Polycom SoundStructure C16 Design Manual page 719

Noise Immunity
Cellular telephone systems often have transmit and receive frequencies in the 900 to 1800 MHz range.
During normal operation of these cellular phones there is regular communication with the closest cellular
towers for status as well as signaling incoming data and telephone calls. For some GSM style phones this
signaling occurs at the rate of one 576 µs pulse every 4.6 ms which represents a 217 Hz signal. Often in
conferencing applications, these cellular telephones are placed on the conference table in close proximity
to the local microphones. Because these microphones often are not designed to be immune to frequencies
in the 900 MHz to 1800 MHz range, that the transmit and receive signal can be easily coupled into the
microphone where it is rectified by microphone's electronics and combined with the local microphone audio
signal. When this happens, a noticeable beeping or chipping sound that sounds modulated such as if it were
Morse code, will be heard at the remote locations.
If this problem is present, the solutions are to move cellular telephones away from the microphones, turn off
cellular telephones, or to use microphones that have improved noise immunity to these high frequencies.
Many manufacturers are now producing microphones with improved noise immunity.
Boundary Microphones
Boundary microphones use the surface the microphone is installed on and the proximity of the microphone
element to the boundary surface to minimize the amount of phase cancellation that occurs when audio
strikes the boundary. The resulting microphone configuration has a higher sensitivity. The pick-up pattern of
the microphone will become half-spherical as the sounds below the boundary are not picked up. For
instance if an omni-directional microphone is placed on a boundary, the pattern becomes semi-spherical. A
cardioid microphone placed on a boundary will become a semi-cardioid pattern with the sound below the
boundary not picked up by the microphone.
Critical Distance
For every audio source in a room there is a distance from that source, called the critical distance, where the
reverberant sound field and the direct sound field from the source are equal in intensity. If a microphone is
placed farther than the critical distance away from the source, typically a local talker, the resulting speech
quality will be considered very poor - characterized by a bottom-of-the-barrel or muffled sound. The critical
distance is a function of both the physical distance from the local talker to the microphone, the directionality
of the source, and the liveliness of the acoustics in the room. More reverberant rooms will have a shorter
critical distance which underscores the requirement to place microphones as close to the talkers as
possible. Increasing the gain on a microphone will not help reduce the critical distance as the reverberation
and noise will be amplified along with the local talker's voice when the gain is increased.
The critical distance can be measured with an SPL meter and noise source. When the measured sound
level doesn't drop by 4 to 6 dB for each doubling of the distance, the critical distance of the microphone from
the noise source has been reached.
As a rule of thumb, for omni-directional microphones, the microphone should be no farther than 30% of the
critical distance away from the talker. A directional microphone should be placed no farther than 50% of the
critical distance.
If due to architectural constraints, or room usage requirements, the microphones must be placed farther
than 50% of the critical distance (for instance with ceiling microphone installations), the users must either
accept the resulting speech quality or increase the effective critical distance by moving microphones closer
to the talkers, moving noise sources away from the microphones, lowering the level of the noise, and
improving the acoustics in the room to reduce the amount of reverberation. Improving the acoustics in the
room can be done by increasing the absorption of surfaces in the room - acoustic paneling, reduced HVAC
airflow speed/noise, carpeted floors, curtains, and other absorptive surfaces wherever possible.
Polycom, Inc. 719