Sonic concealment
Apr 1, 1999 12:00 PM, Duke Ducoff
There is a Monty Python sketch whose hero, Dennis Moore, robs from the rich to give to the poor. Dennis is so proficient that, after a while, the poor are wealthy, and the rich are destitute. When confronted with this problem, Moore delivers the punch line, "This distribution of wealth is trickier than I thought."
Many sound contractors reach the same conclusion when asked to provide a simple masking system for an open office complex. A contractor may say, "No rocket science needed here. Throw some 8 inch (203 mm) loudspeakers in the ceiling, get a noise generator and an EQ, and have enough power for the loudspeakers. There's a special on a 1 RU EQ. Tell the customer payment is due in 15 days, no problem."
The phone rings just as he gets back to the office. The customer is having problems with noise being so loud in some areas that people want it turned off. In other areas, employees can still hear one another's conversations. This is where the fun truly begins. This scenario may be exaggerated, but it illustrates why there is a dedicated group of acoustic consultants that does sound masking almost exclusively.
Did the contractor have a chance? The odds were certainly stacked against him when he was brought into an office complex that was already furnished and laid out. It is important to be included in the design stage. There will usually be several meetings that include the HVAC designer, the architect, the acoustic designer and the owner, all of whom need to tackle such issues as what kind of lighting and sound levels are required, how close or far away the cubicles will be to each other, how the work will migrate through the office, what kind of background noise there will be, and what type of furniture and artwork will be used. Where cubicles are placed and spaced relates to the overall equation.
Industry ratings Many people in our industry have seen noise reduction coefficient (NRC) ratings as they are applied to sound panels. These have an absorption rating for sound at 1/3-octave frequencies between 250 Hz and 2 kHz from 0 to 1 (100%). Many people have been dismayed to see some products rated higher than 1. Higher frequencies are normally more easily absorbed than lower frequencies. The problem with this rating is that it cannot convey how much sound can be stopped or how it will act as a barrier.
NRC is not to be confused with noise criterion (NC) or, as they are sometimes called, room criterion (RC) curves. These curves were developed to gauge how much sound generated by HVAC systems could be tolerated. Ratingsbelow 25 are extremely quiet; 25 to 40 is considered the norm. Anything above 40 is considered too intrusive for the office environment. For speech privacy in the open office environment, an NC rating between 35 and 40 of masking noise is considered ideal. HVAC noise need not be considered for masking situations because it is not steady and turns itself on and off. Music presents similar problems.
Sound transmission class (STC) has been used widely as a fully constructed assembly's ability to reduce transmitted sound (for example, a complete ceiling with tiles, HVAC returns and light fixtures or a floating floor system). Ratings can be anywhere from the teens to the seventies, with higher numbers indicating better isolation. This, however, does not always relate well to an open office plan where fixtures and panels do not always go from floor to ceiling. A better method was needed.
The American Society for Testing and Materials set about coming up with standard ratings for cubicles (based on height and position), ceiling tiles, light fixtures, furniture panels and wall fixtures. These were subject to different ratings for speech privacy. Their committees (and subcommittees) developed real-world results for acoustic data for open office plans. Now we have an articulation class (AC) rating to judge a material's performance in relationship to speech privacy. The rating runs from a low of 100 to a high of 250; the better materials run from 180 to 220. There are separate criteria for ceiling systems (how well the tile blocks reflections in the critical 45degrees to 60degrees range), acoustic barriers (how well they block out sound), and furniture panels and wall systems (how the sound is reflected). What makes this data particularly meaningful is that these tests are conducted in live open office situations. Design consultants can now know what they choose.
So, just pile everything together, and it will sound great, right? It's not that simple. Yet another test must be carried out with a calibrated mic in one location and a calibrated loudspeaker in another (like from one cubicle to another). This is another ASTM standard, called articulation index (AI), which has a range of 0 (unintelligible) to 1 (understood). This tells you how much of the speech can be understood. Anything below 0.35 will give you the minimum privacy necessary.
Loudspeakers First, there is the completely self-contained system. The amp, loudspeaker, EQ, noise generator and enclosure are all in the same package. There are some advantages to this system, the biggest of which is overall cost. These kinds of systems lend themselves to smaller installations. By having their own noise source for loudspeakers, general incoherence is improved. But there are disadvantages. The EQ must be set before the unit leaves the factory, so a change in the layout can prove troublesome. Next, 120 VAC must be run to each individual unit in a conduit. Plenum wire is usually acceptable for most normal, low-voltage loudspeaker lines. Paging or background music is no longer an option. Levels have to be set at each unit's location, requiring more staff. The overall quality or reliability of these units seems to vary greatly with units marketed by audio manufacturers on the higher end of the scale. Units marketed by furniture manufacturers have experienced reliability problems.
Most consultants today are leaning more toward an 8 inch (203 mm) coaxial loudspeaker (smaller sizes can work) in a fiberglass-lined enclosure, in which the assembly hangs in the plenum with the loudspeaker facing up. The fiberglass absorbs the back-wave of the loudspeaker, so most of the energy will go forward. For a less expensive solution, try an empty 5 gallon (19 L) paint can. You can mount ceiling loudspeakers the conventional way through the tile, but the sound can be reflected more easily for a more uniform coverage without hot spots by hanging them in the plenum (see Figure 1). The plenum should be free of obstructions, which must be addressed in the planning stage. The loudspeakers must be placed so that they form a uniform pattern of coverage that extends just beyond the intended zones. Spacing of the individual loudspeakers will vary according to plenum depth and ceiling height. Some consultants use conventional box loudspeakers and fire them sideways or through diffusers, depending on the plenum layout and room design. Also, planar loudspeakers that resemble the ceiling tiles they replace have been used successfully in many situations because of their non directional characteristics.
Amps Amps should be designed for commercial use because the units will have almost continuous use. Most systems use 25 V or 70 V lines, which makes connecting loudspeakers easier. Normally, they will be tapped at 0.5 W. If the loudspeakers will double for paging or background music, double or triple output voltage. You want a minimum of a 10% cushion between the amount of total wattage the loudspeakers are tapped against the total power of the amp. If the loudspeakers' total voltage adds up to about 50 W, use at least a 60 W amp. This should be a hard-and-fast rule for any low-voltage application.
The amp should be fan-cooled or have proper heat sinking and should be located in a ventilated area. A gain control should be available from the front of the amp or from an easily accessed volume control from the amp's loudspeaker outputs.
Noise source We start with white noise, which has equal amounts of energy at all frequencies. When this is plotted, you get a 3 dB/octave rise. White noise is generally used for testing electronics, never for loudspeaker response. The closest approximation you get to white noise is FM radio inter-station noise (when the muting is off). White noise in itself sounds a little too hissy for masking applications, so a pinking filter is applied, which yields a 3 dB/octave roll off. Because it cancels the 3 dB/octave rise, we get noise with equal energy per octave. This gives us the nice flat frequency response we like to see on real time analyzers, and we can better see what the loudspeaker is doing, or with an EQ, we can tell it what to do.
In masking, the noise source needs to be steady and stable despite voltage or temperature fluctuations. There has been some question of whether the noise source should be digital or analog. The digital word length, which refers to the time elapsed before the noise repeats, must be sufficient. Analog noise sources, by nature, never repeat and are so specified in high-security government applications.
When a single noise source is used, there will sometimes be a barely noticeable volume change between the loudspeaker coverage areas. This is sometimes referred to as phasing. One of the growing trends in masking is using dual noise generators for every other loudspeaker in the layout. The most positive aspect of this trend is the reduction of the overall coherency between the two channels and elimination of the potential phasing problems. There is also the redundancy factor-if one channel goes down, at least the other will still operate.
EQs The role of the EQ is to shape the filtered noise to the consultant's desired curve (similar to NC-40 contour shape). Most consultants prefer 1/3-octave EQs. Different EQs must be used if there is a major change in the zone in a ceiling tile or flooring system or a radical room change, or if music or paging is to be sent through the system. When multiple EQs are used in cookie-cutter, multi-floor layouts, EQ settings must be absolutely repeatable. This all but eliminates slider EQs because the accuracy of their settings is ballpark at best. Additionally, 1 RU EQs have especially poor accuracy. Only the precision-knob analog EQ will give the unit-to-unit consistency that consultants demand, but they are costly.
The digitally controlled analog EQ is the most prevalent in the market today. Many companies offer these with built-in noise generators. Some have front-panel control; others work on simple PC programs. The consistency and overall high quality of these units are desirable traits, as well as the aspect of locking out unwanted fingers.
It is of extreme importance that levels not vary more through the areas by +/-2 dB. The uniformity of the sound in all areas is paramount; your customers will notice any deviations. The levels of noise are equally important. Too little noise makes it is easy to hear your neighbor; too much noise becomes intrusive. The preferred noise level falls between NC 35 and NC 40.
Pricing Price is normally figured on a per ft2 basis. Many factors affect the cost. The number of different layouts, the condition of the building structure, unique acoustical problems, workers unions, and varying layouts from floor to floor in a multi-floor layout are all potential contributing factors.
If a self-contained system is used, the pricing should fall in the 95 cents to $1.25/ft2 region. The more common centralized system will vary according to the number of square feet. Spaces smaller than 10,000 ft2 can be as high as $1.50/ft2. The price of these systems, however, can drop dramatically to 60 cents/ft2 for environments of more than 100,000 ft2. There are charts available that track this but, because of all the variables involved, there is sometimes a reluctance to venture a set price. For a general guideline, see Figure 2.
The most important aspect of sound masking is in the planning. Convincing the owner that everyone, especially the architect and audio consultant, needs to work closely to provide a pleasant work environment is essential. If the owner wants to try only one zone, or if he wants to see if it is necessary after everyone moves in, his costs for an after-the-fact masking system will increase exponentially. It is proven that when people are in a pleasant, well-lit work environment that is free from distractions, they are more efficient.
* AC (articulation class): numerical rating created by the ASTM to judge speech privacy acoustical performance.
* AI (articulation index): weighted fraction between 0 and 1 that tells how much of the speech is intelligible.
* ASTM (American Society for Testing and Materials): conducted tests for judging the acoustical standards for many different types of building materials. Different standards are in place for ceiling systems (Standard E111), acoustical barriers (Standard E1375) and many other materials.
* NC (noise criterion curves): curves used to relate the intrusiveness of such noise as HVAC systems in buildings. NC 35 to 40 is optimum for speech privacy. Sometimes referred to as RC (room criterion curves).
* NRC (noise reduction coefficient): rating between 0 and 1 to evaluate how much sound can be absorbed by a material; based on absorption coefficients at 1/3-octave bands between 250 Hz and 2 kHz.
* RC (room criterion curves): see NC.
* STC (sound transmission class): numerical rating of a constructed assembly's ability to reduce airborne transmitted sound, typically ranging from 20 to 70 with the higher number indicating better reduction.
Gregerson, John. "Setting Sounder Standards for Acoustical Performance," Building Design & Construction, April 1992.
Herbert, R. Kring. "Office Soundscapes: Exploring Acoustic Performance," www.facilitiesnet.com/NS/NS3b56d.html, 1996.
Joiner, David. "Acoustical Design for Open Plan Offices," Designers West, October 1984.
Lewitz, Joel, P. E. "Electronic Sound Masking Systems," NSCA Contractor's Conference & Expo 1993, Sales Track.
Sullivan, Edward. "A Closer Look at Elements of a Productive Indoor Environment," www.facilitiesnet.com/NS/NS3bh5b.html, 1995.
Thomas, Marita. "Achieve Speech Privacy with Sound Masking Systems," Facilities Design & Management, July, August 1984.
www.astm.org
www.facilitiesnet.com
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