ANR vs. Passive Attenuation
OK. This is a very timely thread for me as I am also looking into a new headset acquisition. I recently decided that I would NOT be buying an Active Noise Reduction (ANR) system such as Bose or Lightspeed. Rather, I will be looking at the passive noise attenuation systems such as the Clarity Aloft or Quiet Technologies. In no particular order, the performance measures I used for my assessment are:
1. NOISE REDUCTION
I want to protect my hearing and be able to communicate clearly. ANR, as the name implies, uses active (i.e. needs power) electronics to sample and cancel the ambient noise in the cockpit. The cancellation effect is about 20 to 24 dB in a fairly narrow spectral band of 40Hz to 250 Hz. This low frequency region happens to be where most of the cockpit noise is generated, so that's a good thing. Most of the sound energy from human speech is in the 100 to 500 hz region, so the low frequency region is really where ANR systems shine. To illustrate this, if you have a pair of ANR headsets or have tried them on, the thing you notice when you apply the ANR power is that the low frequency rumble fades away. Now you know why. But because nothing comes for free in the world of active electronics, there is very little cancellation outside of the ANR band. In fact in some regions of the spectrum, the noise levels are actually increased by a phenomena known as "overshoot".
Passive Attenuation is better, in my view, because it provides significant (25 to 45 dB) wide band (from 100 to 4000 Hz) noise reduction. It requires no power source, and does not boost noise in any region of the hearing spectrum. The passive systems use a foam earplug to block the noise out. The plug is attached to a sound transducer (like those used in hearing aids) and has a sound channel (Hole) through the center of the plug to allow the sound to reach your ear. Here are the spec numbers from the two passive systems I am looking at:
Clarity Aloft
35 - 45 dB between 400 HZ and 4 KHz
Quiet Technologies
100Hz = 30 dB
125Hz = 30 dB
250Hz = 32 dB
500Hz = 35 dB
1KHz = 35 dB
2KHz = 38 dB
4KHz = 42 dB
So in the low freq regions the passive systems provide about 10 dB more attenuation than the ANR systems. In case you don't know, the dB or decibel system is logarithmic scale for measuring power ratios.
dB = 10log(Power1/Power2)
In simple non-math terms this means that 3dB equals TWICE the sound energy difference and 10 dB equals TEN TIMES the sound energy difference. So 10 dB is very significant.
2. PRICE
The Bose X is three times the price of the Quiet Technologies - or a 4.8 dB cost ratio
'nuf said there.
3. RELIABILITY
ANR = Batteries (Or Aircraft power), Active Electronics, High Sensitivity sampling microphones
Passive = None of the above
4. COMFORT
In analogous terms
ANR Headset = Football Helmet
Passive Earpiece = VAF Ball Cap
For me the decision is a simple one. And being an engineer, I like simple when I can get it!
YMMV