The sound that we hear in a critical listening room is determined by the complex interaction among the quality of the electronics, the quality and placement of the loudspeakers, the hearing ability and placement of the listener, the room dimensions (or geometry if non-cuboid) and the acoustical condition of the room's boundary surfaces and contents. All too often these factors are ignored and emphasis is placed solely on the quality of the electronics and loudspeakers. However, the tonal balance and timbre of a given loudspeaker can vary significantly, depending on the placement of the listener and loudspeaker and the room acoustic conditions. In some cases the differences between different loudspeakers located in the same location in a room can be less than the differences introduced by moving the same loudspeaker to different locations in a room.
The Acoustic Distortion introduced by the room can be so influential that it dominates the overall sonic impression. The causes of Acoustic Distortion are:
1. Modal Coupling: the acoustical coupling between the loudspeakers and listener with the room's modal pressure variations or room modes
2. Speaker-Boundary Interference: the coherent interaction between the direct sound and the omnidirectional early reflections from the room's adjacent boundaries
3. Comb Filtering: the coherent constructive and destructive interference between the direct sound and early reflections
4. Poor Sound Diffusion: the spatial and temporal reflection pattern due to mid and late arriving reflections
We can summarize the potential acoustical distortion problems and possible solutions by dividing the frequency range roughly at 300 Hz. Below 300 Hz we need to focus on the interference at the listening position between the loudspeakers and their adjacent boundaries, as well as the coupling between the loudspeakers and listener with the modal response of the room. These low frequency forms of Acoustical Distortion can be addressed with proper placement of loudspeakers and listening position, as well as the application of dedicated low-frequency absorbers in appropriate locations and low-frequency equalization. Above 300 Hz we are dealing with specular reflection effects which result in comb filtering and the lack of adequate sound diffusion. Comb filtering can be minimized through the strategic application of absorptive, diffusive, abffusive or diffsorptive surface treatment. Sparse diffusion can be improved through the application of efficient diffusive surfaces and a coupling of reflective and diffusive surfaces strategically placed.
| Below 300Hz | ||
| Problems | Solutions | RPG AcousticTools™ |
| Modal Response | Room Dimensions | Room Sizer |
| Speaker / Listener Placement | Room Optimizer™ | |
| Damping | Modex™ | |
| EQ | Digital Equalization | |
| Speaker - Boundary Interference | Speaker / Listener Placement | Room Optimizer™ |
| Damping | Modex™ | |
| EQ | Digital Equalization | |
| Above 300Hz | ||
| Problems | Solutions | RPG AcousticTools™ |
| Comb Filtering | Absorption |
Abffusor® Abflector™ Absorbor™ ProFoam™ ProCorner™ Topakustik, Topperfo |
| Diffusion |
BAD™ Panel Skyline® Diffractal® |
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| Surface Treatment Placement |
Room Optimizer™ CATT Acoustics |
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| Poor Diffusion | Diffusion |
Diffractal® Optiffusor Opticurve™ Digiwave™ Skyline® Omniffusor™ BAD™ Panel |
| Reflection |
Golden Pyramid™ FRG Reflector |
|
| Placement |
Room Optimizer™ CATT Acoustics |
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RPG has published White Papers entitled Minimizing Acoustic Distortion in Home Theaters and Minimizing Acoustic Distortion in Project Studios. These White Papers can be downloaded from the Acoustics Library.
As a quick summary, it is always best to design a critical listening room from the bottom up. Low frequency control is important because once the room dimensions are established, poor modal frequency distribution is fixed and difficult to minimize. Also Speaker-Boundary Interference and modal coupling can be optimized by optimal speaker/listener placement. If it is a new room, then a room dimension analysis should be carried out to minimize problems. RPG will soon offer the Room Sizer, a software program to determine optimum room dimensions from ranges in the length, width and height. There are some spreadsheet programs available, but they do not consider the effects of room absorption, the Q or breadth of the room resonances, nor their overlap. Once the room dimensions are determined, the speakers and the listening position should be determined using the Room Optimizer™. With these two determinations completed the low-frequency response has been optimized as best as possible. There are two remaining tools we have at our disposal if more control is needed. Dedicated low frequency absorbers and equalization. RPG has developed 7" deep, diaphragmatic-membrane, modal-control devices, called Modex™. Modex™ modules offer 100% absorption at the 1/3-octave center frequency bands of 40, 50, 63, 80, and 100 Hz. These Modex™ modules may be used individually for narrow-band, corrective modal modification or combined for broad-bandwidth low-frequency absorption from 40-100 Hz. The absorption range can be extended to 10 kHz, for full-spectrum absorption, by application of 12" deep Modex Plus, which incorporates porous absorption chevrons on the face.
When designing a new room it is a good idea to reserve a 12" deep area around the perimeter of the room for acoustical control. It is advisable to utilize wall/ceiling soffits to address low-frequency control, HVAC, wiring and lighting and a ceiling cloud over the listening area to conceal Modex™ on the ceiling, provide absorption/diffusion, HVAC and lighting. It is also a good idea to determine at the design stage if the acoustical treatment will be completely concealed, completely revealed or some combination of the two. Methods of concealment include stretch fabric systems or perforated metal or wood systems. If the acoustical treatment will be exposed, determine what wood species, finishes and fabrics complement the room decor.
We can begin our room design suggestions by considering each wall separately.
FRONT WALL:
If the room exists, nothing is more important than measuring the frequency response at several listening positions to determine the room response. From this we can determine what type of low frequency treatment may be needed. If the room is being designed, the Room Sizer and/or Room Optimizer™ provide predictions of the room's low-frequency response. They can be used as guides of what Modex™ modules are required. Modex™ modules should be placed on the axial dimensions, i.e. on the front/rear, left/right, floor/ceiling boundaries. Typically we recommend that the front wall be covered with Modex™ modules wherever practical, because the front wall will experience the omnidirectional low frequency sound from the woofers and sub-woofers, resulting in speaker-boundary interference. Front wall treatment can be concealed if desired by curtains, a projection screen, a stretch fabric system or other forms of concealment. Modex™ modules should be selected to provide uniform absorption over the frequency range of interest. If the depth is limited to 7", we suggest Modex™ uniform low frequency control. If 12" depth is available, we suggest Modex Plus, a broad bandwidth absorptive system that extends from 40 Hz up to 10 kHz. It is customary for the screen to be placed against the front wall. Try to locate the screen at least 12" into the room so there is room for acoustical treatment.
REAR WALL:
The rear wall usually is treated with an exposed or concealed wooden Diffractal® system to uniformly diffuse sound across the listening area, thus heightening the surround sensation. If possible the listener should be seated at least 4' (1200 mm) from the rear wall. Additional Modex™ modules can be placed below and surrounding the Diffractals® if necessary. If other forms of diffusion are desired, the Skyline® and Omniffusor™ are good options, especially if the listener is relatively close to the rear wall.
LEFT/RIGHT WALL:
The front of the room, from the screen to the first row of seats should be made abffusive or diffsorptive with our Abffusors® or BAD™ Panels, typically 4" in depth. The rear sidewalls are usually made diffusive, so that the surround channels can be uniformly diffused. This is usually accomplished with 4" Diffractals® or FlutterFree® in a Helmhlotz mounting. For lateral modal control, sidewall Modex™ modules are also important and are usually concealed by a stretch fabric or perforated system.
CEILING:
The ceiling should be diffusive, using our Omniffusors or Skylines, or diffsorptive, utilizing the BAD™ Panel. Another effective approach is to install a diffusive cloud with Omniffusors™ or Skylines® with the Modex™ on the deck above. The cloud is a highly recommended element, if it fits into the design of the room. Typically Omniffusors™ are used when there is the possibility of using a heavy duty-ceiling grid. If a lightweight surface treatment is desired, the Skyline® is the appropriate choice. For diffsorptive treatment the BAD™ Panel may be directly attached to the ceiling or included in a ceiling cloud.
SOFFITS:
Wall/Ceiling soffits are a very useful way to include HVAC, lighting, wiring and low frequency Modex™ control.
DESIGN OPTIONS:
RPG offers five design options in our CineMusic™ Systems. These systems are all priced on an individual basis after reviewing room drawings and evaluating the client's preferences.
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