Many people used to judge the “quality” of a new car by the sound generated when the door was closed. A smooth, solid “ca-thunk” was a sign of a well-built automobile, while inferior vehicles were identifiable by a hollow or “tinny” sounding door close. While this may sound silly, it does reflect an important aspect of sound quality in the environments which we live, work, and play.
Take a moment and listen to the world around you and try to identify individual sounds. For example, if you’re in an office, listen for the clack of a coworker’s keyboard; the beep of the break room microwave; or the howl, hiss, or rumble of the HVAC system. If you’re at home, listen for the hum of the refrigerator; the throb of the dishwasher; or the clatter and groan of the heat or AC kicking on. All of these noises contribute to the overall sound level within a room, but each has a unique sound quality.
What is sound quality? All noise sources have a unique sound signature that is determined by the frequencies generated, their relative strength and how the sound varies over time. Noise that includes sound over many or all of the audible frequencies is considered broadband noise, while noise that consists of one or more prominent frequency peaks is considered tonal noise. When a tonal noise is a higher frequency, it is often described as a whine or chirp. A tonal noise that is in a low frequency is often described as a throb or a rumble.
Why does sound quality matter? Too often, designers focus solely on the overall noise level within an environment. This is commonly expressed in decibels, weighted to account for human hearing, or dBA. By simplifying the background noise to a single number, the sound quality is typically neglected, despite being a significant, if not equivalent, contributor to a user’s perception of sounds.
Obviously, loud noise levels can have an adverse effect on occupants, regardless of any other quality of the sound. However, even if the noise levels are acceptable sound can still provide a significant source of annoyance. While the human brain is very good at disregarding broadband noise, we typically notice and are irritated by tonal noises.
How does knowing this help us to design better environments? It starts with analyzing the intended use of the space, and then examining what items in the space will contribute sound. Quiet is always better, right? Not necessarily! While quieter is typically better in sleeping areas, imagine if your office had no background noise from building systems. In such an environment, you can clearly hear every conversation a colleague has, every time they drag their foot across the floor, and every time they click their pen. In office environments, some background noise is desirable to mask intrusive noises from coworkers.
In both the home and at work, if tonal or unsettling noises are present, occupants will be disturbed. Avoiding or attenuating tonal noises requires evaluating the full frequency spectrum generated by each noise source, not just the overall level. Many HVAC units, especially when located very close or within the occupied space, generate both broadband noise and tonal noise. Unit selection should take this into account. Avoiding devices with strong tonal noise, or attenuating their levels below the background noise level, will improve the occupants’ satisfaction with the auditory environment.
Designing spaces that people want to spend time in requires addressing all of our senses, including hearing. This requires careful analysis of the physical space, its intended use, and all potential noise sources. To produce a well-rounded environment, designers must address not only the overall sound levels in a space, but also the qualities of that sound.