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)

Blumlein(2) Excellent front quadrant pickup Equally 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 capability Splayed angles with super- and hyper- cardiod pickups offer widest stereo image possibilities.

Mid-Side Summary Excellent stereo-to-mono compatibility Flexibility in postproduction remixing Easy 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 Compromise ORTF (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°)