Free Space Listening
If we want to hear sound in our two channel and home theater systems the best way possible, we need to move our stereos and home theaters outside in the mountains somewhere away from civilization. An ideal setting would be a grass land area surrounded on four sides by mountains to shield our sound presentation from the wind. No babbling brook close by just a nice flat area covered by grass at least 6″ thick. We want to keep ambient noise levels as low as possible.
Free Space Sound
This idealistic setting would be termed free space listening. Free space because we are just that, we are in a space that is free from any room boundaries. The only surface we will have to deal with is the earth. We will not have four walls and a ceiling to increase middle and high reflections and cause low frequency pressure build ups which produce resonances or standing waves. We will have just our speakers and a chair. Now all we need is a quiet and steady power source.
Our four walls and ceiling in our listening rooms, home theaters, and professional recording studios produce a sound of their own. It is termed “room sound”. Every room has its own sonic signature. It is not as unique as a fingerprint, but it is close. The size or dimensions of the room contribute to the overall room sound by defining the volume of the room and how that volume or capacity reacts with low frequency energy that is placed inside it by our speakers. The dimension that spans from our left side wall to the right side wall produces a certain set of resonances that are the most powerful and can present the most difficulty in dealing with acoustically. These are axial modes which represent two opposite, parallel surfaces. Tangential modes represent four surfaces and oblique modes involve all six room surfaces. We have distortion everywhere.
Room sound is also influenced by the room materials used to build it. Sound or rather the sounds produced inside of our rooms take on the color or characteristics of the surfaces that they strike. If sound strikes glass, we have “glass sound”. It is a bright and harsh sound that destroys middle and high frequency clarity. A room made mostly of wood will have a far different sound than a room made from drywall. Wood has a warm, harmonic, and richer sound that is very welcome to our ears. Drywall, well drywall, sounds exactly as the name implies. No need to say more.
Our rooms produce a sound of their own because of dimensions, volume, and materials they are constructed with. Each wall contributes its own set of ingredients to our acoustical room soup. Most of our room boundary surfaces effects have an impact on our sound stage. A sound stage is the distance between our speakers, above our speakers, to the left and right of our speakers and even above our speakers. It is the stage in which our electronics portray the music stored inside them. It is our speakers and amplifiers job to portray each and every vocal and instrument on our sound stage. With a properly designed and created sound stage, our speakers sonically disappear and our replaced by the music. If it is done properly, you are brought up out of the audience on to the stage. Don’t forget to bring your instrument.
Walls and Ceiling – 83%
Our room boundary surfaces or walls and ceiling produce 83% of the room sound we hear. The walls deal with many rays and waves of energy striking their four surfaces. Some of this energy is absorbed by the wall, some is not and is reflected back into the room. Energy from our speakers strikes the side wall and we immediately start this acoustical chain reaction which bombards the listening position with wall surface reflections and these reflections are then laced with the wanted direct energy from our speakers.
Direct Sound Please
The wanted, direct energy is the energy that travels the shortest distance to your ears directly without reflection off of any wall surface from our speakers. Draw a a straight line from your speaker cone to your ears. That is the pathway direct sound travels. A properly set up two channel room will have all side wall reflections delayed below 15ms.from the direct sound. This will allow for the correct balance between direct and reflected sound at the listening position to produce a full and balanced sound stage. This can be accomplished using the correct diffusion and absorption technologies.
Side Wall Reflections
Side wall reflections uncontrolled, confuse our hearing mechanism and distort the direct wanted sound. We do not want all direct sound from our speakers in a play back, listening mode. We want approximately 70% direct and 30% reflected or “room sound”. So, reflections must be controlled not eliminated. This is not the case with professional monitoring of recorded music for edit. We want no room in the mix. This is the very reason behind near field monitoring. Side wall reflections properly controlled, can add more width to our sound stage. We can actually create a sound stage that extends more right of the right channel speaker and more left of the left channel speaker with a greater height and depth, by minimizing and balancing reflections at the listening position.
Front And Rear Walls
Front and rear walls can be acoustically extended. With a balance of diffusion and absorption on both the front and rear wall, we can have a sound stage that has more depth that extends from the front of the stage to the rear of our sound stage. With diffusion on the front and rear wall surfaces, we can also realize more separation in our vocals and instruments which provides us with more definition and focus. We must use two dimensions of diffusion on the front and rear wall surfaces to allow for the front and rear walls to acoustically disappear. A series of vertical and horizontally placed quadratic diffusors will accomplish this. One must make sure the listening position is far enough away from the front and rear wall diffusors, so that the diffused wave form has time to develop fully before reaching the listening position.
Our ceiling must be treated with three types of acoustical treatment. We must first have both horizontal and vertical diffusors and with those a proper balance of middle and high frequency absorption. We want our ceiling to acoustically open up or as some say disappear and add more height to our presentation, so that our sound stage sounds much larger. Just as with controlling side wall reflections we widened the sound stage, we can raise the height of our own sound stage by diffusing and absorbing all ceiling reflections at the listening position in a controlled, balance, and measured manner.
Our walls and ceilings contribute to the sound we hear in our rooms. Every room has a sonic signature (room sound) that it imparts onto and into our music playback and recording sources. Understanding what each surface contributes to the total room sound and then applying the proper room acoustic treatment goes hand in hand. You can not successfully have one without the other.