Crown PZM-10 Application Manual page 21

How the SASS Works
Back to the SASS. It uses two small microphones spaced
a few inches apart. Each microphone is on a surface that
blocks sound from the rear, and these surfaces are
angled apart (Fig. 55). In other words, the surfaces make
the microphones directional. So the SASS is like a near-
coincident pair, in which two directional mics are angled
apart and spaced horizontally a few inches.
Fig. 55 – Parts of SASS
The surfaces make the microphones directional only at
mid-to-high frequencies. At low frequencies, the micro-
phones pick up all around them—they are omnidirec-
tional.
The SASS produces stereo in different ways at differ-
ent frequencies. At low frequencies, the SASS acts like
a spaced pair, producing time differences between
channels to make a stereo effect. At high frequencies,
the SASS acts like coincident pair, producing mostly
loudness differences between channels to make a stereo
effect. At mid-frequencies, the SASS acts like a near-
coincident pair, using both loudness and time differ-
ences to make stereo.
This is the same way the human hearing system works.
Our ears are omnidirectional at low frequencies, direc-
tional at high frequencies (because the head blocks
sounds), and are spaced apart a few inches.
Since the SASS hears sounds the same way our ears do,
it produces very natural stereo with easy-to-localize im-
ages. It also gives a pleasing sense of spaciousness, a
sense of the environment in which the sound was re-
corded. Both these attributes can be heard over loud-
speakers or over headphones.
The coincident-pair method gives a narrow stereo
spread over headphones. The spaced-pair method
makes images that are poorly focused or hard-to-local-
ize when heard with speakers (Fig. 56). The SASS has
neither of these problems. It gives accurate, wide stereo
over headphones, and makes images that are sharp and
correctly placed when heard with speakers or headphones.
The SASS Is Mono Compatible
It's important for stereo recordings to be mono compat-
ible. That is, the tone quality must be the same whether
the program is heard in mono or stereo.
With spaced-pair or near-coincident recordings, the
microphones are spaced apart. Sound arrives at the two
microphones at different times. Thus, the left and right
signals are in phase at some frequencies, and out-of-
phase at other frequencies. If the two channels are com-
bined for mono listening, the out-of-phase frequencies
cancel out. This makes dips in the frequency response
(Fig. 57). The non-flat response gives a filtered, colored
tone quality to whatever is recorded.
Recordings made with the SASS do not have this prob-
lem. That's because it's made with a special block of
dense foam between the mic capsules. This foam barrier
absorbs sound. It prevents sound from the right side
from reaching the left microphone, and vice versa. Thus,
the signal is much louder in one channel than the other.
For a phase cancellation to be complete when two chan-
nels are combined to mono, the levels in both channels
must be about the same. But the levels in both channels
are different in the SASS (due to the foam barrier be-
tween capsules), so phase cancellation in mono is rela-
tively slight (Fig. 57). Thus the tone quality stays the
same in stereo or mono with the SASS.
Better Bass Response
All directional microphones have reduced output in the
deep bass. Thus, stereo methods that use directional
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Fig. 56
Fig. 57