Noise Control & Room Modes Acoustics Noise Control & Room Modes

Noise: Good vs. Bad Bad noise is considered unwanted sound; Good noise is a type of sound or noise we can use for our benefit. The picture to the right shows random noise, which is typically an unwanted type.

White and Pink Noise White noise has equal energy at every frequency, giving it an inherent high frequency rise of 3 dB per octave. Pink noise has equal energy per octave, resulting in a flat frequency response.

White and Pink Noise Pink noise is used to measure frequency response because of its flat response. Real Time Analysis is done with pink noise. White noise is generally considered unwanted noise and is found at the output of amplifiers and tape recorders.

Interfering Noise Typical sources of noise in a room are: Traffic or outside sources HVAC systems Electronic equipment (computers, console power supplies, etc.)

Sources of Noise Noise can invade a studio or other room in the following ways: Transmitted through air passages (airborne) Transmitted by diaphragmatic action Transmitted through solid structures

Sources of Noise Airborne noise: noise passing through cracks or openings in the room Structural noise: invades a room by mechanical transmission through solid structural elements like wood, steel, concrete, or masonry. HVAC ducting and water pipes are culprits as well. Diaphragmatic noise: sound energy transmitted directly to a rigid structure like a wall or window.

Sound Insulation A wall, for example, must offer a given transmission loss to sound transmitted through it. An outside noise level of 80 dB would be reduced to 35 dB by a wall having a transmission loss of 45 dB.

Transmission Loss Transmission losses shown below are based on the mass of the material rather than the kind of material. For example, a layer of lead gives 95 times the transmission loss of plywood.

Porous Materials Porous materials such as fiberglass are excellent sound absorbers, but they are of limited value in insulating against sound. The transmission loss for porous materials is directly proportional to the thickness. This loss is about 1 dB (100 Hz) to 4 dB (3,000 Hz) per inch of thickness. In contrast, solid materials like sheet rock yield approximately 5 dB per doubling of thickness.

Sound Transmission Classification An STC rating of 50 dB for a wall would mean that it is better in insulating against sound than a wall of STC 40 dB.

Comparison of Wall Structures

Wall Structures

Wall Structures

Wall Structures Different structural designs result in different amounts of transmission loss. A well-built staggered stud or double wall might have an STC of 50 dB or better.

Windows The sound transmission loss of a window between studio and control room should be comparable to the wall itself. A double window is typical.

Sound Insulating Doors The transmission loss of a door is determined by its mass, stiffness, and air-tightness. Sound locks are commonly used to avoid the expense of special doors.

Sound Insulating Doors Some other door designs

Modal Resonances: Reflections Indoors Outdoors the only reflecting plane may be the earth’s surface. Indoors the sound energy is contained, resulting in a louder sound. The drawing shows a virtual sound image created by the reflecting waves.

Two-Wall Resonance When a loudspeaker radiating pink noise excites the space between the walls, the wall-air-wall system exhibits a resonance at a frequency of f0 = 1,130/2L or 565/L, when L = the distance in feet between the two walls and 1,130 the speed of sound in feet per second. A similar resonance occurs at 2f0, 3f0, 4f0, etc., up through the spectrum. These resonances are called modes.

Room Modes Each axial mode involves only two opposite and parallel surfaces. Tangential modes involve four surfaces, and oblique modes involve all six surfaces.