I understand the basic process of ANC in headphones and earphones. But when an pair of earphones says “42 dbs” of noise cancelling, does it mean sounds louder than that, say 50 dbs, will sound as loud as 50 dbs or 50 minus 42 dbs?
In: 10
The question you’re really asking is less to do with Noise Cancellation, and more to do with “how decibels work” For the full explanation (and one not riddled with my misunderstandings and inept over-simplifications) see [Wikipedia](https://en.wikipedia.org/wiki/Decibel)
TL;DR: You can just take the number of dBs of noise cancelling from the noise level of the background and (ignoring a bunch of details) get the approximate noise level you’ll hear inside the earphones.
Let’s see if I can avoid getting it horribly wrong in the attempt to make it ELI5-able – If anyone else can poke holes in this, please do, I’m not *100%* confident in the job I’ve done 😛
Our ears (and other senses) actually adapt their sensitivity according to the general amount of sound (or light or whatever) around, so quiet things will feel louder if the general noise level is quiet, and even loud things won’t feel that loud if there’s a lot of other noise about. That’s how we can see in the dark, but also function well on a bright sunny day (when the level of light might be 1000s of times more) You can’t hear a pin drop at a concert, or see a [glowing mushroom](https://www.google.com/search?q=bioluminescent+mushroom&tbm=isch) on a sunny day, though.
I’ve used some reference points from [this chart of sound levels](https://ehs.yale.edu/sites/default/files/files/decibel-level-chart.pdf) – “Zero Decibels” is generally defined as the quietest sound perceptible without background noise.*
In general, the *range* of levels you can perceive together is the limiting factor. You can hear a 20dB noise (whispered conversation) in a quiet room (<20dB) but not on a busy city street (~80dB) – I tried to find a number for what range of noise levels we can reasonably perceive at once (somewhere around 50dB, maybe?) but didn’t manage to find one, sadly – somebody chime in if you know the answer 😛
If you’re in an environment with 65dB of background noise (people chatting at the table next to yours in the cafe, perhaps) and your music is at, say 75dB (“Chamber music in a small auditorium”) then you’ll have no difficulty hearing your music clearly, but you’ll still be able to hear your neighbours. If your headphones provide 40dB of noise isolation / cancellation, that will cut the background noise level to something around 20dB (a whispered conversation) and you’d probably struggle to hear it at all over the music. Turn the music off, though, and your ears would turn up their sensitivity and you’d be able to hear it just fine (just like if you took the headphones off and the only noise was a whispered conversation)
People often make a big thing of decibels being logarithmic, but as this deliberately matches how our hearing works, it isn’t really all that relevant (unless you’re doing maths with actual *Watt* power levels) A 40dB sound has 10x the *power* of a 20dB sound, but it only *sounds* something like twice as loud, as you’d expect (at least close enough for our purposes)
It’s also worth mentioning that our ears aren’t equally sensitive to every *frequency* of sound, so more often than not, figures are actually quoted in “dbA”, which under-counts the power of very high or very low frequencies that we can’t hear well giving a better comparison between how loud different *kinds* of sounds appear to our ears. (Bare dB includes sounds we just can’t hear, which is very relevant if you’re building an ultrasound machine, but not so relevant if you’re trying to judge how good your headphones are!)
Of course, if you have frequency specific hearing loss, the adjustment might be off for you. The figures you get from your headphone manufacturers should make it clear exactly what frequency profile is being used, but that’s often not the case, especially at the cheaper end of the scale (where they often use entirely meaningless “PMPO” power figures…)
* Though when you’re talking about gain in amplifiers and the like, 0dB is usually defined as the input signal level – it’s a *ratio* on a logarithmic scale, so you need some kind of reference level.
It means, in this case, that all sound in the operational range is reduced by the 42dB. Which for most ANC headphones is limited to sounds below a fairly low frequency – they work best on low frequency repetitive noise like airplane engines, less so for human vocal range, and least of all for higher frequencies than that.
42 Db attenuation means about 95% of the energy of the sound should be cancelled out (making assumptions here).
Decibels are an attenuation factor – think of it like SPF sun screen.
WhatYouHeard = “ActualSound minus 42 Db”
– which is really “ActualSound times 0.008”
Tricky bit a Db is it’s a **relative** measure, at least most of the time.
DbA vs DbB vs DbSPL (Sound Pressure Level) vs plain-old Db.
– https://en.wikipedia.org/wiki/A-weighting
– DbSPL assigns a value of 20 micropascals to equal 0 DbSPL
– Micro Pascal is 1 millionth of a Pascal https://en.wikipedia.org/wiki/Pascal_(unit)
– Pascal is a derive SI unit based on kg / (meter x second)
Clear as mud?
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