What You See Is What You Hear
If you have ever been in a space with loud mechanical equipment, you likely have looked left and right, and up and down to find the source of the noise. Learn more about the reactions of sound and sight in spaces with this quick 2-3-minute read. This blog post provides a quick overview of how sound is perceived and how people respond.
Evaluating and reducing mechanical equipment noise is a common practice in acoustical consulting, but this practice gets trickier when dealing with noise from outdoor equipment, especially when it is located at ground level. Not only does noise infiltration to a building have to be managed, but cognizance of sound levels at the property line and the impact of the noise on nearby bystanders is critical, as well. Sound data and calculations are always necessary for a thorough acoustical review, but using the principle of “out of sight, out of mind” to preliminarily plan for noise mitigation can go a long way.
Psychoacoustics is the study of sound perception and psychological responses to auditory stimuli. Although, individual perception of sound varies wildly, there are some concepts that generally reign true for most, one of which is the idea that the brain can be tricked into thinking noise is quieter when it cannot visually identify the source. How can this help with mechanical equipment noise? In situations where mechanical equipment must be located outside and on-grade, if line-of-sight to the equipment can be broken, there is a good chance that the noise will be perceived as less disruptive. The two primary methods acousticians look at to accomplish this are sound barriers and sound enclosures.
A sound barrier consists of acoustically treated panels that partially surround mechanical equipment but do not fully encase it. Since sound barriers do not completely close-off the equipment, mechanical noise can still flank around the barriers, but the visual separation creates a perceived sound reduction greater than what would be objectively measured. A sound enclosure, on the other hand, fully encompasses mechanical equipment. Sound enclosures trap equipment noise inside, so while they break line-of-sight to mechanical equipment, they have the added benefit of lowering actual radiated sound levels. Although sound barriers do not objectively perform as well as sound enclosures, there is a lot to be said for the perceived benefit they provide, and they should not be written off. Since barriers require less material, they can be lower cost, so getting a client on board with a sound barrier may be a more attainable goal.
What else can be done to keep equipment out of sight? Locate equipment away from windows and balconies. Just like breaking line-of-sight can cause a perceived noise reduction, creating line-of-sight has the opposite effect. If mechanical equipment is visible from a window or balcony, it might sound louder than it otherwise would. It can also be jarring to exit a quiet building and immediately be exposed to mechanical noise, so keep equipment away from major points of egress. And finally, always feel free to contact your friendly neighborhood acoustician to review your equipment and offer recommendations for your specific scenario.
- Everest, F. Alton, and Ken C. Pohlmann. 2009. “The Perception of Sound.” In Master Handbook of Acoustics, 5th ed. New York: McGraw-Hill.
- Wilson, Charles E. 2006. Noise Control, rev. ed. Malabar: Krieger.