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Comment by rebolek | Hacker News Reader

While the author is technically right, I must argue that in the area of sound work, decibels make sense. Zero is base level, -3db is half loudness, +3dB is double. There may be a better way to describe loudness, but decibel is good enough.

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But, no! -3dB is half power, not half loudness. -3dB down is just slightly less loud.

Exactly. There's no way +3dB is perceptibly twice as loud. That's easy to test for yourself. +10dB is roughly twice as loud.

The BBC used to teach +6dB for twice as loud for music sources, but they've always been a bit conservative.

+10dB is twice of something: it is an increase in loudness which is twice compared to a +5 dB increase.

If +10dB is absolutely twice as loud, it means that 110 dB is 32 times as loud as 60 dB. We are then essentially denying that perception of loudness is logarithmic!

> -3db is half loudness, +3dB is double

It isn't tho. It's close but not exactly. And there's nothing about -3 behing half that makes sense except for familiarity (and it's not even wide-spread familiarity - most people wouldn't know how much louder +3dB is).

It's just an unnecesarilly confusing definition that stuck for historic reasons.

> It isn't tho. It's close but not exactly.

It isn't tho :-). It's not close to double loudness. It's double power, which is 1.41 higher sound pressure, which is only slightly louder.

For practical day to day use in very narrow context, yes.

But, what you're saying is only approximately correct (the factor is a bit less than 2) and there are many related fields, even in areas that would be relevant to the physics of sound reproduction, in which the same notation "dB" means something different.

So on my amplifier, 0db is the loudest and -50db, where I usually listen is what?

-47db is definitely not twice as loud as -50db, of course.

I'm not confident in what I'm saying here, so please correct me if I'm wrong as I'd like to learn:

Human hearing isn't linear in terms of loudness. So a 3db increase in loudness sounds like "an increase", but the pressure is actually double. Hence, it makes sense to use db to describe loudness even in the context of perceived loudness to human-hearing.

This is similar to brightness. In photography, "stops" are used to measure brightness. One stop brighter is technically twice the light, but to the human eye, it just looks "somewhat brighter", as human brightness appreciation is logarithmic, just like "stops" and "db".

I mean, it should be, unless your amplifier is taking its own liberties with the definition of dB.

Again, technically, it is. But ear isn't scientific device, neither is your amplifier. What I was describing is more than an agreement than some precise measurement. 3dB is more or less double the volume but different frequencies have different responses so I really wouldn't want to have some "perfect" way of measuring loudness as it would be so needlessly complicated that it would be useless.

Human hearing perception response is logarithmic. This is part of the reason we use the unit. Our senses naturally work in that domain.

> Human hearing perception response is logarithmic.

It is, but -50db to -47db (+3db) is not double perceived loudness. It's double power. About +6 or even +10db would be double perceived loudness.

Going from -50dB to -44dB is a much louder change than going from -6dB to 0dB.

Human hearing is logarithmic. The dB is measuring ratio of sound pressure level and it's accurate that +/-3dB is almost doubling/halving of the SPL.

That doesn't make sense. -50dB to -44dB and -6dB to 0dB is the same change in power, as a factor. If human hearing is logarithmic, the same factor produces the same increase in loudness.

3dB is “twice as loud” in that it’s twice (or half) as much power.

Part of the reason why the bel is used is that human perception is closer to logarithmic than linear (weber-fechner law), so a logarithmic scale is a better approximation of “loudness” than a linear one.

-47db is double the power. Perceived double loudness would be ~ -40db

You should have a legitimate question as to what dB’s the amp is referring to. What brand and model is it?

First, is it actually specified as dB’s? Most amps I’ve seen display volume on an arbitrary scale, no units specified.

Second, the amp has no way to know dB sound pressure, as that depends on the rest of signal chain.

So is the displayed figure referring to dBu, dB mV, something else, or something totally bunk?

If 0 dB is its maximum, then -50dB is 50 dB down in power from its maximum (so 10^-5). No m V or other post-fix needed.

That’s true but not that helpful for determining what your ears actually hear.

For music, they hear about 10dB is half/twice as loud.

That’s true but not really helpful for understanding what the amplifier is reading (thanks to nonlinearities down the line.)

As someone else alluded to, 3dB is a doubling in power, not perceived loudness. 10dB will be perceived as a doubling in loudness. This is the original unit (the Bel, rather than deciBel) which was I believe derived by testing on human subjects to measure this.

TBH I don't agree with a lot of the article - yes, dB on its own only indicates a ratio, but certainly in the field I work with this is known, and there are qualifiers (dBA, dbFS dbU) which tie the ratio to a known value so you're talking about an absolute, known quantity - even the dBa which is mentioned as if it comes out of nowhere is something which most audio engineers know about and use regularly because it's important to know the difference betweent he signal present and perception of it by the listener.

> -3db is half loudness

Sure, perfect sense.

10*log10(0.5) = -3

That's the math of it.

I know. Well, ≈ -3, anyway.

That's great, but only if 'Base level' is clear to everyone involved. Besides the base level, you also need to define what exactly you are measuring. Including the frequency weighting.

This makes a spec sheet that says "this machine produces X dB of sound" effectively useless.

Any reasonable spec sheet would not say that. I say reasonable, because in the audio world, people pay extra money for oxygen-free copper cables.

I'm looking at quite a few consumer level workshop machines. All of them have horrible spec sheets already. But on sound, even if they do mention it, it is still useless.

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