Dynaudio pmn manual Delaying Monitors, Line-Up& Calibration, Acoustic summation of audio signals

Air_US_rev_111.qxd 16-12-2004 14:18 Page 42

DELAYING MONITORS LINE-UP & CALIBRATION

ITU 775

When using a 5.x set-up with monitors arranged on a circle line (as with ITU 775) the distance from the listener to each of the main monitors is the same.

In case of limited space the three front monitors can be arranged on a straight line. To compensate the center monitor being closer to the listener than the left and right monitor, the center signal can be delayed. To obtain coincident arrival of the sound from all front monitors the delay ranges normally between 0 and 3ms. dependent on the off-set needed.

The speed of sound is approximately 340 m/s or 1130 ft./s.

Approximately 1 ms. of delay will be used per 1 ft. or 30cm, when the monitor has to be "moved" backwards.

Cinema set-up

In the set-up for the cinema the front monitors are always arranged on a straight line. In this case the audience is placed closer to the surround monitors than the front monitor. In order to retain the directional information towards the screen the surround monitors are delayed. Normally the Haas-effect is taken into account. In the listening position the sound from the surround monitors will arrive about 15-20ms after the sound from the front monitors. The delay times involved with cinema processing for the surround channel is dependent on the size of the mixing facility, the cinema, or the home theatre/living room.

The procedure is to calculate the difference in distance between front monitors and the closest surround monitor. Then the time difference is calculated. To this is added 15- 20ms.

Example: From a listening position there are 20 meters to the front monitors and 5 meters to the surround monitors. The difference is (20-15) 15 meters.

The time is 15/340 sec, which equals .044 sec or 44ms. To this is added 15ms. The result is that the surround signal has to be delayed 59ms (or the closest possible).

Acoustic summation of audio signals

The total sound level in the acoustic summation of two sound sources, for example of two monitor monitors, depends upon both the signal and the acoustics. The sound sources can be correlated or uncorrelated.

The listening position (or measurement position) can be either in the direct sound field or in the diffuse sound field. In the direct sound field, there is only one sound direction. This direct field exists either in the open, in a reflection-free room or close to the monitors. The diffuse sound field occurs in a room when you are so far away from the monitors that the portion of direct sound is less than the sum of all the reflections. The distance from the monitors where the direct sound field and the diffuse sound field are equally large is called the critical distance. In a control room, it can typically be 1-3 meters. The near field in front of the monitors can be regarded as a direct field.

Based on all monitors in phase, the sum of the sound pressure from two monitors each playing at the same level is 3 dB, unless the signals are correlated (they are identical) and the sum is measured in the direct field. In this situation the resulting sound pressure level is 6dB higher than the level of the individual monitors.

Acoustic calibration of multi-channel systems Calibration of the acoustic sound levels has been a requirement for many years when working with sound for film, although it has not been particularly common in other branches of the sound industry. However, with the widespread use of multi-channel formats for all forms of music and film presentation in the home, it has turned out to be beneficial to also have the acoustic levels under control here.

It is important to differentiate between production for the cinema and production for 5.1 channel reproduction in the home based upon ITU 775.

42