Broadcasters usually have monitoring rooms of various sizes and shapes. To achieve consistent quality in recording, live productions and on-air transmission, complete control of the audio reproduction quality in all facilities is mandatory. This requires careful installation, loudspeaker system calibration and proper system setup documentation of all production spaces. This article presents practical guidelines to achieve consistency.
Controlling radiation effects
The majority of the audible problems in monitoring quality are due to effects of the control room on the sound radiated by the loudspeakers and subwoofers; therefore, their placement in the room is critical. Loudspeakers and subwoofers radiate long wavelengths at low frequencies. Several cancellation effects and standing waves (resonances) in the room can affect loudspeaker or subwoofer performance.
The radiation space is defined as the solid angle (part of a sphere) into which the loudspeaker is radiating sound. A loudspeaker or subwoofer produces a certain volume flow, which naturally spreads out in all directions at low frequencies. As we limit the space, and simultaneously keep the power constant, the energy density (intensity) in the limited radiation space increases. Hence, every halving of the radiation space doubles the SPL. You can halve the radiation space of a loudspeaker by placing it at a wall. (See Figure 1 on page 12.)
At high frequencies, the loudspeaker does not radiate in all directions, and placing it on the wall does not increase the sound level. However, low frequencies are boosted and the frequency response is no longer flat, causing the loudspeaker to sound boomy. It is important to correct the response of the loudspeaker or subwoofer so the frequency response in the room remains flat.
Walls can cause cancellations
When two identical signals are in antiphase (180 degrees out of phase), they cancel each other, and this results in silence. If the loudspeaker is placed at a quarter sound wavelength away from a reflective wall, the wave reflected off the wall arrives back at the loudspeaker in antiphase and cancels, totally or partially, the original signal at that particular frequency. How complete the cancellation is depends on the distance and the ability of the wall to reflect the sound. This results in the sound level dipping down at the frequencies where the reflected sound is in antiphase. The depth and width of a cancellation dip varies, but in most cases it is quite audible. No loudspeaker equalization will cure this problem; increasing the level of the loudspeaker at the dip frequency also boosts the reflection, but their sum remains low, and the dip is not removed.
The best way to cure cancellations is to flush-mount the loudspeakers in a hard wall. This places the loudspeaker in an “infinite baffle” and can totally eliminate the dipping phenomena because no reflections are present.
The second best cure is to place the loudspeaker close to the wall. This raises the frequencies where the cancellations occur. With small loudspeakers inherently less directional in midfrequencies, the cancellation dips might just move to the low midband and still cause coloration. Fortunately, placing small loudspeakers close to the wall, at distances between 0in (at the wall) and up to 7.5in from the wall, renders the loudspeaker response unaffected by such cancellation dips, in most cases.
The third cure is to move the loudspeaker considerably far away from the wall; the first cancellation frequency goes down far enough below the low cutoff frequency of the loudspeaker. However, now distances to other boundaries in the room can be similar to the desired distance from the wall behind the loudspeaker, and the reflections from these other surfaces might also affect the loudspeaker frequency response in similar ways.
Nearby walls affect free-standing loudspeakers
The reflections from all boundaries strongly affect the performance of freestanding loudspeakers. In general, when positioning the loudspeaker's front baffle farther than 12in from the wall, a reflection of the sound from the wall behind the loudspeaker can cause cancellations in the low frequencies. The bass reproduction quality can be lowered. In some cases, most of the low-frequency response can be absent altogether.
For two-way loudspeakers, low-frequency cancellations in the frequency range of 40Hz-80Hz should definitely be avoided. Cancellations in the frequency range of 80Hz-200Hz should also be avoided where possible. If this is not possible, the overall sound quality will still remain acceptable. Acceptable response flatness can be achieved up to 3ft from the wall. Distances within the 3ft-7ft range also should be avoided.
Large loudspeakers placed at a distance more than 7ft away from the wall may suffer from a cancellation in the very low-frequency region around their low-frequency cutoff, compromising their LF extension. The loudspeaker no longer reproduces low frequencies. In practice, freestanding loudspeakers always suffer from some irregularities in their frequency responses caused by cancellations. (See Figure 2 above and Figure 3 on page 16.)
A new situation occurs with a subwoofer that uses a crossover (85Hz) between the loudspeakers and the subwoofer. The subwoofer should be placed acoustically close to the walls to maximize its efficiency (maximum distance from a wall is 60cm). This eliminates most possible sources of cancellation dips in the subwoofer response.
High-passed main loudspeakers (satellites) do not reproduce low frequencies. They can be placed at a distance where low-frequency notching does not occur in their passbands. The guidelines for placing the satellite loudspeakers are similar to those for the freestanding loudspeakers, with the addition that satellite loudspeakers should not be placed too far from the subwoofer (maximum distance is 6.5ft). If the distances are larger, the tonal balance between the loudspeakers playing with the subwoofer may differ considerably between them due to excitation of different room modes.
Early reflections can color sound and spoil imaging
Early reflections, with high amplitude in relation to the direct sound, can color the sound, smear the coherence of sound images and compromise the localization of sources in the space between loudspeakers. To avoid this, all reflective surfaces between the loudspeakers and the listening position should be minimized.
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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.)
Calibration improves quality and consistency
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.