Audio Recording Methods Revised

Reference|Recommended | The Silent Front
Viewing page 1 of 3 pages. 1 2 3 Next

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 part.
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 the advantages.



1 2 3 Next

The Silent Front - Article Index
Help support this site, buy from one of our affiliate retailers!
Search: