Lab #6 – Room AcousticsPhysics of Music PHY103 Lab ManualLab #6 – Room AcousticsEQUIPMENT- Tape measures- Noise making devices (pieces of wood for clappers). - Microphones, stands, preamps connected to computers.- Extra XLR microphone cables so the microphones can reach the padded closet and hallway.- Key to the infamous padded closetINTRODUCTIONOne important application of the study of sound is in the area of acoustics. The acoustic propertiesof a room are important for rooms such as lecture halls, auditoriums, libraries and theatres. In this lab we will record and measure the properties of impulsive sounds in different rooms. There are three rooms we can easily study near the lab: the lab itself, the “anechoic” chamber (i.e. padded closet across the hall, B+L417C, that isn’t anechoic) and the hallway (that has noticeable echoes). Anechoic means no echoes. Ananechoic chamber is a room built specifically with walls that absorb sound. Such a room should be considerably quieter than a normal room. Step into the padded closet and snap your fingers and speak a few words. The sound should be muffled. For those of us living in Rochester this will not be a new sensation as freshly fallen snow absorbs sound well. If you close your eyes you could almost imagine that you are outside in the snow (except for the warmth, and bizarre smell in there).The reverberant sound in an auditorium dies away with time as the sound energy is absorbed by multiple interactions with the surfaces of the room. In a more reflective room, it will take longer for the sound to die away and the room is said to be 'live'. In a very absorbent room, the sound will die away quickly and the room will be described as acoustically 'dead'. The time for reverberation to completely die away will depend upon how loud the sound was to begin with, and will also depend upon the acuity of the hearing of the observer and the ambient noise level of the room. In order to provide a reproducible parameter, a standard reverberation time has been defined as the time for the sound to die away to a level 60 decibels below its original level. The reverberation time, RT60, is the time to drop 60 dB below the original level of the sound. The reverberation time can be measured using a sharp loud impulsive sound such as a gunshot, balloon popping or a clap. Why use 60dB to measure the reverberation time? The reverberation time is perceived as the time for the sound to die away after the sound source ceases, but that depends upon the intensity of the sound. Tohave a parameter to characterize a room that is independent of the intensity of the test sound, it is necessaryto define a standard reverberation time in terms of the drop in intensity from the original level, i.e., to define it in terms of a relative intensity. The choice of the size of the relative intensity drop to use is arbitrary, but there is a rationale for using 60 dB since the loudest crescendo for most orchestral music is about 100 dB and a typical room background level for a good music-making area is about 40 dB. Thus the standard reverberation time is seen to be about the time for the loudest crescendo of the orchestra to die away to the level of the room background. The 60 dB range is about the range of dynamic levels for orchestral music.What is a good reverberation time for a room? If you are using the room for lectures (speech) thena long reverberation time makes it difficult for the audience to understand words as the echoes interfere. However a long reverberation time adds character to spaces such as churches where organ music is played. Reflective surfaces lengthen the reverberation time whereas absorption surfaces shorten it. A larger room usually has a longer reverberation time because it takes longer for the sound to travel between reflections. Rooms that are good for both speech and music typically have reverberation times between 1.5 and 2 seconds. The reverberation time is influenced by the absorption coefficients of the surfaces in a room, but italso depends upon the volume of the room. A small room would not have a long reverberation time. An example of a large room with reflective surfaces that has a long reverb time (and so is constantly unpleasantly noisy) would be Wilson commons. Although it is visually striking this building is atrocious acoustically. It would be possible to improve the acoustics of this space by hanging artwork made of absorptive materials. The new biomedical and optics building (Georgen) has a similar problem -- all those beautiful glass surfaces are highly reflective acoustically (and so you can hear the growl of thePhysics of Music PHY103 Lab Manualespresso machine from the coffee shop everywhere in the building). It seems that some recently built buildings on campus are designed by architects who have neglected the acoustics of the spaces. I suspect that it might be possible to compensate for visually striking but acoustically reflective building materials with cleverly placed acoustic absorbers hidden in the ceilings behind the lights or boldly in the open as 3D structures on opaque walls. Predicting the reverberation time and Sabine’s formula.Sabine is credited with modeling the reverberation time with the simple relationship which iscalled the Sabine formula:60(0.16sec/ m) (0.049sec/ ft)e eV VRTS S= =(Equation 1)This formula relates the reverberation time, RT60, to room volume and an effective area. You use the 0.16 sec/m coefficient if you are working in meters. You use the 0.049 sec/foot coefficient if you are working in ft. Here V is the volume of the room and Se is an effective area. The effective area is calculated as follows1 1 2 2 3 3...eS a S a S a S= + + +Here each area Si has an absorption coefficient ai. The effective area is a sum of areas, Si , each with its own absorption coefficient ai. These areas are the surfaces in the room (ceiling, walls, floor, seats, people, etc…). Another way to write the effective area is with a sumsurfaces e i iiS a S=�Note you must put the areas (Si ) in the same units as the volume V (meter2 for area and meter3 for volume or ft2 for area and ft3 for volume). When a sound wave in a room strikes a surface, a certain fraction of it is absorbed, and a certain amount is transmitted into the surface. Both of these amounts are lost from the room, and the fractional loss is characterized by an absorption coefficient, a, which can take values between 0