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The Acoustics Laboratory
The Cooper Union Audio Lab (CUAL) at The Cooper Union's prestigious Albert Nerken School of Engineering ranks as one of the finest laboratories devoted to education and research in the field of recorded and reinforced sound. The facility includes a large walk-in, full-surround anechoic chamber, characterized by a cut-off frequency of approximately 140 Hz, and an echo chamber with a reverberation time T60 > 4.5 sec. Instrumentation includes a full complement of Bruel and Kjaer sound and vibration instrumentation, Fast Fourier Transform (FFT) analyzers, signal generators, four Mackie 824 Studio Monitor loudspeakers, numerous test loudspeakers, test microphones, portable DAT, portable binaural broadcast quality microphone systems, and a fully operational DJ booth including a Urei 1620 mixer and three shock mounted Technics SL1200-M3D turntables and two Pioneer CDJ700 CD players. Computer software includes Pro Tools, Sound Forge, and MATLAB. In addition, the laboratory benefits from a high-speed internet connection and a substantial acoustics library which includes a complete set of the Journal of the Acoustical Society of America. A well-equipped tool/storage room provides a site for maintaining and repairing instruments. Also, the Department of Mechanical Engineering operates a complete state-of-the-art machine shop, and offers available resources in electronic circuit design and fabrication, and laser velocimeters to measure sound correlates.
Ongoing research projects include the following:
“Dry” recordings of speech, music, and sound effects made in the anechoic chamber using a remote located Pro Tools system
Design, development and testing of new loudspeaker designs
Visualization and research of the acoustical properties of cymbals and development of new technologies to record their motion and radiated sound
Development of high fidelity wearable sound systems
Measurements of reverberation times of the various interior spaces at The Cooper Union and development and installation of engineering solutions to improve the acoustics
Electronic design of advanced acoustical devices for large scale sound reproduction
Support for art and architectural installations and projects
EID 160 Student Projects – Cooper Union Audio Lab - Spring 2002
Helmholtz resonator
A Helmholtz resonator was constructed from PVC piping. The design permitted changes to the resonator’s volume, neck length, and neck area. The device was excited by a stream of air directed across the neck, then later by a loudspeaker. The design was iterated to optimize performance taking into account effects of viscous boundary layer damping in the neck, radiation resistance of the open end, and frequency of oscillation. Tests were conducted to compare measured results with theoretical predictions, including length corrections required by the neck.
Design, fabrication, and testing of an electronic music mixer
Circuit diagrams were obtained for various DJ (Disc jockey) mixers and tone control circuits. Statistical frequency analysis and spectrograms were conducted on relevant musical program material to determine the center frequency and bandwidths of certain instruments. These results drove the design of specialized controllable parametric filters to be used in a new DJ mixer design. The component circuit was fabricated and tested in laboratory and live music conditions.
Design, fabrication, and testing of a new tri-amplified loudspeaker design
A group of students developed a new design for a single unit loudspeaker system which is intended to rest on the floor or be hung overhead, providing uniform sound coverage over a hemispherical zone around the unit from 50-15,000 Hz. After a literature and product search, 13 component loudspeakers were acquired: 1 woofer, 4 midrange, and 8 tweeters. The mounting for the speakers was designed, and the entire system was fabricated and tested. Testing included determination of relative gains, crossover frequency points and roll-offs
Acoustical characteristics of Hi-Hat cymbals
A closed Hi-Hat cymbal system was constructed and attached to a shaker so vibrational modes of the cymbals could be excited. The cymbals were tested in open, just contacting, and tightly contacting configurations. For each, a swept sine wave excitation permitted determination of mode frequencies and approximate quality factors. By sprinkling blue sand on the cymbal surface while vibrating, the classic Chlandi figures could be clearly observed revealing the mode shapes including the very complex nodal lines at the higher frequencies. This information was utilized to permit accelerometer and laser velocity measurements of correlates of radiated sound.
Improvement of the acoustical characteristics of Room 202
Room 202 of the Engineering building at The Cooper Union was inspected and tested for speech intelligibility, background noise, and reverberation time. It was found that the room suffers from a reverberation time in excess of 3 seconds, with the ideal for a lecture room being less than 1 second. Further, excess background noise was found due to air circulation vents in the window sills. Tests of speech produced at the front of the room and heard at the back confirmed that the resulting acoustic properties of the room’s interior were problematic. Theoretical calculations predicted that if 50% of the ceiling was covered with absorbing foam panels, the reverberation time would be adequately reduced. Small wooden barriers with one side covered in foam were designed to block mid and high frequency noise from the air vents. The foam and barriers were installed and the room was retested. The reverberation time was reduced to .9 seconds and the background noise from the air vents was reduced by 7 dB – the acoustics of the room was greatly improved.
Performance metrics of non-linear electronic devices
Utilizing a high performance dual-channel dynamic signal analyzer, two electronic music mixers were evaluated to determine what non-linear properties were responsible for their relative subjective properties. Typically, no explicitly non-linear metrics are professional electronic audio equipment. Interviews with professional musicians suggested that of the two mixers, one of the circuit designs permitted a greater non-linear summation of different music signals resulting in a superior sound. This led to tests of level dependent frequency response, coherence, and confidence. After the objective evaluation was complete, side-by-side calibrated A/B subjective testing of the two units was conducted. A plan is in place to have a public demonstration in the future for industry professionals.
Wearable full-range sound system
Conventional stereo headphones can be used to recreate the ear reception free-field listening experience down to very low frequencies, but they do not reproduce the sensation of sound produced by low frequency, large wavelength sound applying pressure over the body. Various devices attempt to do this but they all apply the forces locally, unlike actual sound pressure fields at large wavelengths which apply pressures somewhat uniformly over the body’s exterior. A device was developed and tested to achieve this using a circumferentially actuated belt. It successfully produced inward radially-directed pulsating forces spread over a large body area. The effect was to better recreate the real experience of listening to sound in free-field using a wearable system.
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