For too long, too many people have only looked at sound from a not-to-exceed viewpoint. As a result, the systems in our buildings have become quieter and quieter. So quiet, that you can hear every distracting and annoying noise, and know far too many private matters about the people around you. As we move forward with designing buildings for the future, we must view the right level of background white noise as important as any other design feature.

What is background white noise?

Background white noise is what many people call the ambient noise, or background sound level, that is always present in our lives. Few people have experienced silence. It is not a pleasurable state. In fact, silence is quite uncomfortable because people are used to being bathed in sound from even before birth. Whether it is laughter, crickets or traffic, there is always background white noise or ambient sound present in our lives.

Even though many people refer to this background sound as “white noise” or “pink noise,” this is not technically correct. White noise and pink noise are used for certain acoustic tests, but not as background sound. Sometimes, the background sound is loud and intolerable. Other times, background sound feels protective and soothing. It all depends on what the sources of the background sound are and how loud they are.

Inside buildings, the background sound or ambient sound in large open spaces is often established by occupant activities. In private spaces or when occupant activity is minimal, the background sound is established by the building’s mechanical, electrical, plumbing and conveying systems. Unfortunately, the background sound in some buildings, especially in urban settings, is established by environmental noise, such as traffic or airplane flyovers, transmitting through the exterior windows or the roof.

Background sound inside buildings should be intentionally designed just like any other building design element. The alternative is an unintentional and unpleasant listening experience for people every minute they are in the building.

What are the desired characteristics of background sound?

Designing background sound is related to its purpose. Is it meant to aid in way-finding through an eldercare residence, comfort acute care patients, provide auditory access to nature, or help make private speech be less intelligible and distracting in close work quarters?

By definition, background sound is not foreground sound, meaning that you are not purposefully listening to it or gaining information from it. In many cases, background sound is not consciously noticed. Favorable background sound or ambient noise typically has the following characteristics:

• Low to moderate loudness
• Constant and consistent – any changes are very gradual over time and unnoticeable
• Broadband or contains many different pitches across the audible hearing range – tonal noises, such as sirens and alarms, are annoying and disconcerting
• No discernable or complex information, such as speech or musical lyrics
• No strong rhythms

Types of background sound sources

Some people automatically associate background sound with the building’s mechanical, electrical, plumbing and conveying systems, or with electronic sound-masking systems. These are only partially correct, being too narrow in scope to define background sound in its entirety.

Designed background sound can be divided into two basic categories – that which is meant to sooth people or comfort them mentally, and that which is meant to simply make speech less intelligible and distracting.

• Music: compositions specifically created for the purpose of soothing people in times of pain and comforting them mentally. This type of designed background sound is widely used in healthcare facilities of all types.
• Nature: sounds, especially water features such as waterfalls or rain sounds, are used frequently as background sounds in large public spaces including waiting rooms, lobbies, courtyards, atria, and cafes. The sound of the falling water is both soothing and provides excellent speech privacy, so groups of people talking cannot overhear each other. Some water features are designed specifically for the open office environment.
• Mechanical Systems: the background sound level in most buildings is established by mechanical systems and more specifically, the air turbulence noise of the supply-air blowing through the diffuser and the return-air flowing through the grille in the ceiling. This approach to background sound has proven to be unreliable because most engineers only design for maximum permissible noise levels. Even though the measured background sound levels are below the maximum permissible levels, they still vary greatly from room to room. Some are at the upper part of the range near the maximum value and provide acceptable sound masking. But others are far below the maximum value and do not provide sound masking. HVAC systems can be “tuned” to be more reliable, but it takes a constant air volume system and careful selection and specification of air diffusers and grilles. Over time, mechanical systems have become much quieter. They actually need to generate a little more noise to provide a louder background sound level.
• Electronic Sound Masking: What is sound masking and how can it be achieved? Electronic sound-masking systems are what most people refer to generically as white noise. The frequency spectrum, or pitch, of white noise, is not very pleasant. For this reason, electronic sound-masking systems shape the overall sound spectrum to be more palatable to the ear, while still masking noise and human speech for speech privacy. Electronic sound masking is used mostly in commercial office spaces, especially in the open office areas where many people can be disturbed by just one person on a telephone call. Sound-masking systems also are used in private offices to help keep people from being bothered by their colleagues talking loudly in the adjacent offices. Sound-masking systems come with a wide variety of capabilities from individual white noise machines that sit on your desk, to more sophisticated systems that are installed in the acoustic ceiling and provide not only background white noise but also paging functions and background music.

The result of designers not specifying the appropriate background noise masking in their buildings is the common use of earplugs and headphones, while people are going about their daily activities.

How are sound privacy and speech confidentiality defined and measured?

There are far too many different metrics to predict or measure the degree to which eavesdroppers can understand speech. Some methods measure speech privacy and the goal is to maximize the value. Other methods, such as Speech Intelligibility Index (SII), are designed to measure speech intelligibility and lower values mean greater privacy. The metric that is gaining more industry-wide acceptance and use, both by acousticians and in building standards, guidelines, and rating systems, is Speech Privacy Potential (SPP). SPP is favored because it is easy to understand, calculate and specify.

What is most important to know when determining how to block out noise? Sound privacy and speech confidentiality between enclosed rooms is dependent on the sound-blocking capacity of the architecture in relation to the background sound. If the architecture blocks a lot of sound, then the background sound or ambient sound does not need to be very loud. But as the sound-blocking capacity of the architecture decreases, the loudness of the background sound must increase to maintain the same level of privacy.

A good example is spending a night in a hotel room while traveling. The thin walls and gaps around the door to the corridor generally provide a low level of sound insulation. Noise in other rooms and in the corridor can be annoying and disruptive when trying to sleep. However, when you turn on the television, radio, bathroom fan or set the HVAC unit fan setting to “high,” the resulting background sound increases, masking the noise you were hearing previously.

Speech Privacy Potential (SPP) is an industry standard measure of sound privacy. An SPP value of 75 is a minimum goal. At SPP 75, you might be able to tell someone in an adjacent room is talking, but you cannot understand the conversation. At SPP 80, it is difficult to tell that someone in an adjacent room is even talking. To calculate the SPP you simply add the blocking capacity of the architecture to the background sound level.

Selecting the right background sound level

The goal of Speech Privacy Potential (SPP) is 75 to 80. The two variables are the sound insulation rating of the wall (STC) and the background sound level (dBA). Use the table below, based on the performance of the wall, to select the right background sound level.