AES Section Meeting Reports

District of Columbia - April 3, 2009

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3 April 2009 AES-DC/ASA-DC Meeting: The Clarice Smith Performing Arts Center Tour, Plus an Acoustics Lecture
by Fred G. Geil (AES-DC Secretary) and David J. Weinberg (AES-DC Chair)

On 3 April, about 40 members and guests attended a joint meeting of the AES and the Acoustical Society of America (ASA) Washington DC sections at the eight-year old Clarice Smith Performing Arts Center ( at the University of Maryland's College Park campus. The meeting included a tour, a dinner/lecture at a nearby restaurant, and a choice of concerts following the lecture.

Kristine Eckerman, Assistant Manager for Audio Services, shared some history. Kirkegaard Associates ( served as acoustical consultants; Ed McCue, Kirkegaard Associate and Principal Consultant, was the project manager. The center comprises seven halls, each having been designed for a specific type of performance, such as orchestral, chamber music, dance, opera, etc. The underlying philosophy was that while diverse performing groups had to be accommodated, they would be staged in the hall that was appropriate to the production rather than attempting to make each space universal. The facility also houses 30 classrooms, 50 rehearsal and practice rooms, plus 100 faculty studios and offices, all designed with consideration for acoustics, sound isolation, and mechanical noise and vibration control.

Dekelboum Concert Hall.
The Dekelboum Concert Hall, the center's largest at about 860,000ft3 (approximately 148'Lx70'Wx83'H; essentially rectangular with curved ends and a cathedral ceiling) is optimized for orchestral music. It seats 900 in floor and single-balcony seating, plus 200 in the choir loft behind the stage.

This auditorium has lots of wood surfaces, some of them serving as convex diffusers. The very high ceiling increases the room's volume and hence reverberation time, accounting for the liveness heard during the presentation. The room can be damped by up to 30 sound-absorbing curtains, with 12 presets simply recallable. All the curtains were closed during our visit, minimizing RT (because the curtains are mounted against the walls at the interior designer's insistence, they have no substantial effect below mid-frequencies). Even so, the room is not dead enough for optimal speech or operatic intelligibility. Mid-frequency (500, 1kHz and 2kHz octave bands combined) reverberation time in the unoccupied hall can be adjusted from just under 2 seconds to about 2.5 seconds.

The original design called for a horizontal concrete ceiling below the high-pitched roof, but the faculty's request for an organ to be installed above the choir seats caused the architects to adopt the visual vocabulary of a highly vaulted cathedral rather than a classically proportioned shape. As an acoustical compromise, sections of a lighter-weight fabric canopy, stretched between the roof trusses, were designed to more quickly return high-frequency energy to the performers and audience while allowing lower frequencies to take advantage of the whole hall volume.

While no sound reinforcement was in the original design, wiring was installed just in case. This proved providential, as some performances in this hall require electronic support, which led to adding a mixing board in the rear of the seating area. Demonstration of the system revealed fairly natural-sounding speech, although a bit too reverberant.

The hall is dead silent, thanks to the large-volume low-velocity overhead supply vents, with 'mushroom' return vents under the seats.

Eckerman stated that most performing groups like the concert hall's sound. In response to a question about sound levels she noted that while 85dBSPL max is maintained during mixing, the U of MD jazz band reaches 90dBSPL without amplification.

Gildenhorn Recital Hall.
The second performing space we visited was the Gildenhorn Recital Hall, a smaller space with balcony, having 300 seats and a ceiling 60' up.

Diffuser panels are employed at the rear of the hall, behind the stage, and overhead. Reflecting panels are behind the overhead diffuser panels, and the side walls deliver lateral reflections. There are 10 acoustical curtains, not all of which were deployed.

This space is considered ideal for chamber groups and jazz. The area under the balcony is said to suffer from decreased mid- and high-frequencies. There is a microphone-pair permanently hung overhead near the stage edge, which recordists are encouraged to use rather than deploying additional microphones. There is a recording room with video monitoring of the stage. When necessary, a mixing desk can be installed at the rear of the seating area.

Along the back wall in each of these two halls are quadratic diffusors, oriented to diffuse incident sound vertically, not horizontally. By email, McCue explained that this orientation attenuates late returns back to the sending end of the hall from various vertically offset sources (such as soloists downstage, elevated choirs, suspended speakers) more dependably than if the diffusion was horizontal.

Not visited were five additional spaces which include dance studios and two "black box" theaters. All seven spaces can be monitored or tied together for audio and video through the patch-bay room. About 90% of the audio/video staff are students from a variety of technical and non-technical disciplines.

Acoustics Lecture.
During dinner, held at the Seven Seas Restaurant in College Park, MD, Neil Thompson Shade (President and Principal Consultant, Acoustical Design Collaborative,, and professor at the Peabody Institute of the Johns Hopkins University; see photo) presented a tutorial on metrically and aurally evaluating a performance space. Following a succinct history of acoustics from Sabine through Beranek, Schultz, Goettingen, Ando and others, he discussed parameters including strength (a measure of overall mid-frequency loudness in the hall vs the same in an anechoic environment; 5-6dB is considered appropriate in a concert hall), reverberance, clarity (ratio of early reflected energy to late reflected energy; the first 80ms for music, the first 50ms for speech; +2dB to -2dB is considered good), spaciousness (IACC and envelopment, perception of which is level dependent), intimacy (including the ITDG — 25ms is optimum for orchestral music), warmth (bass ratio=level in the 125/250Hz octaves divided by level in the 500/1000Hz octaves), critical distance, reflections, and background noise. (Unfortunately, background noise at the restaurant was quite high). Shade also addressed hall size (the tradeoff between the number of seats -- the amount of audience absorption — vs overall loudness), shape and materials.

Attendee questions centered on shared opinions of several concert halls — which are considered good and which are not. Shade considers the Dekelboum concert hall "too reverberant" and in need of a canopy over the stage, which would also decrease the hall's reverberation. He observed that there is often a compromise between achieving optimum acoustics on stage and in the house. Shade ended his lecture by explaining the subjective theater evaluation form he developed, suggesting we make copies and use it for our own edification.

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