Audyssey and Room Correction Systems for Stereo/Home Theater
No matter how good an audio system you have and how good the speakers are that are attached to it, the room will have a lot to do with the ultimate quality of the sound that you hear coming out of it. Acoustics (the scientific study of sound waves) affects even the best sound by absorbing and reflecting it, sometimes to the point where you can’t even tell what the original sound was. While most rooms don’t have bad enough acoustics to distort sound to that point, it is true that all rooms distort it to some extent.
What makes the room distort sound is a combination of the room’s shape and the materials that are used. Some materials are acoustically “hard,” which means that they reflect sounds easily. Generally speaking, the harder a material is to the touch, the harder it is acoustically. So, the walls of rooms, windows and wood furniture are all acoustically hard materials, reflecting sound. Soft materials, like fabric drapes, upholstered furniture, carpeting and human bodies all tend to absorb sound.
It’s not quite that easy, as the frequency of the sound affects its absorption rate as well. High frequencies tend to be absorbed at a faster rate than low frequencies are. At the same time, each material’s absorption and refraction of sound varies based upon the frequency of sound striking it.
The shape of the room as well as the shape of the objects in the room affect the sound as well, since sound reflects off of most surfaces, especially hard ones. As the sound waves strike the surface, they are reflected off at a similar, opposite angle. So, that means that sound waves which are hitting a wall directly at 90 degrees will be reflected right back to the sound source. However, sounds that hit the wall at 45 degrees, will reflect off at the opposite 45 degree angle, traveling farther down the room.
One final room factor that can drastically affect the sound quality is the relation of the speakers and the listener to one another. If the listener is closer to one speaker than to the others, they will hear more coming out of that speaker. Depending upon how the sound is recorded, that could mean that one type of sound, say from one instrument, overrides the sounds coming from the other speakers.
Compensating for all this is a challenge, one that gives many audio and acoustic engineers headaches. The old way of dealing with these problems is for the sound engineer to use a graphic equalizer. Most people think of graphic equalizers as providing them with a means of “customizing” the sound to their personal preference. Typically, this means boosting the bass, while cutting down the midrange and high frequencies. Sadly, those people are wrong. Graphic equalizers were originally created to give sound engineers a way of balancing a room’s sound.
Unfortunately, that doesn’t help you much for your home theatre. Setting up a graphic equalizer properly, as a sound engineer would do, requires special equipment and knowledge. For most people, it’s just not worth the extra trouble and expense. Instead, they set their equalizer and balance by ear, to give them the sound that they want.
Even when properly set, a graphic equalizer is limited in what it can do to correct the sound in a room. All an equalizer does is to allow adjustment of different frequency ranges of sound, nothing more. Some audio problems are not caused by imbalance of frequency, but by imbalance of volume, delay and other distortion factors.
It doesn’t matter if you are correcting the room’s sound with a properly tuned equalizer or by ear, you are typically limited to only being able to create one “sweet spot” in the room where the sound is the best. Normally, this is the seat that is closest to the center of the room.
This is where digital room correction systems come in. A digital room correction system is a computer-controlled device, which automatically adjusts the sound to match the room’s acoustics. This can be through a combination of active measures, adjusting the tone by frequency range, the balance, delay and even speaker phase.
Like a sound engineer using a graphic equalizer to set up a room, digital room correction does have a setup process. However, this process is largely automated; not requiring the knowledge of acoustics that using a graphic equalizer does. As an automated system, anyone can set up the room.
Besides being automated, the really great thing about this type of system is that it can be set up to provide more than one sweet spot in a room. In the setup process, it allows you to select multiple locations and run the setup procedure for each of them. The system software then calculates the necessary adjustments to provide the best possible sound for all of the locations that have been input.
This eliminates the age old problem of only having one sweet spot in a room, allowing everyone to hear quality sound, regardless of where they are seated. For high-fidelity listening to music or the soundtrack of a movie, this helps ensure that everyone will have the same listening pleasure.
The setup procedure requires a microphone to provide a means for the system to “hear” the room’s acoustics. The system plays a sound, which is actually a combination of all frequencies from 20 Hz to 20,000 Hz simultaneously. The return of this sound, received at the microphone, tells the system everything it needs to know about the acoustics of the room. Once the setup has been accomplished, the information is stored in memory and doesn’t need to be repeated, unless the room has changed significantly (placement or type of speakers, wall coverings, placement of furniture).
Audyssey is the biggest name in digital room correction. Their system was developed based on years of research into acoustics, defining how room acoustics affect the sound heard in a room. While Audyssey provides stand alone systems for digital room correction in home audio systems and home theaters, they also partner with major HiFi audio companies to provide their systems to be built into amplifiers and receivers manufactured by those companies.
R.A.M. Rich Murphy