When an acoustical job fails to meet its soundproofing objectives, the contractor is often blamed for sub-par work. That blame is often misplaced. Designs that ignore flanking sound paths (such as ducts and pipes running through the space into an adjoining room and common surfaces between rooms such as a concrete floor slab) can substantially minimize the effectiveness of a properly sound-proofed wall assembly.

However, by following a new sound transmission standard, designers can ameliorate the condition. The National Research Council’s Institute for Research in Construction (IRC) has produced the rating system that assesses the privacy of meeting rooms. It goes beyond the standard STC ratings because the signal-to-noise ratio measurement accurately predicts the expected level of the audibility or intelligibility of the transmitted speech.

Although the concept of speech privacy has been around for more than 50 years, no standard procedures and acceptable criteria to ensure speech privacy had been developed until the IRC’s rating, simply called Speech Privacy Class (SPC).  It’s not a case of reinventing the wheel, however. “We haven’t produced new solutions to obtaining adequate sound attenuation; we have developed better procedures for more accurately rating speech privacy of rooms,” explains Dr. John Bradley, the IRC’s senior research officer who led the research initiative.

It’s an important development because people are increasingly demanding sound-proof meeting rooms. The IRC worked in partnership with Public Works and Government Services Canada and the RCMP to develop the SPC rating. Until now, the RCMP and federal government had limited success in the field. The work resulted in a new ASTM measurement standard (ASTM E2638) in 2008. It can be used at the design stage to specify design/construction requirements for speech privacy.

The idea, says Bradley, is to achieve more accurate assessments of construction, but he hopes that the new SPC standard will lead to better evaluations of designs so that the “true cause of problems” can be identified.   
Bradley says an STC measures the general properties of a wall. Constructing a wall with a high level of sound attenuation is relatively easy; not so easy with doors. An SPC rating measures weak spots like doors. Even the most expensive acoustical door won’t contain all sounds and if its seals are not maintained its acoustical properties will deteriorate in time. Ideally, a vestibule (double door system) is the sound proofing solution, but the additional space required is often a deterrent.

Bradley says a room’s SPC classification can be based on how confidential the information discussed in the room is. SPC ratings can measure the level of privacy or security of a room on a numeric scale from 70 (speech is frequently intelligible, sound is audible) to 90+ (speech unintelligible, sound is rarely audible). Alternatively, ratings can be based on categories ranging from minimal speech privacy to very high speech security.

The degree of protection, he points out, depends on two key construction-related factors: the sound insulation between the interior of the room and the position of an eavesdropper outside the room; and, the background noise level at the listener position.     

“You need a measurement system that identifies real problems, but doesn’t have problems of its own,” points out Bradley. Positive results from lab tests don’t always translate to positive results in the field. For example, a sound proof wall won’t be sound proof if it is build on a six-inch concrete slab because sound vibrations through the floor minimize the wall’s effectiveness. Moreover, pipes and ventilation ducts crossing from the room to another room act as sound transmission lines if the ducts are lined with insulation or installed with silencers.

Ottawa-based acoustical consultant State of the Art Acoustik Inc. has been involved in the development of the SPC rating.

“We’ve always had clients, especially clients in government, asking us if we can make our walls more secure,” explains the company’s Joey Jraige. “In the past they asked for higher STC ratings, but they found the rooms still allowed sound to pass through the walls.”

Bradley and his team have helped guide clients through the differences between speech privacy and sound isolation, adds Jraige.

Even though STC standards have been around for years, designers and contractors don’t always meet the standard because it can be a complicated process to meet. “Noise is like the best inspector because it will find every leak in a room,” says Jraige, noting that the firm has consulted on projects with poor sound attenuation where a layer of drywall was missing or a duct was improperly sealed.

An acoustical consultant for State of the Art Acoustik, Jraige points out that the problem with STC lab tests on wall assemblies is that conditions often are very different in the field. “Of all the walls I’ve ever tested I have rarely seen one without a penetration, be it electrical boxes, ducts, or doors, but you don’t see that in lab tests.” SPC testing is different because it focuses on points of sound transmission weaknesses in a wall assembly.

Jraige says wall and ceiling contractors don’t always have enough information to know how to stop sound transmission. For instance, installing insulation inside of an electrical box often won’t make a difference in a wall’s overall sound attenuation. However, wrapping the electrical box with a putty pad has proven to be an effective sound-deadening solution.

It is important to run ducts and pipes only through walls with doors, so as to not compromise noise-sensitive walls, he points out. Installing a vestibule with two doors radically improves sound attenuation qualities, but it may require a six- by 10-foot space. If security is the key issue, a vestibule is a necessity. “Without a vestibule you’ll have sound coming out of the doors into the hallway.”

Fine tuning is still required on the ASTM speech privacy standard, the acoustical consultant says, pointing out that it is a big step in the right direction. “When we do our tests we try to follow the guidelines but sometimes we also have to use common sense in the design to make sure it is successful.”

Jraige believes that more corporate and residential clients will seek STC ratings for their projects, but if their sound attenuation needs for meeting rooms go beyond the usual, an SPC rating might be preferable.

In a paper presented at a conference in noise control (Inter Noise 2009), Claude Fortier of State of the Art Acoustik says that the SPC is easy enough to perform, but it is a difficult test to explain vis-à-vis STC tests because it focuses on speech. “There is a narrow frequency range between no speech privacy and excellent speech privacy, which is difficult to convey because it does not form a part of the more common linear interpretation of sound measures such as noise and sound isolation previously being measured.”