Special Report: Sound Masking Part II
Containing the Beast, Isolating Noise

by Art Barkman

Once all sources of necessary noise—the unavoidable day-to-day background buzz of the workplace—are established, it’s time to control, contain and conquer. By understanding the principles of sound, and the sneaky ways it sometimes works, facility managers can implement solutions to mitigate its negative effects and significantly improve the quality of life at the office.

Barriers

We’ve already established that aside from structure born sound, sound travels by three paths: 1) direct, 2) reflected, and 3) diffracted. The direct path is the “line-of-sight” straight path between the source (people, machines, etc.) of the sound and the receiver(s). We block that path by placing barriers between point A and point B. The ability of those barrier systems and materials to block sound is measured by a test which provides performance criteria in terms of STC (Sound Transmission Coefficient). This is a single number that defines the average number of decibels blocked by that material or system. The higher the STC the better a material is at blocking sound.

In an open plan space those barriers are partial height and their barrier performance (STC) is not as critical as in situations where more isolation is required. Properly designed and specified, partial height barriers can provide an adequate degree of speech privacy for typical office usage. When a high degree of privacy and/or noise containment is required, full height walls must be used.

Leaks

The problem with full-height walls is that they can create a sense of false security with regard to the isolation they provide. A wall that appears to be a solid barrier may have weak points and/or flanking paths by which sound can find its way to the other side.

A wall with a STC of 45 will block 45 decibels. That’s a lot of sound being blocked. If that same wall has a 0.3% (less than 1%) line of sight opening, the STC drops to 25. Perceptually, that is a 75% loss of performance. It will be 400% louder on the other side of that wall than if that barrier wall were tightly sealed.

It’s not just clear (see-through) openings that diminish the efficiency of barrier walls. Combinations of weak points and other conditions serve to defeat an otherwise efficient envelope. Examples of these conditions and solutions are, for example:

Ceilings: Unless the wall extends to the slab above, sound will pass through the ceiling on one side of a wall, bounce off of the slab above and pass through the ceiling on the other side of that wall.

Solutions: Specify ceilings with STC equal to the wall. Add materials to the back of the ceiling panels to increase their STC (barrier blankets, trowel-applied coatings, etc.). Install a vertical plenum barrier between the top of the wall and the slab.

Wall corners: Where walls intersect outside window walls there may not be a tight seal at the mullion. Additionally, the filler piece above a perimeter convector unit between the wall and the exterior wall may be mostly cosmetic (a visual barrier).

Solutions: Use caulking. Add additional materials to these filler pieces. The idea is to have the mass (weight per square foot) equal to that of the rest of the wall.

Convector penetrations: Where perimeter HVAC units penetrate common walls, the hole created to pass the duct or pipe through can be substantially larger than that conduit. The unit metal cover (usually with vents) hides that violation from view, but not from sound.

Solutions: Make smaller holes. Caulk or otherwise seal openings with dense material (spackle, lead, etc.).

Doors: Typically, doors do not fit tightly in their jambs. Sometimes small rubber buttons in the door jamb are used to cushion the door to eliminate a “slamming” noise. These assure an air space between door and jamb. To avoid interference, doors are usually cut to provide a clearance of the carpet as they open and close. Gaps of ½- to 1-inch are not uncommon. These leaks work to defeat an otherwise tight, acoustically efficient wall. Hollow core doors often are not adequate.

Solutions: Use gaskets around doors. Use “drop” guillotine-like closures at thresholds. Use solid core doors (check STC data from the manufacturer).

Other penetrations and weak points: Back to back electrical outlets can present a weak point. The base plate of a traditional wall can provide a leak if it does not fit flush to the floor (floors are usually not level).

Solutions: Install electrical outlets two studs part. Use caulking under base plates.

Hint: If you want to find out where some of the weak points are in the acoustical “envelope” of a room, turn out the lights and look for light leaks.

Art Barkman is president of Sound Management Group, Inc., Hillsborough, New Jersey. www.smg-corp.com

Sound Masking Part I: Mission Shhh ... Combating Noise can be accessed in the Articles Archive, click here.