Acoustics & Noise Control in Design Space
Acoustically designed glass wall at a radio station allows light and visual access but separates sound.
In solving noise issues in buildings, it helps to take an objective, systematic engineering approach. Starting with a good understanding of sound, how it travels and how we hear it, we can then proceed to creatively applying a variety of solutions that adhere with building codes and acceptable standards.
Some basics on sound measurement and analysis
Sound is wave energy, and it is measured in two basic qualities: Amplitude and Frequency.
Amplitude is the intensity of sound (in physical terms, it’s the pressure of air being pushed by sound wave energy), measured in dB or decibels. This measurement helps give an idea of how loud a sound is in relative terms. 1dB is the smallest difference in amplitude that the human ear can discern. Most people have trouble discerning 3dB changes in amplitude. To achieve an easily discernible reduction in noise, a decrease of 20dB to 40dB is required.
Points of reference:
0 dB - threshold of hearing
20 dB - whisper
40 dB - quiet neighborhood
60 dB - normal speech
80 dB - busy office
100 dB - city traffic
120 dB - inside a loud dance hall
140 dB - air raid siren
160 dB - jet engine
180 dB - theoretically, a Saturn V rocket
Amplitude measurements are often based on a reference standard for a given purpose (for instance, radar, radio, antenna ratings, etc.). For the purpose of taking into account the sensitivities of human hearing over a wide band of frequencies, the “dBA” measurement uses an A weighting filter, which gives more weight to frequencies we are able to hear. We don’t hear low frequencies as well as we can hear higher frequencies, though they might produced at a similar power. A low 60hz tone @ 10dB, for example, will sound to us much softer than a 1,000hz tone @ 10dB. The dBA scale adjusts for those discrepancies.
Frequency is the pitch of sound. Physically, it is the rate of vibration in a sound wave. Frequency is measured in Hertz (Hz), or cycles per second. For example: 440 cycles per second is the musical reference note ‘A’ on a piano (A=440hz).
Ways to Control Noise
We typically use three classes of physical methods to reduce sound:
• Barriers: Dense materials to lessen sound transmission
• Absorbers: Soft materials to absorb specific frequencies
• Damping: Materials vary to deaden a specific materials’ resonant (ringing) properties
Remember: To make a significant difference in perceived amount of noise, you need to reduce by 20 to 40dB.
Barriers: The key for here is significant mass.
• Brick and concrete are great. Lead, a very dense material, is great. CMU is good.
• Gypsum wallboard (GWB) is standard wall building material, and is not great at stopping sound, but doubling the density can decrease sound transmission by about 6dB. In cases where you need to reduce noise, a second layer of 5/8” GWB is worth it.
• Of the GWB assemblies, double stud walls are the most effective, better than staggered stud walls. You might think stuffing the walls with soft fiberglass insulation helps but actually, it does not absorb sound as well as GWB.
Absorbers: Soft materials in architectural thickness’ help absorb high frequency sound. Different materials have vastly different sound-absorbing characteristics.
• Soft materials on the wall help break up first and second reflections: reverberation. They help make speech more intelligible.
• Acoustic Ceiling Panels (ACP) as a lay-in ceiling help as well. Additionally, the cavity above a lay-in ceiling helps reduce reflected low frequency sound. The same ACP panels glued to walls cannot absorb low frequencies since there is no space behind to mitigate low frequency energy.
• Carpet helps with high frequency sound and deadens footfalls within a space and cuts transmission of sound to spaces below.
Damping: Damping is typically not required in most buildings outside of recording studios and TV & Radio studio environments.
Acoustical standards of acceptability
When a building is put together and the acoustical design is considered, certain acoustical standards of acceptability are used to help ensure the building is built to achieve certain levels of noise reduction.
NC - Noise Criteria Standards
Noise criteria standards have been developed to provide target levels of noise at different frequencies for various types of spaces. Starting with a table such as the one below, you consider what NC level you want to achieve and then refer to charted NC curves (image).
NC 10: Approximate threshold of hearing for continuous noise.
NC 15 to 20: Recording or broadcast studios
NC 20 to 25: Concert hall, bedrooms
NC 25 to 30: Legitimate theater
NC 30 to 35: Courtroom, library, church or office
NC 35 to 45: Classroom, open office, restaurants
NC 45 to 55: Gym, pools, washroom
NC 50 to 70: Manufacturing areas
For instance, an engineer could check the actual measured noise of HVAC ductwork (either measured or provided by a manufacturer) for a bedroom, hospital, or hotel room, and see if it fit within the NC-20 to NC-30 curves. NC curves are the most commonly used set of criteria for setting the levels of noise to be expected from a completed design.
There is another set of criteria called NCB curves or balanced noise criterion, but these are more stringent and not very widely accepted. A good reference for standards is the American Society of Heating, Refrigerating and Air-Conditioning Engineers, or ASHRAE, an organization that helps set these types of standards.
STC (Sound Transmission Class)
STC measures acoustic isolation performance of an assembly, i.e., how much sound gets through typically a wall, but sometimes a ceiling or floor.
STC 25: Normal speech can be easily understood.
STC 30: Loud speech can be understood.
STC 35: Loud speech heard but unintelligible.
STC 42: Loud speech audible as a murmur.
STC 45: Loud speech not audible.
STC 50: Loud stereo faintly heard.
These ratings are determined in a laboratory, but typically out in the field, in order to get to this level of performance, the type of wall or ceiling/floor built should be rated about 10 units higher. Most building materials have STC ratings by the manufacturer, and some manufacturers provide building details that have been sound tested. The US Gypsum company puts out a manual that has GWB assembly ratings for common building assemblies (for example, they list sound ratings for a 2x4 wood stud wall with 5/8” GWB on both sides, a typical wall between two rooms in a house).
A couple other ways to measure sound:
IIC (Impact Isolation Class): These are standards for noise that are created by impact in a structure, like banging or footsteps. These numbers are relatively similar to STC numbers.
NRC (Noise Reduction Coefficient): A rating supplied by materials manufacturers for the absorptive properties of their product.
Environmental noise standards and regulations
Building codes use the varying noise standards to set acceptable levels in buildings. Codes change over time, but generally, these standards reflect what people are comfortable with. The major problem with code set standards is that materials and construction methods can be specified with laboratory tested measurements, but field conditions can make achieving actual noise levels difficult. Remember the rule of thumb to build to standards 10 units higher. This can be costly, so owners should be made aware as soon as possible.
The State of Connecticut regulates exterior noise control regulations through the Department of Energy and Environmental Protection. Local town governments are charged with setting noise ordinances and enforcing their codes. These relate to a person or business emitting sounds from their property to others’ properties. There are three classes: ‘C’ - Industrial, ‘B’ - Commercial, and ‘A’ - Residential. They are very specific about how to classify different types of noise. The criteria are clearly measurable.
Local Authorities usually have subjective regulations for external noise control. Usually a Judge needs to determine if a sound source is a nuisance.
A major source of noise from building components is the HVAC system. Since most buildings have some form of air ventilation or conditioning, noise created by the machines that move air around our spaces is of particular concern. In many types of spaces, it is one of the primary sources of occupants’ complaints. We pay close attention to sound attenuation (reduction) in ducts as well as the actual machinery (fans, pumps, condensers, etc). A few methods are outlined below.
In ductwork, a primary method to absorb sound is the use of parallel baffle modules. They are available at low cost in 3’, 5’, 7’ and 10’ lengths and make significant reductions in the sound of the air system (15 to 50dBA at certain frequencies). They should be considered at the beginning of the mechanical layout (as they take up space) and should always be used at both the supply and return of fans. Simply inform the Mechanical Engineers what Standard of Acoustical Acceptability you want for the building (NR Curves, etc.)
Sound traps (lined ductwork offsets) can reduce fan noise 10-15dBA. The larger the duct, the less the effect.
Centrifugal fans are generally quieter (lower RPM) than axial fans (higher RPM).
Equipment should be installed on isolation mounts. Spring-type mounts work well. If properly selected, they should be compressed 1 to 2 inches at rest. Neoprene pads and mounts work OK for smaller pumps and fans.
Obviously, don’t design quiet spaces to be located next to or under large equipment.
A note on room acoustics and reverberation. Reverberation in rooms generally reduces the intelligibility of speech. Small, controlled amounts are desirable for music. It is controlled with absorptive materials, so carpeting or certain kinds of wall and ceiling treatments can reduce reverberation.
Thanks to Lewis Bell, an acoustic and noise control consultant who presented much of the information here in a seminar. Author of the textbook Industrial Noise Control: Fundamentals and Applications, he has decades experience with aerospace, industrial, and architectural acoustics. He was responsible for the acoustic redesign of the auditorium space used for the presentation at Northeast Utilities’ Berlin facility.