The scientific “unit” we call the decibel • Hacker News Board

Re: The scientific “unit” we call the decibel

#321

Post by bitdivision » Thu, May 22, 2025, 2:38 PM UTC

Aachen wrote: post #270

cesaref wrote: post #127

Generally speaking, the db scale is very useful for many practical situations, and this is overlooked in this critique. As have been pointed out, it's just a power ratio on a logarithmic scale, but this has many benefits, the main one being that chaining gain/attenuation in a system is just a case of adding the db values together. 'We're loosing 4db in this cable, and the gain through this amp is 6db, so the output i…

> this has many benefits, the main one being that chaining gain/attenuation in a system is just a case of adding the db values together. 'We're loosing 4db in this cable, and the gain through this amp is 6db, so the output is 2db hotter than the input'. I'm not a sound engineer, so to check my understanding: would this not be the case for any other scale indicator? If you have a cable that loses 4m and you're sending…

No, because db is logarithmic. That's the key property that allows you to do addition instead of multiplication. Remember these are ratios here as well.

So the original example in linear units would be: We're reducing the signal by 63% in the this cable, and the gain through this amp is 199.5%, so the output is...

0.63 * 1.995 = 1.26

126% which is 20log10(1.26) db or around 2db.

-4 + 6 = 2 is a little easier to do.

Re: The scientific “unit” we call the decibel

#322

Post by viraptor » Thu, May 22, 2025, 2:41 PM UTC

drob518 wrote: post #305

viraptor wrote: post #243

Earlier quoted context omitted.

That only works if you're already familiar with the context and system and assume other people are too and don't care about anyone new to that area. (Good luck coming to the audio equipment datasheet with no experience and figuring out what the dB means in each case) "He's 3" works because of the previous question and because everyone had experience of talking about age. dB for anyone not already knowing the answer i…

Then your complaint is with the dropping of units, not dB.

Yes, that's what the whole article is about...

Re: The scientific “unit” we call the decibel

#323

Post by drob518 » Thu, May 22, 2025, 2:41 PM UTC

sanderjd wrote: post #316

drob518 wrote: post #305

Earlier quoted context omitted.

Then your complaint is with the dropping of units, not dB.

Yes. The point is that for some reason dB seems particularly susceptible to people dropping the units. For instance, I've heard loudness of sounds described in decibels for my whole life, and first saw the actual units people are describing when I read this article and thread today.

That’s because you’re a casual observer. If you’re an audio engineer, recording things, designing microphones, amps, or speakers, then you’d know it. Trust me. I’m a digital electrical engineer (computer engineering, basically). I thought that dBs were weird, too. My dad worked in microwave communications systems for his career and dBs are perfectly natural for him. Ditto my daughter who is an audio engineer. Dropping units when you’re working in a particular field is quite common, as who wants to be needlessly wordy when it’s redundant and everyone in the industry understands it? IMO, this article is just the author raging about his own ignorance.

Re: The scientific “unit” we call the decibel

#324

Post by drob518 » Thu, May 22, 2025, 2:42 PM UTC

sanderjd wrote: post #311

drob518 wrote: post #301

Earlier quoted context omitted.

Just because the author is ignorant of the context doesn’t mean that the engineers working in those fields are. They use it because it all makes sense in context.

This is literally the same as saying that you don't need to explicitly specify units in any field because "it all makes sense in context" to "engineers working in those fields". No. We've painstakingly figured out the right answer to this through the generations of doing science and engineering: You always specify units.

But obviously we don’t. So, there’s your counter proof.

Re: The scientific “unit” we call the decibel

#325

Post by drob518 » Thu, May 22, 2025, 2:43 PM UTC

viraptor wrote: post #322

drob518 wrote: post #305

Earlier quoted context omitted.

Then your complaint is with the dropping of units, not dB.

Yes, that's what the whole article is about...

But it’s not. He’s raging against dB.

Re: The scientific “unit” we call the decibel

#326

Post by modeless » Thu, May 22, 2025, 2:43 PM UTC

severusdd wrote: post #76

While I thoroughly enjoyed reading this piece of internet-rant, I've to argue that dB is still probably the best we have on this! In RF engineering, expressing signal levels in dBm or gains in dB means you can add values instead of multiplying, which definitely appeared like a huge convenience for my college assignments! A filter with -3 dB loss and an amplifier with +20 dB gain? Just add. You can also use this short…

There's nothing wrong with using a logarithmic system, that's not the complaint here. The complaint is that using decibels instead of bels is weird, and also that it's a scale and not a unit but people use it as if it was a unit without specifying a reference point, and also that the scale changes for different base units. Crazy how many people here are missing the point.

Re: The scientific “unit” we call the decibel

#327

Post by BenjiWiebe » Thu, May 22, 2025, 2:44 PM UTC

calmbonsai wrote: post #65

If you're specifying decibels in written form you always include the basis or you're simply being incomplete with your units. I don't understand the complaint there. In casual conversation, the context implies the basis. Dealing with decibels is also another shorthand to know the domain has a wide enough value gamut such that logarithmic values (where addition is multiplication) makes sense. See also, the Richter sca…

Exactly! I have seen temperatures specified as degrees, without specifying Fahrenheit or Celsius. It's a bit like that.

My exposure to dB is mainly the RF field, and it's simple. dB is gain/loss (power ratio, just like the dictionary says!) and dBm is relative to a milliwatt. In software defined radio, you've got dBFS - relative to maximum representable. And they're all labeled correctly.

Re: The scientific “unit” we call the decibel

#328

Post by aimor » Thu, May 22, 2025, 2:45 PM UTC

Yes, dB is a mess and if we could do it again we ought to use something more explicit like just annotating the units with log10 such as log10(W). Then it's easy to use other convenient units like log2(W) or if you want to reproduce dBV: 20log10(V). dB is fine shorthand, but it gets used all the time in places where things should be explicit.

Re: The scientific “unit” we call the decibel

#329

Post by stevage » Thu, May 22, 2025, 2:47 PM UTC

Gosh, I didn't know it was that bad - I didn't know it was used for so many different things.

Re: The scientific “unit” we call the decibel

#330

Post by wl » Thu, May 22, 2025, 2:47 PM UTC

grues-dinner wrote: post #171

IsTom wrote: post #95

Earlier quoted context omitted.

> Decibels aren't units... they are ratios. Until they are a ratio to a specific arcane reference level as mentioned in the article.

It's not that arcane, it's referenced to a sound pressure level of 1 pascal. Which, yes, is still arbitrary in that it all ends up back at the arbitrary values of things like metres, seconds, kilograms, etc. Volts per pascal might make sense in some contexts (like your input buffer power supply), but log volts per pascal makes sense in others, particularly for audio applications where you stack gains and attenuations…

dB SPL is referenced to 20 µPa, not 1 Pa. You might be confused by the fact that the 94 dB SPL (1 Pa) is the default for microphone calibrators and specifying microphone sensitivity.