This chapter is from the book
This chapter is from the book
This chapter is from the book
Acoustics
One of the most impressive aspects of the Cisco TelePresence solution is its audio quality. The sound is spatial (emanates from the direction of the person who is speaking) and is full-duplex. (You can talk over each other with no clipping.) The microphones are directional and have a coverage pattern designed to capture the voice of the participants sitting directly in front of them. The microphones also filter out certain background frequencies. The audio compression board in the system encodes the voice from the participants at 48KHz using the Advanced Audio Coding—Low Delay compression algorithm. On the receiving end, the speakers are specifically designed to reproduce the frequency range and decibel levels of human speech so that it feels life-like, as if the person is sitting 8 feet (2.438 meters) or so away from you on the other side of the virtual table.
Other capabilities within the Cisco TelePresence portfolio exploit the acoustic properties of the Cisco TelePresence system. In multipoint meetings for example, the Cisco TelePresence Multipoint Switch (CTMS) relies on the signal strength of the audio coming from each microphone to determine which segment should be displayed at any given time.
Background noise and reverberation in the room can degrade these acoustic qualities and even disrupt the switching behavior in multipoint meetings. Therefore, careful engineering of the environment must be done to ensure that ambient noise and reverberation levels within the room are kept in check. However, you don't want the TelePresence room to be so flat and sterile acoustically speaking that it feels like you're in a sound chamber or recording studio. The goal is to re-create the experience of an in-person meeting, so some amount of ambient noise and reverberation is tolerable—even desirable. Cisco has defined precise targets and thresholds for ambient noise and reverberation levels within a TelePresence room, providing a comprehensive test methodology for measuring those levels and recommendations for remediating typical sources of higher than desired ambient noise and reverberation.
Measuring Ambient Noise
Ambient noise is everywhere and is generated by numerous things. Take a moment to pause and listen to the background noises around you. Have you ever noticed the sound of the air whooshing through the air-conditioning ventilation vents in your room, the gentle humming of the air-conditioning machinery in the ceiling, the buzzing of fluorescent light fixtures above you, the sound of cars and buses and ambulances passing by on the street outside your building, the people talking in the room next door, or the sounds of phones ringing in the offices and cubicles around yours? You probably haven't because your brain has become accustom to those sounds and unconsciously tunes them out, but those are exactly the types of sounds we are interested in measuring and, to the degree possible, eliminating inside the TelePresence room.
Ambient noise can emanate through thin, hollow walls or through the cracks in the door jamb around the door. It can travel up and over walls from adjoining rooms and corridors and permeate through the ceiling into your room. This section discusses methods for treating the walls, doors, flooring, and ceiling materials to remediate these sources of noise, but first, consider how these noise sources are measured.
Cisco uses a Sound Pressure Level (SPL) meter to measure the level of ambient sound within the room. SPL is a logarithmic measurement of the root square mean (or average power) pressure of sound relative to silence. It is denoted in decibels (dB), with silence equal to 0dB. Sound travels through the air in waves. Therefore, SPL is simply a measure of the strength, or pressure, of that wave.
The SPL of human speech is generally 60dB to 65dB. Because of the way the human ear and brain work, background sound that is 25dB to 30dB less than human speech generally goes unnoticed. Therefore, the goal is create a room where the average SPL of ambient background noise is no greater than approximately 36dB. Noise levels exceeding 42dB are cause for concern, and levels exceeding 50dB can cause significant problems with the TelePresence experience.
When measuring the SPL level of a room, the average measurement across the entire environment (that is, throughout the room) is used. This establishes a baseline measurement referred to as the noise floor average. However, because sound dissipates over distance, when measuring the SPL of a specific source, such as an air conditioning vent or fluorescent light fixture, the SPL is taken from a defined distance from the source (for example, SPL = 30dB at 1 meter away from the vent).
As with the lighting measurement techniques discussed in the previous sections, Cisco divides the room into sections, or zones, to measure the ambient noise floor average at various points within the room. Figure 8-37 illustrates the acoustic zones of a CTS-3000 room.
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Figure 8-37 CTS-3000 acoustic zones: top down view
Within each of the six zones, the ambient noise is measured with the decibel meter approximately 5 feet (1.5 meters) from the floor using a slow sweeping motion to capture the average SPL for that zone. These measurements are done using an A-weighted test. A seventh measurement is taken using a C-weighted test within the middle of the room (front of zone 5) to capture the C-weighted average SPL for the entire room. Note that the C-weighted target is approximately 52dB, compared to the A-weighted target of 36dB mentioned previously. Finally, specific measurements are taken of any particular source of noise, such as each of the air conditioning vents in the room, at a distance of 3 feet (1 meter) from the source, using an A-weighted test. You should be concerned with any A-weighted measurements that exceed 36dB, a C-weighted measurement that exceeds 56dB, or any specific source such as HVAC vents that exceed 36dB at 3 feet (1 meter) distance from the source. For all these tests, you should choose a time of the day that represents the high average, ideally, when the HVAC is actively producing air flow through the vents.
Measuring Reverberation
Reverberation is essentially a measurement of how long a sound continues to bounce around the room before decaying to the point that it can no longer be heard. The measurement used to denote reverberation is called RT60, which is a measurement of the time required for a sound to decay by 60dB. For example, if you have a source generating sound at 65dB, and it takes 200ms for that sound to dissipate to 5dB, the RT60 measurement for that sound is 200ms. The more the sound can reflect off of walls, ceiling, flooring, and other surfaces, the longer it will take for that sound to decay. Figure 8-38 illustrates this concept.
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Figure 8-38 Reverberation illustrated
The ideal reverberation level for a Cisco TelePresence room is 150ms to 300ms. Levels ranging from 300ms to 500ms are cause for concern, and levels exceeding 500ms can cause significant problems with the TelePresence experience. Reverberation is measured for each of the following frequency ranges independently: 125Hz, 250Hz, 500Hz, 1kHz, 2kHz, and 4kHz. Different frequencies of sound reflect off of (or are absorbed by) wall, ceiling, and flooring surfaces differently. Although the human ear cannot discern all these frequencies, the microphones of the TelePresence system might. Therefore, measuring the reverberation of all these frequencies ensures that we understand the acoustic behavior of the room for all types of sounds, from the low frequency sounds generated by building machinery, through the frequencies of human speech and music, up into the higher pitched sounds generated by electronic devices. For each of these tests, you want to measure from the center of the room, as illustrated in Figure 8-39.
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Figure 8-39 Reverberation zone: top down view
To measure reverberation, place the decibel meter in RT60 mode in the center of the room on a table surface approximately 5 feet (1.5 meters) off the floor. Use a tone generator and amplified speaker to completely fill the room with > 70dB of white or pink noise for several seconds and then instantly silence the tone generator. The decibel meter measures the time it takes (in milliseconds) for the noise to decay by 60dB. Repeat the test for each of the six frequency levels. For accuracy, several measurements should be taken with the tone generator and amplified speaker at different locations within the room for each of the frequency ranges to ensure that your measurements represent a true average for the room.
Targeted and Maximum Ambient Noise and Reverberation Levels
Table 8-4 summarizes the targets and thresholds for ambient noise and reverberation.
Table 8-4. Target and Maximum Ambient Noise and Reverberation Levels
|
Measurement |
Target |
Maximum |
Notes |
|
Ambient Noise Floor Average (A-Weighted) |
36dB |
42dB |
Within each of the six zones |
|
Ambient Noise Floor Average (C-Weighted) |
56dB |
62dB |
In the front of zone 5 |
|
Specific noise source (@ 1 meter from the source) |
36dB |
42dB |
Air-conditioning vents, light fixtures, or any other specific device such as the fan on a UPS or Ethernet switch |
|
Reverberation (RT60) |
150ms–300ms |
500ms |
For each of the six frequency levels |
Controlling Ambient Noise and Reverberation Levels
The primary method of controlling ambient noise and reverberation levels within the room is to use the appropriate wall, flooring, and ceiling building materials. All types of building material have ratings associated with them for the following three acoustic properties:
- Noise Reduction Coefficient (NRC): The NRC is a rating that represents the amount of sound energy absorbed upon striking a surface. An NRC of 0 indicates perfect reflection; an NRC of 1 indicates perfect absorption. NRC generally pertains to sound within the room and, therefore, applies to wall, flooring, and ceiling surfaces. The target NRC for a TelePresence room is .60.
- Sound Transmission Class (STC): The STC is a rating that represents the amount of sound energy required to transfer through a surface or structure. An STC of 40 requires greater than 40 decibels of sound energy to transfer through the structure. STC generally pertains to sound leaking into the room from adjacent rooms and corridors and, therefore, pertains to wall and ceiling surfaces and items such as doors and windows that can leak audio. The target STC for a TelePresence room is 60 for internal walls, doors, and windows and 90 for external walls, doors, and windows.
- Impact Insulation Class (IIC): The IIC is a rating similar to STC but pertains specifically to flooring surfaces. IIC measures the resistance to the transmission of impact noise such as footfall, chairs dragging, and dropped items. This measurement is especially important in multifloor buildings and with plenum flooring. The IIC represents the amount of sound energy required to transfer sound through a surface or structure. An IIC of 40 would require greater than 40 decibels of sound energy to travel through a surface or structure. The target IIC for a TelePresence room is 60.
Table 8-5 summarizes the target and maximum ratings for common construction surfaces within the TelePresence room. The Notes column provides examples of the types of materials you can use to achieve these ratings.
Table 8-5. Target and Maximum NRC, STC, and IIC Ratings
|
Material |
Acoustic Property |
Target |
Maximum |
Notes |
|
Walls |
NRC |
.40 |
.30 |
Acoustic fabric on gypsum or moderate-weighted curtains |
|
STC |
60 |
40 |
1/2-in. gypsum drywall on both sides with heavy insulation or acoustic panels |
|
|
Flooring |
NRC |
.40 |
.30 |
Padded carpeting over cement |
|
IIC |
60 |
40 |
Standard commercial construction practices |
|
|
Ceiling |
NRC |
.80 |
.70 |
Commercial acoustic ceiling tile |
|
Tile |
STC |
60 |
40 |
Commercial acoustic ceiling tile |
|
Doors |
STC |
60 |
40 |
Solid core door with gasket on top, bottom, and sides |
|
Interior Windows |
STC |
60 |
40 |
1/4-in. double pane windows or acoustical treated coverings |
|
Exterior Windows |
STC |
90 |
70 |
Location near high traffic or airports might want highest ratings |
Scenarios for Mitigating Ambient Noise and Reverberation
This section concludes with a few common scenarios for how these ratings apply and what type of remediation tactics you can use.
First, by far the most common problem encountered by customers is the noise created by the HVAC registers. The challenge is that because the TelePresence equipment and the human bodies within the room produce so much heat, either a high level of air flow or a low temperature air flow is required to achieve a comfortable temperature within the room. Finding the proper balance of temperature, air flow, and SPL can be tricky. On one hand, increasing the air flow generally causes the SPL of the register to go well above the maximum of 46dB, either as a result of the air flow through the register or the machinery noise created by the motors and fans traveling through the ducting. On the other hand, decreasing the temperature can cause the air flowing out of the register to be uncomfortably cold for people who happen to stand or sit directly beneath it. It is recommended that you consult an HVAC specialist for assistance in finding the proper balance for your room. However, one general piece of advice is to always use NC30-rated air registers, which diffuse the air flowing out of the register to reduce the air flow noise. The next section "HVAC" reviews the BTU requirements and recommended types and locations of HVAC registers in greater detail.
The second most common area of problems are ambient noise and reverberation levels caused by low NRC and STC values of walls, doors, and windows. Noise can also come up and over the walls from adjoining rooms and corridors and permeate through the ceiling. Figure 8-40 illustrates some of these common scenarios.
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Figure 8-40 Example NRC and STC scenarios
Simple tactics for remediating these issues include increasing the thickness of the walls (for example, installing a second layer of gypsum drywall to double its thickness), adding sound-absorbing insulation within the walls, installing an acoustic blanket or foam tile inserts above the ceiling to eliminate the sound traveling up and over the wall, and using ceiling tiles with high NCR and STC ratings. Figure 8-41 illustrates some examples of these materials.
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Figure 8-41 Example ceiling materials used to increase NRC and STC ratings
For rooms that exhibit high levels of reverberation, the best remediation tactic is generally to cover the wall surfaces with acoustically dampening materials, such as small fabric panels placed in strategic locations on one or more walls in the room. Refer back to the "Wall, Floor, and Ceiling Surfaces" section earlier in this chapter for additional considerations.