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)
(sin  )  (cos  )  1
2
2
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 hypercardiod 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°)