Title means "Quietness", "Comfortable Sound" and "Excellent Acoustics"

Nagata Acoustics News 05-02 (No.206)
Issued : February 25, 2005

Tomioka Cultural Center "Manabi no Mori" Opens

by Masaya Uchida

Concert style stage view
Proscenium style stage view
Audience area view
From Tokyo's Ueno train station, a three hour-ride on the Joban Line's express train brings the traveler to Tomioka, a coastal region town of 16,000 people situated midway between the larger Fukushima Prefecture cities of Iwaki and Haranomachi. Tomioka is famous for its Yonomori Park, where more than 2,000 yoshino cherry trees blossom along a 2.5 km. (1.5 mile) path. When the cherry trees come into full bloom each year, they form northeastern Japan's most admired tunnel of flowers. Visitors also come to Tomioka to enjoy the embankments full of azaleas near the Yonomori JR train station. Tomioka is also home to the Fukushima Daini Nuclear Power Plant.

Tomioka's new cultural center, named "Manabi no Mori" (literally, "The Forest for Learning") opened in October 2004. The facility includes a large and small hall, as well as a library, local history resource center, classrooms and assembly spaces, providing a multipurpose lifelong learning venue for the community. The project architect was Kume Sekkei and a consortium of Nishimatsu, Tanaka and Futaba construction companies served as general contractor. Nagata Acoustics provided acoustical design and consulting for the room acoustics, noise control, and stage sound system of the facility, with particular focus on the acoustical design and implementation for the Large Hall.

<< Manabi no Mori's Large Hall >>

The Large Hall is a 500-seat, multipurpose hall. For classical music concerts, we provided deployable sound reflection panels. When the hall is configured with the sound reflection panels, the lighting spaces at the front sides of the stage can be closed, so that both the ceiling and the sidewalls of the auditorium present a seamless continuity between the stage and the audience seating area. For theatrical performances, symposia, and other events that need a proscenium stage, a portion of the side sound reflection panels can be deployed together with proscenium panels suspended from above the stage to create the theater configuration's proscenium arch. The entire audience seating area slopes upward towards the rear of the hall at a somewhat sharp angle, ensuring easy viewing of the stage from every seat. At the upper portion of the right sidewall, a large window faces onto the hall's foyer and allows natural light to filter through the foyer into the Large Hall.

<< The Large Hall's Interior and Acoustical Design >>

The architect's interior design for the Large Hall called for smooth surfaces and a simple appearance. To accommodate the visual design approach, we limited the placement of sound diffusing elements to the lower portions of the audience area's sidewalls and, for proximity to the performing musicians, at the lower portions of the wall behind the stage. We designed the upper portions of the audience areafs sidewalls to incline inwards in the direction of the center of the auditorium as they rise towards the ceiling, so as to avoid the negative acoustical phenomena that sometimes arise from parallel walls.

We located the hall's sound absorbing surfaces on the wall behind the audience seating (where we used perforated wallboard backed by glass wool and cement excelsior board) and at the portion of the ceiling that contains a cutout section for a lighting space (using perforated wallboard backed by glass wool). When the stage is configured for concert hall use, the Large Hall's reverberation time measures 1.5 seconds and when configured with the proscenium stage, the reverberation time is 1.1 seconds (both measurements taken at 500 Hz with the hall unoccupied).

<< The Large Hall's Sound System >>

Our sound system design for the Large Hall focuses on providing functionality for two main objectives: (1) amplification of human speech; and, (2) amplification of recorded music. For these purposes, we installed loudspeakers in the ceiling above the stage proscenium, at the sides of the stage and at the front of the stage.

In order to successfully locate the main loudspeakers above the proscenium, we needed to take into consideration the importance of the area around the proscenium for obtaining early sound reflections. Loudspeakers with large apertures become sound-absorbing surfaces and their placement at the proscenium could have an undesirable effect on the hall's acoustics. To mitigate this possibility, we selected loudspeakers with rotatable horn so that they can be placed either vertically or horizontally. We installed three of these loudspeakers in horizontal style along the loose inclination of the ceiling and as close to the ceiling as possible. In this way, we minimized the impact of the proscenium loudspeakers' apertures. In addition, for the side loudspeakers, we built a setback into the sidewalls up to the height needed for the loudspeakers so that the angle of the loudspeakers minimizes the aperture surface areas facing the audience seating. We installed the same loudspeakers in vertical style for the side loudspeakers.

Sound system designs typically use the same loudspeakers at the proscenium and at the side of the stage, because this approach results in consistent sound quality. In this case, the ability to rotate the loudspeaker's horn and easily customize its positioning proved to be a most valuable feature. During the construction of the hall and sound system installation, we checked the placement angles of the loudspeakers, and we also verified that the cloth material and the furring strips used to cover the front of the loudspeakers would not impede the loudspeakers' sound output. When the hall's construction was finished, we directed the fine-tuning of the sound system, achieving the client's desired clear and natural sound quality and even sound pressure distribution for amplified sound in the hall.

<< The Small Hall Multipurpose Programming and Sound Isolation >>

The Small Hall has a flat floor and is intended to be used for mini-concerts and rehearsals, plus a wide range of other events that the hall can accommodate by opening the Small Hall's large wood doors and joining the hall with an outdoor terrace. In addition, the Small Hall is located a short corridor's distance from the right side of the Large Hall's stage, so that when the Large Hall has school music festivals and other events involving multiple performing groups, the Small Hall can be used as a waiting area and rehearsal space for the Large Hall's performers. Because of the proximity of the halls, and to maximize the ability to use the halls simultaneously, we installed two soundproof doors and a soundproofing shutter so that, together with a sound lock, we achieved a sound isolation performance level of 80 dB (at 500 Hz). Barring the exception of some unexpected, extremely loud sound, this sound isolation performance level enables both halls to be used simultaneously without any mutual interference.

The new Tomioka Cultural Center's Japanese-language website shows a number of events on the spring calendar. The website URL is http://www.tomioka-manabinomori.jp/.

Thoughts on the Acoustical Design of Home Theaters - Part II

by Makoto Ino

In this second installment of several articles on home theaters, I will share some details about our acoustical design for Mr. K's home theater in his new residence, as well as the thought processes that led to our design decisions. The first decisions that need to be made in designing any home theater are the size of the room and its geometry. In last month's article, I mentioned the unique geometry of Mr. K's home theater, and in this article I will turn first to decisions about the size and placement of equipment in the defined space. Then I will return to a consideration of the room's acoustical objectives and size.

For the plan view design of the loudspeakers in the room, we began by following the recommended ITU-R BS.775-1 standard for the angles of loudspeaker placement relative to the listening location. Because the recommended standard for loudspeaker placement matches the placement used by TV and video editing studios, following the ITU standard ensures that the home theater will incorporate all of the basic conditions required for the home theater's reproduction of surround sound entertainment content. In addition, a recent report documents that following the ITU standard results in sound images that give an excellent sense of orientation and feelings of expansiveness. (See the Japan Audio Society's Journal, 2004, Vol. 44, No. 10.)

<< Screen Size and Viewing Distance >>

After determining the size of the room and its geometry, the next decisions that need to be made for the home theater are (1) how much space is needed between the loudspeakers and the listener/viewers and (2) how large should the screen size be. First, I will consider the screen size and the distance between the screen and the listener/viewer. The dynamic power and immediate presence viewers feel when watching a film or video is determined more by the view angle of the person(s) watching the screen than by the sheer size of the screen. While individual preference accounts for some portion of the perceived force and impact of screens, we can use the center seating areas of cinema theaters as examples of view angles that offer the desired relationship. The center seating area of cinema theaters typically offer view angles in the range of 35 - 55 degrees.

The ITU standard specifies a view angle of 48 degrees for HDTV screens with an aspect ratio (width-to-height ratio) of 16:9. Following the ITU standard clearly ensures that the viewer will always experience the same power and immediacy as in the most premium cinema theater seats.

The resolution capability of the human eye is determined by the view angle. Therefore, another way to find the best viewing distance from a screen is by moving away from the screen or display panel until the scanlines of the screen and the picture source become no longer distinguishable to the eye. This point is the best viewing distance.

Yet another approach to determining the viewer's distance from the screen can be derived from the ITU standard. For the current HDTV models, the standard puts the viewer at a distance slightly greater than twice the height of the screen.

<< Loudspeaker Distance from Listener/viewers >>

Unlike the methods for determining the distance of the screen from the viewer, the optimal distance between the listener/viewers and the loudspeakers depends on the characteristics of the loudspeakers. While output capacity is certainly one factor to consider, the best distance from the listeners will also depend on whether the tweeters are dome type or horn type, as well as on differences in the apertures of the loudspeakers' woofers.

In general, the larger the loudspeaker unit, the more distance is desirable between the listeners and the loudspeakers. Small, bookshelf-size loudspeakers need about a 2 m. (7 ft) distance from the listeners. Larger, freestanding models should be listened to from a distance of more than 3 m. (10 ft). Determining the best distance for a particular set of loudspeakers requires trial-and-error listening at difference distances, and the importance of hands-on adjustments of the distance to the listener is probably the most important aspect to keep in mind. Of course, finding the best distance between the loudspeakers and the listener/viewers is also dependent on the assumption that the selected loudspeakers are appropriate for reproducing the entertainment content's sound qualities and sound images (i.e., sense of scale or expansiveness).

<< Mr. K's Loudspeakers and Projection Screen >>

Our design plans for Mr. K's home theater included installing large loudspeakers. Therefore, we allowed for more than 3 m. (10 ft) from the loudspeakers to the location where Mr. K and others will sit, and we selected a screen that measures 130 in. diagonally. At the same time as we made these equipment selections, we evaluated how we wanted the amplified sound to be heard and what impacts the room's size and geometry would have on our goals for the room's acoustics.

We identified five sets of objectives for the home theater's size and geometry:

<< Impact of Size and Geometry on the Home Theater's Acoustics >>

Fig.1 Traditional arrangement
Fig.2 Mr.K's Home Theater
When considering the geometry of the room's footprint, if we were to follow the prevalent theory for audio listening rooms, as developed, for example, by the International Electro-technical Commission in IEC-29B, the room geometry would be the kind of rectangular configuration shown in Fig. 1. With this geometry, the loudspeakers must be placed near the sidewalls and the proximity of the loudspeakers to the sidewalls would necessitate that sound absorption measures be added to the sidewalls to mitigate the strong impact of the near sidewalls on the sound reflections.

In addition, with the traditional rectangular arrangement, there is a comparatively large area of space behind the listener/viewers' sitting position, and this space is likely to be considered wasted space. The wasted, or unused, space leads to attempts to shorten the length of the room. When the length of the room is shortened, the room becomes more of a square than a rectangle, and the changed geometry leads to the need for low register sound absorption measures to prevent a strong "booming" phenomenon from occurring. Also, in a more square-shaped room, there would be the tendency for sound reflections to bounce back and forth between opposite walls as well as between the floor and the ceiling, resulting in high-register flutter echoes and low-register "booming." Either or both of these phenomena would impair the quality of the home theater's acoustics. The closer two dimensions of a room approach a ratio of 1:1, the more the frequency of standing waves overlap, resulting in increasingly disturbing impacts on a room's acoustics.

<< Good Ratios for Home Theater Dimensions >>

In order to maximize the benefits of the home theater's sound reproduction capabilities, the approach should not be to add sound absorbing measures to control negative sound phenomena. Rather, the room's dimensions should be determined based on a ratio that will not result in sound reflections bouncing back and forth and overlapping in undesirable ways. For example, if the room's length, width and height have respectively a ratio equal to the cube root of 2 to the power of "n" (n = 0, 1, 2), then the room's dimensions will be good from the perspective of achieving good acoustics. The resulting ratio is approximately 1:1.3:1.6 (or any integer multiple of this ratio).

As shown in Fig. 2, for Mr. K's home theater, we designed a room configuration with a length-to-width ratio of 1:1.3. With this more-wide-than-long footprint, the surround-sound loudspeakers fit well into the wide space along the rear wall of the room (the wall behind the listener/viewer) and the main left and right loudspeakers stand more than 1.5 m. (5 ft) from the sidewalls. In addition, for the relationship of the room's width to the distance between the left and right main loudspeakers, we achieved nearly a 2:1 ratio. In our experience, this ratio of room width to distance between left and right loudspeakers produces clear sound reproduction with minimal distortion. I am of the opinion that the reason this placement ratio works well is related to the relationship of the distribution of the standing waves to the location of the sound source, but this theory has not been scientifically tested. With the room geometry and size finalized and the size and placement of loudspeakers and viewing screen selected, our design for Mr. K's home theater has made good progress towards completion. In the next issue, I will continue the discussion with additional information on this case study.

Nagata Acoustics To Design Acoustics of Mariinsky Theatre's New Opera House

by Yasuhisa Toyota

Russia's Mariinsky Theatre (often known by its earlier name, Kirov Opera, when performing outside Russia), in the city of St. Petersburg, plans to build a new opera house and has selected Nagata Acoustics to be the acoustical consultant for the project. The new opera house will seat an audience of 2,000 and will be built on the opposite side of a small canal from the current Mariinsky Theatre, which holds a similar 2,000-seat capacity. The new opera house will be used for opera and ballet performances, and will also be designed for the orchestra to perform musical concerts from the opera house stage.

<< Maestro Valery Gergiev and the Mariinsky Theatre's Global Fame >>

Under the baton and direction of Artistic Director Valery Gergiev, the Mariinsky Theatre ranks as one of the world's most active opera companies. Since Maestro Gergiev assumed his current position in 1988, the Mariinsky Theatre's performance schedule at home and abroad has grown prodigiously. At the same time that the Mariinsky Theatre's opera, ballet and orchestra companies perform on overseas tours, they simultaneously stage performances at home. They accomplish this by maintaining two opera companies, one ballet company and two orchestras, all under Maestro Gergiev's leadership. At present, all of the home performances converge on the one, historic Mariinsky Theatre opera house in St. Petersburg, and the single venue can no longer accommodate the many performance schedules. Maestro Gergiev's vision for the new opera house is for "Mariinsky Theatre to become the performing arts focal point for the artistic and culturally-rich city of St. Petersburg, developing in a comparable fashion to the role of Lincoln Center in New York City." The Mariinsky Theatre new opera house project is a major step towards the realization of the maestro's vision.

<< The New Opera House's Architectural Design Competition >>

Exterior view by courtesy of DOMINIQUE PERRAULT ARCHITECTURE
Interior view by courtesy of DOMINIQUE PERRAULT ARCHITECTURE
In January 2003, as part of the new opera house's concept phase, the Russian government announced that it would hold an international competition to select the architect for the project. The Russian government's decision to open the competition to international participation was almost without precedent and certainly the first competition of its kind for a project in St. Petersburg. The Russian Minister of Culture and Maestro Gergiev, in his capacity as the Mariinsky Theatre artistic director, invited five Russian architects and six architects from other countries to participate in the Mariinsky Theatre new opera house international design competition. In July of the same year, the proposal of French architect Dominique Perrault won the competition and the project proceeded to the architectural renderings phase. Mr. Perrault named Nagata Acoustics as his choice for the project's acoustical consultant and we were awarded this role.

<< The New Opera House Project Overview >>

The new opera house complex will include the 2,000-seat opera house that can also be used as a classical music concert hall. In addition to the main stage, two side stages and two rear stages are planned, for a total of five stage areas. In addition, the project plans call for a smaller scale hall that can be used both as a concert hall and rehearsal hall, and for a number of support rooms and other facilities. Overall, the new complex has a planned area use of some 40,000 sq. m. (10 acres).

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Nagata Acoustics News 05-02 (No.206)
Issued : February 25, 2005

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