Reducing HVAC Noise in the Classroom

By Consulting Specifying Engineer Staff December 22, 2004

Unwanted noise in school classrooms is a serious problem. The most obvious victims of noise pollution are students with hearing disabilities, but they are not the only ones. Noise impairs learning for students with other types of learning or behavioral disabilities or those for whom English is a second language. An often overlooked factor is temporary hearing impairment—on any given school day as many as 25% of the students in a class may be “hearing disabled” due to ear infections or other medical conditions.

According to the Acoustical Society of America (ASA), many U.S. classrooms have so much background noise that the speech intelligibility rating is 75% or less. This means that children with normal hearing can hear clearly fewer than three out of four words spoken. The adverse impact this has on learning is particularly severe for young children, who are less able than older students and adults to put what they hear in context and fill in the gaps. Once younger students miss a few words, they are likely to miss the whole message. The ASA report on classroom acoustics is available for free download at .

HVAC systems are notorious contributors to nuisance noise. Attempts to reduce energy use often aggravate acoustical problems in classrooms. Many schools rely on in-room heaters or portable air conditioners and heat pumps to cut energy consumption. This puts the source of equipment noise right next to, or even inside, the classroom. In the worst cases, this reduces to a negative number the signal-to-noise ratio, a number that defines how much of the teacher’s speech can be heard above ambient room noise.

The American National Standards Institute (ANSI) has developed a standard specification and design guideline that can help eliminate acoustical problems in the design stage. The standard, ANSI S12 60-2002, acts as a resource for architects, school superintendents, audiologists and even parents by identifying the minimum requirements for an effective learning environment. (See “ANSI S12.60: Practically Speaking,” CSE 08/04 p. 15) Industry experts will find it a helpful tool for selecting the right equipment and accessories for efficient acoustical design.

According to the ASHRAE TC 2.6 Noise & Vibration Committee, “Fiber glass duct systems insulation continues to be the most cost-effective solution to noise control in most HVAC air duct systems.” Fabricating ducts from fiber glass duct board or lining sheet metal with fiber glass duct liner insulation is an especially attractive option for schools. It is less expensive than other noise control solutions and requires no additional space. It is effective in reducing the transmission of equipment noise and the noise of cross-talk between classrooms. Unfounded fears about health issues have been laid to rest by a definitive determination issued by the International Agency for Research on Cancer, an arm of the World Health Organization.

Ducts with interior fiber glass insulation provide significant noise reduction. A common reference for measuring sound absorption of duct liner is the Noise Reduction Coefficient (NRC), an average of the sound absorbed at four different frequencies. For example, one-inch thick fiber glass duct insulation from one manufacturer has an NRC of 0.70, which means it effectively absorbs 70% of the sound at the most common frequencies. Two-inch thick duct liner from the same manufacturer has an NRC of 0.90, absorbing 90% of the noise. Fiber glass achieves this premium acoustical performance at a cost of pennies per foot. In the case of fiber glass duct board, the final installed cost is competitive with uninsulated sheet metal, with the added bonus of very low air leakage with normal installation techniques.

The benefits to schools of lining air ducts go beyond acoustical improvements. The thermal insulating properties of fiber glass help reduce energy consumption and promote comfortable temperatures. This thermal protection also prevents condensation from forming on the outside of metal ducts, eliminating a potential source of water damage and possible mold growth.

Educators and architects are beginning to understand the importance of classroom acoustics and the impact of unwanted noise on the ability to learn. The HVAC system can be a large contributor to noise pollution in schools. Investing time in the design stage and a few extra dollars during construction to reduce the noise from the air handling system can pay off handsomely in improved student performance.

For more information about how using fiber glass duct board or liner from Johns Manville can help reduce classroom noise, click here .