The ABCDs of architectural acoustics
There are four main things to consider when controlling sound moving between spaces.
Absorb
The first and perhaps most obvious method of controlling sound energy is through absorption. This controls how sound energy is reflected within an enclosed space. This is generally accomplished with soft materials in an office space. Examples include carpeting, acoustic ceiling tiles, furnishings, and wall-mounted fiberglass panels. The presence of people and even some plants can also contribute.
While these materials will absorb sound, they do not attract sound energy—making absorption difficult because energy travels in all directions. Absorption only works on the energy that reaches the material, and it may not absorb all the energy and thus, allowing some to pass through.
Ceiling tiles in particular have two functional roles for architectural acoustics.
- They help absorb energy bouncing back and forth between walls in an enclosed room, which makes the room echo less and increases vocal clarity within the space.
- They absorb some sound energy radiating upward that would otherwise reflect off the hard deck back down into the room. However, this is not the tile’s primary purpose, and is a measure of its ceiling attenuation (CAC), rather than its noise reduction coefficient (NRC).
Block
Another obvious way to stop sound from reaching unintended ears is to block it—or more accurately, bounce it back into the room from where it is trying to escape. Blocking materials include walls, doors, windows, cubicle dividers, furnishings, and glass panels.
If an object is not an absorber, it is likely reflecting sound energy back into a space. As sound energy travels in all directions and bounces across the room, it will find its way through every gap, crevice, and hole in the blocking material. Sound energy contained within the room is called reverberation, and will bounce around until it escapes, decays, or is absorbed. This reverberation can be especially troublesome for teleconferencing systems since it makes the voices sound more distant to those on the other end of the call. (The ‘Auto Echo Cancellation’ feature does not apply to this kind of echo.) No surface is a perfect reflector either, and some energy will travel through the solid blocking material.
In simplified terms, STC is a measure of how much sound energy is blocked versus how much is transmitted through the material when averaged across a frequency range of roughly 125 to 5000 Hz. The higher the STC number the more energy blocked by the assembly. However, when working from the numbers specified for particular construction products, the entire assembly must be taken into account. For example, a very expensive STC-50 door will not function to specification if it is installed into an STC-30 wall. When working with modern demountable walls, that a manufacturer may provide a laboratory-tested specification of STC-45, but the field-tested performance will depend on the quality of installation labor and the attention to detail of the complete wall assembly installation. If any of the fit or finish is off-kilter and does not sit quite right, air gaps and uneven seals will decrease the STC rating.
Cover
Covering sound energy with more sound to increase privacy and reduce distractions can seem counter-intuitive, but it is very effective and is already happening in many office spaces to some degree. ‘Sound masking’ is the process of adding low-level background sound to an environment to promote speech privacy and freedom from distractions.
When using the photocopier, one may be unable to overhear conversations, but again discern discussions when the machine stops. This same principle applies when speaking in a loud environment. When background noise is loud, people raise their voices above it to be heard. This difference in sound energy between speech and background noise is the signal-to-noise ratio.
Normal conversational speech measures between 60 to 65 dB, while typical background noise levels
of a modern office space are about 30 to 35 dB—a difference of 30 to 40 dB between the signal (speech) and the noise. Lower signal-to-noise ratios mean it is harder to distinguish the signal from the noise, and higher ratios mean it is easier. A signal-to-noise ratio of only 15 dB represents near-perfect speech intelligibility, and 30 to 40 dB is fantastically clear—better than an emergency notification system would be expected to achieve. It is important to remember a 10-dB change is 10 times the sound energy, so 30 dB is 1000 times more acoustical energy.
For offices, the goal is to have very low signal-to-noise ratio across the critical range of human hearing. Introducing sound masking to increase the normal background office noise from 30 to 35 dB up to between 42 to 48 dB, and specifically tuning the masking noise to the human range of speech, can have a huge impact on increasing privacy. The key difference between covering speech versus absorbing or blocking it is masking covers the sound energy that has already escaped other attempts at containment. Essentially, sound masking anticipates the shortcomings of attempts to absorb or block sound.
A common misunderstanding when considering sound masking is its placement. For instance, placing the speakers inside the room to be masked is incorrect. Masking relies on the voices to be weakened by distance and other obstructions. Therefore, the sound masking speakers should go where the overhearing ears are (Figure 2). The sound is already weaker at this location so a little increase in background noise makes it unintelligible. To mask the sound from the source the location of the person speaking would have to be masked. The background noise would have to be louder than the person speaking, and then nobody would be able to hear him or her—even inside the conference room. Instead, the sound masking is positioned to prevent the unintended listeners from hearing something they do not want, or should not, hear.
