Masking Sound

In a busy room full of people, so much noise is generated in the frequency range of human speech that only the closest, most attentive listener can understand what you say. A spy will turn up the radio before holding a conversation in a possibly bugged room for the same reason. Background sound that is close to the frequency of speech reduces the intelligibility of speech.

Noise that carries information reduces the productivity of office workers. What we hear depends on the level of attention to what we are doing and to the in-

trusiveness of the outside sound. In a very quiet space with no background noise, any sound is distracting. With a constant ambient sound level in the listener's room, sound transmitted from another room is masked, becoming inaudible, or simply less annoying.

Where it is too costly or too difficult to treat a build- i( ing for a persistent or distracting noise source, low-level masking noise may help. Masking sounds are also useful in rooms that are so quiet that heartbeats, respiration, and body movement sounds are annoying, as in a bedroom where small noises disturb would-be sleepers. Natural sounds, like waves against a shore, wind through trees, an open fire crackling, the sound of rain on a roof, or a brook or fountain splashing are sometimes used. Sometimes, a slightly noisy ventilation system works, but most systems run irregularly and can themselves be distracting.

Because heating, ventilating, and air-conditioning (HVAC) system background sound levels rarely provide the consistency and spatial uniformity necessary for speech privacy, practically all open-plan office installations use carefully designed electronic masking systems to provide uniform background sound at the proper level and with good tonal characteristics. These masking systems are usually operated at or near the upper limits of acceptability for average building occupants, around 50 dB. Higher masking sound levels make the masking sounds themselves a source of annoyance.

A masking system consists of a signal (noise) generator, an equalizer for shaping the signal, an amplifier and controls, and a distribution system for feeding the speakers, which can be hung above a suspended ceiling, mounted in the ceiling, or wall-mounted. Volume is controlled remotely and can be automatically reduced after-hours to a level that won't bother the few remaining workers.

The sound produced by an electronic masking system is white noise, which has been described as the noise of air rushing through an opening, the noise of water in piping, or a whooshing sound. It can be tailored to the user's preference with filters in the equalizer. The sound is usually tuned to emphasize lower frequencies, which avoids high-frequency hissing noises.

For use in offices, electronic sound-masking units are often hung above the ceiling where they are completely out of sight. The sound masking fills the plenum area and then gently filters down through the ceiling tiles into the office space below to unobtrusively raise the background sound level. The speakers can be adjusted to the individual acoustical comfort requirement in any given area. The units are about 15 cm (6 in.) in diameter and about 20 cm (8 in.) tall with a chain for

hanging (Fig. 54-4). The number of units required depends on the size of the area to be masked. On average, one unit can cover approximately 21 to 23 square meters (225-250 square ft). The units are easy to install, maintenance free, and have negligible operating costs. Sound-masking systems cost about $1 per square foot of space covered.

Some sound-masking systems also offer paging and music, but this is not recommended, especially if the masking sound shuts off during announcements, as it will be noticed when it is turned back on. The masking sound should blend into the background, and its hidden quality should not be disturbed.

Suspended ceiling systems are now available that not only incorporate wireless systems for office communication, but also include sound systems that deliver sound masking, paging, and music simultaneously, without shutting off the masking sound. All three modes are delivered through the same set of speakers that blend invisibly into the ceiling plane. This eliminates the need for redundant systems.

The Public Building Service of the U.S. General Services Administration (GSA) sets criteria and standards for the design, specification, and evaluation of systems and components for open office spaces in federal buildings. The GSA's speech privacy potential (SPP) rating is a summary of background sound level and attenuation between typical source and listener locations. You may encounter this rating when doing work in federally owned buildings, and when specifying materials that are marketed to government agencies.

Ceiling-mounted masking system loudspeakers b should not be visible, as they attract interest and eventually become annoying. They can be placed face-up in a plenum to increase dispersion and improve uniformity, but should not be mounted face down in the ceiling. Most ceiling tiles will allow masking sound to penetrate to the office area below.

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