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The scientific “unit” we call the decibel

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Re: The scientific “unit” we call the decibel

#461

Post by bigiain »

timewizard wrote: post #97
bigiain wrote: post #23

Earlier quoted context omitted.

> > That said, I don’t know how to pronounce “3e5” > "Three to the exponent of five." Or "Three Exponent Five." Or "Three Exp Five." Somehow, you need to distinguish between 3^5 (=243), 3 x e^5 (=~445.24), and 3 x 10^5 (=300,000). I'd pronounce "3e5" and "three times ten to the 5" in most cases.

> 3^5 (=243) Three to the power of five. > 3 x e^5 Three times the fifth power of e. Or Three times e's fifth power. > 3 x 10^5 (=300,000). Three to the exponent of five. A calculator user once suggested "decapower." I think exponent and "EXP" are comfortable and easy to say and are ingrained to most old school calculator users. Which is also why I think "e's fifth power" can be a more natural sequence.

> Three to the exponent of five.

I think that's still ambiguous. The base of the exponent is implied, and there are decent arguments or fields in which the assumed base of 10 isn't universal. In your "Three times e's fifth power." version, you needed so specify e. I think for accuracy it's also needed to specify 10 in that "Three to the exponent of five." case.

(Having said that, there are certainly cases where the 10 as the base of the exponent will be clear/unambiguous from the context. )

Re: The scientific “unit” we call the decibel

#462

Post by davrosthedalek »

perching_aix wrote: post #418
davrosthedalek wrote: post #413

Earlier quoted context omitted.

I believe the point is that in "centimeters squared" you also square the cm. So it's 1/100 of a meter, then squared. While the mega in megapixels applies to the area. So presumably there are two length multiplied to be a pixel, and then you have a million of those. I don't think it's a very deep point, and I would say the mega is not squared, but the centi is squared.

What finally clicked it into place for me was trying to perform a unit conversion. It's kind of annoying cause we don't really use mega with anything squared usually (I don't recall anything at the moment at least), which added to the confusion. When one converts from square kilometers (km2) to square meters (m2), one needs to undo the kilo (x1000) not once, but twice, accounting for both dimensions. So as you say, i…

Yes, and I think it just depends where how you read the square. For cm^2, we mean that we square a centimeter, not that we have a centi of a meter^2. But a hectare is really a hect of ares, so 100 ares. Here the square "is baked in" into what comes after the modifier.

Re: The scientific “unit” we call the decibel

#463

Post by davrosthedalek »

mcnamaratw wrote: post #460
mitthrowaway2 wrote: post #455

Earlier quoted context omitted.

Even then, you'll get different results if it's 10 dB of voltage gain or 10 dB of power gain. You need to know what the actual units are.

No, dB are always power gain.

Exactly, and that's why a voltage gain of factor 10 is 20dB, because it's a power gain of factor 100.

Re: The scientific “unit” we call the decibel

#464

Post by davrosthedalek »

sanderjd wrote: post #453
davrosthedalek wrote: post #385

Earlier quoted context omitted.

But dB without reference makes sense in many many occasions. Either because the reference is implicit (not ideal, but we have many implicit assumptions in communication), or because it's genuinely a ratio. Attenuation, gain. If you every find an "official" written document that uses dB not as attenuation/gain and is not specifying the reference (at least in a footnote), it's written either by idiots or for idiots, or…

No it doesn't. It's always bad for the actual unit to be implicit.

The unit of a gain/attenuation is [1]. There is no implicit unit in that case.

Re: The scientific “unit” we call the decibel

#465

Post by deepsun »

fouronnes3 wrote: post #36

When I worked on a radar project, my fellow radar engineers (I'm software) used dB a lot. A lot of them would actually agree with the article, but historical sometimes wins even when you're aware of its shortcomings. Aren't we the same in software anyway? The email protocol, terminal escape sequences, the UX of git command line, etc... Each of those could have an "X is ridiculous" blog post (and I would enjoy every s…

Statistics once got me off-guard because that historical baggage. I thought I knew the probability theory pretty well to carry it to Statistics, but man, Statistics is a very different beast because it developed independently of Mathematics.

Re: The scientific “unit” we call the decibel

#466

Post by nothacking_ »

formerly_proven wrote: post #18

Decibels make sense and it’s usually only laymen who use only „dB“. I’d be surprised if you’d find „-45 dB“ as the specification for a microphone. Here’s two random examples: Sensitivity at 1 kHz into 1 kohm: 23 mV/Pa ≙ –32.5 dBV ± 1 dB Sensitivity: -56 dBV/Pa (1.85 mV)

Except finding '-45 dB' is frustratingly common. Have a look at this random microphone datasheet from DigiKey: https://www.sameskydevices.com/product/resource/cmm-3125at-4...

They do state the test conditions "at 94 dB SPL, 1 kHz", but don't specify the units attacked to the actually measurement. It's given as a ratio to an unspecified quantity.

Re: The scientific “unit” we call the decibel

#467

Post by BenjiWiebe »

bitdivision wrote: post #381
BenjiWiebe wrote: post #320

Earlier quoted context omitted.

So what I learned today, via another comment and confirmed with a dictionary, is that decibels (unspecified reference) is NOT pure ratio - it's a _power_ ratio. So it will be Watts, not Volts.

But that's why it's so confusing. Did they purposefully leave off the reference to indicate Watts, or did they mean volts (as I'd guess audio engineers typically do when talking about amplifiers).

No, it isn't Watts if there's no reference. It's a ratio of powers. i.e. amplification or attenuation.

To have an actual amount of power, you need to reference it to some particular amount of power, like dBm for power-ratio-relative-to-milliwatt.

Re: The scientific “unit” we call the decibel

#468

Post by klysm »

StableAlkyne wrote: post #380
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. Sometime not as in megabytes. Even worse is Mega in Megabytes could be 1,000,000 or 1,048,576 and it's more or less up to you to know what's what (Yeah, there are formally megabytes/mebiibytes/MiB/MB, but I honestly cannot recall the last time I heard anyone use anything other than just "megabyte" for 2^20 bytes... Or even wanted to refer to exactly 1,000,000 bytes. Other…

I see MiB in usage a lot and I always make the distinction myself

Re: The scientific “unit” we call the decibel

#469

Post by somat »

sgarland wrote: post #441
tzs wrote: post #352

Earlier quoted context omitted.

> Look at Fahrenheit, for example. I’m American, and I still think it’s absurd, but because I’m extremely used to it, it feels natural. What do you find absurd about Fahrenheit? If it is that its 0 point is not absolute 0 then I think you can make a good case, at least for scientific work. It's a bit harder to make the case for absolute 0 being the 0 point a scale for ordinary day to day use since all temperatures mo…

For the same reason that every other Imperial measurement is absurd – they’re completely arbitrary. 1 inch is 3 barleycorns, which can have very different sizes. 12 inches to a foot, because a human foot is a decent measurement, I guess? 3 feet to a yard, 5280 feet to a mile… these make sense for their time, but we are no longer in that time. I understand the argument for Fahrenheit having better granularity with who…

As compared to to what? where C and K are 273.15 apart.

But joking aside, While you do point out some of the worst offences, 5280 foot in mile is a special sort of stupid. I do wish we would have metriced around base 12 like a lot of the old measurements were instead of base 10.

Base 10 sort sucks for quantities. I mean, we are all used to it and it works well enough, and having a proper base system is far far better than the alternative coughs roman numbers. but base 10 is a quirk of chance, we very nearly ended up with base 12, and I think we would have been slightly richer for it.

And before you give me the tired ol "BuT yOu HaVe TeN FiNgErS", no, you have 8 fingers and 3 bones per finger, a very common early way of counting for them who had to actually count large numbers(sheep herders) was to use your thumb to mark the spot and count on your finger bones, 12 on one hand, and 12 on the other, this is why 144 (a gross) is so common.

Update: I take back what I said about the mile.

    /usr/games/factor 5280
    5280: 2 2 2 2 2 3 5 11
It was clearly an enlightened choice, again too bad we are not employing base 12 to really take advantage of it.

Re: The scientific “unit” we call the decibel

#470

Post by nayuki »

cb321 wrote: post #186

Abbreviation confusability is relative to { in fact one might say measured by ;-) - number? entropy? etc. } the listener/reader's knowledge/exposure, much as sound levels need a reference distance. I have heard "bare K" refer to a great many different things, not just kilobits (transmission) or kilobytes (storage) or kilograms (drug trade) or kilometers (foot races) and on & on, but pages or items or etc. The fundame…

> I have heard "bare K" refer to a great many different things

The worst of the worst is when it refers to kilometres. Like, "I'm selling my car on the used market and its odometer is 150k", where they mean "150k km", but stacked prefixes are not allowed in metric, so the correct notation is "150 Mm" or "150 megametres".

I get immense pushback whenever I point this out - the response is usually along the lines of "but you know what I meant", and it inherently assumes that km is the universal unit for talking about distance traveled by car.

I point out that this is as ridiculous as saying "the hard drive is 4k GB" because they grew up with gigabytes; no, the correct notation is either "4000 GB" or "4 TB".

Also regarding bare k or kilo (especially キロ in Japanese), you can say something ridiculously ambiguous like "My scooter goes up to 30k for 90k and weighs 20k" (respectively km/h, km, and kg).

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