Good Design for Architectural Acoustics
Good design for architectural acoustics can be relatively straightforward when applied in the design phases of the project. Acoustics problems can be very difficult to fix, however, after a facility is built.
Rather than consult with a qualified acoustics consultant in the design phase, an owner or architect often will call in an insulation contractor to try to fix noise and sound isolation problems by applying fiberglass or mineral wool insulation to noisy ducts or piping or above a drop ceiling. This approach—“sticking some acoustics” on a problem—is rarely successful. This article discusses what can-and cannot-be accomplished using insulation in noise control.
The Properties of Sound
Sound is a pressure wave. It may be helpful to visualize sound waves like a slinky. Imagine a column of air traveling through space, alternately compressed and rarefied.
The speed of sound is approximately 1,130 feet per second. Compare that to the average speed of air in a ducted system, on the order of 1,100 feet per minute, or 1/60 the speed of sound. Sound travels about as well upstream as it does downstream within ductwork, and fans are perceived as noisy on both the supply (downstream) and the return (upstream).
Sound waves have size, too. Though people normally talk about sound in terms of frequency (Hz), the wavelength (size) of a particular sound partially determines the means needed to treat it. The performance of sound-control treatment depends on the dimensions of that treatment relative to the wavelength of the sound.
For example, high-frequency sound—such as the high-pitched hiss one might hear as air leaks from unsealed ductwork—has a short wavelength. The dimension of the wavelength of this sound is about the size of the seam in the duct. Noise in this frequency range can easily be treated with mineral fiber insulation.
On the other hand, low-frequency sound—such as the rumble one hears when standing next to a large fan or air-handling unit—has a long wavelength. About the size of the machine itself, this cannot be successfully treated using mineral fiber alone.
The loudness, or amplitude, of a sound wave is usually expressed in decibels (dB). Sound measurements in dB are typically made in each of eight “octave bands” covering the entire audible range from very low to very high frequency. A particular sound-level measurement typically yields eight separate dB values corresponding to the loudness in each of the eight octave bands. These eight octave-band measurements are simplified into a combined single-number representation known as dB(A). A particular eight-octave band sound measurement describes a sound spectrum, and the single-number simplification of dB(A) provides a fairly easy way to show how loud various noises sound to the human ear. It should be noted, however, that dB(A) is a simplification used for convenience.
Other Useful Terms
Structure-borne Versus Airborne Noise
Audible noise frequently results from vibration transmitted into the building structure and re-radiated out elsewhere as noise. This is structure-borne noise. Noise that results from structure-borne vibration cannot be effectively treated using insulation materials, or even using additional massive construction. This class of problem can only be treated using vibration-isolation and/or
Transmission Loss (TL) and Sound Transmission Class (STC)
TL is a measurement applied to common structures and components used for containing sound-including doors, walls, windows, floors, and ceilings—and to composite external wrapping of equipment, piping, and ductwork commonly referred to as lagging. Both TL and STC are ratings of the amount of sound blocked from passing through a structure. They are most often applied to wall construction, and door and window types.
Sound travels wherever air can travel; unless a wall is not airtight, sound does not really go “through” it. The mechanism works more like this: Airborne sound strikes one side of a solid wall, the wall responds by vibrating, the vibrating backside of the wall becomes a radiator of sound, and sound becomes audible on the other side of the wall.
STC is a single-number simplification of the amount of sound transmitted through a structure. It is strictly limited to speech frequency and is not at all applicable when discussing noise problems related to mechanical equipment, which typically occur at lower frequencies. When comparing STC ratings of various partitions, it should be noted that equivalent STC ratings do not imply equivalent performance at all frequencies.
Here are some common STC ratings:
Sound Absorption and Noise Reduction Coefficient (NRC) Rating
NRC rating is commonly applied to acoustic tile ceilings and fabric-wrapped acoustic wall panels.
When a sound wave strikes a surface, one of three things can happen: The sound is reflected away from the surface, is transmitted through the surface, or is absorbed by the surface. The NRC rating of a material is a measure of the amount of sound absorbed by that material in a particular installation and within a certain range of sound frequencies. The higher the NRC rating, the more sound is absorbed by that material.
Noise criterion (NC), room criteria (RC), and dB(A) are all single-number simplifications that describe background noise level. Each is a representation of the sound spectrum, and NC and RC are often used as design targets for sound-sensitive spaces. The higher the number is, the louder the background noise levels. Here are some examples:
In addition, dB(A) often is used for outside noise and in municipal ordinances to describe maximum permissible noise level at a property line. The chart below shows some common equipment noise levels. Note which frequency dominates in each example.
The dominant audible noise frequency of the transformer appears in the 125 Hz octave band. Fiberglass insulation is most effective in treating noise at 500 Hz and above. The transformer does not generate much noise in the upper frequency bands, so it is difficult to treat its noise using fiberglass alone. The airborne noise level of 70 dB at 63 Hz usually can be contained by normal construction methods. However, the transformer should be mounted on vibration-isolation mounts to keep the 125 Hz hum out of the building structure.
The dominant frequency of the screw chiller appears in the 250 Hz octave band. Mineral fiber insulation alone is not particularly effective in the 250 Hz frequency band, so wrapping a chiller in mineral fiber will not do much good. One can install a chiller in a room lined with a 2-inch-thick insulation system and achieve significant noise reduction in the room. No amount of treatment within the room will keep sound energy at 250 Hz from transferring through the building structure and reappearing elsewhere as noise, however.
In any sound-sensitive application, a chiller like this must be supplied with vibration-isolation mounts. In addition, structural isolation of chiller rooms often is required in sound-sensitive applications.
The dominant frequency of the rooftop air-handling unit appears in the 63 Hz octave band. Fiberglass alone is ineffective in treating noise at 63 Hz. However, sufficient length of 2-inch internal fiberglass duct lining can provide significant noise reduction in all of the upper frequencies, and it makes an enormous difference in the amount of audible duct-borne fan noise transmitted by an air handler like this.
Areas Where Fiberglass Insulation Is Effective In Treating Sound
Insulation contractors are often faced with the challenge of solving equipment noise problems, improving sound isolation, or managing other acoustic problems in buildings after design and construction are complete. Unfortunately, most acoustics problems are difficult to address postconstruction. Of those that can be dealt with at this point, only a few can be successfully addressed by an insulation contractor. These are identified and discussed below.
Sound Treatment of Mechanical and Electrical Equipment Rooms
The noise output from multiple sources in mechanical and electrical rooms can reach unacceptably high levels. This noise can be easily transmitted through the walls and break through duct walls. It is also transmitted via conduit and piping as vibration. Lining the interior surfaces of the mechanical room with an absorptive material can be beneficial. Normally, 2 inches of 3 pound/cubic foot density fiberglass board is recommended. Typical application would consist of 50-percent coverage evenly distributed on the walls and 100-percent coverage on the ceilings. If a room is subject to abuse (where the fiberglass board would be damaged), the wall finish might include cementitious wood-fiber panels on a C-40 mounting over the fiberglass board.
Improving Interior Room Acoustics and Speech Audibility
Similar to (and more familiar than) sound treatment of mechanical rooms, mineral fiber treatment of occupied spaces provides control of echo and reverberation in a room. It also greatly improves speech audibility in these spaces. Products used in these spaces include high-NRC acoustic ceiling tile, fabric-wrapped acoustic wall panels, and occasionally mineral fiber batt insulation in the ceiling cavity. These materials are not typically provided by the insulation contractor, however.
Improving Performance of Wall Construction
There is a relationship known as mass law that states that the TL performance of a wall is related to the mass of the wall (among other factors). The end result of this relationship is that one achieves a 5-dB improvement for every doubling of the mass of the wall.
So, for example, at 1,000 Hz, a 4-inch dense poured concrete wall provides roughly 44 dB of transmission loss. An 8-inch dense poured concrete wall provides 49 dB. A 16-inch dense poured concrete wall provides 54 dB, and so on. If constructed strictly of dense poured concrete, a high-performance wall quickly starts to become massive.
If, on the other hand, a lighter-weight double wall is provided, with a mineral fiber-filled airspace in between two 8-inch solid core concrete block walls separated by an 8-inch airspace (and not structurally connected), one would have 10 to 15 dB better performance at 500 Hz and above than a single 12-inch, solid-core concrete block wall.
There are other tricks to this type of construction, but the end result is that the internal airspace provided with absorptive material such as mineral fiber will “decouple” the two walls and radically improve performance of the partition. This type of wall construction has to be designed into the project, though. There is little chance of solving a sound-isolation problem in an existing wall via the introduction of fiberglass.
An additional, often overlooked step in providing the best possible sound-isolation performance in wall construction is the proper airtight sealing of all service penetrations. This includes ductwork; sprinkler piping; domestic and circulating water; waste lines; and conduit penetrations in walls, floors, and ceilings. All of these penetrations can transmit airborne noise and structure-borne vibration, depending on what they are connected to.
All penetrations in acoustically sensitive areas, as well as mechanical and electrical equipment rooms, should be acoustically sealed airtight to minimize noise and vibration transmission. Most pipes and conduit less than 1½ inches in diameter can be grouted airtight into the wall, but other penetrations should be sleeved so that they are ½ to 1 inch larger than the object penetrating the structure. The resulting clearance should be packed loosely with glass or mineral fiber and should be caulked on both sides with a nonhardening, resilient acoustic caulk.
Although these penetration details typically fall to the general contractor, there can be a fair amount of mineral fiber packing required for proper sealing of penetrations in acoustically sensitive applications.
Duct Liner and Sound Attenuators
Attenuation of duct-borne fan noise can be accomplished by installing a combination of prefabricated sound attenuators and internal duct lining. Standard coated 1- and 2-inch duct linings should be used in all low-pressure ductwork serving acoustically sensitive spaces. Half-inch duct lining should never be used, for acoustical reasons. The lining should be coated and have a density between 1½ and 3 pounds per cubic foot.
As noted earlier, because the speed of sound is much higher than normal system velocities, sound travels upstream almost as well as it does downstream. Thus, fan noise control is as important in return ductwork as it is in supply ductwork. Attenuators and duct lining are usually recommended on both the supply and return sides of the fan system.
Although this is a good example of how fiberglass is used well to control noise, fiberglass duct liner and fiberglass-filled prefabricated sound attenuators are typically provided by the sheet metal contractors.
External Wrapping of Noisy Pipes and Ducts
Wrapping noisy ducts and pipes with fiberglass alone is almost never successful in treating noise problems. This is because the noise problem is almost always a mid- to low-frequency problem, with a wavelength that is too large to be treated by an inch or two of fiberglass. However, coupling the insulation with an external massive barrier can have significant results. This is an approach insulation contractors can take when faced with a similar noise problem. See the example below.
This may be the ideal situation where an insulation contractor can provide a useful reduction in noise after construction is complete. The technique can be applied for noisy circulating water piping, waste lines, rain leaders, and even duct work. When lagging duct work, an airtight, multilayer gypsum board soffit should be substituted for the loaded vinyl external jacket.
Areas Where Fiberglass Insulation Is Not Effective in Treating Sound
Areas where the installation of fiberglass insulation is not effective in treating sound are discussed below.
Mechanical Room Doors and Penetrations
No amount of insulation mounted to the walls of a mechanical room will overcome the effects of a poorly selected or ungasketed door. Where doors to mechanical rooms adjoin acoustically sensitive spaces, they must be gasketed at the head, jambs, and threshold. Such doors must not be supplied with louvers, undercuts, or transoms.
In addition, air intake and exhaust air openings must be located where mechanical noise cannot reenter the building through doors, windows, ventilators, or smoke vents; and where it will not affect use of outdoor spaces.
As in the case of installation of both screw chillers and large air-handling units, mechanical room noise is often intense enough to excite the mechanical room surfaces. If these surfaces are structurally continuous with those of acoustically sensitive or critical spaces, noise and vibration may be transmitted to those spaces. In such cases, there must be structural separation between mechanical rooms and acoustically sensitive or critical spaces. If structural separation and massive double construction (including floating concrete floors) are required, simply applying insulation will not work.
Noisy Grilles, Registers, and Diffusers
Mid- to high-frequency noise at the face of a supply air diffuser or return air grille is usually generated at the device. High-velocity air flowing over the face of the diffuser or grille makes noise. The more restrictive the design, the more noise it generates. There is little that can be done by the insulation contractor in these situations, other than to suggest reselecting the grilles and diffusers.
Although much depends on the design of the diffuser or grille, the following recommendations are for air velocities in supply and return branch ducts for various target-background-noise levels.
Vibration Isolation of Mechanical and Electrical Equipment
Almost anything that has moving parts or handles the flow of gas or liquid vibrates to some degree. Rigidly attaching any vibrating machine or conduit to a structure will transmit that vibration into the structure. This vibration will likely manifest itself as audible noise above, below, adjacent to, or perhaps (surprisingly) far away from the offending piece of equipment.
Good candidates for vibration isolation include:
- Air-handling units
- Large circulating water piping (connected to pumps)
- Rooftop air handlers and fans
To isolate equipment vibration, mount the equipment on properly selected spring or neoprene isolators, then attach all connected services (ducts, pipes, and conduit) resiliently so that the main piece of equipment is free to “float” on the isolators. Often when there is a loud rumbling or tonal noise in a space adjacent to a piece of equipment like this, the problem is with the vibration isolation of that equipment. In this situation, all an insulation contractor can do is point out to the owner or architect that structure-borne vibration may be the source of the noise due to direct coupling of the equipment to the building structure.
Insulation materials are often used to solve acoustics problems. They can be useful in controlling room acoustics parameters like reverberation and echo, and for controlling the buildup of sound, or loudness, within a space. They also can help reduce the loudness of sound within a mechanical equipment room or improve speech intelligibility within a classroom. Insulation materials can improve sound isolation between adjacent spaces, and quiet noisy pipes and ductwork, but this is usually effective only when used in combination with more massive materials.
Mineral fiber insulation materials are of no use, however, in the control of generated noise problems at grilles, registers, and diffusers, or for the control of structure-borne noise and vibration. If a contractor, owner, or architect suspects a vibration-isolation or structure-borne vibration problem as the source of a noise problem, he or she should get advice from an experienced acoustics consultant.