Background noise measurements are an important part of determining the quality of presentation in a listening space. winRTA uses the Noise Criteria standard to evaluate noise.
AcoustX microphones are designed to strike a balance between high frequency accuracy during tuning and low noise during background noise measurements.
The AM-2 is a ¼” (6 mm) microphone. This size microphone has become the de facto standard for room tuning. It has good high frequency characteristics and can measure background noise
in the range of NC 25 to NC 27. It is pressure response calibrated, which means that its frequency response is flat when the sound is at a 90 degree angle of incidence.
So the microphone should be pointed straight up, toward the ceiling when tuning.
The earlier AM-1 microphone is slightly smaller, approximately 3/16” (5 mm). Because of its smaller size, the high frequency characteristics are excellent,
but the noise floor is slightly higher, in the range of NC 27 to NC 29. The AM-1 is also pressure response calibrated.
The CM-1 (flat plastic microphone) is slightly larger than ¼”. The noise floor is similar to the AM-2. This microphone is pressure response calibrated.
The AM-30 is has a larger diaphragm. It fits into a standard ½” calibrator. It is free field response calibrated, which means that the frequency response is flat
when the sound source is on axis. It has a significantly lower noise floor than the other microphones and is designed primarily for background noise measurements.
It can measure in the range of NC 20.
In a cinema, noise typically originates from the ceiling where the HVAC equipment is located, especially higher frequency noise emanating from the diffusers.
NC is typically measured with the microphones in the tuning position.
This means that the microphones are usually pointed toward the noise source. For greater accuracy in high frequency noise measurements, winRTA (V2.4) added an option to emulate
free field response for this type of measurement. When measuring NC in a theatre using the AM-2, we recommend that you use the “Pressure to free field” option. Note that if this option
is used with the AM-1, response will be slightly overcompensated, whereas with the CM-1, it will be not quite enough. In any case, the measurement should still be more accurate
than without the conversion.
In most cases, the NC measurement can be made with the microphones in the same position as used when tuning the room. In a very large room, it may be useful to position the
microphones so that they are equally spaced the room, putting one in each quadrant of the room.
In small to mid-sized rooms, placing the microphones together in the center of the room is also an option. In this case, each microphone should be put at a 45 degree angle,
and pointing into one of the four quadrants of the room. Then select pressure to free field conversion. This should effectively measure the entire room. When utilizing
the AM-30, no response conversion is necessary, because it is free field response calibrated.
A single microphone can also be used, for example when using one AM-30 to measure a very quiet room.
In this case, the microphone should be first be plugged into input #1, and a single mic measurement taken. Then move the mic to input #2, rotate it 90 degrees, and take
another single mic measurement with mic 2 selected, etc. This will give the equivalent of a complete four microphone NC test, and measure the entire room.
Measuring the background noise in very quiet rooms presents its own set of challenges. The quieter the room is, the quieter the microphone must be.
This typically means a larger capsule, which is more directional. More care must be taken aiming larger, more sensitive microphones to achieve accurate high frequency