Specifications

18
Boundary-Layer Capsules
Boundary-Layer Capsule
BLM 3g
– microphone capsule (pressure transducer) designed
for placement at an acoustic boundary
– when the pickup angle changes, the sound color
does not change
– excellent reproduction of low frequencies
– often preferred for orchestral recording
Boundary-layer recording offers unobtrusive micro-
phone placement possibilities; no stands or hanging
fixtures are required. When pressure transducers are
used, the result is a hemispherical directional charac-
ter istic that is independent of frequency; thus it is not
necessary to aim the microphones, and if the angle of
sound pickup changes, the sound quality remains
essentially the same.
The technique is based on an effect in physics by
which sound pressure doubles along a sound-reflecting
surface. If a transducer is placed directly on such a sur-
face, or is built within it as with the BLM 3, its sensitiv-
i ty to direct sound will be twice that of an equivalent
transducer in a free sound field. Its response to diffuse
sound remains unchanged, however, causing a notable
increase in the clarity of pickup for a given miking dis-
tance.
In theory an ideal transducer of this type would have
a membrane of infinitesimally small size, and would be
mounted flush with an infinitely large, perfectly sound-
reflecting surface. In practice, the plate in which the
small transducer of the BLM 3g is mounted assures
reflection of only the high and medium frequencies.
To reflect sound at lower frequencies (greater wave-
lengths), the dimensions of the surface must be corre-
spondingly larger. So the microphone should be placed
on a floor, wall or ceiling, or mounted on another sur-
face large enough to reflect the lowest frequencies of
interest.
Since the plate of the BLM 3g is only 5 mm thick
and the transducer is not mounted at its center, the
edges of the plate have very little effect upon the
sound pickup. The BLM 3g does not interfere with the
natural sound field; off-axis sounds thus encounter
very little fre quency discrimination. Sound coloration is
low, even for moving sound sources, because the fre-
quency response is essentially the same for all angles
within the hemispherical pickup pattern.
Microphones used near sound-reflecting surfaces
ordinarily produce comb-filter effects. This does not
occur with the BLM 3g, since at the boundary in which
the transducer is mounted there can be no phase dif-
ference between the direct sound and its reflection.
Ordinary pressure transducers show a difference in
sensitivity between direct and diffuse sound at high
frequencies. In a properly placed boundary-layer micro-
phone, however, this difference occurs at all frequen-
cies. The sound pressure for direct sound is doubled at
an acoustic boundary (6 dB increase), while the essen-
tially random phase relationships of reflections in the
BLM 3g
with CMC 6 microphone amplifier
Frequency range: 20 Hz – 20 kHz
Sensitivity: 19 mV/Pa
Equivalent noise level:
A-weighted*: 12 dB-A
CCIR**: 23 dB
Signal-to-noise ratio
(A-weighted*): 82 dB-A
Maximum SPL: 128 dB (0.5% THD)
Frequency response curve BLM 3g + CMC 6
Polar diagram
from outer
to inner:
up to 2 kHz 4 kHz
8 kHz
16 kHz
20 50 100 200 500 1k 2k 5k 10k 20kHz
+10
0dB
-10
-20
*IEC 61672-1, **IEC 60268-1
COLETTE
modular
hemisphere