By: Zachery L'Italien, MCH Senior Acoustical Consultant
All occupied interior spaces require some degree of Heating, Ventilation, and Air Conditioning, i.e., HVAC. One downside to these systems can be their noise generation, so this early consideration is important for all spaces sensitive to noise.
Mechanically speaking, these systems require that sufficient clean air quantities are supplied and returned (or exhausted), and that air distribution is provided throughout the space.
Where speech, music, and the like must be clearly intelligible and enjoyed, HVAC noise must be appropriately controlled to avoid disruptive masking. HVAC noise control begins with these considerations:
1. Equipment Selection
Selecting quiet equipment is often one of the most effective means of noise control, and makes subsequent recommendations easier, with much less emphasis on noise control devices. For example, we often recommend reselection of air handling units if the sound power level data is very loud in the low frequencies. A reselection can often minimize the need for lengthy silencers.
2. Equipment Mounting
Mounting equipment to stiff structure is key to providing effective vibration isolation. It is also important to consider mount equipment in locations minimize impact to noise-sensitive spaces. For example, mount equipment above a bathroom, rather than directly over an audience chamber or conference room.
3. Airflow Velocities
Ductwork and other devices can generate noise by way of airflow turbulence. In the course of design, airflow velocities should be carefully considered, as well as well-considered ductwork layouts in order to minimize turbulence. See my earlier blog, which summarizes airflow turbulence.
4. Internal Duct Lining
Internal duct lining is among the most cost-effective tools at our disposal. While largely ineffective in low frequencies, it provides considerable attenuation in the middle and high frequencies.
5. Volume Dampers
These are noise-generators. Generally, integral dampers on diffusers/registers should be avoided where quiet is required. Instead, dampers should be placed several feet (at least) away from diffusers/registers. Typically, lining or acoustical flex duct between dampers and diffusers/registers should be employed. In spaces with high sensitivity to noise, such as recording studios or theaters, it is advantageous to size ductwork such that the system is “self-balancing,” thereby minimizing the need for dampers.
6. Duct Silencers
Silencers are useful in mitigating HVAC noise, especially where challenges to providing sufficiently quiet equipment arise. Silencers should be used judiciously, such as when low-frequency noise control is needed, and also should be placed such that they do not generate excessive noise or static pressure. Worth noting is that silencers are often used to address sound isolation as well.
Takeaway
This blog covers only the tip of the HVAC noise iceberg. All systems have specific noise control challenges, each with unique solutions. Excessively loud HVAC systems can have negative impacts on intelligibility, occupant comfort, and enjoyment.
To save time and costs, consult with your project acoustician early in design to ensure that your project’s noise goals are met.
