Expert Column: Developing a Sound Home Theater
Aug 1, 2007 12:00 PM, By Sean Bowman
Make entertainment spaces sound their best with proper acoustics.
Once reserved for the rich and famous, home theaters have entered the mainstream. Like any other construction in your customer's home, such rooms can run the gamut from very simple to very elaborate, depending largely on the project's budget and the room design. The most important thing to remember when building a home theater is that even with high-end equipment, sound quality will suffer if acoustics are neglected. A room with proper acoustics and mediocre gear can easily outperform a room featuring high-end equipment without the proper acoustical treatment.
The acoustics of a home theater is one of the topics most often overlooked by do-it-yourselfers. Most enthusiasts get caught up with how their theater looks and what gear it houses, while neglecting to properly address potentially serious acoustic issues. This can compromise system performance. Critical factors for the acoustician to consider include where the home theater is located within the house, the room's dimensions and construction, what furnishings and equipment will be in place, listening and speaker positions, treatment type and location, and how many people the room is expected to seat.
One of the biggest acoustical concerns clients need to address is the transmission of noise — either external sounds leaking into the theater, or vice versa. There are many in-wall acoustical materials and techniques that can be used to help address sound transmission, including layered drywall, mass-loaded vinyl, mineral-fiber insulation, and/or resilient channel.
Addressing isolation in the floor usually means decoupling all or part of the floor from the rest of the structure. There are many products available for “floating” the floor or decoupling the speakers. These are designed to interrupt the path of vibration and minimize transmission into the floor joists that are shared by neighboring rooms. Keep in mind that surface-mounted treatments such as absorbers and diffusors do not help with sound transmission, just with sound quality. Structural vibration between rooms needs to be addressed at the design and construction phase of each project.
Once we take care of proper wall/ceiling/floor isolation, another common concern is the HVAC system. Loud ducts and noisy air vents raise the overall noise floor (measured in dB to identify how quiet the room is) when the unit is running, which can negatively affect your listening experience. (Check out the American Society of Heating, Refrigeration, and Air-Conditioning Engineers at www.ashrae.org for expert advice on this, or for some self-study, read Noise Control in Buildings — A Practical Guide for Architects and Engineers by Cyril M. Harris, or Noise Control Manual for Residential Buildings by David A Harris. Both are good references.) These issues are difficult to tackle post-construction and should also be addressed during the design phase of the project.
Ergonomics, or space use, can change the way home theaters sound. This includes everything from the composition of the floors, walls, and ceilings to equipment and audience placement. Weigh the physical area of the space against the number of seats desired. Ideally, listeners should be at least 3ft. away from any wall or corner. Not taking this into account can lead to inconsistent low-frequency response and excessive reflection at the listening position from boundary proximity and room modes (based on the relationship between the room's dimensions).
Other space factors to consider are flooring material (hardwood, carpet, second-row riser, etc.), doorway location (along the rear wall or side wall), windows (if needed, use a thick fabric curtain or blackout drape), and equipment placement (specifically speaker location, the size of the screen vs. the room size, projector throw distance, etc.).
The complete viewing experience must be taken into account when looking at the room acoustically. For instance, the distance to the screen is vital. For an 8ft.-wide projector screen, the viewing distance should be a little more than 12ft. for most projectors.
Rooms with carpeting and padded furniture are often better choices than rooms with hard floors or paneling if you are going for a more controlled overall sound. There are several best practices regarding speaker placement, but each room is innately different; there is no definitive answer for the best location in every case.
Dolby has a general speaker layout plan and good advice for placement in surround listening spaces (www.dolby.com/consumer/home_entertainment/roomlayout.html). For improved sound quality in your customer's specific room, consult an acoustician or try listening to the system in a few different configurations. Listen to the way sound reflects off surfaces, balance between left and right channels, articulation from the center channel, low-frequency consistency, etc. Proper placement of the front channels (LCR) is one of the most critical aspects of the overall sound quality. In general, keep subwoofers out of corners, because corners are notorious for creating standing waves. Some prefer placement of the subwoofers at the front of the room, while others prefer placement toward the rear of the room. This is largely dependent on the number of subs in the system, the implementation of bass management, and the geometry of the room.
Keep in mind that all materials are “acoustic materials,” meaning that every surface has an acoustical property that will affect the overall sound of the room. Some knowledge about typical treatment practices can help balance the amount and quality of these materials to achieve the optimal combination to fit the client's goals.
TREATING THE ROOM
Using a combination of surface-mounted materials to absorb, scatter, or reflect sound can enhance the sound of the home theater.
The best place to start with treating a room is with bass trapping. Low-frequency waves are long, and therefore strong, and thus are the toughest to control. This is true whether you're attempting to block their transmission to a neighboring space or trying to absorb them to clean up the low-frequency response within a room. Controlling low-frequency sound is harder than controlling mid- or high-frequency sound and generally requires more effort and expense.
To calculate the frequency ranges and location where bass trapping is needed, identify room modes based on the relationship between length, width, and height (there are several good room mode calculators available online). Room modes are essentially distances/dimensions that correspond to wavelengths. These specific wavelengths (frequencies) are heard at inconsistent levels throughout the room depending on listening position. Most room-mode programs will show you the strength of the mode and its axis of origin (X, Y, or Z; i.e. length, width, or height), allowing you to identify the proper location for bass trapping. Another common technique for determining bass trapping location is to listen to a familiar music sample and walk around the perimeter of the room. Areas that you hear a buildup of bass (often corners) are good locations to add bass trapping.
Most typical room construction consists of carpeted floors with drywall/gypsum walls and ceilings. In general, drywall is fairly reflective at the mid-high frequency range. Our ears blend the reflected sound with the direct sound we hear from the speaker, causing coloration or even a distinct flutter or echo problems. There are two main ways to control that reflection — absorption (subtracting that energy from the room) or diffusion (evenly scattering it back into the room).
A widely used treatment technique for many regular rectangular rooms is to absorb reflections along the front and side walls and diffuse reflections toward the rear of the room. This helps control reflection by accentuating clarity from the front sound stage (LCR) and optimizing the diffuse effect in the surround field (especially helpful for use with bipole and dipole surrounds). Alternate approaches include absorption treatment along all walls, while primarily using diffusion along the ceiling. This would help you achieve more accuracy with similar treatment of front and surround channels, but you will want to make sure you are not absorbing one frequency range more than another. Either of these treatment philosophies (or even a combination of the two) can be the right answer for your client, depending on the sound they are going for, the system they are using, and the geometry of the room.
Absorbent treatments are commonly concealed behind panels of acoustically transparent fabric, giving the room a nice tailored/upholstered look. Fabric-wrapped panels are available in a variety of finishes, styles, and colors, giving the homeowner several options that fit the desired décor of the theater.
USE OF DIFFUSION
With the advent of multichannel audio, the need for proper acoustical treatment has never been greater. The majority of today's modern home theaters are equipped with 5.1 or 7.1 surround sound. This affects the way a room is treated acoustically, as a certain amount of diffusion is needed to maximize the surround sound experience.
The surround channels help create an ambiance that puts the viewer/listener in the center of the movie. Proper diffusion can even out the response in the room, broaden the sweet spot for listening, compensate for some geometrical irregularities with floor plans, and make decay times more uniform across the frequency range.
Some may suggest covering your walls 100 percent with absorbent treatment to eliminate all reflections. However, covering all surfaces in the home theater with an absorbing material usually causes more acoustical problems. Typically a well-treated, dedicated theater will have about 40 percent to 50 percent of the wall/ceiling surface area treated with absorption and the remaining areas treated with diffusion or reflection to keep the room from getting too dry at the high-frequency range. This helps create an optimal acoustical environment with a natural response for accurate audio. An overly dry space is uncomfortable to be in, and obviously not ideal for a high-performance theater. A balance of bass trapping, absorption, and diffusion should be considered in each project.
THE HOME THEATER EXPERIENCE
Whether your customers are watching a feature film, a home movie, or cable television, the home theater is a place where the family can gather for peace and sanctuary, isolated from the stresses of everyday life. Make their home theater look and sound its best. Consult an acoustical engineer at the very beginning of construction for optimum sound quality. It is much easier, and in the long run cheaper, to address acoustical problems in the beginning stages of a project.
Sean Bowman is an acoustician/application specialist for Auralex Acoustics. Check out www.auralexelite.com for more acoustical insight and product information.
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