Consistent audio
Jul 1, 2009 12:00 PM, By Christophe Anet
Equipment selection and calibration are key.
Figure 3. Frequency domain notches and distances from the single wall behind a free-standing loudspeaker and its front baffle.
Select image to enlarge.
Symmetrical positioning of the loudspeakers and all equipment reflecting sound is essential. Even after this has been done, some reflections will remain, so everything possible should be done to remove reflective surfaces from the vicinity of the acoustic path. Also note that the smaller the loudspeaker physically is, the less directional it is and the more the loudspeaker is influenced by its surroundings.
The often-compromised center loudspeaker should be placed above video screens or TV monitors. One should always ensure that the center loudspeaker does not suffer from first-order ceiling reflection. If the ceiling is low, some absorbing materials should be placed over the ceiling surface near that center loudspeaker.
Equipment placement affects sound quality
Loudspeakers should be placed as far as possible from reflective surfaces. This keeps the reflection-related problems in the frequency response to low frequencies and also improves the imaging. In the presence of many reflecting surfaces (such as tables, computer screens, etc.), loudspeakers can be placed slightly above the listening level and then tilted down (maximum 15-20 degrees) to point toward the listening position.
High-frequency response and loudspeaker orientation
Multilistener control rooms are commonplace in broadcast environments. Because several operators can be present simultaneously, the loudspeakers are frequently poorly placed and aimed.
High-frequency information is of the utmost importance for the listener to evaluate subtle movements and variations in the audio stage. If room reflections are too high compared with the direct sound, the imaging is smeared and quality is poor. Loudspeakers should have a well-controlled directivity. It leads to a high direct-to-reflected sound level ratio and reduces the effects of nearby sound-reflecting boundaries. This helps the operator to hear the actual program material content and reduces the room effects.
Loudspeaker design can control the radiation angle of the tweeter and midrange drivers such that the detrimental diffractions from the loudspeaker enclosure and room surfaces are minimized. The localization, imaging and flatness of the frequency response are then improved, irrespective of the loudspeaker location. (See Figure 4 on page 19.)
Calibration improves quality and consistency
Figure 4. Typical freestanding monitor frequency response featuring a directivity control waveguide and associated directivity curves (off-axis response); the lower curve is the power response of the system measured in an IEC-approved reverberant chamber.
Select figure to enlarge.
Every monitoring system should be calibrated in its final installation to provide the best possible reproduction quality and consistency across monitoring rooms. Today, DSP processing is integrated in monitoring loudspeakers. The most important benefit of such technology is the possibility for extensive automated calibration of a loudspeaker system within a given room. An automatic calibration tool can measure and determine the system response and calculate all the correct acoustical compensations and correction parameter settings for each loudspeaker and subwoofer. The automatic system determines precise acoustical settings to give a flat frequency response at the listening position (or over an area via spatial averaging) using notch and shelving filters available in each loudspeaker and subwoofer. It also aligns loudspeakers in time for equal delay from all loudspeakers to the primary listening position, aligns output levels of loudspeakers, and sets the subwoofer crossover phase. The entire calibration process takes less than five minutes for a full 5.1 system.
More and more small rectangular rooms with strong modal resonances at low and midrange frequencies, low ceiling height and nonsymmetrical equipment layout are used as broadcast monitoring and production rooms. As a consequence, the need for proper loudspeaker placement and consistent monitoring system calibration is more essential than ever before.
A well-engineered monitoring system, containing DSP equalization and supported by a fully automated equalization method, can bring these difficult and challenging environments close to the quality of properly designed control rooms. Even then, however, correct loudspeaker and subwoofer placement is essential.
Christophe Anet is technical editor at Genelec Oy.
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