Quietness, Comfortable Sound and Excellent Acoustics NAGATA ACOUSTICS

News 08-05 (No.245)

Issued : May 25, 2008

[ Japanese Version ]

Iwaki Performing Arts Center Special Issue
Overview and Room Acoustics of Iwaki "Alios" Phase 1

by Toshiko Fukuchi

On April 8, 2008, in Fukushima Prefecture's Iwaki City, the Iwaki "Alios" Performing Arts Center opened its doors to the completed first phase of this new facility. For older people in Japan, Iwaki City evokes memories of how a major coal mine became transformed in 1966 into a destination resort named Joban Hawaiian Center, now the Spa Resort Hawaiians. Japan's younger generations more likely associate Iwaki City with the 2006 film "Hula Girls," a fictionalized dramatic comedy that chronicles the tribulations of Japanese coal-miner daughters who decided to learn the hula and become dancers at Japan's first ever spa resort.

Counterintuitive to the Hawaiian theme of Iwaki City's famous resort, the city is located in Japan's Tohoku Region. From downtown Tokyo, the Japan Railroad's Super-Hitachi express train travels north for two hours and 15 minutes to arrive at Iwaki City. The current city boundaries encompass areas that were five cities, four towns and five villages until they merged into one municipality in 1966. Iwaki City occupies the southeastern-most portion of Fukushima Prefecture, with a Pacific Ocean coastline forming its eastern border and the Abukuma Mountain Range rising to the west. The combination of ocean and mountain surroundings makes the area rich in natural beauty.

<< Project Overview >>

Iwaki Alios seen through the adjacent park's trees. The large hall wing is at the left end of the building, the medium-size theater wing (under construction) is on the right, and, between them is the central concourse section of the building.
Iwaki Alios seen through the adjacent park's trees.
The large hall wing is at the left end of
the building, the medium-size theater wing
(under construction) is on the right, and,
between them is the central concourse
section of the building.

The Iwaki Alios project used a Private Finance Initiative (PFI) to fund the construction of this large-scale performing arts center. When both phases are complete, the center will have a 1,705-seat large hall, a medium-size theater and a small theater, a small classical music hall, one large and one medium-size rehearsal room and four studios. Currently, the medium-size theater is under construction, and the center has planned a grand opening for the spring of 2009, when this second phase will be complete.

The project's design management, construction and construction management team, Iwaki Bunka Koryu Partners Co. Ltd, was selected through a design-build proposal competition. The team's participating companies responsible for the design management included Shimizu Corporation, Naomi Sato Architects, Nagata Acoustics and Theatre Workshop. The team's companies responsible for the project's construction were Shimizu Corporation, Joban Kaihatsu Company Ltd., Fukuhama Daiichi Kensetsu, KYB Co., Ltd., Marumo Electric Co., Ltd. and Yamaha Sound Technologies Inc. (The small hall for classical music was a renovation project and the PFI team was responsible for only the design management. Kajiwa Corporation constructed this hall.)

The Iwaki Alios site occupies one corner of the Taira Central Park located adjacent to the Iwaki municipal office building. The old Taira Civic Auditorium occupied the site until it was torn down to make way for the new facility. The recently renovated park also benefited from a PFI which paid for it to be transformed from a densely wooded place with a dark atmosphere into a park full of people, sun and vitality. The park flanks the north side of the entire Iwaki Alios structure. The large hall's wing is at the east end, the medium-size theater is under construction in the west wing and between these two wings is a section of the building that houses the large and medium-size rehearsal rooms and the small theater, as well as the center's main entrances and lobby that connects the east and west wings. On the park side, the Iwaki Alios building exterior has verandas constructed with wood columns and decks designed to blend well with the park's appealing new landscape.

<< Room Acoustics Planning for the Large Hall >>

The large hall's first-floor seating and three balconies
The large hall's first-floor seating
and three balconies

The large hall has three tiers of balconies along the sidewalls and at the rear of the hall. Fundamentally, the hall has a shoebox shape, with one key modification. It would have been very difficult to build a tiered hall with a seat-count of 1,705 using a pure shoebox configuration and achieve the fine acoustics we desired. Our modification to the shoebox shape slopes the hall's ceiling progressively upward towards the rear audience seating.

While the client's programming specified the performance of classical music as the large hall's primary purpose, the project's requirements also stated that the large hall's acoustics be appropriate for stage performances and popular music concerts. We often encounter this kind of concept in the programming planning for multipurpose halls. What made this project extraordinary was that the client's requirements included rigorous standards and specifications that typically would be implemented in a specialty hall for one or another of the multiple genres, but that are not usually included in a multipurpose hall.

Specifically, the stage apparatus, lighting and acoustical equipment are the equal of the equipment in a theater designed solely for drama performance and the hall's shape and the materials selected and designed for the interior walls, ceiling and finishes have the same level of acoustical performance characteristics and functionalities that we implement in classical music concert halls. Given sufficient space and budget, there is nothing impossible about this approach. Nevertheless, during the design and construction phases of the project, we naturally encountered situations where a difficult decision needed to be made and we needed to prioritize either the concert hall characteristics or the theater characteristics of the hall. At each of these junctures, the design team found itself engaged in lengthy debate before choosing among competing paths forward.

<< Designing the Room Acoustics for the Large Hall >>

The client's stated objectives for the large hall's acoustics included target performance levels for sound volumes, sound imagery localization, and the expansiveness of the sound heard by the audience. The client also required that the hall be an easy environment for the musicians (meaning primarily that they can hear themselves and each other) and have appropriate reverberation times and sound clarity.

Most of these client objectives would be significantly influenced by how the hall's sidewalls and ceiling reflect sound back into the audience when the sound reaches these surfaces from the instruments of the musicians performing on stage. This reflected sound is known by the term "early sound reflections." Recognizing the important role of the early sound reflections in achieving the client's objectives, during the early stages of the design process we proactively focused on using computer simulations to consider various room configuration options. During this part of the design work, we developed dozens of computer simulations and held meetings with the architect to discuss each result.

The words architect Naomi Sato's initially used to describe his design concept expressed a desire to create a configuration that would spatially envelope the audience in the hall. A concave, embrace-like shape produces the undesirable phenomenon of sound focusing. From the room acoustics perspective, it is a configuration to be avoided. We worked closely with Mr. Sato's proposals for the hall's interior shape, identifying how they could be made to have beneficial effects on the room's acoustics. Mr. Sato also proposed sidewall surfaces with protrusions or other uneven treatments that would promote sound diffusion, taking into consideration the acoustical requirements for the hall. Nagata Acoustics evaluated each proposal using computer simulation techniques until we collaboratively arrived at the chosen hall shape.

<< Classical Music Performed on the Large Hall Stage >>

In a multipurpose hall, classical music performances require that the stage have a system of sound reflection panels. This project's requirements included providing sound reflection panels for a large orchestra configuration that fills the stage to a depth of 13 m.(43 ft). We created a system that works well for large orchestras and can also be adapted for medium-size orchestras that use a stage depth of 9 m.(30 ft). The stage sound-reflection system adjusts in conjunction with the adjustments to the height of the proscenium, which can be set to between 13.5 m.(44 ft) and 15 m.(49 ft) to suit a range of orchestra sizes and music programs.

The large hall stage, set up for a large orchestra
Large orchestra mode
The large hall stage, set up for a medium-size orchestra
Medium-size orchestra mode
The large hall stage, set up to for an expanded orchestra
Expanded orchestra mode

In addition to being adjustable for large and medium-size orchestras, the stage sound-reflection system can also be used when seating is added to the stage area to increase the hall's seat-count to 1,840. When these seats are added, the sound-reflection system can be set up with a depth of 16 m (52 ft). This setup can also be used for performance repertoires that need expanded orchestras. When Iwaki Alios held its Phase 1 opening concert, Maestro Kenichiro Kobayashi conducted the NHK Symphony Orchestra in a performance of Beethoven's Ninth Symphony, joined by local residents singing the choral part. The hall used the expanded orchestra setup for this performance, with the chorus standing in the space where the additional audience seating will usually be placed.

<< The Large Hall's Concrete Interior Walls >>

Sound-diffusing wall elements in the large hall
Sound-diffusing wall elements
in the large hall

The large hall's sidewalls are constructed of a series of stacked, three-dimensional, board-like pieces. Each piece protrudes more widely at its base and is tapered at its upper edge, so that the base of each piece overhangs the tapered portion of the piece below it. At first glance, these pieces appear to be wood planks. In fact, they are precast concrete. The right angles formed along these sidewalls are very important, because we designed these surfaces so that when they reflect sound, the sound will reach the central audience seating.

We chose concrete for the sidewalls because its rigidity ensures abundant low frequency reflections, but our decision also required us to address concrete's tendency to reflect higher octave sounds too strongly, which results in harsh tones. To prevent such harsh tones from occurring, we specified two kinds of randomly implemented protrusions and incisions in the surfaces of the concrete pieces.

The large hall's precast concrete sidewalls represent a noteworthy example of project team collaboration. When I presented the acoustical effectiveness of the concept at a design session, I was well aware that I was asking for something difficult to accomplish. Nagata Acoustics' design was not a simple request to form large slabs of concrete. Instead, our design required the careful preparation of multiple forms to achieve the detailed random variations in surface texture we would specify. Nevertheless, Shimizu Corporation's passion, determination and technical skill came through for us, making the precast concrete precisely to our specification. While we have no way to compare the results of our use of concrete with what would have been achieved using a hypothetical, more easily produced alternative material in the same hall, I was able to confirm that our design achieved the project's desired results the first time I listened to a concert in Iwaki Alios' large hall.

<< Adapting the Large Hall for Theater and Popular Music Events >>

The large hall's retractable sound absorption curtains
The large hall's retractable
sound absorption curtains

To enable Iwaki Alios to shorten the large hall's reverberation time for events other than classical music performances, we installed sound absorbing curtains that can be draped in front of broad sections of the upper sidewalls, covering as much as about 350 sq. m. (3,767 sq. ft) of wall surface. The sound absorbing curtains enable the hall's reverberation time to be shortened by 0.2 ~ 0.3 seconds. Additionally, the hall's reverberation time changes by, at most, 0.5 seconds depending on whether the stage sound-reflection system is set up or not. (Both of these measurements are reverberation times at 500 Hz). With these two ways to vary the hall's reverberation time, separately and in combination, the hall will be able to provide acoustics that have the appropriate reverberation time for classical music concerts, rock concerts, plays, brass band performances and more.

<< Acoustics of the Small Theater >>

The small theater, seen from the audience seating
The small theater,
seen from the audience seating

At the rear of the small theater, storage space allows all of this hall's movable, tiered seating to be stored out of sight, so that the room doubles as both a flat-floored event space and a theater. The small theater is intended for parties and music recitals and other performances, as well as for plays. The hall's deployable seating creates a theater that seats 233.

When the small theater is configured as a flat-floored space, its rectangular shape requires measures to prevent flutter echoes. We added box-shaped, sound diffusing objects along three of the theater's walls to prevent the occurrence of flutter echoes. (The rear wall does not have these protrusions.) We intentionally designed the dimensions of the sound diffusing elements to have a variety of depths and be a variety of sizes.

Our design for this hall included the goal of excellent sound clarity and to achieve this objective we gave attention to controlling low-frequency sound reverberations. At numerous locations along the walls, we created air spaces behind the walls and sound absorbing structures, which we located along the walls in an irregular pattern.

Sound Isolation at Iwaki "Alios"

by Ayako Hakozaki

Cross-section view of Iwaki Alios Performing Arts Center
Cross-section view of Iwaki Alios Performing Arts Center

The key goal of our sound isolation planning for the Iwaki "Alios" project was to enable simultaneous use of the large hall, in particular, and the building's other rooms. We devised a two-pillar approach to achieve this goal.

One main focus of our sound isolation design strategy provided structural acoustic separation between the wing with the large hall and the middle section of the building where the small theater, rehearsal rooms and studios are located, and also between the middle section and the wing with the medium-size theater. The means used was installation of acoustically effective expansion joints. This design eliminated the possibility of solid-borne noise passing between one of the wings and the middle section of the performing arts center.

The second key component of our sound isolation strategy implemented an anti-vibration and sound isolating structural design for each room of the building's middle section. Structurally, the floors, walls and ceilings of the small theater and the large and medium-size rehearsal rooms are all supported by anti-vibration structural materials. In the walls and ceilings of these rooms we used triple layers of 10 mm (0.4 in.) fiber-reinforced Gypsum panels.

During the design phase of the project, our working name for the middle section's four studios was "the rock band practice rooms" in recognition of the client's intention that these studios be used by musicians practicing on electrically amplified instruments. The musicians who will use these studios can be expected to generate very loud sound volumes. For these studios, we devised an even stronger sound isolation structure. We used a bottom layer of anti-vibration material to support a floating floor foundation on top of which we added a totally independent steel frame. To add anti-vibration and sound isolating performance to the walls and ceilings, we specified quadruple layers of 10 mm. (0.4 in) fiber-reinforced Gypsum panels.

The four studios have glass windows that overlook the lobby. To minimize sound leakage from the studios into these central areas of Iwaki Alios, we installed windows made of three different kinds of glass, each of which has a different thickness.

<< Measured Sound Isolation Results and Iwaki's Taiko Drummers >>

A taiko drum ensemble performs in the middle rehearsal room
A taiko drum ensemble performs
in the middle rehearsal room

As a result of the strategies I described above, we achieved very robust sound isolation performance that effectively isolates the large hall from sound produced in the building's small theater, two rehearsal rooms and four studios, even when the sound isolation performance is greater than 90 dB (at 500 Hz). Nevertheless, as my colleague Ms. Fukuchi wrote in the May, 2006 issue of this newsletter, regardless of the measured sound isolation values, it is very difficult to create a space that isolates sound from live rock music and taiko drums to the level of quietness needed for classical music concerts.

Soon after the completion of Phase 1 of the Iwaki Alios project, one of Iwaki City's taiko drum ensembles contacted the Iwaki Alios office and requested permission to use the new building's medium-size rehearsal hall. The center took advantage of this opportunity to organize a live listening test of the center's sound isolation performance during a taiko drum performance. The city's three taiko ensembles participated in the test. While the drummers performed in the medium-size rehearsal room and the small theater, 40 volunteers from among Iwaki Alios management and technical staff, and members of the architectural and construction teams stationed themselves throughout the performing arts center to test whether they could hear the sound of taiko drumming leaking into various locations in the building.

This test's taiko drumming produced sound equating to sound pressure levels of 120 dB (at 125 Hz) and 110 dB (at 63 Hz) and reached maximum sounds 10 dB louder than these numbers. The live listening test confirmed that the taiko drumming in the small theater and medium-size rehearsal room could not be heard in Iwaki Alios' large hall. As one of the members of the design team who participated in this listening experiment, I both breathed a sigh of relief and gave myself a virtual pat on the back. More importantly, I was truly glad to witness the happy faces of the center's management and operations staff, who thereafter expressed confidence in their ability to successfully promote the use of every room at the Iwaki Alios Performing Arts Center.

<< Noise Mitigation in the Large Hall >>

The client's written objectives for this project included meeting or exceeding a quietness level of NC-15 inside the large hall. At NC-15, a room is so quiet that the human ear does not hear any sound.

To achieve this project objective, we applied a comprehensive strategy to all of the HVAC equipment and all of the other building machinery. We specified floating floor structural designs for all equipment rooms in the vicinity of the large hall, effectively preventing the transfer of solid-borne sound.

We also took care to isolate the large hall from sound produced outside the performing arts center. We specified double-layer exterior walls and a double-layer roof for the large hall's wing. Our design successfully achieved the target NC-15 level of quietness in the large hall.

Nagata Acoustics Inc.
Hongo Segawa Bldg. 3F, 2-35-10
Hongo, Bunkyo-ku, Tokyo 113-0033 Japan
Tel: +81-3-5800-2671, Fax: +81-3-5800-2672

(US Office)
2130 Sawtelle Blvd., Suite 307A,
Los Angeles, CA 90025, U.S.A.
Telephone: (310) 231-7818
Fax: (310) 231-7816

E-mail: info@nagata.co.jp

[ Japanese Version ]