Setting up a Recording Studio in Your Home. Part 3 — Standing Waves.
Standing waves comprise another type of problem encountered in small studios. Standing waves often result in “booming” sound in the frequencies below 300 Hz, and also cause microphones placed in specific locations in the room to not hear some notes or frequencies (nulls), or to pick up other notes too loudly (peaks). In stereo (or multi-channel) recording, they can also cause the apparent sound source to jump around on the virtual stage, as the ear is particularly adept at locating sounds sources.
Some Definitions
Sound propagates via waves that travel approximately 1100 feet per second. Waves follow various paths as they bounce around in a room. Paths can be cyclical, wherein waves repeat a given path. An example of a simple cyclical path is formed when a room has two parallel reflective surfaces such as opposite-facing walls. In this case, the path bounces off two walls and doubles back on itself. A more complex path is a cycle that bounces off all four walls before repeating. A standing wave is formed at a certain frequency when the total path (cycle) length is a multiple of the wavelength at that frequency. For example, a path length of 11 feet would create a standing wave at 100 Hz, because sound travels at 1100 feet per second, and the wavelength of a 100 Hz tone is 11 feet. The same path would also form standing waves at multiples of 100 Hz, so we would expect to see them at 200 Hz, 300 Hz, etc. as well.
Physical Manifestation
When a standing wave has formed, points along the path take on peculiar characteristics of nulls and peaks, according to their distance from a reflecting surface along the path. Precisely how the peaks and nulls distribute along the path is beyond the scope of this article, but suffice it to say that the phenomenon creates undesirable spots in the room at which to place a microphone, as it would pick up too much or too little signal at the characteristic frequencies, compared to other frequencies.
Treatment
There is no perfect room, though different room sizes, shapes, and aspect ratios have different merit. Hence, some form (or forms) of treatment are usually needed. First task, as usual, is diagnosis. Make a test recording of the piece you wish to record, play it back and listen for any of the following:
- Notes that are too loud
- Notes that are too soft, or inaudible
- Notes that sound like they’re coming from a different location in the room than other notes
Make an honest assessment about whether the performer has actually played the note(s) too loudly or too softly, or if the performer has moved in the studio while playing, and then try any of the following treatments in the studio’s physical setup:
- Move the microphone(s)
- Move the performer
- Open/close doors
- Add or move other reflecting surfaces (baffles)
- Move to a different room
After exhausting the above techniques, you can apply remedies to the recorded waveforms:
- Use an equalizer to adjust frequency ranges
- Manually edit each anomalous note (tedious, but feasible)
Alternatively, you can re-record after coaching the performer about which notes to emphasize/de-emphasize (performers hate this).
Further Reading:
Steve
Kindig, Room Acoustics
Matt Bellingham, Room
Acoustics
Small Room Acoustics De-Mythologized (PDF)
Small Room Acoustics (Sound Theory)
Article link: http://www.syllaria.com/StudioArticlePart3.htm
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