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Microphones and Room Effects

Microphones and Room Effects: 
Audio Exemplars and Some Recommendations for Enhancing the Quality of Oral History Recordings

by Brad Rakerd

Department of Communicative Sciences and Disorders

One of the most important determiners of the success of any oral history project is the quality of the audio record that is ultimately produced. Producing a satisfying recording requires two things: (1) recording methods that are sound and sensitive to the situation at hand; and (2) equipment that is adequate for the task. This essay offers audio examples and guidelines regarding both of these factors, particularly as they pertain to the effective use of microphones. Also demonstrated here is the audibility of the room environment in which a recording is made. The room can affect both the intelligibility of the speech that is recorded and the overall ambiance of an interview session.

An ideal environment for an oral history interview would be a carpeted room of moderate size with curtains on the windows, well away from any intrusive noise sources such as computer fans, air conditioners, passing traffic, or (worst of all) other people who may be talking. An ideal arrangement of the interviewer and interviewee(s) within this room would be one in which each maintains a fixed position and speaks directly into a nearby microphone. But the available alternatives are often far from ideal. The best available room may be large or poorly sound treated. The room may have one or more sources of background noise. And the talker(s) of interest may be mobile and/or far from the nearest microphone.

A series of audio-video clips are presented below. They do two things. First, they demonstrate negative listening effects associated with various recording challenges. And second, they show how good decision-making about microphone type and/or microphone use can mitigate, if not eliminate, many of these negative effects.

MICROPHONE DIRECTIONALITY

One of the most common challenges faced when recording an oral history interview is the presence of an interfering noise source (or sources) either within the recording room or somewhere nearby. The extent to which this noise presents a problem will depend in part on the type of microphone that is used to record the interview.

An omnidirectional microphone responds approximately equally to sound arriving from all directions. It therefore detects wanted and unwanted sound sources equally. A unidirectional microphone (sometimes referred to as a cardioidal microphone) favors sound arriving from a single direction and attenuates sound arriving from other directions. It can therefore give emphasis to a desired sound source, so long as it is aimed at that source and away from any competing sources.

The following audio-video clips were recorded in a carpeted conference room with a fan blowing. The fan was located directly behind the microphone and 0.7 meters away. The microphone was placed at the center of a conference table. It directly faced the talker and it was 1.3 meters away.  In the first clip the microphone was omnidirectional and the noise is quite noticeable in the recording. In the second clip the microphone was unidirectional and the interfering effect of the noise is notably reduced.

Comparison:  Microphone Directionality

    •  Omnidirectional Microphone 1.3 meters from the Talker (noise source present)

  •  Unidirectional Microphone 1.3 meters from the Talker (noise source present)

MICROPHONE PLACEMENT

A microphone should always be placed as close to the talker as possible without it becoming intrusive. This increases the power of the talker’s speech relative to the power of all unwanted sound, including any room reflections of the talker’s own speech. The advantage of close placement can be especially high for directional microphones. Several comparisons in which the distance between a microphone and the talker was varied are given below. The first (Comparison A) is for the omnidirectional microphone that was previously placed 1.3 meters away from the talker, with an active noise source (a fan) directly behind. That recording is repeated here. It is then compared to a recording made with the same omnidirectional microphone moved one meter closer to the talker (and one meter further away from the fan). Increasing the proximity of the microphone is this way measurably reduced the audible interference from the noise.

Comparison A:  Distance to an Omnidirectional Microphone in a Noisy Room

    •  Omnidirectional Microphone 1.3 meters from the Talker (noise source present)

  •  Omnidirectional Microphone 0.3 meters from the Talker (noise source present)

Comparison B (below) is the same as Comparison A, except the microphone is now unidirectional. The benefit of increased microphone proximity is again audible, and the joint benefits of microphone directionality and proximity can be heard by comparing these recordings to the same ones made with the omnidirectional microphone.

Comparison B:  Distance to a Unidirectional Microphone in a Noisy Room

    •  Unidirectional Microphone 1.3 meters from the Talker (noise source present)

  •  Unidirectional Microphone 0.3 meters from the Talker (noise source present)

Finally, Comparison C shows the effect of varying distance to the microphone in a quiet conference room (no interfering noise), and Comparison D shows the effect of varying the directionality of the microphone in that same room. The reverberant effect of the room can be heard in all of these recordings, but it can also be heard to be reduced by reducing the distance to the microphone (comparison C) or by increasing the microphone directionality (comparison D). In rooms that have a greater amount of reverberation than this conference room the advantages afforded by microphone proximity and directionality can be important to achieving a satisfying quality in the finalized recording.

Comparison C:  Distance to an Omnidirectional Microphone in a Quiet Room

    •  Omnidirectional Microphone 1.8 meters from the Talker (quiet conference room)

  •  Omnidirectional Microphone 0.3 meters from the Talker (quiet conference room)

Comparison D:  Microphone Directionality in a Quiet Room

    •  Omnidirectional Microphone 1.8 meters from the Talker (quiet conference room)

  •  Unidirectional Microphone 1.8 meters from the Talker (quiet conference room)

ROOM REVERBERATION

All other things equal a room of larger volume will be more reverberant than a room of smaller volume. And a room bounded by hard surfaces and containing furniture made of hard materials will be more reverberant than a room with soft surfaces and soft contents. The audible and potentially very negative effects of room reverberation are shown in this comparison of recordings made in two rooms that had markedly different reverberation characteristics. In both cases, the recordings were made with a directional microphone that was in good proximity to the talker. But even with this favorable microphone type and distance the effects of the highly reverberant room were strong enough to be intrusive. In such a case it would be important to move to a more favorable recording environment or to modify the existing environment by introducing as much sound absorbing material into it as possible.

Comparison: Rooms with Widely Differing Reverberation Characteristics

  •  Speech recording made in a quiet conference room

  •  Speech recording made in a large, highly reverberant room

MULTIPLE TALKERS

If the speech of multiple talkers – say an interviewer and an interviewee – is to be recorded during a session, a common strategy is to use a single, shared microphone and to place it somewhere near the middle of the conversational space. Ideally the microphone used for this purpose would be unidirectional and it would be rotated to face the different talkers as the conversational focus shifts. But doing this is cumbersome, so the typical strategy is use an omnidirectional microphone and to position it at a fixed location. The first example below shows such a case.

An omnidirectional microphone was placed at the center of a conference table, about midway between two talkers, and it was used to make a single-channel recording that captured both of their voices. Because the microphone was omnidirectional and because it was a good distance from both talkers, the resulting recording was vulnerable to room effects and to interference from any noise that may have been present in the room. It the example, room effects are especially audible.

An alternative approach to recording multiple talkers – one that would assure a “cleaner” recording of all of the voices involved in the conversation – would be to give each talker a dedicated directional microphone. This approach carries with it the added requirement that the individual microphone signals must be handled properly.  One possibility is to electronically balance and mix them, with the combined signal then recorded onto a single audio track. Another is to record each microphone signal onto its own audio track, and then combine the tracks at the time of audio playback. In the second example shown below, each talker has been fitted with a dedicated and directional lavalier microphone, with the individual microphone signals then recorded onto separate audio tracks. The resulting recordings are clear, lively, and much less influenced by room effects.

Comparison:  A Single Shared Microphone vs. Multiple Dedicated Microphones

    •  Two Talkers Share a Single Omnidirectional Microphone

  •  Each Talker Wears a Dedicated Directional Microphone (Lavalier)

SUMMARY RECOMMENDATIONS

(1)  As a general rule, the use of a proximate and directional microphone is advocated whenever making speech recordings.

(2)  If two or more talkers are involved in an interview each should have a dedicated and proximate microphone if at all possible, with the individual microphone outputs either recorded onto separate audio tracks or electronically mixed prior to the making of a single-track recording.

(3) The reverberation characteristics of a room should be considered when deciding whether or not the room is a good venue for conducting an interview. If reverberation is too great the room should be “softened” with sound absorbing material, or an alternative location should be found.

Citation for Article

APA

Rakerd, B. (2012). Microphones and room effects: audio exemplars and some recommendations for enhancing the quality of oral history recordings. 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/microphones-and-room-effects/.

Chicago

Rakerd, Brad. “Microphones and Room Effects: Audio Exemplars and Some Recommendations for Enhancing the Quality of Oral History Recordings,” 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/microphones-and-room-effects/

http://ohda.matrix.msu.edu/2012/06/collection-management-systems/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.

Permanent link to this article: http://ohda.matrix.msu.edu/2012/06/microphones-and-room-effects/

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