Perception of Sound and Earplugs

[shaunnutley]

Earplugs can be rated according to the number of decibels they reduce sound by, and this could be NRR or SNR. I have noticed on websites that earplugs can be used to block loud industrial noise. However, my question relates to a equal decibel level by the ear and a sound much louder but coming from much further away and wether this alters whether the sound could still be heard.
If earplugs block a sound at a certain frequency at 30dB directly by the ear, and there is a sound of 90dB that has attenuated by 60dB by the time it reaches your ears then surely the 90dB sound would be louder even though it is still only 30dB? My point is obviously the same sound can be perceived much louder than what it actually is depending on the distance and its original volume. As earlugs can block 30dB at this pitch then would both sounds not be able to be heard?
Manufacturers seem to state that 30dB would be 30dB, regardless how loud the original sound is.

[rojash]

That's because 30dB is 30dB! The attenuation due to the earplugs) takes place at the ear, so it is the sound pressure level at the ear that counts. What the SPL is or was at some other point is irrelevant.

[tubeway]

I think that it depends on what sort of dB measurement is used. Most dB scales are weighted as the decibel is given in respect of another value (ie a ratio rather than a true measurement). There are several dB scales and all will have different values of the same sound - and will all have different variation depending on the frequency of the soud. Just off of the top of my head I can think of dBSPL, dB(A), dB(B), dB(C), dB(D), dBSWL and dBSIL scales. If you want a scale that gives an idea of how loud a sound will sound then the Phon or dBSIL are weighted for loudness perception. The unit of percieved loudness is actually the sone which has variations which depend on the frequency and how the sound is presented. Also a persons dynamic range would affect their perception of loudness, as could conditions such as hyperacusis. The manufacturers are trying to give a general (average) rule for somethink that has too many variables and a subjective element too, which renders absolute measurement impossible. Even giving attenuation figures in a more basic pascal measurement would be difficult.

[thechymist]

Shaun,

Yes as the previous guy said 30dB is 30dB.

A louder sound coming from further away makes no odds if it is 30dB when it hits the ear or ear plug, however its frequnecy is key.

The ear can only hear certain frequencies, 30dB of an audible frequency will always be perceived as louder than 90dB of sound that you can't hear, but maybe dogs and cats can.

The sound intensity from a point source of sound will obey the inverse square law if there are no reflections or reverberation.

Ear plugs do not block all frequencies though so certain frequencies will seem louder post ear plug i.e. on the timpanic membrane and downstream of it.

The normal range of human hearing is from about 20 Hz to 20 000 Hz.

If your ear plugs block a particular frequency (lets call it A) within this band and two frequncies (A) & (B) hit the ear plug then (B) is experienced at 30dB and (A) is either absent or much reduced.

I would very much like to know whether you are speaking from a in interest in acoustics or from interest in the health and safety side?

I can maybe help from the health and safety side, there are much more effective ways of reducing the impact of industrial noise.

If this is the case then your starting point must be a noise survey.

This will identify the exact frequency your offending equipment is generating then you can decide if ear plugs are the best approach or an acoustic enclosure or a rubber mat or sound proofing for the walls etc.
It could be ear defenders that are the better solution.