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July 4, 2006 by Devon
Cooke and Mike Chin
A controversy in the
forum discussion for one of our recent articles sparked some introspection
about how "accurate" the audio recordings of the products we review
can be. SPCR has made audio recordings of nearly every product we have reviewed
in the past two years. During that time, we have become intimately familiar
with the nuances of our recording system and the way it captures sounds. A description of this audio recording system appears on page 2 and 3 of SPCR's Test / Sound Lab: A Short Tour.
Originally, the recordings were intended to be brief "snapshots"
that provided as much detail as possible, allowing the subjective sound to be conveyed in a way that words cannot. However, they were not
intended to be pure reproductions of what we hear when we review a product
such a goal is impossible. Detailed as it is, each recording is only a snapshot
from a single position.
With few exceptions, all of the recordings we have made in the past
two years have been made with the microphone positioned three inches from the
subject, with the head positioned 45° off the axis of the noise. This position
was chosen for a number of reasons:
- High Detail Three inches is close enough to pick up a lot of detail
from even the quietest noise sources.
- Near-field Emulation The recording distance emulates the kind
of near-field listening that we often do when listening critically in person.
- Low Background Noise The microphone is close enough to the sound source to make it
much higher in level than the ambient background noise and the electronic noise in the recording system. The resulting high signal-to-noise ratio allows even very quiet sounds to be heard above the background noise in the recordings.
This close mic position also has several drawbacks, some of which we were aware
at the start, and others that have come to light since then. First and
foremost, the recordings rarely represents how a product sounds
from the one meter distance that we use to take our SPL measurements.
More importantly, they often do not capture our subjective impressions from the one meter
distance. As a result, our comments about some products did not always
reflect what could be heard in the recordings that accompanied them. It was
this issue that generated the controversy referred to earlier. Some of the other disadvantages of the recording
setup are listed below.
Not all of these disadvantages can be addressed. Some, notably the fidelity
of the playback system, are inherent to the recording process, and are guaranteed
to introduce distortion to no matter how carefully we make our recordings. What
is clear, though, is that some of the disadvantages can be addressed
by examining what our recordings should represent and taking steps to improve
the accuracy of that representation.
DIRECT CONSEQUENCE OF OUR RECORDING PROCEDURE
OUT OF OUR CONTROL
The "Point of Origin" Problem It is not always clear what part of the
product should be the "origin" from which the three inch position
should be measured. This problem is very pertinent for complete PC systems with
multiple noise sources. Sometimes, a three inch distance from one part of
the system sounds different from a three inch distance from another
Low Frequency Emphasis Because of the way sound attenuates over
distance, bass frequencies were disproportionately emphasized.
Ambient Conditions Even when calibrated, the recordings do not
convey how loud the sound is in comparison to the background noise.
Playback Fidelity The fidelity of the recordings depends heavily
on the fidelity of the sound system they are played back on something that
SPCR has no control over.
Difficult Playback Setup Accurate playback volume is
dependent on how well the sound system is calibrated to our reference
noise recording. To obtain the original volume, each user must perform a careful setup procedure of his sound system.
In-System Conditions Because products are recorded on an
open test bench, the recordings did not convey how the products would sound
in an actual system.
No matter how much we insist that our recordings do not fully represent what
we hear during our tests, we know that most casual readers will not catch this
nuance and will continue to listen to our recordings as though they are "perfect
representations" of what we heard. Rather than fight this tendency, we have decided
to make adjustments to our recording system instead. While we still insist that
our subjective analysis is the most important part of any review, improving
the quality of our recordings certainly can't hurt!
The question was, how could we improve the recording system?
Our first instinct was to tinker with the recording distance perhaps
positioning the microphone half a meter from the source would provide a better
recording? It quickly became apparent that such an approach was beyond the capability
of our equipment, and the reason for choosing the three inch distance in the
first place was brought home to us: Detail in the quietest recordings quickly
got lost in the background noise if the distance was increased.
A NEW MIC
The next step was to change the microphone. Luckily, we happened to have another
one that fit the bill perfectly. Enter the Sennheiser
ME 66 shotgun microphone. Unlike the T.H.E.
KP-6M that we have used for the past two years, the ME 66 has a highly directional
(the technical term is "super-cardioid") pickup pattern that intentionally
isolates the subject from the background noise. It is intended for use in the
film and broadcasting industries, where it is a favorite among independent and
documentary filmmakers. (Editor's Note: Devon's other obsession
is making motion pictures.)
A new microphone allows us to make detailed recordings from one meter.
Despite numerous theoretical reservations about frequency response and the
difficulties of dealing with a directional microphone, we decided to test it
out, and were pleasantly surprised with the results. The single biggest difference that
we noticed was the amount of line noise the
ME 66 has a significantly lower noise floor than the KP-6M a testament
to Sennheiser's "low inherent self-noise".
The lower noise means that we can now make recordings from a one meter distance
while still getting enough detail to make the recording worthwhile. Testing
showed that the microphone is about as sensitive as our own hearing from one meter.
If a noise source is audible from one meter, it can be recorded from one
meter with roughly the same amount of detail that we can hear.
For most noise sources, this works well enough, but what about noise sources
that we can't hear from one meter? If we can't hear them, neither can the microphone,
but we still want our readers to hear what we hear. (Also, not everyone sits
a full meter away from their system.) The solution is to move the microphone
closer, allowing more detailed recordings at the risk of reintroducing some
of the distortions that we wanted to get rid of in the first place.
Eventually, we reached a compromise: Two recording distances will be used:
- One meter so that "nominal" volume, audibility, and sound character can be judged.
- One foot (or 30 cm if you will)
to capture all the details from even the quietest noise sources.
Once this methodology was formulated, we reviewed how it addresses
the disadvantages of the previous recording system and how it affects
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