Achieving Good Audio Recording Levels
by Doug Boyd
It is of paramount importance to monitor recording levels during the digital recording of an oral history interview. The dynamic range of a recorder or a microphone is the range between the highest level and the lowest level (the noise floor) of sound that can be captured. Recording a signal with levels too close to the noise floor will result in a poor signal-to-noise ratio with hiss and hum being quite evident. To achieve optimal recording levels, you should set your recording levels as close to the permitted maximum level (PML) as possible, without reaching or exceeding that limit. Reaching or exceeding the PML will result in distortion of the signal. Digital audio can come very close to the threshold for PML without distorting; however, clipping will occur once the peak has been reached.
Clipping is usually indicated with a red light. If utilizing colored level meters, green is acceptable, yellow is still acceptable but approaching PML, and red indicates clipping. After the interview, it is possible, with audio editing software, to boost the levels if originally recorded too low. A weak signal-to-noise ratio, however, will raise the noise floor and boost the noise during the recording. At the other extreme, clipping is very difficult to fix, so it is best to record the interview with a certain amount of headroom, or the space between the peak signal during the interview and the PML. Interviews falling between -12 dB and -6 dB usually allow enough headroom to account for intermittent spikes in peak levels.
The difficulty with oral history recording is its dynamic and unpredictable nature. Conversation can be very quiet at times and then quickly escalate in volume. Because of this, you need to evaluate and set the levels at the beginning of the interview but also monitor recording levels throughout the recording to make sure they remain at optimal levels. You should not, however, “ride” the levels, constantly turning them up and down according to the sonic dynamics of the interview.
- Record as strong a signal as possible without clipping
- Locate a comfortable range for your peaks (usually between -12 dB and -6 dB) at the beginning of the interview. A recording that has average peaks under -16 dB will normally have a greater amount of noise when the levels are boosted to optimal levels. I prefer to stay away from averaging in the -3 dB range because of the unpredictability of an interview. If clipping occurs, don’t panic, but do gently back the levels down.
Adjusting Levels On Your Recorder
Manual level control
Manual level control involves the operator adjusting the levels by use of the input level or recording level controls. When recording with manual level control it is best to use a limiter to protect against clipping.
A limiter sets a threshold above which the signal will be gently pushed down in order to prevent clipping. This is preferred over ALC or AGC (see below) as it allows the operator to set optimal levels and minimizes noise while still protecting the recording from clipping. Limiters are not foolproof however, and good levels must still be determined by the operator.
Automatic Level Control/Automatic Gain Control (ALC/ALG)
ALC and AGC are circuits in a recorder that determine an average optimal level. Use of one of these will minimize the risk of clipping but typically not produce as high a quality of recording as manual level control because they boost quiet moments in the recording up to record level and thus boost background noise.
It is also recommended that operators utilize headphones, at least in the beginning of the interview, in order to monitor the sound quality of your recording. Levels will not indicate if there is a minor buzz, or the introduction of extra noise from a ceiling fan or an air conditioning unit for that you may not have noticed during setup. A microphone can greatly exaggerate noise unnoticed by the ear and can often be rectified by a different (typically closer) microphone placement (See microphone section). Monitoring through headphones at the beginning of a recording can greatly improve sound quality and, on occasion, prevent the loss of an entire recording.
Citation for Article
Boyd, D. A. (2012). Achieving good audio recording levels. In D. Boyd, S. Cohen, B. Rakerd, & D. Rehberger (Eds.), Oral history in the digital age. Institute of Library and Museum Services. Retrieved from http://ohda.matrix.msu.edu/2012/06/achieving-good-audio/.
Boyd, Douglas A. “Achieving Good Audio Recording Levels,” in Oral History in the Digital Age, edited by Doug Boyd, Steve Cohen, Brad Rakerd, and Dean Rehberger. Washington, D.C.: Institute of Museum and Library Services, 2012, http://ohda.matrix.msu.edu/2012/06/achieving-good-audio/
This is a production of the Oral History in the Digital Age Project (http://ohda.matrix.msu.edu) sponsored by the Institute of Museum and Library Services (IMLS). Please consult http://ohda.matrix.msu.edu/about/rights/ for information on rights, licensing, and citation.