AcousticsAcoustics

Noise Control Noise Control & Room & Room ModesModes

Noise: Good vs. BadNoise: Good vs. Bad

• Bad noiseBad noise is is considered considered unwanted sound; unwanted sound; Good noiseGood noise is a is a type of sound or type of sound or noise we can use noise we can use for our benefit.for our benefit.

• The picture to The picture to the right shows the right shows random noise, random noise, which is typically which is typically an unwanted an unwanted type.type.

White and Pink NoiseWhite and Pink Noise• White noiseWhite noise

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

White and Pink NoiseWhite and Pink Noise

• Pink noise is used to measure frequency Pink noise is used to measure frequency response because of its flat response. Real response because of its flat response. Real Time Analysis is done with pink noise.Time Analysis is done with pink noise.

• White noise is generally considered White noise is generally considered unwanted noise and is found at the output unwanted noise and is found at the output of amplifiers and tape recorders.of amplifiers and tape recorders.

Interfering NoiseInterfering Noise

• Typical sources of noise in a room Typical sources of noise in a room are:are:– Traffic or outside sourcesTraffic or outside sources– HVAC systemsHVAC systems– Electronic equipment (computers, Electronic equipment (computers,

console power supplies, etc.)console power supplies, etc.)

Sources of NoiseSources of Noise

• Noise can invade a studio or other Noise can invade a studio or other room in the following ways:room in the following ways:– Transmitted through air passages Transmitted through air passages

(airborne)(airborne)– Transmitted by diaphragmatic actionTransmitted by diaphragmatic action– Transmitted through solid structuresTransmitted through solid structures

Sources of NoiseSources of Noise

• Airborne noise: noise passing through cracks Airborne noise: noise passing through cracks or openings in the roomor openings in the room

• Structural noise: invades a room by Structural noise: invades a room by mechanical transmission through solid mechanical transmission through solid structural elements like wood, steel, concrete, structural elements like wood, steel, concrete, or masonry. HVAC ducting and water pipes are or masonry. HVAC ducting and water pipes are culprits as well.culprits as well.

• Diaphragmatic noise: sound energy Diaphragmatic noise: sound energy transmitted directly to a rigid structure like a transmitted directly to a rigid structure like a wall or window.wall or window.

Sound InsulationSound Insulation

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

Transmission LossTransmission Loss• Transmission losses shown below are based on the Transmission losses shown below are based on the

mass of the material rather than the kind of mass of the material rather than the kind of material. For example, a layer of lead gives material. For example, a layer of lead gives 95 95 timestimes the transmission loss of plywood. the transmission loss of plywood.

Porous MaterialsPorous Materials

• Porous materials such as fiberglass are Porous materials such as fiberglass are excellent sound absorbers, but they are of excellent sound absorbers, but they are of limited value in insulating against sound.limited value in insulating against sound.

• The transmission loss for porous materials The transmission loss for porous materials is directly proportional to the thickness. is directly proportional to the thickness. This loss is about 1 dB (100 Hz) to 4 dB This loss is about 1 dB (100 Hz) to 4 dB (3,000 Hz) per inch of thickness.(3,000 Hz) per inch of thickness.

• In contrast, solid materials like sheet rock In contrast, solid materials like sheet rock yield approximately 5 dB per doubling of yield approximately 5 dB per doubling of thickness.thickness.

Sound Transmission Sound Transmission ClassificationClassification

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

Comparison of Wall StructuresComparison of Wall Structures

Wall StructuresWall Structures

Wall StructuresWall Structures

Wall StructuresWall Structures

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

WindowsWindows

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

Sound Insulating DoorsSound Insulating Doors

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

Sound Insulating DoorsSound Insulating Doors

Some other Some other door designsdoor designs

Modal Resonances:Modal Resonances:Reflections IndoorsReflections Indoors

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

Two-Wall ResonanceTwo-Wall Resonance

• When a loudspeaker radiating When a loudspeaker radiating pink noise excites the space pink noise excites the space between the walls, the wall-air-between the walls, the wall-air-wall system exhibits a wall system exhibits a resonance at a frequency of fresonance at a frequency of f00 = 1,130/2L or 565/L, when L = = 1,130/2L or 565/L, when L = the distance in feet between the distance in feet between the two walls and 1,130 the the two walls and 1,130 the speed of sound in feet per speed of sound in feet per second.second.

• A similar resonance occurs at A similar resonance occurs at 2f0, 3f0, 4f0, etc., up through 2f0, 3f0, 4f0, etc., up through the spectrum. These the spectrum. These resonances are called resonances are called modesmodes..

Room ModesRoom Modes

• Each Each axialaxial mode involves only two opposite and mode involves only two opposite and parallel surfaces. parallel surfaces. TangentialTangential modes involve four modes involve four surfaces, and surfaces, and obliqueoblique modes involve all six surfaces. modes involve all six surfaces.