Noise Cancelling and Hearing

[poss]

I've been wondering for a little while on just how well noise cancelling protects your hearing. I understand that to cancel a sound wave an equal and opposite wave must be created to in turn "phase" the other out.

However, to my knowledge, this just cancels the actual sound out but what about the remaining sound pressure levels that accompany all sound? Won’t they have an effect on your hearing as well?

[gijoe]

I believe that your physics is sound but my understanding is that the 'extra' cancelling signal does not reach you and you hear only the resultant reduced noise signal.

Try the Bose or Lightspeed - I would wager that you will be amazed and will not look back.

[mnehpets]

As you allude to, sound is transmitted by pressure waves. The pressure waves cause the instantaneous pressure at your ear to vary above and below the ambient pressure. The pressure variations cause your eardrum to vibrate, and hence are the reason you actually hear sound.

Let's suppose you're in a single engine piston aircraft. The combined noise might create pressure variations that range from -0.5 pascal to +0.5 pascal. If we could measure the instantaneous variation caused by the noise and if we could create another variation that was the exact opposite value (so when the noise is causing a variation of -0.3 pascal, we would want to produce a +0.3 pascal variation), then the net effect would be zero pressure variation - the pressure at your ear would stay at ambient pressure.

This is more or less how noise cancellation works. It cancels out the physical phenomena that causes noise, rather than just your perception of it.

- S

[rsuggitt]

But.... what do noise cancelling headsets do with sound at very low or very high frequencies, outside the usual audible range ?

[mnehpets]

But.... what do noise cancelling headsets do with sound at very low or very high frequencies, outside the usual audible range ?

Good question. Active noise cancelling in headsets is only really effective from about 50 Hz to 2 kHz. At the low frequency end, effectiveness is probably going to be limited by the headset's speaker and feedback mike. The upper bound is there because the wavelength becomes small enough that it's no longer possible to accurately create a negative sound wave at your eardrum. So the ANC doesn't do anything to sound outside of audible frequencies. The passive noise cancellation in headsets, however, is quite effective for frequencies at 1kHz and up, so you want a headset that combines both.

Note, though, that in a single engine piston aircraft, the bulk of the noise energy is at low multiples of the engine RPM. The most important multiples are 2x and 3x - this related to the number of blades on the prop, and the number of cylinders. So, if you're flying at 2400 RPM, then you should worry mostly about noise at around 80Hz - 120Hz.

- S

ps: I'm not really an expert on this, but since this is pprune, I will quite happily pretend to be authoritative by occasionally inserting long words.

[poss]

Thanks for the info!
I use a Lightspeed Zulu set at the moment and think it's noise reduction is fantastic but I'm very concious of my hearing as I have slight tinnitus in my ears and am eager to prevent further damage to my hearing.