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The scientific “unit” we call the decibel

lcamtuf.substack.com

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Re: The scientific “unit” we call the decibel

#471

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…

> "Plain" decibels are simply (power) ratios. / "Flavored" decibels are in reference to some power quantity.

I think this is analogous to https://en.wikipedia.org/wiki/Affine_space . If I understand correctly, an affine space has absolute points and relative vectors.

In terms of types: point ± vector = point; point + point = illegal; point - point = vector; vector ± vector = vector.

Similar with datetimes - you have absolute datetimes (e.g. 2025-05-23T05:16:35Z) and relative offsets (+1 minute, -1 day, etc.). You cannot add two datetimes together.

A plain decibel would be a vector, and a flavored decibel would be a point.

Re: The scientific “unit” we call the decibel

#472
post #188
post #177

Earlier quoted context omitted.

Ratio is just a single number. 4:3 can be expressed as 1.3333~ and it just says how much bigger one number is compared to another. RE: Yes, I was able to read and understand the article. I also have 8 years of EE. Ratio is still a single number in the end. You can have an actual size of a monitor 1600 x 1200 and the ratio of sides is 1600/1200, which is just a single number. You can express it in multiple ways. You s…

Sure, and 10db means "10x more power". But: 1. 10x more power than what? It changes, and you Just Have to Know. 2. It's 10x more power; so if you're measuring power, like pascals, then 10db means 10x more pascals. But if you're measuring something like voltage, then it's not 10x more voltage, it's something else. 3. And if you're talking about sound, you may be talking about objective power; or you might be talking a…

> Sure, and 10db means "10x more power". But: 1. 10x more power than what? It changes, and you Just Have to Know.

That gives me an idea - explicitly state the basis and multiply. So the current notation "3 dBm" should instead be "1 mW × 3 dB".

Furthermore, any addition in the logarithmic domain must be grouped, like: "3 dBm + 5 dB" --> "1 mW × (3 dB + 5 dB)".

Re: The scientific “unit” we call the decibel

#473
post #469

Earlier quoted context omitted.

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…

> I do wish we would have metriced around base 12

Please clarify what you mean by this. Let me call your system altmetric for clarity.

Surely, you want altmetric to use prefixes that are powers of 12 - okay, fair enough. I see the analogy with the fact that 1 foot = 12 inches, and how base-60 is used in minutes and seconds (also arcminutes and arcseconds). (But why no thirds and fourths?)

But do you want altmetric to require all numbers to be expressed in base-12? If no, then your system does not allow easy conversion. In real metric, the fact that 1.234 kg = 1234 g is a trivial conversion, and it turns a calculation problem into a mere syntactical transformation. If yes and you require base-12, then you've basically alienated everyone. It would be about as weird as telling construction workers and doctors and drivers to use hexadecimal. But at least it makes unit conversions as trivial as base-10 metric.

Let's say you have your altmetric utopia with prefixes based on powers of 12, regardless of whether you require numbers to be expressed in base-12 or not. What do you do about the rest of the world which uses base-10?

You're the head chef for a cruise ship, and the upcoming voyage has 572 people for 14 days. (Imaginary) guidelines say that to keep people happy, you need to provision an average of 800 g of food per person per day. In metric: 572×14×800 g = 6406400 g ≈ 6406 kg ≈ 6.4 Mg (tonne), a simple calculation.

In altmetric, you still get 6406400 g, but now you need to start dividing by 12 repeatedly to form larger groups. Let's just say alpha = 12^3 and beta = 12^6. So 6406400 g ≈ 3707 alphagrams ≈ 2.15 betagrams. That doesn't make life any easier.

Or let's take a somewhat different example. When buying stocks on the market, you specify how many shares you want to buy and the price you want to buy at. But you can't say "I have $X, buy as many shares as possible without exceeding $X". So say you just received a $30000 bonus (after tax) and your favorite stock has an asking price of $68.49 per share for an unlimited quantity. In decimal math, this is easy to figure out - $30000/($68.49/share) = 438.02 shares, so you round down to 438 shares and place your order.

But suppose you're in some F'd up world where you have to specify your stock order in stones, pounds, and shares ("ounces"), where 1 stone = 14 pounds, 1 pound = 16 shares. So your order of 438 shares becomes 1 stone + 13 pounds + 6 shares. You had to do an excessive amount of busywork just to fit into that non-decimal system. And along the way, you might have to think about things like the fact that it's also $15341.76/stone, $1095.84/pound.

You're not the first person I've come across who wants measurements to be grouped/divided into units by some factor other than 10, usually 12. I did a lot of thinking about this, and my conclusion is that if you make an altmetric system where prefixes are not powers of 10, then you lose a huge benefit of the metric system. (The other huge benefit is coherent derived units, like 1 joule = 1 newton × 1 metre.)

> I take back what I said about the mile.

It looks like the derivation of the English statute mile is this: 1 mile = 8 furlongs, 1 furlong = 10 chains, 1 chain = 4 rods, 1 rod = 5.5 yards, 1 yard = 3 feet. You can confirm that 8 × 10 × 4 × 5.5 × 3 = 5280.

https://en.wikipedia.org/wiki/Mile#Statute , https://en.wikipedia.org/wiki/Furlong , https://en.wikipedia.org/wiki/Chain_(unit) , https://en.wikipedia.org/wiki/Rod_(unit)

Re: The scientific “unit” we call the decibel

#474
post #208

Earlier quoted context omitted.

I think the difference is that if you write a blog post complaining about the silliness of these labels, you don't get people telling you that no, you don't get it, it's totally fine, these are just aspect ratios.

I remember a recent post about the ambiguous nature of a pixel, and extended to aspect rations, that garnered VERY similar responses to what you described.

Yeah, it did get similar responses. https://www.nayuki.io/page/pixel-is-a-unit-of-length-and-are... , https://news.ycombinator.com/item?id=43769478

Re: The scientific “unit” we call the decibel

#476

It's a bit like "percent". A shorthand for dealing with ratios, often with hidden assumptions.

Insert mandatory complaint about percentages being misused : their point is the approximation : 1.02×1.03~1.05 +2%+3%~+5%, which only holds up to low tens +/-%.

There is a solution for you - log points! https://en.wikipedia.org/wiki/Relative_change#Logarithmic_ch... , http://a-loonie-saved.blogspot.com/2008/08/log-points.html

An example of deceptive arithmetic: A mutual fund's 5-year history of annual returns is −70%, +30%, +50%, +20%, +40%. Naively, the total return for the 5-year period looks like +70%. But in fact it's −1.72%, because the reciprocal function makes the −70% very “heavyweight” compared to the positive changes.

Using log points instead, the 5-year sequence is approximately −120.4, +26.2, +40.5, +18.2, +33.6. The sum of these rounded numbers is −1.9 (nowhere near +60.0), which agrees well with the correct result of about −1.735 log points (i.e. −1.72%).

Re: The scientific “unit” we call the decibel

#477
post #122

Earlier quoted context omitted.

How else would you describe what occurs other than 6dB/octave?

Notice how both dB and octave are ratios, but for some reason for frequency ratios we don't use bels or decibels. An octave is a ratio of 2, a bel is a ratio of 10.

For frequency, a decade is a ratio of 10, akin to a bel. And in photography, a stop or an EV is a factor of 2, akin to an octave. https://en.wikipedia.org/wiki/Logarithmic_scale#Common_uses

Re: The scientific “unit” we call the decibel

#478
post #99

Decibels are not anywhere near as ridiculous as Marketing Kilobytes introduced in the 1990s. Pot, meet kettle.

This is one of those cases where marketing is correct.

Oh tell me, if your CPU processes 1 byte in 1 cycle, and it runs at 800 MHz, how many bytes does it process in 1 second?

The answer is 800 million bytes, or 800 (real) megabytes. It cannot be 800 mebibytes. (Equal to 763 mebibytes.)

Similarly, let's say we have a 1-bit Boolean attribute for each person in the world, and the world population is 8 062 000 000 billion people. How many bits do we need in our database? It's 8.062 gigabits, not 8.062 gibibits. (Equal to 7.508 gibibits.)

The telecom industry has always used power-of-1000 prefixes on bits and bits per second. You have a gigabit Ethernet LAN, and assume no protocol overhead. How long does it take to transmit a 4.7 GB (real gigabytes) DVD image? Multiply by 8 to convert from bytes to bits, so that's 37.6 Gb, so that will take 37.6 seconds to transmit. But how long does it take to transmit a "700 MB" (actually MiB) CD image? Well, it's 734 MB (real megabytes), so 5872 Mb, which is 5.872 seconds.

The problem with the abusively overloaded definition that 1 kilobyte = 1024 bytes, 1 megabyte = 1048576 bytes, etc. is that it fails to align with the rest of the metric system, or even how we group decimal numbers into thousands and millions. The computer industry is wrong here.

And now you have the problem that you can't fit a memory dump of "16 GB" of RAM onto a "16 GB" flash memory card, because the former is actually GiB but the latter is real GB.

Re: The scientific “unit” we call the decibel

#479
post #433

Earlier quoted context omitted.

As a European if somebody tells me the diagonal size of a display in cm or meter, I'm simply not able to "grasp" it. I need to whip out the calculator and divide by 2.54, turning it into inches. It's just how monitors are measured in my head at this point.

When the Euro was introduced, I've heard people say things like "older generations will never adapt to the new currency, they'll always refer to the old denominations in their head" – which turned out to be completely untrue. Adapting to new units is very possible, but it needs a concerted effort. Absent a good inherent reason or anybody capable of artificially creating one, it won't happen on its own.

It was a true concern, I know people who never got used to it. But were 70+ at the time and dead by now, so it eventually gets fixed, yeah.

Re: The scientific “unit” we call the decibel

#480
post #478
post #99

Decibels are not anywhere near as ridiculous as Marketing Kilobytes introduced in the 1990s. Pot, meet kettle.

This is one of those cases where marketing is correct. Oh tell me, if your CPU processes 1 byte in 1 cycle, and it runs at 800 MHz, how many bytes does it process in 1 second? The answer is 800 million bytes, or 800 (real) megabytes. It cannot be 800 mebibytes. (Equal to 763 mebibytes.) Similarly, let's say we have a 1-bit Boolean attribute for each person in the world, and the world population is 8 062 000 000 billi…

> Oh tell me, if your CPU processes 1 byte in 1 cycle, and it runs at 800 MHz, how many bytes does it process in 1 second?

An interesting metric, used by noone in the Universe except you for the sake of this discussion. But let's entertain this: if the actual CPU speed is 838,860,800Hz, how many bytes does it process in 1 second?

> Similarly, let's say we have a 1-bit Boolean attribute for each person in the world

I have no problem with definition of bits.

> And now you have the problem that you can't fit a memory dump of "16 GB" of RAM onto a "16 GB" flash memory card, because the former is actually GiB but the latter is real GB.

Remarkable circular reasoning, since the Marketing Kilobyte was defined in the 1990s precisely to inflate actual storage sizes without getting class action suits.

> it fails to align with the rest of the metric system

Look, byte is not derived from fundamental units. It is thus not a part of metric system so SI has zero business regulating information storage. On the other hand you can't buy a computer that does not address memory in anything other than powers of two. Nor you could ever buy a 1000 million bytes RAM chip, because they don't ever exist for basic reason that binary computers use 2^n addressable space.

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