Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

The scientific “unit” we call the decibel

lcamtuf.substack.com

Post reply on HN
Print view View original

81–90 of 511 posts

Re: The scientific “unit” we call the decibel

#81

Post by cycomanic »

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.

Re: The scientific “unit” we call the decibel

#83

Post by baxuz »

kristjank wrote: post #33

This seems exceedingly ignorant of the work decibels do in telecommunications, RF and fibre engineering. The voltage vs power relationship is something that exists and is a core memory of beginner blunders in the field, but it boils down to a simple 10 vs 20 division operation. Besides that, the decibel simplifies a lot of multiplying very small and very big numbers to summing of two-digit numbers that you can do in…

You haven't addressed any of the points the author made. You only show habituation and status quo bias.

Re: The scientific “unit” we call the decibel

#84

Post by rusk »

svara wrote: post #62
kristjank wrote: post #33

This seems exceedingly ignorant of the work decibels do in telecommunications, RF and fibre engineering. The voltage vs power relationship is something that exists and is a core memory of beginner blunders in the field, but it boils down to a simple 10 vs 20 division operation. Besides that, the decibel simplifies a lot of multiplying very small and very big numbers to summing of two-digit numbers that you can do in…

> makes me really doubt that the OP has any practical experience about the things they're talking about. Maybe not, but you can get used to many odd things given enough experience. I totally share the authors view. I don't usually have trouble grasping the definition of a unit, but dBs are just hilariously overloaded. The same symbol can literally mean one of two dimensionless numbers, or one of who knows how many ph…

dB for sound in particular aligns with human experience. The (10 -) 1-10 on a volume knob typically aligns with a logarithmic scale because we hear differences in loudness at an order of magnitude.

A linear volume knob would be frustratingly useless as you would have to crank it many many many times the higher up you want to go. Presumably hundreds of times. A traditional pot couldn’t do that of course but maybe you could satisfy your curiosity with a rotary encoder?

Re: The scientific “unit” we call the decibel

#85

Post by rusk »

baxuz wrote: post #83
kristjank wrote: post #33

This seems exceedingly ignorant of the work decibels do in telecommunications, RF and fibre engineering. The voltage vs power relationship is something that exists and is a core memory of beginner blunders in the field, but it boils down to a simple 10 vs 20 division operation. Besides that, the decibel simplifies a lot of multiplying very small and very big numbers to summing of two-digit numbers that you can do in…

You haven't addressed any of the points the author made. You only show habituation and status quo bias.

[flagged]

Re: The scientific “unit” we call the decibel

#86

Post by topspin »

neepi wrote: post #37
eru wrote: post #32

Earlier quoted context omitted.

Obviously there are better ways. The author mentioned several. Eg you could clean up the naming of the variants of the unit, and make it no less useful for RF work, and strictly better in general.

Sorry but no. Absolutely no one cares about the mathematical purity of a unit like the decibel. It's a hammer that turns log stuff into easy to work with linear stuff. It's x10 because that turns into reasonably easy to use numbers with enough precision to use casually. Otherwise it's just a log ratio. Any suffix is a log ratio to a reference value for the entire system (dBv / dBm etc). Say in a 50 ohm system I have…

This is the correct take. These logarithmic scales, in all their various forms, reduce the math involved to something tractable. The reduction is so great that there is no substitute. Griping about the numerous arbitrary reference values and their obscure and ambiguous notations is fine, I suppose, and perhaps fixing this has some value. But ultimately, logarithmic scales are a tool that will be utilized regardless of whatever frustration they create among amateurs. Also, there is certainly the risk that whatever "fix" one might imagine will only produce yet another, parallel "standard."

Since you deal in RF, I thought I'd mention an interesting device frequently found in such systems: the logarithmic amplifier, or log amp. I'm certain you are familiar with these, but others might find this interesting. These devices are often used in Automatic Gain Control (AGC). Real signals in RF span a very wide range of power, and stages of an RF system need the power to fall in a narrow range to function optimally. A log amp computes an output signal that is some logarithmic function of its input. The output typically serves as feedback to control the gain of an amplification or attenuation stage. The latter stage can then maintain tight control over the level of the source signal as it rapidly fluctuates in real world conditions.

So there you have a logarithmic solution in a physical device, as opposed to some model abstraction.

Re: The scientific “unit” we call the decibel

#87

Post by thaumasiotes »

badlibrarian wrote: post #71

Horsepower is a unit of measurement. There's hp and bhp and hpE and... "Other names for the metric horsepower are the Italian cavallo vapore (cv), Dutch paardenkracht (pk), the French cheval-vapeur (ch), the Spanish caballo de vapor and Portuguese cavalo-vapor (cv), the Russian лошадиная сила (л. с.), the Swedish hästkraft (hk), the Finnish hevosvoima (hv), the Estonian hobujõud (hj), the Norwegian and Danish hestekr…

> "Other names for the metric horsepower are [...]"

I'm not clear on what point you think you're making. Is it interesting that the same thing might have different names in different languages?

That subsection of the article, by the way, is obviously lying:

> The various units used to indicate this definition (PS, KM, cv, hk, pk, k, ks and ch) all translate to horse power in English.

cv and ch translate to steam horse, which isn't hard to see even if you only speak English. What does "vapor" mean to you?

The opening of the article does suggest some problems, though not problems that wouldn't apply to the word "ounce". But you seem to have pulled an extended quote describing a completely expected state of affairs, while ignoring this:

> There are many different standards and types of horsepower. Two common definitions used today are the imperial horsepower as in "hp" or "bhp" which is about 745.7 watts, and the metric horsepower as in "cv" or "PS" which is approximately 735.5 watts. The electric horsepower "hpE" is exactly 746 watts, while the boiler horsepower is 9809.5 or 9811 watts, depending on the exact year.

I don't get it.

Re: The scientific “unit” we call the decibel

#88

Post by [deleted] »

bigiain wrote: post #23
timewizard wrote: post #12

> That said, I don’t know how to pronounce “3e5” "Three to the exponent of five." Or "Three Exponent Five." Or "Three Exp Five." > Seeing this, some madman decided that 1 bel should always describe a 10× increase in power, even if it’s applied to another base unit. This means that if you’re talking about watts, +1 bel is an increase of 10×; but if you’re talking about volts, it’s an increase of √10× This is power vs.…

> > 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.

[deleted]

Re: The scientific “unit” we call the decibel

#90

Post by baxuz »

neepi wrote: post #60
fouronnes3 wrote: post #53

Earlier quoted context omitted.

However you must concede that the perspective of an outsider can be refreshingly eye opening sometimes, even to experts. Especially when it reveals the arcane practices that make a field difficult to learn - for example here the point about the base being too implicit is very valid. The article is perhaps ignorant of a few things, but its criticism shouldn't be dismissed outright IMO. Accepting constructive criticism…

That would assume that the entire world hasn't already tried this a thousand times over. Measurements are standardised communication tools. If you start changing the definitions, things fall out of the sky and on your head.

This mindset is why the US is still using barley seeds as a standardized unit of measure.
Post reply on HN