1 Acoustic Characterization of 41 Academic Spaces

Jacob Fern

Abstract—The acoustics of ’s new academic way that we actually hear different frequencies, although building were tested for two different acoustical parameters: it is a very simplified version. Figure 1 shows the standard reverberation time and ambient sound levels. These mea- 1 surements were compared with ANSI/ASA S12.60-2010. A-weighting curve . Based on the overall ambient sound pressure level and rever- beration time measurements, Cooper Union’s new academic building does not meet ANSI standards for academic acous- tical characteristics. Although most rooms were just below the maximum limit for reverberation times of 0.7 seconds, almost every room that was tested failed to fall below the ANSI standard for the ambient sound levels of the room. In many cases, this was due to external noise from the city, and in almost all cases, the HVAC system seemed to be at fault.

I. Background A. Sound Level Measurement The intensity of sound can be quantified by measuring the pressure of the sound wave, commonly measured in Pascals. A microphone is an instrument which converts a changing sound pressure into a voltage. This voltage can Fig. 1. A-Weighting Curve then be read by a sound level meter, computer, or other device which then displays the voltage as a sound pressure. The chart shows how to adjust sound pressure level mea- The sensitivity of a microphone is commonly expressed in surements according to frequency. If, for example, a 100 units of mV/Pa, which describes the voltage output of the Hz tone was measured to have a sound pressure level of microphone compared with the pressure of the sound wave. 60 dB (unweighted), then the A-weighted sound pressure Sound levels are often measured with the units of deci- would be: bels (abbreviated dB). The decibel is a logarithmic scale, SPL = 60 − 19 = 41dBA which works well for sound measurements, since our ears A also sense sound logarithmically. To convert a sound pres- Often, when referencing standards for maximum allowable sure into logarithmic scale, the following equation is used: sound pressure levels on job sites or in schools, the value is expressed as an A-weighted measurement. Other weighting   prms schemes (such as B, C, and D weighting) also exist; how- SPL = 20 log10 (1) pref ever, they are generally for specific industries, and are gen- erally not commonly used for standard room acoustics. For where prms is the root mean square pressure (measured −6 example, D-weighting was specifically designed for mea- in Pascals), pref is a reference pressure of 20 × 10 Pa suring aircraft, and has frequency adjustments specifically (roughly equal to the faintest sound that the average hu- designed for jet engines. man can hear), and SPL is the resulting Sound Pressure Level, expressed in decibels. A.1 Octave Band Levels Human hears are not equally sensitive to all frequencies. Sounds can be broken up into different frequency bands. Generally, most people cannot hear frequencies lower than One common method of dividing frequency bands is into 20 Hz, or higher than 20 kHz. Weighting schemes have “octave bands”. An octave band consists of a range of been developed which attempt to correct sound pressure frequencies surrounding a center frequency, f . The limits level measurements to account for the variable sensitiv- c of the band can be calculated using the following equations: ity to different frequencies. The most common weighting scheme is the A-weighting. Because we are not as sen- fc flow = √ sitive to low frequency noises, these frequencies have a 2 lower weighting, while the most sensitive frequencies (in the 2 kHz range) are weighted higher. This weighting and √ scheme was developed to more accurately represent the fhigh = fc × 2

A. The Cooper Union for the Advancement of Science and Art: 1 Everest, F. Alton; Pohlmann, Ken C. “Master Handbook of Department of Mechanical Engineering. [email protected] Acoustics.” 5th ed. 2009: McGraw Hill. pp. 30-31 2

Commonly, center frequencies begin at 31.5 Hz, and each II. Testing Methods and Procedure successive center frequency is equal to twice the previous A. Ambient Sound Pressure Level Measurement center frequency. When plotted on a logarithmic scale, the center frequencies will be equally spaced. Octave band Ambient sound pressure levels were measured with both levels are often split further into third octave bands, which A and C weightings. This was achieved using LabView split the octave bands into three sub-bands. software with the Sound and Vibration Suite. A micro- phone was placed near the center of the room, away from B. Reverberation Time any reflective objects such as walls or desks. The A and A key measure of the sound quality of a room is the C weighted sound pressure level was recorded for ten sec- reverberation time of the room. The reverberation time onds, and the average value of that period was recorded. (expressed in seconds) is the amount of time that it takes The sound pressure level was recorded with a sample rate a sound to decay by 60dB on an unweighted scale. For this of 51.2 kHz, to effectively capture the entire audible fre- reason, the reverberation time is often abbreviated as T . quency range. A schematic of the setup is shown in Figure 60 3.

Computer Sound NI 9233 SPL B&K Type Analog to with Stopped 4189 Free Digital LabView Field Converter Ambient SPL Microphone

60 dB

Reverb. Time

Time Fig. 3. Ambient Sound Pressure Level Measurement Setup

Fig. 2. Non-ideal Reverberation Time Measurement Sound pressure levels were recorded with A and C weightings to compare with the standards set by ANSI S12- 60, which states that core learning spaces with a volume of Because of equipment limitations, it is not always possi- less than 20,000 ft3 should not have ambient sound pres- ble to measure time it takes for a sound to decay by exactly sure levels which exceed 35dBA (55dBC), and core learn- 60dB. If the background sound level of the room is approx- ing spaces with a volume of greater than 20,000 ft3 should imately 60dB, then the sound would have to be 120 dB in not have ambient sound pressure levels exceeding 40dBA order to decay by a full 60dB,which would be roughly as (60dBC). loud as an air raid siren. If measuring in an indoor space, this is obviously not practical. Therefore, it is common B. Reverberation Time to measure a smaller drop (30-50dB), and extrapolate the The reverberation time of various rooms was measured slope of the decay to determine how long it would have using two different methods. The first method used an im- taken to decay by a full 60dB, as shown in Figure 2. pulse sound source, and the decay of the impulse was mea- High reverberation times can be desirable in some situ- sured. The second method used a sustained white-noise ations, such as music performance. In this case, reverber- signal. The signal was suddenly stopped, and the decay ation adds fullness to the music, and can make the perfor- time was measured. Both methods used a combination of mance sound better. Often, reverberation times of large Matlab and LabView to perform the measurements. concert venues can be as high as 1.5-2.5 seconds2. For academic spaces, however, reverberation should be kept B.1 Impulse Measurement to a minimum. Longer reverberation times make it far more difficult to understand speech, and can be detrimen- The easiest method of measuring the reverberation time tal to learning. ANSI S12.60 recommends that reverbera- of a room is using the impulse method. With this method, tion times in academic core learning spaces be no longer an impulse sound source is introduced, and the decay time than 0.7 seconds. Furthermore, it states that any academic of the impulse is measured. To be effective, the impulse space should have the ability to reduce reverberation times should be very loud (greater than 120dB), and should con- to 0.35 seconds, if necessary. tain a broad spectrum of frequencies. Ideally, the impulse source would contain all audible frequencies; however, this 2 ibid. p. 378 is rarely possible with common sources. Common impulse ACOUSTIC CHARACTERIZATION OF 41 COOPER SQUARE 3 sources include gunshots, large capacitor discharges, and B.3 Octave Band White Noise Measurement balloon pops. For convenience and safety, balloons were used as impulse sources for this experiment. White noise reverberation time measurement ensures During the testing process, a microphone was set up in that all frequencies are excited; however, it does not in- the center of the room, away from any tables or walls (to dicate which frequencies are contributing the most to the reduce the effects of direct reverberations). A LabView VI reverberation in the room. To find this information, an oc- was used to record the sound pressure level at a sample tave band reverberation time measurement was performed. rate of 51.2 kHz. Two different sized balloons were used Although the general setup was the same as previous rever- during the testing procedure: a small water balloon with beration time measurements, instead of playing pure white a 5” diameter, and a standard sized “party” balloon with noise, octave filters were applied to the noise. Each set of an 11” diameter. filtered noise was played individually, and the reverbera- tion time for each octave band was recorded. After recording the sound pressure from the balloon pop, custom Matlab scripts were used to convert the sound pres- The octave band filter set was generated using Matlab, sure to an instantaneous sound pressure level. The peak as shown in Figure B.3, such that the upper cutoff fre- value of the sound pressure level was identified, and a lin- quency of the first band intersected the lower cutoff fre- ear regression was automatically fit to the decay. Since quency of the second band at -3dB. This was done for all measurements were less than 60dB above the ambient a total of 9 center frequencies, covering the entire audi- sound pressure level of the room, the regression line was ble spectrum. White noise, generated at a sample rate extrapolated to calculate the final reverberation time of of 48kHz was filtered through each octave band. Each the room. This procedure was repeated using a smaller band was played individually, and the reverberation time balloon, so that the two values could be compared. was recorded. This process was repeated ten times, and an average reverberation time for each octave band was recorded. B.2 White Noise Measurement

Although the impulse method of measuring reverbera- tion time is the easiest, it is extremely difficult to excite all audible frequencies equally. Generally, only a small range of frequencies are excited. Although this may be accept- able for a rough estimation of the reverberation time, it does not fully characterize the performance of the room. To excite a broader range of frequencies, white noise can be used instead of an impulse source. With this method, white noise is played loudly, and is allowed to fill the room with noise. Ideally, the sound would be loud enough that the sound pressure level would be the same regardless of where the measurement was taken. This situation is known as a diffuse sound field, and is ideal for reverberation times measurements. After the sound filed has become diffuse, the noise is suddenly stopped, and the decay is measured. A microphone was setup in the center of the room, and a pair of studio monitors were placed on one side of the room. Matlab was used to generate a white noise signal at a frequency of 48 kHz, which ensured that all audible Fig. 4. Octave Band Filter Bank frequencies would be excited. The noise was generated while simultaneously recording the sound pressure in the room using LabView. After a period of 5 seconds, the Because the original white noise signal was filtered so white noise was stopped, and the decay was recorded. The many times, the resulting octave band signals had far lower setup was similar to the schematic shown in Figure 3 overall acoustic power. Therefore, larger speakers were After recording the decay with LabView, the data was necessary to produce high enough sound levels to accu- imported into Matlab, where the decay was automatically rately measure the reverberation time. Rather than us- identified, and a line was fit to the curve. The maximum ing the Mackie HR824 Studio Monitors, which were used sound pressure level achieved using this method was ap- for previous white noise experiments, larger JBL MP418S proximately 95dB, with an ambient sound pressure level speakers and a Crown XLS402 power amplifier was used. of approximately 65dB. Since the sound pressure level was Because of the large size of the speakers, amplifiers and not 60dB above ambient, the slope was automatically ex- other equipment, the procedure was only performed in the trapolated to calculate the reverberation time. acoustics lab. 4

C. Software attention was given to sound transmission during the de- sign of the building. Extensive Matlab scripts were written to help with the There are two possible methods of lowering the ambient data acquisition and analysis process. The goal was to sound pressure level: add absorptive materials on the in- provide a set of scripts that would allow any user to per- side of the room, or reflective materials on the outside of form acoustic tests with little or no knowledge of Matlab. the room. The first method would not reduce the amount For additional information about the Matlab scripts, the of noise that gets into the room; however, it would reduce following commands can be used: the reverberations of that noise, reducing the amount of >> help balloon_pop_analysis time the sound stays in the room, and also the overall am- >> help octave_band_reverb_time bient sound pressure level. This method would also reduce the reverberation time of the room, if necessary. III. Results and Discussion The second method would tend to reflect any sounds A. Ambient Sound Pressure Level from outside of the room back at the source, reducing the amount of noise that ever reaches the room. Although The ambient sound pressure level of various classrooms, effective, it may be difficult to do this approach, since the laboratories, study spaces, and common areas is shown in building has already been constructed. Furthermore, this Figure 5. method would not help reduce reverberation times in the room.

B. Reverberation Time The reverberation times of various room and common areas was measured using the impulse method. A balloon was used as the impact source. Often, two different size balloons were used. This is due to the different frequency components of the impulse of each source. Larger balloons generally have a wider range of frequency components, while smaller balloons have smaller ranges. Furthermore, larger balloons have higher total acoustical energy, and are able to produce louder impulses3, which made them suit- able for larger spaces (such as the Grand Stiarcase and the Rose Auditorium). The reverberation times various rooms in the building are shown in Figure 6, where “Broad Band” excitation corresponds to the large balloon pops, and “narrow band” excitation corresponds to the smaller balloon pops. More Fig. 5. Ambient Sound Pressure Level of Various Rooms [dB] narrow band measurements were taken, since smaller bal- loons were more readily available. From this information, it is clear that almost none of Figure 6 shows that although most core learning spaces Cooper Unions core academic spaces or ancillary spaces fall within ANSI standards for reverberation time (with conform with the ANSI standard for ambient sound pres- the exception of rooms LL210 and 801), most rooms are sure levels. For many of these rooms, especially those with on the upper end of the standard, and are nearly be- a large number of windows, the ambient sound pressure yond the acceptable maximum reverberation time. The level was heavily influenced by noise from the outside. rooms which were beyond ANSI standards could be con- These recordings were take late at night, when traffic is sidered non-standard classrooms (although they are still relatively light; however, in the middle of a day, sound commonly used as core learning spaces). levels may rise considerably. Room 801 is officially considered a conference room, and Another significant source of noise was the HVAC sys- has glass windows covering approximately 50% of the walls. tem. This noise tends to be quite variable, and the ambient Since glass is much harder than drywall, it reflects sounds sound can change by as much as 10-15 dBA with the cycles much more, leading to an increased reverberation time. of the HVAC. For example: room 504 (which had the low- Room LL210 also had higher than recommended reverber- est ambient sound pressure level) did not have any HVAC ation times. Again, this may have been due to the floor to turned on during the measurement. All other rooms did, ceiling windows on one of the walls of the classroom. which caused the ambient sound pressure level to rise. Reduction of reverberation time is a key component in Finally, although all rooms were tested without any speech intelligibility, and therefore, academic success. It other people present in the room, students’ activities in is essential that reverberation times do not exceed ANSI other rooms were clearly audible, and influenced the av- 3 Patynen, Jukka; Katz, Brian F. G.; Lokki, Tapio. “Investigations erage sound pressure levels. Music and speech could be on the Balloon as an Impulse Source.” Journal of the Acoustical clearly heard through the walls, indicating that improper Society of America 129 (1), January 2011. ACOUSTIC CHARACTERIZATION OF 41 COOPER SQUARE 5

greatest. This information can also be useful when deter- mining how easy it will be to hear and understand speech. A spectrograph of human speech reveals that most of the sounds that we can produce lie between 100-4000 Hz, with most of the sound concentrated between 1000-3000 Hz4, although some speech can even reach frequencies of up to 8000 Hz5. Therefore, if speech intelegibility is the primary concern in classroom acoustics, it is important to have low reverberation times for these frequency ranges. Figure 7 does not show any reverberation times that are drastically above the standard of 0.7 seconds; however, the peak reverberation occurred for the octave band centered at 2,000 Hz, which directly corresponds to the average fre- quency of human speech. Ideally, the opposite trend would be observed, with the lowest reverberation time at this oc- tave band. Reverberation times were originally also recorded for the Fig. 6. Reverberation Times of Various Rooms [seconds] 31.5 Hz, 63 Hz, and 125 Hz, octave bands; however, these measurements were found to be extremely variable, and unable to produce consistent results. Even at the 250 Hz standards; therefore, it is highly recommended that sound band level, the deviation from the mean is extremely wide, absorbing treatment be applied to the interior of highly ranging from 0.35 seconds to nearly 0.7 seconds. Addi- reverberant rooms. This treatment will not only reduce tional measurements would have reduced this error; how- the reverberation time, but may also reduce the overall ever, most of the error was due to imprecise curve fitting ambient sound pressure level in the room as well. and extrapolation of the decay slope. This is likely due to the fact that the low frequency signals were only able C. Octave Band Reverberation Time of raising the overall sound pressure level of the room by An octave band reverberation time analysis was per- approximately 20 dB, requiring a large amount of extrap- formed in the acoustics laboratory (Room 710). In this olation to determine the full reverberation time. Because analysis, the reverberation time of the room was calcu- of the overall low sound level, the signal to noise ratio was lated for seven octave bands with center frequencies at 250 also lower, increasing the chances of random error in the Hz, 500 Hz, 1 kHz, 2 kHz, 4 kHz, 8 kHz, and 16 kHz. measurement. The frequency dependent reverberation times are shown in Figure 7. D. Anechoic Chamber Status The anechoic chamber was recently completed, and is now available for use by the Cooper community. A brief timeline of the of the progress is outlined below: Installation of Floor and Wall Wedges: Installation of a large portion of the floor and wall wedges was com- pleted during the summer of 2010. This was the fastest stage of the process, since the floor wedges did not require any special mounting brackets, and the wall brackets had already been purchased and in- stalled. At this point, the chamber was also being used for storage of some of the remaining sound absorbing wedges, preventing access to the chamber. Finish Installation of Wall and Ceiling Wedges: Construction of the chamber ceased for approxi- mately 6 months while waiting for the delivery of special mounting brackets for the remaining ceiling and wall wedges. These brackets were manufactured to work with the existing mounting system that the sound absorbing wedges used. These wedges Fig. 7. Average of 10 Octave Band Reverberation Time Measure- ments 4 Everest, F. Alton; Pohlmann, Ken C. “Master Handbook of Acoustics.” 5th ed. 2009: McGraw Hill. p. 74 5 Lord, Harold W.; Gately, William S.; Evensen, Harold A. “Noise The purpose of this study was to determine the frequen- Control for Engineers.” 1987: Krieger Publishing Company. pp. cies which effected the reverberation time of the room the 47-53 6

were moved from the anechoic chamber in the old engineering building at 51 . Although the ceiling is sloped, allowing the wedges to be higher at one side than at the other, the wedges were all installed at the same elevation. Modify Existing Wall: Before a door could be installed in the chamber, the existing opening for the door was widened to allow for the non-standard size of the chamber door. Additionally, portions of the opening were closed off, since the door was installed approxi- mately 2.5’ above ground level. Installation of Doors: Two doors are used on the ane- choic chamber. The outer door is constructed with a metal shell with insulating material on the inside, and is approximately 2” thick. This door forms a tight seal with the frame, preventing outside noises from Fig. 9. Anechoic Chamber Fire Suppression entering the chamber. An interior door is also in- stalled, which is constructed with a light metal frame, and has sound absorbing wedges installed on the face. Data lines and power outlets were also included inside This door reduces internal reflections that would occur the chamber, allowing for computers and other equipment off of the metal exterior door, and further attenuates to be used inside the chamber. The single data connection sounds entering from the exterior. is currently hanging from the ceiling of the chamber, while Lighting in the anechoic chamber is provided by a single the power is supplied on two lower corners of the room. incandescent bulb (shown in Figure 8), hanging from the The anechoic chamber was finished in mid April. Al- ceiling at the center of the chamber. Incandescent lighting though extensive testing has not yet occurred, initial re- is important, since florescent bulbs tend to produce an au- verberation time measurements were performed in the ane- dible hum. Other forms of lighting, such as LED lighting, choic chamber. Ideally, the anechoic chamber should have are sometimes used in anechoic chambers; however, these zero reverberation time (since no echos or reverberations lighting systems are often more expensive and complicated should occur in the chamber). Initial measurements pro- to install. Additionally, LED lighting requires transform- duced reverberation times which were less then 0.01 sec- ers to provide DC power, which can also produce audible onds. This is the lowest detectable reverberation time hums. with the current calculation algorithm, which calculates instantaneous sound pressure levels in 0.01 second inter- vals. Therefore, the actual reverberation time of the room was unmeasurable using current equipment and software.

IV. Conclusions Based on the overall ambient sound pressure level and re- verberation time measurements, Cooper Union’s new aca- demic building does not meet ANSI standards for academic acoustical characteristics. Although most rooms were just below the maximum limit set for reverberation times, al- most every room that was tested failed to fall below the ANSI standard for the ambient sound levels of the room. In many cases, this was due to external noise from the city, and in almost all cases, the HVAC system seemed to be at fault. In order to bring the classrooms, laboratories, and study Fig. 8. Anechoic Chamber Lighting areas into compliance with ANSI standards, it is recom- mended that sound absorbing panels be installed in the A fire suppression system was also included in the new most reverberant rooms. This would help lower the re- anechoic chamber. Although the sound absorbing wedges verberation time in the room, and also reduce the overall are not flamible, a sprinkler was installed in the center of ambient sound levels. the room, near the light. The sprinkler (shown in Figure 9) will cause small reflections of high frequencies; however, V. Suggested Future Work since the sprinkler is mostly hidden inside a sound absorb- The goal of this project was not only to characterize the ing wedge, the effect should be minimal. building’s acoustical properties, but also setup a system for ACOUSTIC CHARACTERIZATION OF 41 COOPER SQUARE 7 future testing. This project focused on developing a soft- ware system that would allow acoustical measurements to be made by users without significant knowledge of acous- tics or programming. In order to get an accurate representation of the acous- tics of the building, it is important that all core learning spaces be tested for both reverberation time as well as ambient sound pressure level. This can be accomplished using the software that has been created for this project, or by using sound level meters. Current sound level me- ters available in the Cooper Union Acoustics Lab have not been calibrated recently, and do not have the capability of measuring below 60dB. Therefore, in order to make accu- rate and meaningful measurements, it would be necessary to purchase a new sound level meter capable of performing the measurements described in the report. Additionally, it would be useful to perform octave (or even third octave) band frequency reverberation time mea- surements on each of the rooms. This process could shed light on the specific frequencies that are problematic, and determine whether these frequencies interfere with human speech. It can also help with the selection of sound absorb- ing panels (if necessary), since the effectiveness of most of these products is frequency dependent.

VI. Acknowledgements I would like to thank my advisors: Professor Wei and Professor Baglione for their guidance and assistance with this project. I would also like to thank Jody Grapes and the staff of buildings and grounds for their help with the completion of the anechoic chamber.

VII. References [1]ANSI/ASA S12.60-2010/Part 1: Acoustic Perfor- mance Criteria, Design Requirements, and Guidelines for Schools. [2] Everest, F. Alton; Pohlmann, Ken C. “Master Hand- book of Acoustics.” 5th ed. 2009: McGraw Hill. [3] Patynen, Jukka; Katz, Brian F. G.; Lokki, Tapio. “Investigations on the Balloon as an Impulse Source.” Journal of the Acoustical Society of America 129 (1), Jan- uary 2011. [4] Lord, Harold W.; Gately, William S.; Evensen, Harold A. “Noise Control for Engineers.” 1987: Krieger Publishing Company. pp. 47-53