Back to Stereo:Stereo Imaging and Mic

Techniques

Huber, Ch. 4

Eargle, Ch. 11, 12

Localization/Phantom Images

Two loudspeakers recreate a stereo sound stage, or stereo field.

Sonic images appearing between the speakers are referred to as Phantom Images.

Localization cues: Interaural Phase (Time) differences Interaural Intensity (Loudness) differences Pinnae, head, and shoulders

More Localization

Phase typically more important for low frequencies.

Intensity more important for high frequencies.

Discrete time delays at high frequencies also a factor in localization.

Brain processes all cues, creates a perceptual image.

Stereo Miking Techniques

The use of two (or more) microphones to obtain a coherent stereo image.

Coincident; Near Coincident; Spaced Coincident yields excellent stereo image (individual

placement of sound sources). Spaced yields a more spacious-sounding result. Near Coincident techniques attempt a compromise.

Coincident X/Y Arrays

Two directional microphones (same make and model) placed as close together as possible without touching. (coincident technique)

Generally at angle of 90° to 135°.Midpoint faces towards middle of source.Directionally comes entirely from amplitude

differences.No phase problems.Cardiod or bi-directional

Blumlein Array

Eargle refers to this as the “most notable” example of an X/Y array.

Crossed bi-directional, 90° angle.Pickup advantages result from relationship

of sine and cosine angles.Same math as with a panpot (panoramic

potentiometer)

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(sinθ)2 + (cosθ)2 =1

Blumlein(2)

Excellent front quadrant pickupEqually good back quadrant pickup, but

out of phase with front.Shuffling: introduction of in-phase

crosstalk between two channels, above 700 Hz. (to match high-frequency image localization due to phase)

Blumlein Summary

Produces excellent stereo stage lateral imaging (due to panning aspect of pickup)

Creates an excellent sense of acoustic space (due to pickup of reverberant signals)

Can create difficulty in mic placement. Wide array of performers will require a placement distance that can compromise presence.

More X/Y stuff

90° angle produces too much apparent center stage, with wide sources appearing much narrower in recorded image.

Widen up to 135° to compensate, but image can still appear narrow.

Super- or hyper-cardiod pickups at 135° angle produce better stereo image results, plus add a bit of room reflection. (but less than Blumlein)

X/Y summary

Crossed cardiod produces center-oriented stereo image.

Wider image when mic angle is wider (up to 135° - “splayed”)

Excellent mono capabilitySplayed angles with super- and hyper-

cardiod pickups offer widest stereo image possibilities.

Mid-Side Summary

Excellent stereo-to-mono compatibilityFlexibility in postproduction remixingEasy to implement in field

Spaced Microphone Arrays

Recreates phase differences for localization.

Blurred, soft-edge image (preferred by many, like a soft-focus photo)

Omni’s and subcardiods preferred.Distances larger than 1m should

incorporate a center microphone to avoid a center hole (Decca Tree).

Spaced Summary, 2 omni mic’s

Microphones usually placed no further than 1 m apart.

Array can be place relatively close to performers (intimacy)

Used when precise imaging not preferred.

Spaced Summary, 3 omni mic’s

Wide spacing creates ambience through inclusion of early reflections

Center mic fills in hole, but not precisely.Lots of room pickup (reverberation and

reflections) without losing significance of direct sound.

Near-Coincident

CompromiseORTF (French Broadcasting)

17 cm separation of cardiod pickups, with 55° outward angle

NOS (Netherlands Broadcasting) 30 cm separation of cardiod pickups, with 45° outward

angle.

ORTF is more accurate for Lc and Rc sources; NOS creates a wider Lc and Rc image.

Near-Coincident Summary

Combines image specificity of coincident arrays, with spatiality of spaced arrays.

Allows for considerable choice in splay angle and mic spacing.

Head (Binaural) Microphones

Two pressure (omni) microphones at ear position in an artificial head (dummy).

Intended solely for headphone reproduction.

Shuffling can modify recording for speaker playback.

Stereo Listening Setups

Loudspeaker-listener angle should be kept moderate 45° or 50° in Eargle Often use the rule of equilateral triangle (60°)