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An absorption coefficient of 1.00 represents a surface that reflects 100% of sound.
An absorption coefficient of 1.00 represents a surface that reflects 100% of sound.
False
Closed-cell insulating foams, with interconnected pores, make effective porous absorbers.
Closed-cell insulating foams, with interconnected pores, make effective porous absorbers.
False
A perfect reflector has an absorption coefficient of 1.0.
A perfect reflector has an absorption coefficient of 1.0.
False
Panel and resonant absorbers are broad-band in their absorption characteristics compared to porous absorbers.
Panel and resonant absorbers are broad-band in their absorption characteristics compared to porous absorbers.
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Marble, with an absorption coefficient of 0.01, absorbs or transmits 99% of the sound energy.
Marble, with an absorption coefficient of 0.01, absorbs or transmits 99% of the sound energy.
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Panel and resonant absorbers are best for controlling sounds that have a broad frequency range that is difficult to predict.
Panel and resonant absorbers are best for controlling sounds that have a broad frequency range that is difficult to predict.
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A suspended ceiling tile with an absorption coefficient of 0.80 reflects 80% of the sound energy.
A suspended ceiling tile with an absorption coefficient of 0.80 reflects 80% of the sound energy.
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Panel and resonant absorbers can be tuned to be effective at the unwanted frequency by adjusting the absorber’s weight, stiffness, or geometry.
Panel and resonant absorbers can be tuned to be effective at the unwanted frequency by adjusting the absorber’s weight, stiffness, or geometry.
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Materials with higher absorption coefficients are typically more smooth.
Materials with higher absorption coefficients are typically more smooth.
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A room constant depends on the surface absorption coefficient and the density of the room.
A room constant depends on the surface absorption coefficient and the density of the room.
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Porous materials are often more absorbent at lower frequencies than middle frequencies such as those found in speech.
Porous materials are often more absorbent at lower frequencies than middle frequencies such as those found in speech.
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Panelized assemblies like gypsum board over stick construction are typically more sound absorbent in high speech frequencies.
Panelized assemblies like gypsum board over stick construction are typically more sound absorbent in high speech frequencies.
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Thicker materials will always have a higher absorption coefficient than thinner ones.
Thicker materials will always have a higher absorption coefficient than thinner ones.
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Materials with an absorption coefficient of $\alpha$ greater than 0.50 are generally considered sound-reflective materials.
Materials with an absorption coefficient of $\alpha$ greater than 0.50 are generally considered sound-reflective materials.
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Materials with lower absorption coefficient values are typically smooth, dense, flush-mounted, and massive.
Materials with lower absorption coefficient values are typically smooth, dense, flush-mounted, and massive.
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We typically perceive an absorption coefficient change of less than 0.10 to be considerable.
We typically perceive an absorption coefficient change of less than 0.10 to be considerable.
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Porous absorbers are the most narrowband of the absorber types, and are therefore specified to deaden a room.
Porous absorbers are the most narrowband of the absorber types, and are therefore specified to deaden a room.
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Fibrous absorbers, including glass fiber and mineral fiber, are a subset of porous absorbers.
Fibrous absorbers, including glass fiber and mineral fiber, are a subset of porous absorbers.
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Porous absorbers translate acoustic energy to electrical energy at low frequencies.
Porous absorbers translate acoustic energy to electrical energy at low frequencies.
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At higher frequencies, sound energy is damped because of the friction encountered when incident sound waves weave through the interconnected pores of the absorber.
At higher frequencies, sound energy is damped because of the friction encountered when incident sound waves weave through the interconnected pores of the absorber.
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Absorption effectiveness of porous absorbers is only a function of thickness and density.
Absorption effectiveness of porous absorbers is only a function of thickness and density.
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The 'room constant' is measured in units called decibels.
The 'room constant' is measured in units called decibels.
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Total room absorption is calculated by summing up the absorption coefficients of all surfaces, regardless of their areas.
Total room absorption is calculated by summing up the absorption coefficients of all surfaces, regardless of their areas.
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Replacing 100 square feet of gypsum board with 100 square feet of a porous absorber having a much higher absorption value will greatly increase the total sound absorption in a room.
Replacing 100 square feet of gypsum board with 100 square feet of a porous absorber having a much higher absorption value will greatly increase the total sound absorption in a room.
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Adding 100 square feet of partial-height gypsum board partitions to a room with 112 sabins of absorption, where the partitions have α=0.04, will increase the total absorption to about 116 sabins.
Adding 100 square feet of partial-height gypsum board partitions to a room with 112 sabins of absorption, where the partitions have α=0.04, will increase the total absorption to about 116 sabins.
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A room with a higher room constant will have more sound reflection as it is more reflective.
A room with a higher room constant will have more sound reflection as it is more reflective.
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The average sound absorption in a room can be determined by arithmetically averaging the absorption coefficients of all materials present.
The average sound absorption in a room can be determined by arithmetically averaging the absorption coefficients of all materials present.
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If you placed a small tile of $\alpha = 0.80$ in a room with a large marble floor with $\alpha = 0.01$, the average room absorption will be closer to 0.40.
If you placed a small tile of $\alpha = 0.80$ in a room with a large marble floor with $\alpha = 0.01$, the average room absorption will be closer to 0.40.
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When calculating the area-weighted average absorption coefficient, you need to consider the surface area of each material relative to the total surface area.
When calculating the area-weighted average absorption coefficient, you need to consider the surface area of each material relative to the total surface area.
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The total surface area of a room is represented by the symbol $s_{total}$
The total surface area of a room is represented by the symbol $s_{total}$
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If a room has 100 square feet of wood floor with an absorption coefficient of 0.06 and 500 square feet of gypsum board with an absorption coefficient of 0.04, the area-weighted average sound absorption coefficient will be 0.06.
If a room has 100 square feet of wood floor with an absorption coefficient of 0.06 and 500 square feet of gypsum board with an absorption coefficient of 0.04, the area-weighted average sound absorption coefficient will be 0.06.
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Replacing a portion of gypsum board with a porous absorber will decrease the area-weighted average sound absorption coefficient of a room.
Replacing a portion of gypsum board with a porous absorber will decrease the area-weighted average sound absorption coefficient of a room.
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A recording studio typically has an average absorption coefficient around 0.02.
A recording studio typically has an average absorption coefficient around 0.02.
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Adding absorption to a room increases reverberance and sound energy within the space
Adding absorption to a room increases reverberance and sound energy within the space
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Sound-absorbing materials are used to increase reverberance in concert halls.
Sound-absorbing materials are used to increase reverberance in concert halls.
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Club music benefits from rooms with more sound reflections.
Club music benefits from rooms with more sound reflections.
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Variable acoustics can be achieved by using panels that either expose a sound-reflective surface or a sound-absorbing surface, or both.
Variable acoustics can be achieved by using panels that either expose a sound-reflective surface or a sound-absorbing surface, or both.
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The noise reduction coefficient (NRC) is used to summarize performance across several octave bands.
The noise reduction coefficient (NRC) is used to summarize performance across several octave bands.
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A higher NRC indicates a less absorbent surface.
A higher NRC indicates a less absorbent surface.
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The NRC is calculated by averaging absorption coefficients at octave bands from 125 Hz to 4,000 Hz.
The NRC is calculated by averaging absorption coefficients at octave bands from 125 Hz to 4,000 Hz.
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The average NRC for heavy carpet, as given, is 0.35.
The average NRC for heavy carpet, as given, is 0.35.
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Carpet is considered an effective sound absorber according to its NRC value.
Carpet is considered an effective sound absorber according to its NRC value.
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Absorption coefficients at 63 Hz are commonly provided in absorption coefficient tables.
Absorption coefficients at 63 Hz are commonly provided in absorption coefficient tables.
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The absorption coefficient at 125 Hz for heavy carpet is stated to be α125 = 0.08.
The absorption coefficient at 125 Hz for heavy carpet is stated to be α125 = 0.08.
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The NRC calculation provides a detailed understanding of the material's sound properties across the entire frequency spectrum.
The NRC calculation provides a detailed understanding of the material's sound properties across the entire frequency spectrum.
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Study Notes
Sound Absorption
- Sound absorption is the process of reducing the intensity of sound waves.
- Some surfaces absorb sound, while others reflect it.
- Absorption is measured by the absorption coefficient (α), a number between 0 and 1.
- A higher value indicates greater absorption and less reflection.
- An open window has an absorption coefficient of 1.00 because no sound is reflected.
- A perfect reflector has an absorption coefficient of 0.00 because all sound is reflected.
Absorption Coefficient
- The absorption coefficient (α) measures the proportion of sound energy absorbed by a surface.
- Higher values of α indicate more sound absorbed and less reflected.
- Materials with greater than 0.50 α are considered sound-absorbent; less than 0.20 are considered sound-reflective.
- Absorption coefficients vary across the frequency spectrum.
- The higher absorption coefficients are associated with more porous, less smooth, and less weighty materials, and those mounted over airspaces.
Types of Sound Absorbers
- Porous materials: These materials, including glass fiber, mineral fiber, and open-celled foams, absorb sound by converting it to heat.
- Porous absorbers perform similarly to wall-mounted absorbers, and offer sound absorption from both sides.
- Fibrous materials: These materials absorb sound through friction within interconnecting air pockets.
- Membrane absorbers: These materials absorb sound through changes in air pressure and acoustic impedance mismatch.
- Panel absorbers: These materials absorb sound by converting sound energy into mechanical vibrations and damping it in an airspace behind the panel absorber.
- Volume resonators: These systems comprise voids or cavities that absorb sound at specific frequencies.
Room Constant
- The room constant is the total amount of sound absorption in a room, measured in sabins.
- It's calculated by summing the product of the absorption coefficient and surface area of each surface.
- A larger room constant indicates more sound absorption.
Room Average Absorption
- The average absorption isn't simply the arithmetic mean of absorption coefficients.
- Area-weighted averages consider the relative surface areas of different materials to provide a more accurate representation of a room's overall absorption.
- Calculating the area-weighted average is done by multiplying each absorption coefficient by its corresponding surface area, summing the results, and dividing by the total surface area.
Noise Reduction Coefficient (NRC)
- The NRC is a single-number rating encompassing speech frequencies that indicate a material's absorption efficiency.
- It's calculated by averaging the sound absorption coefficients across the octave bands from 250 Hz to 2000 Hz, rounded off to the nearest 0.05.
- Common building materials' absorption coefficients can be looked up to determine NRC values for design purposes.
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Test your knowledge on sound absorption coefficients and materials. Explore the characteristics of different absorbers, their effectiveness, and how they can be used to control sound in various environments. This quiz is perfect for acoustics enthusiasts and professionals alike.
