Section 2: Passive vs. Active
Understanding and comparing cancellation specifications
1 of this series dealt with different components that influence
The focus of this session is to show what passive and active attenuation
are and how they affect the total cancellation.
Before going any farther, let me cover what we mean when using the terms
"Passive" and "Active" attenuation. We use these words to describe how much
noise is eliminated by the 'physical' characteristics of the headset
(passive) and how much is reduced via the 'electronic' cancellation (active)
generated within each dome. This cancellation can be measured and
graphed...typically using noise levels (in dB) on one axis and frequency on
the other. These characteristics are unique to each headsets
style/shape/design. These two attenuations, when graphed and overlaid, form
the basis for any claims of 'total' cancellation.
Before we address the issue of advertised claims on various models, let's
look and compare data on cancellation for different products. Below is a set
of attenuation graphs for three different headset designs. For each
platform, the graph lines show Active performance, Passive only, and a third
line showing the combined or Total Attenuation.
There are several interesting things worth noting about these results.
Headset graph #1 with the greatest active performance is actually the WORST
for Total Attenuation. Headset graph #2 provides the lowest Active
performance yet yields the BEST overall quieting! We'll cover the reasons
for this later on in this tutorial. The headset on graph #3 is quieter than
#1 at frequencies above 300Hz but the passive performance was severely
compromised by the conversion to an active product.
What about just adding the active and passive claims together to
determine which is quieter?
This is a misleading practice and leads to false conclusions. The reasons
for this were covered in some detail at the end of Section 2 of ANR 101. In
summary, the active number is typically a single dB number that represents a
frequency where active reduction is maximized. This ignores the breadth of
cancellation provided and overlooks both positioning of the cancellation and
boosting issues (both covered in some detail in Section 1 of ANR101).
Typically manufacturers advertise a 'range' of peak cancellation as a
reference. Depending on the test fixturing and measurement techniques, there
can be substantial differences in results. Remember (from ANR101, Sec 1) the
importance of good correlation between the sensing mic and your ear canal.
The positioning of this mic effects the measured results for cancellation.
It might cancel great at the microphone but not as much in your ear! In the
final analysis, those numbers are only somewhat helpful in comparing
different brands but ONLY for their active performance.
Passively, the story gets even more confusing. First, a bit of history on
the passive noise reduction 'standard'. In the traditional 'passive' headset
market, there has been a longstanding test standard set up by the EPA as an
industrial standard for measuring hearing protection...the "Noise Reduction
Rating" (NRR). This was the most popular means of comparison for passive
headsets in the 80's and 90's. Today, most passive headsets make some claim
about passive noise reduction. In reality, less than half of the models sold
actually have test data to support their claim. Many publish a number like "
24dB" but make no "NRR" claim. Others do imply some "NRR" rating but have no
specific, by Model, test data. Recently others have published a "SNR"
number...a attenuation rating system gaining popularity in Europe. While not
inappropriate, these numbers become misleading since "SNR" tests routinely
will produce higher dB reduction numbers than traditional "NRR"
This information (or lack there of) becomes important when trying to
assess the passive performance of the ANR models available on the market. As
of this printing, no supplier had actually published "NRR" data for the
passive performance of their ANR headset. One reason might be that the
passive performance goes down NOTICEABLY when a passive headset is modified
to become an active one. We'll cover the acoustic reasons for that in more
detail in Section 3. Suppliers might be reluctant to acknowledge that
reduction openly...it wouldn't help sell headsets!! Instead, those that had
a passive model will reference their claims for passive performance without
actually saying that it IS the passive performance. i.e. "We've added 14 dB
of active performance to our 23dB NRR passive headset" Notice they don't
actually SAY their ANR model has 23dB NRR rating...but it's implied.
Test data between Active and Non-Active versions of identical platforms
(covered in the next section) prove this point. At specific frequencies, the
deviation can exceed 10dB. The reduction in passive attenuation of the ANR
version will depend on how the active electronics module is implemented
inside the dome(s). On average the Noise Reduction Rating (NRR) of the
active version would be 2-6dB less than it's passive version.
I'm sure you're beginning to see why confusion abounds in adding active
and passive numbers. The active number...while technically accurate,
provides only a partial indication of how effective the overall ANR
electronics are. The passive numbers are flawed because there are no actual
NRR ratings and then for some, the rating referenced wasn't even an actual
measure of the passive platform! Without being redundant, the only sure way
to get good comparative data is to try on several models and compare them
side by side. That can be done at major trade shows, or some of the larger
pilot supply stores, or possibly by meeting up with fellow pilots who
already have ANR headsets.
3 of this series deals with the acoustic tradeoffs relating to Active
and Passive performance. It specifically deals with Ear cavity volume and
Ear Seal design/comfort.
Go to Section 3 >>