Acoustic for Recording Studio

Ideal Acoustic treatment for commercial users & home users.

There are two very different and unrelated aspects of acoustic treatment used in recording studios: One is sound isolation, which attempts to minimize the leakage between rooms and also between a room and the outside. The other is acoustic treatment within a room, to minimize reflections that cause reverb, echoes, and standing waves. It is the treatment within audio mixing rooms that will be addressed here.

If you walk into an empty room and clap your hands, you'll hear a series of closely spaced echoes. Often these echoes also possess a discernible musical pitch, called ringing, especially if the room is small. Echoes and ringing are caused by sound striking the walls, and then bouncing back and forth between the opposite walls. Besides the obvious intrusion of echoes in a room designed for playing and mixing music, the ringing also causes certain frequencies to be emphasized. The time between the echoes and which frequencies are emphasized depend on the room's shape and dimensions.

To avoid these problems, professional mixing rooms are designed to eliminate most reflections. Deadening the room helps you to hear any reverb and other effects being added to a mix, without being influenced by natural ambience within the room. It also kills the ringing along with the echoes, thereby minimizing the need for 1/3-octave equalizers. (See the sidebar Fine Tuning the Control Room.) But proper acoustic treatment involves more than just eliminating the audible echoes and ringing, which impact only the midrange and upper frequencies. Unless your recording is limited to voice-overs and narration, it is just as important to eliminate the reflections that occur at low frequencies.

Many home-studio owners install commercial acoustic foam on their control room walls, mistakenly believing that is sufficient. After all, if you clap your hands in a room treated with foam (or fiberglass or heavy blankets), you won't hear any echoes or ringing. But these products do nothing to control low frequency reflections, and hand claps won't reveal that. Basement studios with walls made of brick or concrete are especially prone to this problem - the more rigid the walls, the more they reflect low frequency energy. Indeed, simply building a new sheet rock wall a few inches inside an outer cement wall can help to reduce low frequency reflections. The wall vibrates, thus absorbing some of the sound energy instead of reflecting it all back into the room. But this alone is inadequate for a serious mixing room, and you'll get much better results using resonating boxes designed specifically to absorb low frequency energy. These boxes are called bass traps, and they absorb the lowest frequencies where fiberglass and foam stop working. The bass traps I have found most effective are built from plywood panels, and designed to vibrate over a broad range of bass frequencies. Fiberglass is mounted behind the panels to damp the vibration, thus absorbing the bass energy from the room.

When bass frequencies bounce around in a room they generate standing waves. Standing waves are pressure nodes created when a sound wave reflected from a wall collides with the direct sound emanating from the loudspeaker. At some frequencies the reflections reinforce the direct sound, creating an increase in level at that location in the room. And at other frequencies the reflections tend to cancel the direct sound, lowering the volume or in some cases eliminating it altogether. (Standing waves can be reduced with non-parallel walls and an angled ceiling, but such construction is too costly for most home studios.) The variation in bass response caused by standing waves is perhaps the single biggest obstacle to mix down satisfaction for home-studio owners. You create what you think is a terrific sounding mix in your studio, only to get complaints that it sounds either boomy or thin everywhere else.

Standing waves can also occur at mid-range frequencies, but they are less intrusive there because most musical material does not contain sustained single notes as much as in the bass region. Further, mid-range wavelengths are short enough that moving your head even a few inches will bring back a canceled tone. However, it is possible for a sustained note on a flute, French horn, or clarinet to create a standing wave. For this reason, sine waves are never used when measuring the frequency response of monitor speakers in a mixing room. Instead, pink noise is played through the loudspeakers because no single frequency is present in pink noise long enough for a standing wave to develop.

Courtesy

Cineview Projects

Rahul Srivastava