4-Acoustics: Background

Transcription

1 4-Acoustics: Background In the past acoustics for buildings especially concert halls was done by luck. A design would be done that worked well and then it would be mimicked. Other times awful designs emerged and the concert hall would have to be torn down. Acoustic calculations did not exist until the Harvard University's Fogg Art Museum was built in Soon after the museum was built it was determined that its lecture hall had absolutely atrocious acoustics. A young physics professor, Wallace Clement Sabine, was asked to do some research on the lecture hall. Over the next three years, Sabine did scientific testing on the room using a stopwatch, organ pipes and a number of seat cushions. Fogg Art Museum was never fixed but Sabine was able to develop some useful equations from his research. He formulated an equation for reverberation time, relating it to room volume and materials (T 60 = 0.161*V/A where V is the room volume in cubic meters, and A is the total absorption in square meters). The unit for a material's sound absorption, the Sabin, is named after him. The unit of sound absorption is square meter, referring to the area of open window. This unit stems from the fact that sound energy traveling toward an open window in a room will not be reflected at all, but completely disappears in the open air outside. The effect would be the same if the open window would be replaced with 100 % absorbing material of the same dimensions. One sabin is the absorption of one square foot of open window, and one metric sabin is the absorption of one square meter of open window. Since the days of Wallace Clement Sabine many advances have been made in the realm of acoustics. Various branches of acoustics exist that deal with different aspects of sound and hearing including bioacoustics, physical acoustics, ultrasonic, and architectural acoustics. In the area of architectural acoustics most of the developments have been made in regards to concert halls. As this data was collected it was verified how important acoustics are in all areas of a building. The rest of this section will focus on acoustical items that will apply to The Florida State University Ringling Conservation Center library area

2 Human Hearing Humans can not hear all sounds produced in a building, therefore only sounds in the audible range must be accounted for. The upper and the lower limits of the audible frequency (the number of cycles that the periodic signal completes in one second) range is generally considered to be 16 Hz to 20,000 Hz. The diagram below shows the typical frequencies at which different sounds are heard. The audible frequency range depends on many factors such as the setup of the measurements and the age of the listeners. The human hearing system is more sensitive to frequencies in the range of 1000 Hz-4000 Hz. Also, the human hearing system is unable to distinguish between two separate sounds with frequencies too close to each other. For this reason acoustical measurements and calculations are done in octave bands. An octave band is the interval between two frequencies having a ratio of 2:1. Figure 4.1 Human Hearing Sound levels are measured in decibels. A decibel is the unit used to measure the loudness of sound. The decibel is a measure of sound intensity as a function of power ratio, with the

3 difference in decibels between two sounds being given by db=10 log 10 (P 1 /P 2 ), where P 1 and P 2 are the power levels of the two sounds. The change in the sound pressure level (SPL) must be more than certain value in order to be noticeable by the human hearing system. In the chart below it can be seen how the human hearing system responds to varying decibel levels at different frequencies. It should be noted that the decibel values of sounds can not be added to obtain the value of the resulting sound. Instead, the combining levels are turned into intensities, added together and then turned back into decibel values. Figure 4.2 Speech Levels Acoustic Materials The sound absorption coefficient of a material has is measured in sabins as stated above. It is the ratio of the absorbed sound energy to the incident energy. For architectural purposes, sound absorbing materials and constructions can be divided into four types of materials depending on the way the absorption is mainly performed: 1. Turning the sound energy into heat such as fiberglass and carpet

4 2. Vibrating with a specific frequency when the sound hits the surface such as lightweight panels and 5/8" gypsum board. (These materials absorb the sound effectively on a narrow band of frequencies) 3. Turning the sound energy into heat in the neck of the cavities (Helmholtz resonator) such as sound blocks. (This construction has a good absorption on low frequencies) 4. Allowing the sound to go through such as some types of grid systems and lay-in ceiling with sound leakage above it. The most common way to measure sound absorption coefficient is to lay a piece of the material in a reverberant room and then measure the Reverberation time so the coefficient can be derived from Sabin equation. Reverberation time is the time required for the sound level in the room to decay 60 db, or in other words, it is the time needed for a loud sound to be inaudible after turning off the sound source. Reverberation time of a room can be controlled by changing the size or the materials of the room. The optimum reverberation time for different rooms depends on the volume of the space, the type of the room, and the frequency of the sound. Often NRC (Noise Reduction Coefficients) is used to evaluate materials absorptive quality. The NRC is the arithmetic average of the sound absorption coefficients at 250, 500, 1000, and 2000 Hz. This average is rounded to the multiples of Noise Paths Noise in buildings may take many paths. The following figures illustrate the possible paths

5 Figure 4.3 Transmission Paths Figure 4.4 Transmission Paths--Partition Rating Systems NC - The Noise Criteria values are determined from the measurements of the octave-band sound levels in an occupied room when the air-conditioning system is on. The measured values are then compared to standard NC curves to determine the values

6 RC - The Room Criterion is mostly used for acoustical design of HVAC systems. The RC criteria take into account the noise components at the lowest and the highest frequencies. The measurement values should be taken in an unoccupied room. NCB The Balanced Noise Criterion has been recently standardized and has concepts similar to NC and RC. The sound level measurements for NCB should be taken in an occupied room. The RC and NCB ratings include procedures for checking different factors such as the rumble compliance (excessive noise at frequencies below 500 Hz) and the hiss compliance (excessive noise at frequencies above 1000 Hz). STC - Sound Transmission Class is a single number used to characterize the air-borne isolation properties of a partition. The STC is determined from the measured Transmission Loss (TL- sound loss through walls/barriers) of a partition at different frequencies. These measured values are then compared with standardized STC contours. IIC - Impact Isolation Class is another single-number rating system for a solid-borne noise (floor-ceiling structure). The higher the IIC rating, the more efficient the construction will be in attenuating the impact sound within the frequency range of the IIC Library Acoustics The acoustic design of internal spaces has a major bearing on the successful operation of a building. Consideration of privacy, intelligibility and all aspects of room acoustic design allow a building to be used to its full potential. Libraries fall in the speech category and typically are dead spaces (reverberation time is fairly low) and should fall in the range of 0.6 to 1.4. It is important that the reverberation time in a library is low because it directly affects the articulation loss (the ability to understand to speech). This occurs because it is able to keep each sound event separate rather than running them together

7 FSU Ringling Conservation Center Library Acoustics: There was not any documentation on the acoustics in The Florida State University Ringling Conservation Center. This section will focus directly on the library area as it is an important area for good acoustics. Twenty rooms were analyzed in all. These rooms and there volumes and wall areas are listed in Table 4.1. The reverberation times in these rooms were calculated to see if they fall below the recommended levels for the specific type of room. Also the sound transmissions were calculated for rooms with critical sound transmission characteristics. The calculations were done using the sound pressure levels of a normal speaking voice at 1 meter. The rooms that were analyzed for sound transmission were all the offices, conference rooms and study rooms. Table 4.1 Acoustically Analyzed Rooms Room Volume (Ft 3 ) Exterior wall Area (Ft 2 ) Interior Wall Area (Ft 2 ) Floor/Ceiling Area (Ft 2 ) Window Area (Ft 2 ) 1000 Periodicals Periodicals Library Office Conference Conference Study Rm Study Rm Corridor Lobby Library Periodicals Library Library Library Conf Office Multi-purpose Library Conf Sitting

8 The main room of concern in this list is 1185 Library. This room has a very large volume. It is a two story room. From the rendering in Figure 4.5 you can see that there are books in this room that will help keep the reverberation time down but half of this room is study tables which will not. Also the floor in this room is terrazzo which is a reflecting material instead of an absorbing material. Figure Library FSU Ringling Conservation Center Library Acoustics Conclusions: All calculations can be seen in Appendix D for this section. Reverberation Times All of the reverberation times are expectable except for 1185 Library and 2080 Conference where the reverberation times are too high in certain octave bands. Some of the other rooms have too low of reverberation times. This results in poor speech intelligibility. Since it is a library, and little talking will be done unless at a close range, low speech intelligibility is expectable. The

9 reverberation time in 1185 Library at 500 Hz and 1000 Hz is over 2 seconds. If this room was empty a person talking in this room would bounce off the walls and be reflected back to a listen for over 2 seconds. This is not good in a library, especially when there is a large potential for someone to drop a book. However, the reverberation time in this room will probably in actuality less than 2 seconds. This is because the presence of books in the room was not accounted for. The reverberation times in this space will most likely still be too high though. In 2080 Conference the reverberation time in the 250 and 500 octave bands is over 1.0 seconds. This is to high for a conference room where speech intelligibility is necessary. It is recommended to add acoustical panels or acoustical clouds to these rooms. In 1185 Library if the reverberation time can be reduced with these panels down to around 1.4 seconds then with the addition of the books damping the sound the acoustics in this room will be much approved. In 2080 Conference the reverberation times in the 250 and 500 Hz octave bands needs to be reduced below 1.0 seconds. Pictures of these suggested panels can be seen in Figures 4.6 and 4.7. Cost data for these panels and clouds can be seen in Appendix C. These panels were selected from Acoustical Services, Inc. The sound absorption qualities of these materials can also be seen in Appendix C. The selected option is to use the Echo Eliminator, an acoustical panel, because it is the cheapest solution as well as the material is made from recycled cotton therefore it is considered a green material. The panels come in a Blue (Jean Material) or White, Light Gray, and Charcoal. The price of the blue panels is cheaper ($1.75/ft 2 ) because they are covered with jean material and do not need to be died another color. To achieve proper reverberation times in 1185 Library 40 of these panels are needed and would cost $560 in blue and $688 in any other color ($2.15/ft 2 ). To achieve proper reverberation times in 2080 Conference 3 panels are needed and would cost $42.00 in blue and $51.60 in any other color

10 Figure 4.6 Acoustical Clouds Figure 4.7 Echo Eliminator Sound Transmission The sound transmissions that were calculated turned out all to be satisfactory. The sound pressure levels in the receiving room were compared to the RC rating sound pressure levels for that the receiver room minus 5dB. This is because the RC rating distinguishes the amount of sound that is expectable in that type of room. If the received levels are below the RC level minus 5 db they are expectable. Therefore all of the rooms are considered expectable as designed