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Sound Masking Systems by Ashton T aylor , Hoover & Keith Inc. for Atlas Sound A technical guide to achieving effective speech privacy in open-plan offices and other environments.
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . 4 WHA T IS SOUND MASKING? THE ECONOMIC BENEFITS OF SOUND MASKING 4 DEFINITION OF TERMS (ALSO SEE APPENDIX A) . . . . . . . . . . . . . . . . . . 4 PURPOSE OF THIS P APER . . . . . . . . . . . .
P AR T 5 - MASKING LOUDSPEAKERS AND SELF-CONT AINED MASKING UNITS MASKING LOUDSPEAKERS . . . . . . . . . . . . . . . . .24 Upwards Loudspeaker Orientation . . . . . . . .24 Downwards Loudspeaker Orientation . . . . . .25 Horizontal (Sideways) Loudspeaker Orientation .
A sound masking system emits low-level, non-distracting masking noise designed to reduce speech intelligibility and thereby improve speech privacy . This improvement in speech privacy can be of great value in open-plan offices, doctors’ examination rooms and other environments where confi- dentiality is important.
Applications for Sound Masking Systems Open-Plan Offices Definition of T erms (also see Appendix A) In this paper , the term “talker” refers to a person. The term “speaker” refers to a loud- speaker . The term “listener” refers to anyone hearing sounds, whether or not they intend to hear those sounds.
Buildings near Major Roads, Railroads, and Airports In most buildings, it is not feasible to com- pletely mask higher-level noises like those from heavy trucks, trains, or aircraft. However , sound masking can soften the impact of these noises. If a client wants masking to cover up these sounds, make sure their expectations are not too high.
Benefits of Masking to the End User Cost-Effective Speech Privacy Normal (not confidential) privacy can usual- ly be achieved with floor-to-ceiling walls between workspaces. However , sound masking allows normal privacy to be achieved in an open-plan office with simple partitions between cubicles.
Three Steps to Successful Sound Masking Carefully planned acoustics, combined with masking sound, make it possible to achieve the goal of increased speech privacy between workstations.
ground sound levels typical in an open-plan office. Thus, Part A shows a high speech-to- noise ratio in every octave band resulting in high articulation and no speech privacy . Part B shows a lower speech-to-noise ratio and a more desirable level of speech privacy achieved with partitions, absorptive surfaces and masking sound.
Attenuation of Direct Sound The direct sound is speech from a talker that arrives directly at the ear of a listener without being reflected. Figure 3 shows the direct peak sound levels for male and female talkers at a distance of one meter . FIG. 3 - Octave-band speech peak sound levels for male and female talkers at a distance of 1 meter .
Sound T ransmission Class Sound transmission class (STC) is a standard way to specify the attenuation of sound through a wall, an open-plan office screen or other bar- rier . A higher STC is better . A screen with a high STC rating will attenuate the sound more than a screen with a low STC rating.
Layout Simple layout changes can often improve speech privacy in an open-plan office. And, even though these changes will disrupt daily routine in an existing space, clients with severe privacy problems are usually willing to comply .
Ceiling The ceiling in an open-plan office affects speech privacy more than any other acousti- cal element. A hard ceiling reflects sound from one workstation to another , bypassing the sound barrier provided by the worksta- tion screens. This problem is worse when the angle of reflection is between 40º and 60º.
Lighting Fixtures T ypical ceiling-mounted fluorescent lighting fixtures have flat plastic lenses flush with the ceiling. These fixtures reflect speech frequen- cies between workstations, “short-circuiting” the acoustic privacy provided by the worksta- tion partition screens.
and the masking sound coverage will be uneven. These are very undesirable results. Lighting fixtures with open grid diffusers can cause similar problems. Other Causes of Unwanted Reflections Ceilings aren’t the only source of reflected sound problems in an open-plan office.
faces such as shelf covers and drawer faces. Of course, workstation partition screens must be highly absorptive. Hard walls, doors and windows can serious- ly degrade speech privacy in both open-plan spaces and in standard offices.
Ambient Noise T o the extent possible, keep building and office equipment noises below the level of the masking system. The heating, ventilat- ing, and air conditioning (HV AC) system makes a sound similar to an electronic masking sound.
The electronic sound masking system creates a “blanket” of background noise carefully controlled in level, spectrum, and coverage. Masking sound should not call attention to itself in any way . It should merely seem to be part of the general building noise.
the equipment. Ensure the rack has adequate ventilation for uninteruppted usage 24 hours a day , 365 days a year. For an existing space, include the cost of an electrical subcontractor to provide dedicat- ed AC circuits hardwired into the rack.
with a pseudo-random sequence of at least several seconds. T est equipment noise gen- erators usually repeat too frequently to be acceptable for sound masking. Some masking sound generators have computer controls that gradually reduce the normal daytime masking sound to a pre- set nighttime level.
V ery simple masking systems, with only the bare minimum of components, are fairly rare. More commonly , a masking system includes a two-channel generator , signal monitoring for troubleshooting and some- times even paging or background music.
Zone Level Controls Larger masking systems may cover more than one workspace in a building. Unless the workspaces are acoustically very simi- lar , each deserves its own masking sound level control. Even in a single large room, it may be useful to provide separate level controls for open areas, walled offices, con- ference rooms and corridors.
Paging Sound Level T o calculate the paging sound level of a masking loudspeaker at the listener , first gather the following information: * S = loudspeaker sensitivity (from the manufacturer’s data.
Masking Loudspeakers Masking loudspeakers are special assem- blies designed for installation in ceiling plenums. A typical assembly consists of a 4 or 8-inch cone speaker , a 70.7-volt speaker line transformer , a metal enclosure with baffle, and a hanging/mounting hardware kit.
Downwards Loudspeaker Orientation Roof decks (above the ceiling on the top floor of a building) usually have sprayed-on thermal insulation that is also an efficient acoustical absorber . In this situation, mount the masking loudspeakers high in the plenum and point them downward as shown in Figure 14.
conduit, make the loudspeaker connections inside the electrical box or inside the enclo- sure to avoid violating local building safety codes. Always comply with all state and local codes, as well as the National Electrical Code, for any masking loudspeak- er installation.
“Commissioning” the masking system takes place in two steps: 1. Confirm the proper installation and function of all system components. 2. Adjust the system level, spectrum and coverage to the design specification. Good coverage simply means every listener hears masking sound at the desired level and spectrum given in the initial specification.
Masking Spectrum Figure 16 shows a typical masking spectrum compared to typical “quiet” building sound, pink noise, and white noise. FIG. 16 - Octave-band sound pressure levels of typical masking .
Masking Spectrum 2 Masking Spectrum 2, given in the table below , and charted in Figure 18, is appropri- ate for good open-plan spaces (screens 4 - 5 feet high, some reflective surfaces, and moderate furniture absorption).
Comparison of All Three Masking Spectra Equalizing the System: The Equalization Process After selecting one of the three masking spectra, equalize the system as follows: Use a 1/3-octave spectrum analyzer , measuring microphone, sound level meter (SLM) and oscilloscope.
Coverage With the system in normal operation, walk the space, using your ears and a meter to assess the masking level and spectrum throughout the entire space. If necessary , fix problems and readjust the system as follows: * While walking the space, listen and observe the meter to find any “hot spots.
As originally discussed, the goal of most mask- ing systems is to increase speech privacy . A well-planned sound masking system achieves this goal by reducing speech sound energy and increasing background sound (with masking sound).
Normal Privacy Normal is usually the best level of speech privacy achieveable in an open-plan office space. A masking installation that achieves normal privacy will reduce distractions from nearby conversations, footfall noise, and the sounds of office equipment.
Masking Improves Speech Privacy in a Quiet Space In the open-plan offices of a major oil com- pany , nighttime ambient noise levels were very low . As a result, employees could carry on normal-voice line-of-sight conversations over distances greater than 60 feet and speech privacy was effectively impossible.
speakers were aimed downward because of the thermal insulation applied to the under- side of the deck above. The contractor could not install boots above the fixtures because there were a great many fixtures, the electrical conduits were in the way , and service personnel would have removed the boots the first time they serviced a fixture.
between two examination rooms and the area just outside these rooms. The improvement in speech sound isolation was immediate and dramatic and the doctors had no problems using their stethoscopes or other instruments. Prior to the test system installation, doctors and patients could hear every word from the next examination room.
Basic masking systems are relatively simple to design, install and commission but results are greatly influenced by the acoustics of the ceilings, screens, furniture and interior finishes. When designing a masking system,remember these three steps: 1.
ambient noise - The background noise in a given environment, usually composed of many sound sources from many direc- tions, near and far . articulation class (AC) - The sum of the weighted sound attenuation values in the one-third octave bands between from 200 Hz to 5000 Hz.
noise - (1) Any disagreeable or undesired sound. (2) A random sound or electronic signal whose spectrum does not exhibit clearly discernable frequency components. noise criterion (NC) curves - A series of curves of octave band sound spectra used for rating the noisiness of an occupied indoor space.
T o evaluate the acoustics of an office space, use the worksheet titled “Sound-Masking, Octave-Band, Articulation-Index W orksheet” found at the end of this Appendix.
Page 41 Section A Instructions The worksheet gives data for three voice levels: Raised, ANSI Standard, and Normal. These levels are peak levels for male voices.
Page 42 Section C Instructions This section reduces the values entered in Section A to compensate for the normal attenuation of direct sound at increasing distance from the talker . Measure the direct path from the cho- sen talker to the chosen listener .
Page 43 Section D Instructions This section modifies the values entered in Section A to compensate for the acoustics of the furniture, walls, and carpet.
Page 44 Section E Instructions This section modifies the values entered in Section A to compensate for the acoustical effect of screens and ceiling. These two elements are grouped together because the effectiveness of one depends upon the other .
Page 45 Section F Instructions Hard surfaces can reduce speech privacy by reflecting sound energy from the talker’s workstation into adjacent spaces.
Page 46 Section H Instructions Enter the background sound levels (including masking sound) in the “Y our V alues” row of Section H. Measure the actual background sound level or choose one of the data sets in the table. Choose the first row of data for an office with no masking.
Page 47 Section J and Section K Instructions Section J calculates the actual articulation index (AI). First multiply the values from Section I by the AI weighting factor given in Section J for each octave band. Enter these results in the bottom row . Then, add all of the numbers in the bottom row to calculate the AI.
Page 48 W orksheet Example 1 Open Plan Environment Part 1 - No Speech Privacy Assume a client wants a masking system in an open-plan space with standard office furniture, 4-foot high screens, commercial padded carpet, mineral fiber ceiling tile, and workstations spaced approximately 8 feet apart.
Page 49 Part 3 - Substitute 6-Foot-High partition Screens Even with masking, there is still no speech privacy . Next, recalculate the worksheet replacing the 4-foot high screens with 6-foot high screens as follows: Octave Band Center Frequency 250 Hz 500 Hz 1000 Hz 2000 Hz 4000 Hz A.
Page 50 Part 5 - Install a High Articulation Class (AC) Ceiling Part 4 results in marginal speech privacy even with most of the elements correctly implement- ed.
Page 51 Summary and Conclusions In this case, masking sound achieves confidential speech privacy with no other changes to the building. This example shows that sound masking can have many benefits to clients other than those with open-plan office spaces.
Page 52 Sound Masking W orksheet Copy Before Using B. Select talker orientation to listener and enter values (or interp olated values) in last row of table: Octave Bands 250 H z 500 Hz 1000 Hz 2000 Hz.
Page 53 E. Select screen/ceiling c ondition and enter values (or inter polated values) in last r ow of table: Octave Bands 250 H z 500 Hz 1000 H z 2000 Hz 4000 Hz No screen, hard clg.: +2 +2 +2 +2 +2 No screen, abs. c lg.: +1 +1 +1 +1 +1 4 ′ screen, hard clg.
Notes.
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1601 Jack McKay Blvd. Ennis, T exas 75119 T el: 800-876-3333 Fax: 800-765-3435 www .AtlasSound.com PN 484016 A T000369.
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