order to measure the volume in the container, you measure the height of the water and multiply by the area of the vessel's bottom.

If the bottom and mouth have the same area, those cancel out and you can just specify the rain height regardless of the size of the container. That is, if you have 1ml of rain falling per square mm over 24 hours, it will produce the same height of water if you set out a big container or a small container, as long as the containers have straight walls.

If they don't have the same area, then you can't use the handy mm unit.

Re: The scientific “unit” we call the decibel

#395

Post by [deleted] »

perching_aix wrote: post #379
teknopaul wrote: post #369

"This is nuts: it’s akin to saying that the milli- prefix should have different meanings depending on whether we’re talking about meters or liters." Were were here recently with "mega": Sometimes mega is squared as in megapixels. Sometime not as in megabytes. No biggie. Db in audio is a relative scale and that makes perfect sense. If you mixer goes + or - 6db that makes sense but can't be measured as power, your mixe…

> Sometimes mega is squared as in megapixels. Is that right? A pixel is a 2D object already. It's not like e.g. with centimeters, where it's a 1D unit, so it becomes centimeters squared to form a 2D unit.

[deleted]

Re: The scientific “unit” we call the decibel

#396

Post by davrosthedalek »

perching_aix wrote: post #379
teknopaul wrote: post #369

"This is nuts: it’s akin to saying that the milli- prefix should have different meanings depending on whether we’re talking about meters or liters." Were were here recently with "mega": Sometimes mega is squared as in megapixels. Sometime not as in megabytes. No biggie. Db in audio is a relative scale and that makes perfect sense. If you mixer goes + or - 6db that makes sense but can't be measured as power, your mixe…

> Sometimes mega is squared as in megapixels. Is that right? A pixel is a 2D object already. It's not like e.g. with centimeters, where it's a 1D unit, so it becomes centimeters squared to form a 2D unit.

it's (cm)^2, not c(m^2). If there would be the one-dimensional equivalent to pixels, say pix, then 1pixel = 1pix^2, and then 1 megapixel = (kpix)^2

Re: The scientific “unit” we call the decibel

#397

Post by QuarterReptile »

timerol wrote: post #296
49pctber wrote: post #284

The idea that helped make decibels click for me is that they're a way to quickly do both dimensional analysis and gain/attenuation calculations at the same time. "Plain" decibels are simply (power) ratios. These can describe multiplicative changes in power. These are positive for gains (like in a power amplifier) or negative for attenuations (like path loss). They are unitless quantities. Decibels add. A ten 10 dB ga…

> 2 mW / 1 mW = 2 = 10^(3/10) = 3 dBm It's worth noting that this is wrong, in exactly the way that makes decibels confusing. 3 dBm is an absolute power figure (about 2 mW). 2 mW / 1 mW is a ratio of 2 (about 3 dB). 2 mW / 1 mW = 2 = 10^(3/10) = 3 dB. 2 mW = 2 * 1 mW = 10^(3/10) * 1 mW = 3 dB (1 mW) = 3 dBm.

Similarly, but in a very different context:

I have to teach non-engineers C programming for an undergrad course, which is basically trying to teach very explicit attention to punctuation (among many other things). "Watch those double quotes!", "single quotes, not double!", "where's your semi-colon??", and so on.

Then, three weeks into the course, we're passing values by reference with &, and I get the question, "isn't that scanf missing the and-sign in front of the string name?", and I'm forced to answer, "that punctuation doesn't matter, this time," because the C standard makes & do nothing in front of a string specifically because so many people were confused about that fact that a string's variable name is already passing by reference.

Re: The scientific “unit” we call the decibel

#398

Post by timeinput »

klodolph wrote: post #234
sanderjd wrote: post #200

Earlier quoted context omitted.

> In every context where decibels are used, either the unit they qualify is explicitly specified, or the unit is implicity known from the context. The author's whole point is that this is not true. To adapt your analogy, it's not like being mad at the number three, it's like being mad about people not attaching any units to the number three, arguing that it's clear in context. It isn't!

Sure, but the author is wrong. “How old is your son?” “He’s 3.” Clear in context. People write things like dB SPL (A-weighted) in spec sheets because spec sheets benefit from being unambiguous. Most of the time it’s really clear, like you’re talking about insertion loss or amplifier gain and there’s only one reasonable way to interpret it.

But for insertion loss and amplifier gain it is "just" dB, it's the ratio of the input to the output. The amplifier has a gain of 35 dB means its output is 35 dB higher than the input. If the input is -30 dBm the output is +5 dBm, etc. The reference for an amplifier, or insertion loss is clear in context since you're talking about the gain / loss of a device, and isn't referenced to any fixed scale like db relative to 1 mW, SPL (A-weighted), or 1 volt.

On detailed spec sheets they list the gain of amplifiers as xxx dB.

Re: The scientific “unit” we call the decibel

#399

Post by tadfisher »

jameshart wrote: post #378
regularjack wrote: post #241

Earlier quoted context omitted.

There is no such thing as full volume, and there is not such thing as zero volume.

‘There is no such thing as zero volume’ - can you expand on that? Surely constant unchanging air pressure has zero volume? Volume is the range of variation in air pressure, right? That can surely go down to zero. It can also only meaningfully go up to about double the absolute mean air pressure, before what you are talking about becomes shockwaves of overpressure.

Air pressure is a statistical measure; given a room with zero net variation in pressure, I can always find a volume in that room with positive pressure, down to measuring Brownian motion of atoms. Now try to design a measurement apparatus that can only sample a small volume to measure variations in pressure, and you can understand why (sound) volume can never go to zero.

Re: The scientific “unit” we call the decibel

#400

Post by kazinator »

cycomanic wrote: post #81
kazinator wrote: post #57

The confusing thing about decibels is not the Watts versus Volts thing. It is the following. If you mix two identical signals (same shape and amplitude) which are in phase, you double the voltage, and so quadruple the power, which is +6 dB. But if you mix two unrelated signals which are about the same in amplitude, their power levels merely add, doubling the power: +3 dB.

That has nothing to do with decibels at all, that's the fundamental physics (or mathematics if you want) of adding waves, i.e. interference.

Sure, but people who are confused by the voltage vs power thing (also fundamental physics) will likely be quadruply confused by this.
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