Quietness, Comfortable Sound and Excellent Acoustics NAGATA ACOUSTICS

News 08-06 (No.246)

Issued : June 25, 2008

[ Japanese Version ]

Sound Systems for Iwaki Alios' Large Hall and Small Theater

by Masaya Uchida

After devoting the May issue of this newsletter to articles about the Iwaki Alios project's room acoustics and sound isolation, this month we will conclude our discussion of the Iwaki Alios project with this article about the sound systems we designed and installed in the building's large hall and small theater.

<< The Large Hall's Sound System >>

Large hall's stage with stage curtains
Large hall's stage with stage curtains

We began the sound system design of the Large Hall by understanding the character of the hall and the goals the sound system would aim to achieve. At Iwaki Alios, the large hall can be configured to suit a diverse range of performance genres through modifications that include adding or removing the acoustical reflection panels on the stage and changing the height of the proscenium. The sound system of this hall must provide both high quality amplified sound for dramas, musicals and some popular music concerts, and deliver clear voice amplification for the pre-concert announcements, talks and other spoken communications that accompany concert performances.

>>> Loudspeaker Locations

We used a left-center-right (LCR) loudspeaker arrangement for the large hall's proscenium speakers, with mechanisms that raise and lower the speakers depending on the hall configuration being set up. We placed the side-of-stage loudspeakers at locations physically low enough to lower the sound images' locations.

We also installed supplemental loudspeakers. These loudspeakers cover the areas where the proscenium and side loudspeakers could not be relied upon to provide coverage, specifically the seats closest and farthest from the stage, the seats in the third and fourth balconies, and the side balconies which may be outside the coverage area of on-stage loudspeakers added for specific performances.

We embedded the monitor loudspeakers in the side surfaces of the on-stage acoustical reflection panels. In addition to accomplishing their primary purpose of providing feedback to musicians during performances, these loudspeakers can be used for speech amplification during rehearsals or setting up the risers for choirs.

>>> Loudspeaker Selection Criteria and Performance Results

Inside view of the side-of-stage loudspeaker's space
Inside view of
the side-of-stage
loudspeaker's space

Side-of-stage loudspeakers covered by acoustically transparent netting
Side-of-stage loudspeakers
covered by acoustically
transparent netting

We used a number of different criteria to evaluate which loudspeaker models to select for the large hall. The proscenium, side and supplemental loudspeakers need to be able to amplify on-stage dialog and the most pianissimo of instrumental sounds with natural sounding output that harmonizes well with the hall's acoustics. The loudspeakers also needed to be able to output powerful, impactful sound for reproduction of sound effects and popular music amplification. We considered other criteria as well, such as how smaller and flexible the loudspeakers would be for their installation and how they would blend with the architectural design of the hall.

To prevent environmental factors near the installation locations from causing deterioration of the output loudspeaker sound, we decided to create a design to keep the loudspeakers free of any cabinetry that would negatively affect the loudspeakers' acoustics. In particular, for the loudspeakers located at the sides of the stage, we covered the audience-facing sides of these loudspeakers with acoustically transparent netting and we opened up apertures in the rear walls of the loudspeaker spaces. We installed fabric panels from the floor to the top of the stage behind these loudspeakers. We specified that the hall's sound-diffusing sidewall finishes continue into the stage area beyond the locations of the side loudspeakers, to both prevent these nearby wall surfaces from generating the undesirable strong interference of mirror-like reflections of the loudspeaker outputs and obtain desirable sound reflections for classical music performances into the hall.

As a result of our careful selection and installation of the loudspeakers, the sound they deliver in the hall achieves their maximum performance potential, producing extremely natural and clear amplified sound. Because the architect understood the importance of the acoustical conditions near the loudspeakers and worked cooperatively with us, the hall's architectural design minimizes the visual presence of the exposed speakers without compromising the sound system or room acoustics of the hall.

>>> Other Aspects of the Large Hall's Audio System

Sound control room with widely opened window
Sound control room with widely opened window

The large hall's audio control booth is located at the center of the main floor's rear wall. It has windows that open 1.5 m. (4.9 ft) wide, so that the output from the side-of-stage loudspeakers reaches the sound engineer's ears directly. This design allows the sound engineer to hear the same sound as the audience while monitoring and adjusting the sound system settings. The equipment in the audio control booth includes a mixing console for sound amplification, a matrix console (with a custom-ordered fader) and a recording console. We designed the sound system to be very flexible and also to minimize operator error.

In order to prevent deterioration of audio signals transmitted across distances within the sound system and to prevent noise, we installed digital transmission lines to carry the sound from the A/D converter at the stage wings to the mixing console to the amplifiers. We also used A/D converter technology above the ceiling for the hall's suspended microphone for high quality recording capability.

<< The Small Theater's Sound System >>

Small hall's removable main loudspeakers
Small hall's removable main loudspeakers

Sound control booth opened to the audience seating
Sound control booth
opened to the audience seating

The small theater's loudspeaker system design provides maximum adaptability to performance needs with a removable LCR main loudspeaker attached to the stage's front loudspeaker rigging and multiple suspended mounts and portable loudspeakers located throughout the theater. We installed loudspeakers from the same manufacturer and series that we selected for the large hall, so that the loudspeakers can be borrowed between the halls and still maintain consistent sound quality.

The small theater has an open sound control booth located at the rear of the main audience seating. We equipped the small theater with a mid-size digital mixing console and installed digital transmission lines between the mixing equipment and the amplifiers.

<< Stage Communications and CCTV Systems >>

As part of the sound system design of the Iwaki Alios project, we created stage communication systems and feature-rich CCTV system for use by the hall's crew during performance set-up and take-down activities and to support efficient crew communications during performances.

We equipped the large hall with wired and wireless staff intercommunication systems and a hot line system for stage managers and conductors. We also equipped the hall with a talk back system that can be used during rehearsals by directors, agents and other observers to speak to the performers, a paging system for the green room, cue lamps and a stage director's console to operate many functions of the systems mentioned above.

The CCTV equipment in the Iwaki Alios large hall and small theater are connected to a video matrix switcher, enabling easy addition of a roving camera or TV monitors. At the request of the stage crew, we enabled the crew in each hall to be able to use the CCTV to have the view of the other hall's stage and the general areas provided by the facility's security cameras.

In most projects, stage communication and CCTV equipment costs were part of sound system budget. The diversity of these equipments needed for today's many performance genres continues to grow along with producers' various dramatic presentations. Hall security now requires expansions of stage communication and CCTV systems to ensure the safe use of advanced and complex stage mechanisms. These ancillary aspects of today's sound systems impinge on the portion of sound system budgets that can be spent purely on devices that deliver amplified sounds.

<< Noise Mitigation for the Sound System >>

We used separate electrical transformers and grounds (electrode) for the sound system from all other equipments. We also used electrical isolators to separate the racks installed the sound system equipment from the building structures, creating an electrical isolation shield between the sound system equipment and other equipment.

In addition to using metal conduits for the sound system wiring, we used metal conduits for the wiring to the stage mechanisms and the stage lighting, as these can also be noise sources. Our careful attention to noise mitigation resulted in sound systems free of noise problems. This kind of approach is now an essential part of sound system design for halls and theaters, regardless of the scope of the project.

In Iwaki Alios project, the adjustment between the sound system, the architectural design, the room acoustics and the other equipments was very serious issue, as well as the sound system design itself.

After the first phase opening in this April, many events and activities are holding in the large hall, the small theater and the other rooms. In the neighborhood of this new facility, the construction of the medium theater is in progress for the grand opening in the next spring. The people concerned are struggling every day continuously.

Sound Isolation Design-Part 5:
Sound Causes the Walls to Shake-Some Sound Isolation Structure Design Fundamentals

by Chiaki Ishiwata

In Part 4 of this series , I presented examples of how sound isolation considerations may influence the placement of auditoriums and other rooms in a facility's layout. On many projects, site and layout constraints limit the room placement options. Also, external conditions such as proximity to noise-producing transportation may exist. In these kinds of situations, room placement alone cannot fully solve the project's sound isolation needs.

This Part 5 of the article series on sound isolation will introduce the reader to examples of sound isolation solutions other than creating distance between two rooms. I will preface those examples with an explanation of the fundamental mechanisms that make these sound isolation solutions able to achieve their intended goals.

<< Determining the Needed Sound Isolation Performance Level >>

Figure 1 provides examples of the perceived level of sound heard in a "sound receiving" room when sound is emitted in a "sound source" room. The perceived transfer of sound depends on the sound isolation performance level between the rooms and also on the type of music or event generating the sound in the sound source room. (The data assumes a quietness level (background noise) of NC-20~25 in the sound receiving room.)

Fig. 1: Perceived sound isolation performance levels by sound source type
Fig. 1: Perceived sound isolation performance levels by sound source type

As the data in the chart shows, when the sound source consists primarily of amplified lectures and similar speaking events, if the sound isolation performance level between the rooms is at least 50 dB (at 500 Hz), then sound transferred into the sound receiving room will not be noticed because it will be sufficiently masked by the room's normal background noise. However, the chart's data also confirms that music performances and amplified concerts in a sound source room dictate the need for stronger sound isolation performance levels between the sound source room and adjacent sound receiving rooms.

<< How Sound Isolation Mechanisms Work >>

Fig. 2: Sound isolation mechanism
Fig. 2: Sound isolation mechanism

Figure 2 shows the mechanism of sound isolation. This drawing shows what occurs when a sound emitted from the left side of the drawing comes in contact with a wall. When sound is emitted in a room, sound pressure fluctuations propagate and impact the wall that is in their path. Depending on the specifics of the sound pressure fluctuations, when they impact the wall, the wall is either slightly pulled or pushed, causing the wall to vibrate. The wall's vibrations cause the air on the other side of the wall to vibrate, resulting in the propagation of sound waves. The perceived impression is that the sound has transmitted through the wall.

If the same amount of force is applied to two objects and one is heavy while the other one is light, the heavier object will move less. The more weight a wall has, the less likely it is to vibrate, which means that the amount of wall vibrations and resulting sound waves propagated in opposite side will be small. Therefore, since materials with greater surface density are also heavier, the sound isolation performance characteristics of materials vary proportionally based on the following formula: surface density per unit of area = thickness x specific gravity. As the surface density of a material increases, its sound isolation performance also improves. For each doubling of the surface density of a material, the sound isolation performance increases by approximately 5 dB. In acoustics, this relationship is known as the "mass law."

<< Sound Isolation Structure Design >>

Figure 3 shows various sound isolation design options that can be used to improve the sound isolation between rooms that are separated by a single layer concrete wall. Without added sound isolation measures, the sound isolation performance level provided by the concrete wall (assuming standard concrete of 150 mm. [5.9 in.] thickness) would be on the order of 50 dB (at 500 Hz).

Fig. 3: Sound isolation structures and their sound isolation improvement performance levels based on a concrete frame structure
Fig. 3: Sound isolation structures and their sound isolation improvement performance levels
based on a concrete frame structure

In the Figure 3 examples, I added sound isolation designs such as a double slab floor, a concrete or concrete masonry unit (CMU) double wall, or a floor floated on a glass fiber mat. Each of these methods increases the sound isolation performance by 10~15 dB (at 500 Hz). The improvement in sound isolation performance achieved with one of these designs roughly equates to the sound isolation performance level achieved with a layout that places a corridor between two rooms.

Designs that use a double slab floor, a concrete or CMU double wall, or a floor floated on a glass fiber mat improve sound isolation performance by adding weight (density) to the sound isolation layer. When gypsum board or other drywall supports are added to a wall or ceiling as an anti-vibration measure, we similarly expect a 10~15 dB (at 500 Hz) increase in sound isolation performance.

To increase sound isolation performance levels by more than 10~15 dB (at 500 Hz), we combine two or more of the above framing methods to create a total floating frame, also referred to as an anti-vibration and sound isolation frame. With this kind of framing design, we can achieve an increase in sound isolation performance on the order of 20~30 dB (at 500 Hz). When we develop a sound isolation design for a specific project, we consider factors such as the expected sound frequency characteristics of the sound sources, and the room placement opportunities and constraints, and we use these inputs to determine the appropriate combination of sound isolation framing methods to implement.

<< Adding a Second Concrete Wall on a Slab >>

Fig. 4: Comparison of single concrete wall and walls on slab with flanking transmission
Fig. 4: Comparison of single concrete wall
and walls on slab
with flanking transmission

Figure 4 depicts the solid borne sound phenomenon of flanking transmission. Illustration (a) depicts a uniform, single-layer concrete wall that provides sound isolation performance for 50 dB. Illustration (b) depicts the theoretical situation of two walls separated by a cavity of air. The two walls seem to be independent of each other so that the sound waves transfer first through one wall and then through the other wall. One might therefore assume that the sound isolation performance level for this design would equal 50 dB + 50 dB = 100 dB. However, what actually occurs is depicted in Illustration (c) which shows the two walls supported by a horizontal slab.

The sound isolation performance of the design using two independent walls on a slab will be affected by the sound isolation mechanism I described earlier. The sound waves that cause the wall to vibrate will also cause the slab to vibrate, due to flanking transmission of the sound waves. In addition, the sound generated in the sound source room will directly transmit incident energy to the slab. The slab in the sound receiving room will cause the air in the room to vibrate, resulting in the propagation of sound waves and adding additional transferred sound to the sound transferred by the walls' vibrations. For rooms that require a high level of sound isolation performance, the flanking transmission phenomenon cannot be ignored. When designing sound isolation solutions, even the simple assumption 1 + 1 = 2 may not always be valid!

When flanking transmission transmits energy through slabs and other structural elements, as the transmission distance increases, transmission loss occurs. For projects that include both a large and small hall, if we can separate the two halls by at least 20 m. (65.6 ft), we can expect a sound isolation performance level of 80 dB (at 500 Hz) between the halls. If the distance between the two halls is insufficient to achieve the desired sound isolation performance level, we can use acoustical expansion joints (one of the options shown in Figure 3) to interrupt the flanking transmission energy and overcome the lack of sufficient distance between the two halls.

<< Floating Structures (Anti-vibration and Sound Isolation Framing) >>

Fig. 5: Floating floor system with elastic, anti-vibration, sound isolating material connecting ceiling and walls to main structure
Fig. 5: Floating floor system with
elastic, anti-vibration,
sound isolating material
connecting ceiling and walls
to main structure

Floating structures are a design technique that enables us to achieve a high level of sound isolation performance without needing to locate two halls 20 m. (65.6 ft) apart. The "special" ingredient that makes this design successful is the elastic material (typically a rubber spring or glass fiber mats) installed between the structural frame of the building and the supports of the room's floating framework. For readers who are familiar with the kind of framing used to prevent buildings from shaking during earthquakes, floating structure designs for sound isolation can be visualized as a room seismically upgraded within a building to prevent the room from shaking.

Both anti-earthquake framing designs and floating structures rely on selecting materials with the appropriate elastic performance characteristics known to intercept sound or vibration waves of certain frequencies. It is also worth mentioning the precision required when implementing this kind of high performance sound isolation solution. One rebar rod incorrectly connected or a little seepage of some undesired material between the floating floor system and the main structure of the building can cause flanking transmission and grave damage to a floating structure design's sound isolation performance level. Projects that implement floating structures need to keep in mind that the elastic material between the floating structure and the main structure must substantially decrease energy that otherwise would require a distance of 20 m. (56.6 ft) to dissipate.

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 ]