Isolation and attenuation

i realize that this is not on topic at all, but i have to know what are those in your picture? Because they are flippen sweet!

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Originally Posted by sonOFwiggum 

i realize that this is not on topic at all, but i have to know what are those in your picture? Because they are flippen sweet!

If you're seeing what I'm seeing, here.
 

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Originally Posted by Anaxilus 

Quick question.  Anyone know if noise reduction is an additive property?  e.g.  I put ear plugs in at 20db NR then cover them w/ 20db NR ear muffs.  Do I get a theoretical 40db NR or is it a diminishing return?  My gut is leaning towards diminishing returns especially considering sound can be transmitted through bones like the jaw.  Thanks in advance.

I'm no expert, but I think it would give you -40dB. Isolation is a diminishing return anyway, because the scale is logarithmic. So -10dB is half volume of 0dB, -20dB is half of -10dB. So if 0dB is 20 units of a made-up unit "volume", -10dB would be 10 volume and -20dB would be 5 volume. That second -10dB only reduces volume by 5 instead of 10.

The sound that reaches the ear plugs is already attenuated by -20dB. The ear plugs then attenuates that signal by -20dB. If the noise is 90dB, it'll be 70dB by the time it reaches the ear plugs. It would be like listening to a 70dB noise without ear muffs, so the ear plugs would do what they always do and reduce it to 50dB. That's, of course, disregarding bone conductance.

i...must...own...those!

Quote:

Originally Posted by Head Injury 

I'm no expert, but I think it would give you -40dB. Isolation is a diminishing return anyway, because the scale is logarithmic. So -10dB is half volume of 0dB, -20dB is half of -10dB. So if 0dB is 20 units of a made-up unit "volume", -10dB would be 10 volume and -20dB would be 5 volume. That second -10dB only reduces volume by 5 instead of 10.

 

The sound that reaches the ear plugs is already attenuated by -20dB. The ear plugs then attenuates that signal by -20dB. If the noise is 90dB, it'll be 70dB by the time it reaches the ear plugs. It would be like listening to a 70dB noise without ear muffs, so the ear plugs would do what they always do and reduce it to 50dB. That's, of course, disregarding bone conductance.

Not an expert here either, but I always thought power in dB was additive in the logarithmic, as you pointed out. So -20dB paired with -20dB should only cut out -23dB. I think it's a little like putting 2 resistors in series - you double the resistance, halve the power, but halving the power is -3dB.

One way to achieve -40dB, which is 100 times less power than -20dB, is to use attenuation that is 40dB NR in the first place (in which case the 20dB protection layer becomes insignificant, since log(101/100) has an additive effect of -0.004dB).