Aidan Beckett, Leon Li, Katarina Lidrbauch
The Balch Arena Theater is a theater-in-the-round style arena surrounding a circular stage. There is a catwalk style balcony above the entry level, and various entrances and exits on the entry and stage levels. It has the capacity for 212, including standing room and seats on the entry level. There are six main materials on the surfaces of the floor, walls and ceiling. Three of the materials are fabric-based, one is vinyl, one fiberglass, one is concrete and the ceiling is vented. There is plexiglass along some of the railings. There are several doors throughout the theater, most of which are covered in steel and have some rubber sealing. The stairs are plastic and lightweight. Sets are often present on the stage, most of which include some fabric or absorptive materials such as furniture or curtains. There are curtains that line most of the wall surface area around the perimeter of the theater. The ceiling is two stories above the entry level, and features piping and lighting fixtures, some of which are covered by quilted absorbers.
The Balch Arena was designed to be a space for varied types of performances. The space features complex overhead theatrical lighting, and a sunken stage a level below the entry. The most common uses of the room are for theater groups, classes, and speaker events. An average size of a theater group is about twenty people, while musical products often feature an orchestra pit of 20-30 players. The room is capped at 212, although sometimes audience members are allowed to stand along the railings for more popular shows. Rows of chairs on descending steps face the stairs in a circular fashion, with aisles dividing them into four sections. The doors are usually closed for the duration of performances, and the entrance has two sets of doors. Speakers, comedians, actors and even the pit musicians usually use microphones to project in this space. The stage, plexiglass and walls in the room provide reverberation, but the curtains on the walls and risers, the absorptive materials in the second level, and the audience members provide absorption.
Photo from Tufts Department of Theater Dance and Performance Studies source
In order to characterize the acoustic properties of the room, we made both qualitative and quantitative assesments. We listened to each others voices from different locations in the room, as well as a harmonica being played from stage. We took measurements on both the physical dimensions and acoustical impulse response of the room. We used an omnidirectional microphone as a reciever and baloons as an impulse source to measure an impulse response recorded from two different locations in the space. One of us stood in the center of the stage with the balloon and the other sat in the center of the audience area with the microphone. We recorded 2 balloon pops from one microphone location, and then the reciever moved to another location in the audience, this time off to stage left and higher up. We ran into some issues with mechanical background noise as well as the sound of the balloon hitting the floor after being popped, but tried to mitigate these issues by moving away from the source of the noise and recording multiple pops.
In order to measure the physical dimenions of the space we used a tape measurer to measure the smaller components and then used the geometrical aspects of the room to extrapolate the larger values such as total diameter and height. For example, we measured one of the secions of the walls to be about 2.51 meters long, and there are 26 sections making up the circular shape of the room, so we could approximate the circumference of the room as 66 meters.
The Sabine Equation was used to measure the RT60 of the room using the MATLAB code in the dropdown menu below. It can be noticed that the Sabine values for reverberation were significantly higher than the measured values using T20 from the impulse response data. This is most likely due to the approximations we had to make about the geometry and materials in the upper part of the arena. Above the catwalk was very tall and dark, which made it difficult to see how that space was used.
For materials, we separated the components of the space and furniture into the lower level and seating, the balcony, and the upper lighting section. The lower level of seating included empty seats, linoleum, curtain, and painted plywood. The lighting section included ¾ foam, ¼ hard wall panels, and cinder block. The balcony contained curtains, carpet, and linoleum. For the auralization section, we chose to refine the materials and measurements to correct for the unrealistically long reverberation times by scaling down the upper part of the arena and including more absorptive material in the section. We also use seating on more of the surface area of the balcony since the linoleum would be covered by seating rather than include both materials as we did when calculating the sabine reverberation.
Absorptive covering of HVAC pipes
Carpet
Linoleum flooring
Curtains
Absorptive paneling
%% Sabine
d1=0.9411;
r2=19.52;
r1=r2+1.6256;
r3=r2-5*d1;
theta=pi/2;
h1=0.4572;
h2=1.1176;
h3=h2+5*h1;
a2=0.9144;
h4=3.048;
h6=2*h4;
seat_part=theta/2/pi; %portion of the circle without seats
% lower level (seating)
s_floor=pi*r3^2+(pi*r2^2+pi*r3^2)*seat_part; %unseated area on the outer rim + inner stage area (floor)
carpet_floor=(pi*r2^2+pi*r3^2)*(1-seat_part); %seated area's floor area (linoleum)
carpet_side=(h1*pi*2*(r3*5+d1*10)+h2*pi*2*r2)*(1-seat_part); %side of the seating area (linoleum)
curtainlow=(h3*pi*2*r2)*(seat_part); %side of the seating area (linoleum)
hallway_floor=a2*(r2-r3)*3; %floor of the 3 hallways (floor)
hallway_side=(r2-r3)*5*h1/2*6; %sides of the 3 hallways (painted plywood/linoleum)
%balcony
ceiling_balcony=pi*r1^2-pi*r2^2; %Ceiling of the balcony (?)
side_balcony=pi*r1*2*h4; %Side walls of the balcony (half hard wall, half curtain)
floor_balcony=pi*r1^2-pi*r2^2; %Floor of the balcony (carpet)
%Top Lighting Part
ceiling_top=pi*r1^2; %Ceiling of the lighting area (?)
side_wall=pi*r1*2*h6; %Side panels of the lighting area (3/4 foam 1/4, hard wall)
s_flooring=s_floor+hallway_floor;
s_linoleum=carpet_floor+carpet_side; %/sqrt(2) to be seating area
s_wood=hallway_side;
s_curtain=1/2*side_balcony+curtainlow;
s_hardwall=1/2*side_balcony+1/4*side_wall;
s_carpet=floor_balcony;
s_foam=3/4*side_wall;
s_ceilings=ceiling_balcony+ceiling_top;
%Volume
hi1=r2/(r2-r3)*h3;
hi2=hi1-h3;
v1=pi*r2^2*h3/4+(pi*r2^2*hi1/3-pi*r3^2*hi2/3)*3/4;
v2=pi*r1^2*(h4+h6);
v=v1+v2;
theory=[1.347639295 1.060559141 0.950891634 0.863391883 0.83241526 0.911386871];
We plotted energy decay below and calculated T20, EDT, C50, and C80 values from our data. We recorded and analyzed data from two locations in the room. If we compare the graphs from the closer location with those from the further location from the sound source, we can see that the data from the closer location included more realistic values for reverberation time when looking at the raw unfiltered data. This is most likely due to the HVAC and balloon popping sounds that are included in our recording, and the length of the sample we used.
IR Energy Decay and Schroeder Curve at the 125 Hz Octave Band
250 Hz Octave Band
500 Hz Octave Band
1000 Hz Octave Band
2000 Hz Octave Band
4000 Hz Octave Band
125Hz 250Hz 500Hz 1kHz 2kHz 4kHz
T20 =
1.0389 0.7482 0.7214 0.6822 0.7555 0.6441
EDT =
0.4960 0.4998 0.5009 0.5820 0.4690 0.4161
C50 =
6.7233 5.9378 4.6811 6.0194 7.0720 7.9132
C80 =
9.9472 9.7819 9.9842 10.1567 10.4979 11.4127
BR =
1.2732
Signal to Noise Ratio
The SNR generated shows that for the majority of the spectrum above 82 Hz, the energy of the signal is large enough to ignore the noise while making calculations.
At 88 Hz (cutoff of the 125Hz octave band), the energy of the signal is 10^(13.1126/10)=20.4767 times of the noise.
125Hz 250Hz 500Hz 1kHz 2kHz 4kHz
T20 =
7.3613 1.0304 0.7874 0.7408 0.7945 0.7046
EDT =
1.3696 0.6595 0.6918 0.9079 0.7827 0.6789
C50 =
-0.7196 6.2646 4.6477 1.9872 3.8291 4.0422
C80 =
2.9775 7.6712 7.6560 4.1870 6.4632 7.4104
BR =
5.4912
Data Filtering and Noise Reduction
The raw data seems unrealistic. Over 7 seconds of T20 is just impossible. Upon reviewing the Schroeder decay curve, two methods were taken to reduce the effect of noise on T20 and EDT issues.
The first method is to use a modified t20 calculation method by setting the threshold at -18dB instead of -25dB. The linear (log scale) decay region is short enough due to the presence of noise that setting the threshold at -25dB is not going to be capable of capturing the characteristics of the decay. However, it seems like that the T20 value is still a bit too high due to the energy being added to the entire spectrum, which messes up the C50 and C80 calculations as well.
Another way was to use the noise cancelling algorithm provided by Adobe Audition, with the specs that are listed above. While the signal to noise comparison shows that the filtered signal is not far off from the original at higher frequencies, there are still of a couple of more peaks that are visible in the FFT diagram of the noise print. It is likely that the enclosed space with a lower ceiling could be a source of standing wave with the frequencies between 100Hz and 200Hz, as it correlates to the fundamental standing wave generated by parallel surfaces that are approximately 2-3 meters apart. This may affect the calculation as the signal to noise ratio is much lower at these peaks at lower frequency bands.
Using T13
125Hz 250Hz 500Hz 1kHz 2kHz 4kHz
T20 =
1.6757 1.0228 0.7295 0.6427 0.8297 0.6861
EDT =
1.3696 0.6595 0.6918 0.9079 0.7827 0.6789
BR =
1.9665
Using T13 and Noise Filtering
125Hz 250Hz 500Hz 1kHz 2kHz 4kHz
T20 =
0.8211 0.7939 0.7022 0.6558 0.7474 0.6753
EDT =
1.0936 0.5725 0.6674 0.9028 0.7557 0.6702
C50 =
-0.6722 7.1915 4.8455 2.1146 3.9858 4.1149
C80 =
4.1213 8.7025 7.9654 4.3287 6.7112 7.5310
BR =
0.8402
Spot 1: STFT Spectrogram showing high frequency spike
Spot 2: STFT Spectrogram showing mid frequency spike
We noticed there were a couple of extra peaks observed through the STFT (short-time fourier transform) diagram with LabView and similar plots of Adobe Audition. Audibly the only sound that is physically provided by actions of the test is the sound of the popped balloon making a contact with the ground. The other quieter frequencies could come from the reflections off of the harder walls around the higher position of the measurement. Whether this could be an echo effect is uncertain as the amplitude of the sound peaks are not large enough to be audible.
Upon plotting the time domain response of the higher frequency bands, it is also discovered that there are multiple peaks occurring possibly due to direct reflection from the wall at the back. The 3.3ms spacing also matches the roughly 0.566 m distance between the microphone and the wall.
Balch Arena has a relatively short reverberation time, which makes for a dry and dampened auditory experience. Sources and listening experiences in different parts of the space have different results, as the materials used in the treatments and furnishing of the room are not symmetric. There is a lot of noise from HVAC and lighting in the upper third of the space. We were able to hear more acoustical support when the sound source was closer to the bare wall rather than the side of the area covered by thick curtains, which we anticipated. We found ourselves having to speak loudly to hear when communicating from the stage area to the main level, and similarly between these levels and the upper catwalk. From attending shows in this space we remembered most using microphones, and with a full audience we would anticipate even more absorption during a live performance since the seats are only somewhat absorptive when empty. The space is much larger than it appears when looking all the way up to the ceiling above the HVAC pipes, so the sounds of our voices at a normal volume level have to travel much farther in this direction, for example speaking from the main level up to the catwalk, and reflect off surfaces that produce mainly diffuse reflections on this upper level.
When thinking about clarity, we noticed it varied dramatically depending on the direction of the sound source. We also noticed by comparison to lower and higher pitched sounds, mid frequencies around the vocal range seemed to project relatively clearly. This is supported by the C50 and C80 values at both locations, because the lower octave bands yield less clarity while the greater octave bands yield greater clarity before dropping off at the highest bands. This makes sense given that most performances are vocal and speaking performances in this space.
The musical listening experience of a single instrument would be different from a larger ensemble, but is still a good representative of the room acoustics. An instrument with a lower volume inside this dry room feels very little support and could not be heard very well in this situation. Articulation is clear, but the overall volume is quite low.
Our first impression of Balch Arena was its surprisingly dry acoustic response considering the size of the space. When listening to a speaker on stage from the audience section, the sound of their voice is not supported by the room, you can mostly just hear the direct sound with very little mid and high frequency reverberations. We did however notice low-frequency sounds tended to linger and accumulate in the room, making it difficult at times to distinguish individual words or sounds.This qualitative assessment lines up well with our measurements. With an average T20 of 0.765 seconds across the six octave bands measured, the sound energy in the room decays at a rapid pace throughout a broad band of frequencies. At lower frequencies we calculated relatively high reverberation measurements. For each calculation at both microphone locations the highest T20 value was found at the 125 Hz octave band, with a bass ratio at the closer position of a whopping 1.96. The T20 measurements at the closer position could be a result of the enormously large space on the top of the room as well as the lack of materials that are absorptive on the lower frequencies in comparison to the higher frequencies. The initial test at the further position still showed an insane T20 value at lower frequency ranges. Our best hypothesis to explain this phenomenon is the fact that there was loud low frequency background noise present during the recording at the farther position. This caused a high ambient sound energy in the low octave bands, influencing our T20 predictions. Since EDT and T13 is not as affected by ambient noise, the EDT and T13 values were more reasonable.
As for clarity, it was very easy to pick out discrete sounds from accross the room. When talking to a group-mate in the balcony from the stage, the group-mate could understand what was being said without trouble. This was supported by the C50 and C80 values, with an average of 6.4 and 10.4 dB respectively from the first listening position.
Compared to the measured T20/T13, the calculated sabine reverberation time is longer by a margin of larger than 10% for all frequency ranges. Some room of error could include measuring error, room shape estimation, scattering by the lighting section and volume calculation error. The room was not a square-shaped room, making it much harder to measure. Our take was rather simple by measureing the point to point distances, which may create quite some error when counting in the curvatures. The room is also not a perfect square or an oval. Our estimation was to calculate mean distances throughout different locations for approximating the overall shape to be circle. The top section of the room is a huge empty space in our calculation, where we made the assumption that the lighting section is acoustically transparent. However, the mesh and large amount of lighting may have created a lot more scattering than we anticipated. This could be reflected in the simulations as well as scattering in increased. The last error that may occur is the volume calculation. The arena is extremely tall, and there was not a simply way of measuring the height as it's not only unreachable but extremely dark up top. Our estimation of the total height is based on normal floor heights, which could lead to a major source of error.
Having a high bass ratio in a performance space is generally not desirable as it can negatively impact the overall sound quality of the space. A high bass ratio means that there is an excessive amount of low-frequency sound in relation to the mid and high-frequency sounds. This can result in a boomy and muddled sound, where individual notes or sounds are difficult to distinguish. It can also make it harder for performers and presenters to hear themselves clearly, which can negatively affect their performance. Additionally, a high bass ratio can make it more difficult for sound engineers to achieve a balanced mix, as the low-frequency sounds can overpower the other sounds in the mix. Therefore, it is important to strive for a balanced frequency response in performance spaces to ensure optimal sound quality for all types of performances. We would reccomend methods to decrease low frequency reverberations such as helmholtz absorbers and more elements to scatter low frequency sound.
In conclusion, while the Balch Arena has a dry acoustic environment that can be suitable for certain types of performances, its high bass ratio and relatively high ambient noise levels may limit its versatility and potential for creating optimal listening conditions for a wide range of performances. Therefore, there is room for improvement in the form of balancing the frequency response and reducing extraneous noise to enhance the overall acoustic quality of the theater.
The original T20 values are overall dry. Due to the purpose of the room, it is understandable to have a drier room for speech recognition and more modern music genres. However, the T20 times are also skewed towards the lower end of the frequency spectrum. This could well be welcoming feature for classical music genres that are composed and performed at a time period when bass instruments were not well developed. For the purpose of modern genre music where amplification is largely involved and when the sound engineer usually monitors the sound directly from the mix (room acoustics not involved), it would be great to have a flatter T20 curve across the frequency spectrum in this case. The main goal thus becomes to reduce the T20 on the lower frequencies and the opposite with higher frequencies.
In order to balance the frequency response and improve the acoustical performance of the Balch theater, we propose two possible changes which can be made to the room. The first solution is to replace the heavy curtains which surround the walkway with helmholtz absorbers tuned to the 125 Hz frequency band. This would decrease the low frequency energy reflected back into the room while also removing some of the high and mid range dampening effects of the curtains. This would be a relatively cheap way to have an immediate effect on the room's sound quality.
Our second proposed solution is to add a row of seating to the upper balcony area. There is currently a small balcony surrounding the stage but it is acoustically transparent and not accessible by the general public. If seating were to be added to this balcony, that would increase low frequency scattering and absorption, while also having the added benefit of increaing the limited capacity of the space.
We developed a model of the Balch theater in order to simulate acoustic properties of the space and test how changing design parameters would affect these properties. We used the same measurements and material absorption coefficients we took for the sabine calculations to create the model in sketchup shown below:
We first simulated an impulse response in our original model of the space so we could compare the simulated results to the measured results. We used an omnidirectional source in CATT and placed the source on stage approximately where we popped the balloon and the reciever where we placed our microphone in the audience. Shown below is a comparison between the measured and simulated T20 and EDT values:
Shown below are comparisons of the T20 and clarity values between our three models:
In acoustics, auralizations are a way of simulating the way that sound behaves in a physical space. They involve a virtual rendering of the space in three dimensions, as well as a computer algorithm that models the way sound waves propagate in the virtual space. The model uses the way the waves interact with objects, walls, floors and ceilings, as well as the material properties and absorption coefficients assigned to those materials.
Once the sound wave propagation is modeled, the software generates an audio file of the simulated response. The audio file can then be played back, usually accompanied by a visual representation of the space. Auralizations are used in architectural acoustics to simulate and propose changes to the design of concert halls, recording studios, and other rooms.
We used the software CATT acoustic to create auralizations of a person speaking in the space as well as a jazz band playing in the orchestra pit. Jazz band is a very nice example of modern genre music with the involvement of a more presence rhythm section as well as amplified bass. Male speech voice was chosen due to the wider range of frequencies involved. Both sound sources are ideally placed in the middle of the stage. The two reciever positions are: in the audience section right in front of the performance area, and higher up in the audience section on the left side.
For the directivity of the sources we used the "talker" directivity for the male voice and the "wide cardioid" for the jazz band.
To the right are images of the source and receiver locations. The jazz band is modeled by sources A1 and A2, which are located where the orchestra pit would play for a musical performance. The male talker is modeled by source A0, which is near the center of the stage projecting outward into the audience.
Receiver 01 is located where our measurements were taken, in the center of the audience seating. Receiver 02 is located in the proposed balcony seating, as shown for Model 3, and for Model 1 and 2 in the original seating directly below.
Key: Model1 is OG room, Model2 is helmholtz, Model3 is balcony seating, last number is reciever position
Male speaker with original room source and receiver position 1
Male speaker with original room source and receiver position 2
Male speaker with helmholtz absorbers and receiver position 1
Male speaker with helmholtz absorbers and receiver position 2
Jazz band with original room source and receiver position 1
Jazz band with original room source and receiver position 2
Jazz band with helmholtz absorbers and receiver position 1
Jazz band with helmholtz absorbers and receiver position 2
Male talker with balcony seating and receiver position 1
Male talker with balcony seating and receiver position 2
Jazz band with balcony seating and receiver position 1
Jazz band with balcony seating and receiver position 2
Based on the resulting T20 and clarity values of the simulations, it is clear that the adding the helmholtz absorbers has a large effect on the frequency response of the room. The low frequency reverberation is greatly reduced while the mid frequencies are more supported. This is also clearly observable in the auralizations as well.
Although adding the Helmholtz absorbers did block some of the reverberation in lower frequencies, the overall clarity of the sound is still somewhat muddy for the jazz band convolution. This is probably due to the absence of early reflections.
It is reasonable to propose additional seating in the balcony since the structure is already present. This may pose safety risks and require changes to the evacuation plan of the arena, but would be worth exploring since the space has faced issues with exceeding capacity and turning away audience members for popular shows. Adding Helmholtz absorbers to all of the walls currently covered by curtains is somewhat less realistic since they can be expensive to manufacture for custom frequency blocking. This would also take up some additional space in the walkway around the arena, which could make the space more crowded.
All and all, adding both the Helmholtz resonator and seating on the balcony makes the most ideal improvement by adding clarity and mid-range frequency strength in the auralizations which are associated to the "Model 3" of the auralization clips.The auralization was based on a stereo track (jazz band) and a mono track (male voice). The simulation of the male voice clip rendered completely normal, but the convolved audio stereo track of the jazz band actually sounds really skewed to the left. Upon checking the IR, it appears that the IR on the left channel is a lot larger in amplitude than that on the right channel. While in the room, this was not such a big issue, but could be on a pair of in-ear monitors possibly due to the limitations of two seperate channels and the directivity of the simulation. This issue is less presented when wearing the over-head headphones. This could also be a problem associated with earphone tunings.
While these auralizations attempt to simulate how the room would sound with the proposed changes, they include many assumptions and estimations. For example the area of the seated balcony is modeled as absorptive seating, although there would likely be some reflective materials in the structure of the balcony if it were to be constructed. Another assumption that is carried through all of our measurements and simulations is the estimation of the upper volume of the arena. As we mention earlier, it was difficult to take measurements and see above the HVAC and lighting equipment, so we used our best judgement to fill in these gaps.
If we were to iterate on this design or propose further improvements to the space, it would be beneficial for us to obtain a floor plan or more precise measurements. We would also want to broaden our understanding of scattering, which would allow us to make better estimations for the upper part of the arena since the lighting section likely has a lot of scattering resulting from the reflective surfaces that we omitted in our model. Finally, combining the elements that improved our auralizations into one refined improved design would require features that increase early reflections and decrease low frequency reverberation. In these initial simulations we focused on two distinct improvements in order to see how they changed the acoustic response of the space independently, but combining these would give a better result as far as acoustic measurements and listening quality for the audience.