Audio Recording Methods Revised

We listen to our visitors and forum members. We really do. We can’t always give you what you want (we get enough requests to triple our workload), but sometimes it takes an outside view to show us how we can improve.

And improve we have. We’re unveiling a new methodology for making audio recordings and new equipment to make them with. The end result will be recordings that do a better job of capturing what we hear during our reviews so that you can hear them better, too.

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.

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
the advantages.

ADVANTAGES of the New Recording Procedure

The "Point of Origin" Problem Solved —
By increasing the recording distance to one meter, the exact spot where the
microphone is pointing is now much less crucial. Our new system should do
a much better job of capturing system noise where there are several simultaneous
noise sources.

Lower Background Noise — The Sennheiser mic introduces
less electronic noise to the signal. The one meter recording distance means
that ambient noise will now play a greater role, but this is a good thing,
since ambient noise makes a good reference level against which recordings
can be judged.

Lots of Detail — Our new system is capable of picking
up a significant amount of detail from one meter — almost as much as
the previous system could at three inches. When the noise source falls below
ambient at one meter, a distance of one foot provides more detail with less
distortion than previously.

Minimal Low Frequency Emphasis — The longer mic distances stop noise sources from developing chesty FM announcer voice.

Ambient Noise Reference Added — By including sections of ambient
noise in every recording, it is possible to judge how audible a
product is likely to be. This is made possible by the lower line noise of the Sennheiser mic. Of course, it is possible that your ambient conditions
may be quieter than ours, but we think that most are likely to be louder…

Difficult Setup Ameliorated — The ambient noise recordings allow the relative
volume to be more accurately judged. The cost is a sacrifice in absolute accuracy:
We no longer expect people to hear our recordings at the same volume that
we heard them.

Still Beyond Our Control

Playback Fidelity — The fidelity of the recordings depends heavily
on the fidelity of the sound system they are played back on — something that
SPCR still has no control over.

In-System Conditions — Because products are recorded on an open
test bench, the recordings do not convey how the products would sound in an
actual system. There’s no way around this without doubling the time spent
on each review, something we’re not willing to commit to.

Overall, we are pleased with our new methodology. Many existing disadvantages
have been addressed, and, to our knowledge, only one — the loss of super near-field
detail — has been introduced. We expect to learn more as we get
used to the new system, but we do not expect to run into any serious issues. As it is, our
existing recording system served us well for two years and was already the best resource of its type among hardware web sites; hopefully our improved audio recording system will
last at least as long.

The rest of the recording system remains unchanged, as shown below.

Audio recording system: Modified Shuttle Zen PC running a P4-2.53 and suspended Samsung 40G 2.5" notebook hard drive, with single channel M-Audio Tampa mic preamp and M-Audio Firewire 410 external digital sound interface feeding the signal from the microphone. This PC measures less than 17 dBA@1m, a touch below the accurate sensitivity of our SLM.

Mic Preamp & Sound Interface: The top item is a M-Audio FireWire 410, a FireWire-compatible audio/MIDI interface with 4-in/10-out configuration complete with preamps. However, for best results, I use the M-Audio Tampa professional microphone/instrument preamp which has lower noise and higher performance than the preamp built into the FireWire 410. The Tampa integrates a 96-kHz / 24-bit A/D converter, which means the signal remains in digital format from the Tampa onward. Both instruments were purchased directly from M-Audio.

NEW REFERENCE RECORDINGS

Quite apart from the advantages and disadvantages associated with our new methodology
is a specific drawback related to changing our methodology. Recordings made with the new system are not
comparable to older recordings, making it difficult to compare
old and new products.

Re-recording all of our samples is out of the question — many of our recordings
are of products we no longer have access to, and recording the ones we do
have would take days.

Instead, we have re-recorded a selection of the products that we consider best
in their class. These are reference products that set the standard for low noise,
ones that we come back to time and time again. Hopefully, these new recordings
should help bridge the gap between old and new.

In addition to the new reference recordings, the How to Listen &
Compare box that we include wherever there are recordings has also
been updated to reflect the new methodology.

We’ve worked hard to make sure that our new recordings sound as close to the
actual products as possible. Not surprisingly, we think our recording system
is the cat’s pajamas… but if you disagree, feel free to take an active role
on SPCR and tell
us about it in our forums! We’d love to hear your comments and suggestions.
After all, we wouldn’t have made the change in the first place if it weren’t for several vocal forum members!