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The same drum set is recorded throughout this chapter.

Example 6.1: A single mono overhead microphone, centered on the drum set.

Example 6.2: An XY coincident pair of overheads, centered over the drum set.

in a similar way to an XY coincident pair – it is a coincident pair after all. There are no time arrival differences between the coincident capsules, so as with XY coincident pair techniques, only amplitude difference information is recorded and reproduced on playback.

Bi-directional microphones really reject sound sources positioned to their sides. This results in a lot less overlapping pick-up between the mics. Blumlein pairs can produce very natural, realistic images that are wider, more spacious, and lack the narrow mono tendencies of XY coincident pair techniques. Mono compatibility remains excellent because there are no time arrival differences between the two mics. Again though, no microphone is directly on-axis with the center of the sound source.

Sound from behind the microphones will be picked up due to the bidirectional mic’s rear pick-up lobes that are as equally sensitive as the forward pick-up lobes. This can be useful in a good sounding natural acoustic – room reflections and reverb can really energize the sound!

But it can be problematic if the room sound is not desirable or there are sources of unwanted spill, or walls or ceilings too close behind the mic array.

Mic Arrays and Stereo Images @

Example 6.3: A Blumlein pair of overheads, centered over the drum set. You can hear more of “the room.”

Fi g u r e 6 . 2 A Blumlein pair of microphones. Sound sources are shown at various positions in the array’s pick-up, and their perceived playback locations are shown between the loudspeakers.

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6.4 NEAR-COINCIDENT PAIR TECHNIQUES

A near-coincident array uses two directional microphones with the capsules positioned a small distance apart. The mics face away from each other, resulting in less overlap of their pick-up patterns than an XY coincident array. The angle of incidence between the mics is usually between 90° to 110°, as shown in Figure 6.3.

How does this technique work?

A sound coming from the center of the sound source labeled “1” in the figure, is equally off-axis and equidistant to both microphones. Therefore, both microphones pick up identical sound at the same time. With the mics hard panned, both loudspeakers reproduce identical sound simultaneously. Each of the listener’s ears receives identical direct sound and identical inter-aural crosstalk, causing a phantom image to be perceived in the center, directly between the loudspeakers.

Sound “2,” located far to the left of the array’s pick-up area, is not equidistant to both mic capsules. It is closer to the left mic, which picks up the wavefront a fraction of a millisecond before the right mic – so the left loudspeaker reproduces it first. Additionally, this sound is on-axis to the left mic and beyond 90° off-axis to the right mic – so it is reproduced louder and with no off-axis coloration by the left loudspeaker.

Sound “3,” located somewhat to the right of the array’s pick-up area, is also not equidistant to both mic capsules. The right mic picks up the sound slightly before the left mic, so the right loudspeaker reproduces it slightly before the left loudspeaker. This sound source is on-axis to the right mic, and quite off-axis to the left mic, so the sound is reproduced with slightly more amplitude and less off-axis coloration by the right loudspeaker.

Fi g u r e 6 . 3 A near-coincident pair of microphones. Sound sources are shown at various positions in the array’s pick-up, and their perceived playback locations are shown between the loudspeakers.

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matching the original sound source.

• A less muddy and confused sound due to the reduction in common off-axis pick-up between the mics.

• Sources in the center of the image might still be slightly muddy due to their off-axis position – no mic is pointed directly at the center of the sound source. However, the reduced pick-up pattern overlap between the mics decreases this tendency.

• A slight decrease in mono compatibility because the small time arrival differences between each capsule can cause some phase cancellation and comb filtering when the mics are summed together for mono playback. The mono compatibility decreases dramatically as the distance between the two capsules increases.

• The perceived image can be made wider or narrower by increasing or decreasing the angle of incidence between the mics, but as with an XY coincident array, increasing this angle puts each mic further off-axis with the center of the source the array is pointed at, further muddying the center image sound, particularly if the mics have poor, colored, off-axis response.

• Using hyper-cardioid mics instead of cardioid mics will widen the image, however any sound coming from the center of the source will be more off-axis to each mic and consequently picked up with increased off-axis coloration and slightly less amplitude.

• It may be possible to decrease the angle of incidence when using hyper-cardioid mics and maintain a similar image width to using cardioid mics – with the advantage of the hyper-cardioid mics being less off-axis to the center of the sound source.

This list of mainly positives makes the technique seem an always preferable alterna-tive to XY coincident techniques. But when mono compatibility is a major concern, or an expansively wide image is not desired, XY coincident techniques may still be preferable.

It’s important to use the “mono” monitoring button before recording, to see if much sound does disappear, or if any strange phasey comb-filtering artifacts are heard when the image is summed to mono. Slight adjustments to the spacing and angle of incidence can reduce these artifacts and increase mono compatibility.

Specific near-coincident pair techniques include the following:

ORTF

Developed by the Office de Radiodiffusion-Télevision Français, the ORTF technique uses two cardioid mics set at an angle of 110° with the capsules spaced 17 cm (6.7 in) apart. With cap-sule spacing similar to the ears on your head, this technique produces a satisfying, transparent stereo image with relatively good mono compatibility.

NOS

Developed by Netherlands Radio, the NOS technique uses two cardioid mics set at 90° to each other with the capsules 30 cm (12 in) apart. Due to this wider spacing, mono compatibility is not as assured as with an ORTF array but due to the narrower angle of incidence between the mics, they are less off-axis to the center of the sound source. The increased phase differences introduced by the wider spacing results in a slightly bigger and more immersive image than a typical ORTF image.

DIN

Standardized by the German Deutches Institut Für Normung organization, the DIN technique positions two cardioid mics 20 cm (7.8 in) apart, angled at 90°. The mics are not as off-axis as in an ORTF array, but are spaced slightly further apart. The spacing is significantly less than in a NOS array. This results in a good balance of time arrival and amplitude differences that is particularly effective at shorter distances.

Fi g u r e 6 . 4 An ORTF array.

Fi g u r e 6 . 5 A NOS array.

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