Dynaudio pmn Line-Up& Calibration - Background Info, Calibration of 5.1 in an ITU-775arrangement

Air_US_rev_111.qxd 16-12-2004 14:18 Page 43

LINE-UP & CALIBRATION - BACKGROUND INFO

Calibration of 5.1 in an ITU-775 arrangement

In a 5.1 system based on the ITU arrangement, all main channels have in principle the same conditions: there is one monitor per channel and each is placed the same distance from the listener. Internationally, there is however neither agreement on the level nor on the bandwidth for the noise signals that is used for acoustic calibration. Pink noise is good since it includes all frequencies, however it is impractical due to its "unsettled" character, which makes it difficult to measure at low frequencies.

Surround Sound Forum

Surround Sound Forum is a German interest group composed of the VDT (sound technicians), the IRT and the SRT. The SSF has prepared guidelines, which are generally accepted in Europe. Three test signals are specified in it, which are recorded at -18 dBFS (RMS).

	Measurement Signals			Listening Level
Signal		PPM level	PPM level	RMS level		Sound pressure	Sound pressure
(only in one channel)		t <0,1 ms	t <10 ms			level		level
						SLOW		SLOW
		[dB]	[dB]	[dB]		[dB]		[dB(A)]
1 kHz sine		-18	-18	-18
Pink noise 20 Hz-20 kHz		-9	-13	-18		82	78
Pink noise 200 Hz-20 kHz		-11	-15	-20		80	78

SMPTE

The corresponding standard from SMPTE (RP155) uses a standard of -20 dBFS for the reference level. Here, the C- weighted sound pressure level of a 500 to 2 kHz noise signal ends up at 83 dB.

Calibration of cinema systems

In a cinema, the listeners are sitting far from the monitors. Presumably, the majority sits in the diffuse sound field. In any event, attempts are made to establish a diffuse sound field from the surround monitors. Hence when the sound pressure is measured inside the cinema or in a mixing theatre it must be averaged over many different measurement locations. The typical basis for the majority of the standards is at least four locations. If there are different areas for the audience, for example main floor and balcony, measurement should be made in at least four locations in each. Before this measurement can be performed, the system's frequency response must also be in order.

How to measure the sound pressure?

Sound level meter or microphone

If you are used to perform acoustic measurements you probably already own a sound level meter. This can be used for the level calibration of the monitors.

If you do not have a sound level meter, there still is another possibility:

Take an omni-directional microphone. (This is also called a pressure microphone). A studio condenser microphone is normally preferred but the microphone must not be too big. Then a simple clip microphone can be used instead as it

does not influence the sound field. The microphone should have a flat frequency response.

Place the microphone on a stand positioned at the listening seat. Leave the microphone pointing upwards in order to avoid sound pressure build-up in front of the diaphragm and to provide equal conditions to monitors in all directions.

Connect the microphone to a mixing desk or a recording machine that has a meter built in.

Play back a track containing pink noise on the CD you got with the monitors. (or use one of the other tracks as instructed to during set-up and calibration.

Adjust the gain in the microphone channel until the meter deflection reaches a point 10 dB below full scale. This is now your reference for your measurements. Be sure that you have marks on the scale that makes it possible to read steps of 1 or 2 dB on either side of the reference deflection. If 10 dB below full scale is not an adequate point on your meter you may choose another one, just be sure to have a reading on a adequate resolution.

If you use this microphone set-up as described, it can be used for the level comparison between the individual monitors.

Absolute sound pressure levels can be measured if you:

a)are using a well calibrated microphone channel

b)know the sensitivity of the microphone.

43