Acoustical Analysis of McKinnon Theater

Laboratory project for PHYS-583, Applied Acoustics Laboratory
peformed during the Spring 2000 term by:
Nate Dau (AP '01), Ya-Jiang Bemmen (AP/ME '00), and Eric Kendall (ME '00)
surpervised by: Dr. Dan Russell
Kettering University, Flint, MI.

Some preliminary sound recordings were made during the 1998-1999 academic year by Kettering University's Society of Physics Students.

The contents of this web page (text, images, and sound files) are ©2001 Dan Russell

This web report is still under construction: some text and images may not yet be complete. Last updated: March 1, 2001

Description and photos of the theater

 McKinnon Theater is a 400 seat auditorium on the campus of Kettering University, Flint, MI. The theater is used primarily for student assemblies, special speakers and events, small musical performances, plays, and showing movies. The acoustics of the theater have some serious flaws, including a remarkable flutter echo, and student groups have been seeking funds for improvements in the sound reinforcement system. However, as we will show in this web-report, there are some acoustical characteristics of the room itself which cannot be corrected with a sound system. During the 1999-2000 school year the school received a sizable donation to fund a cosmetic renovation of the theater interior. The renovation was to take place during July 2000 and included replacing the seating and carpet, and the installation of a new sound system. As part of our Acoustics Laboratory course during the Spring 2000 term, we took the opportunity to make detailed measurements of the flutter echo and of the reverberation time of the theater before the renovation took place. After completing our analysis, we made some recommendations to the administration for modifications which might fix the flutter echo problem, with hopes that our suggestions might be incorporated into the renovation project, resulting in an improved acoustic performance of the auditorium. A follow up measurement will be made by another group of students after the renovations have been completed.

The photo at right is a composite image showing the view of the auditorium from the stage looking out into the audience. Current condition of theater is result of renovation completed in 1980. Wall sections along the sides of the auditorium are covered with acoustic tile panels and wall sections are angled slightly inward (the walls are not quite parallel). At the rear of the auditorium (visible as the seven cones of light at the back) is a slightly concave curved wall, which is not covered with any acoustically absorbent material, but instead is left as a hard, smooth, reflecting surface.

Click on the image to see a larger version

The photo at right shows a close up view of the problematic concaved curved back wall. Notice that the wall is smooth and hard with absolutely no sound absorption at all. The fact that the wall is smooth and hard means that it reflects sound just as a mirror reflects light. The fact that the wall is concave means that a spherically expanding sound wave originating on stage will be reflected back towards the source rather than reflecting and continuint to spread/expand throughout the rest of the auditorium as it would from a flat surface. The concave shape tends to focus reflected sound back towards the center of the stage rather than redistributing it throughout the rest of the auditorium.

Click on the image to see a larger version

The photo at right is a composite image showing the view of the front of the auditorium while standing at the very back. Visible at the front of the auditorium is the large movie projection screen. Behind the screen is a hard brick wall. As our measurements will show, the current flutter echo problem exists only when the curtains covering the screen are open, exposing the screen. When the curtains are closed, the flutter echo problem completely disappears. However, one of the primary uses of the auditorium is for showing movies which requires that the curtains be opened and the screen exposed. As a result, the intelligibility of sounds in the auditorium noticeably decreases when the curtains are opened.

Click on the image to see a larger version

The photo at right shows a close up view of the chairs in the theater prior to renovation. The chairs have metal backs and frames with cushioned seats.

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Measurements of the Flutter Echo
Standing on stage while facing the rear of the auditorium and clapping one's hands reveals a remarkable acoustic feature of McKinnon Theater. When the curtain in front of the movie screen is open, a hand clap produces a very strong flutter echo as the sound bounces back and forth between the back and front walls. When the curtain is closed, the flutter echo is completely eliminated. This brings out two immediate observations: first is that curtains serve as effective sound absorbers for reducing unwanted reflections, and second is that parallel, hard, reflecting surfaces at opposite ends of a room can cause problems, especially when one of the surfaces is slightly concave.

The photos at right show the stage area with the curtains open (top photo) explosing the movie screen and hard surface, and with the curtains closed (bottom photo). Click your mouse on each photo to hear what a hand clap sounds like with the curtains open and closed. The sound files were recorded with a Tascam DA-P1 DAT recorder using a matched pair of AT4051 audio microphones located in the middle of the audience seating area, with a hand clap from center stage.

The flutter echo is also noticeable for less impulsive sounds, like human speech. The effect can be disconcerting for the speaker on stage, who hears his/her own words reflecting multiple times.


The images below show the time signals recorded for a handclap with the curtain open and closed. The flutter echo is plainly visible in the Curtain Open signal.
  1. On stage widely separated
  2. On stage at close together at center
  3. Middle of seating area widely separated
  4. Middle of seating area close together at center
  5. Back of auditorium widely separated
  6. Back of auditorium close together at center
To listen to each of these recordings, click on the desired microphone location (numbered blue squares) on the floor plan below.

Measurements of the Reverberation Time (RT60)

 We measured the reverberation time (time for the sound pressure level in the room to decrease by 60 dB) using a Stanford Research Systems SRS785 frequency analyzer set to display octave bands on a waterfall plot. We used the internal source to produce a 1 second pink noise burst followed by several seconds of silence. The noise burst was played through a large horn loudspeaker driven by a 100 W audio amplifier. The noise level was measured with a microphone placed in the center of the auditorium about 5 rows from the back curved wall. The analyzer was programmed to take a total of 800 measurements, one every 4 milleseconds, for a total collection time of 3.2 seconds. Using the display features of the octave band analyzer, we were able to display the response of the room as a function of time for each octave band. Results are shown in the images below. The reverberation times were calculated from the slopes of the response curves.
Reverberation Time (TR60) with curtain open and closed.
250 Hz 500 Hz 1000 Hz
2000 Hz 4000 Hz 8000 Hz

This data reveals that opening the curtains significantly increases the reverberation time, especially in the frequency range most important for human hearing and speech perception (1000 Hz to 4000 Hz).

Recommendations for the back curved wall.