News 07-02 (No.230)
Issued :February 25, 2007
[ Japanese Version ]
Scale Model Testing for Two European Projects
by Dr. Keiji Oguchi
This past autumn, for two European projects, the Salle Philharmonique Radio France and the Helsinki Music Centre Concert Hall, Nagata Acoustics began acoustical testing in 1/10th-scale models. The Salle Philharmonique project adds a new concert hall (1,500 seats) to the French Radio Broadcasting Corporation, Radio France's campus located in the 16th arrondissement along la Seine at the same time that the entire interior of the corporation's main structure, an architectural landmark, undergoes complete renovation. The architect of this project is Paris architectural firm Architecture-Studio. Helsinki Music Centre project is the new concert hall (1,650 seats) for the Finnish Radio Symphony Orchestra and the Helsinki Philharmonic being constructed as part of the structure that will also house Sibelius Academy, Finland's national music university. The architect of this project is the Finnish firm Laiho-Pullkinen-Raunio in Turku, Finland. This project's building will occupy a site equidistant between architect Alvar Aalto's Finlandia Hall and Steven Holl's Kiasma Museum of Modern Art.
While these two projects began at very different times, the final stages of their design phases coincided, resulting in the scale model acoustical testing for both projects starting at nearly the same time. In this article, I will use these two examples to present an overview of the construction and equipment of scale models built for acoustical testing, as well as the key benefits of scale model testing.
<< Scale Model Construction >>
In constructing a scale model for acoustical testing, we use materials for the model's floor, walls, ceiling, audience seating and "audience" that effectively simulate each of these acoustical characteristics. As a general rule, concert hall floors, walls and ceilings are sound-reflecting surfaces, so we construct their model versions using painted thick plywood. We also install the 1/10th audience seating and "audience", which had been specially developed by Technological Research Center Tokyu Construction Co., Ltd.
Salle Philharmonique Radio France
Helsinki Music Centre Concert Hall
<< Overview of the Aims and Timing of Scale Model Testing >>
Both the Paris and Helsinki halls have seating that surrounds the stage in arena-style configuration, but, if I were to describe these two halls' shapes as succinctly as possible, the French hall has a rounded shape and, by contrast, the Finnish hall has a angular shape, making them extremely different configurations. We determined the basic shape of each of these halls through a rigorous design process using geometrical acoustics-based computer modeling.
In acoustical scale model testing, by emitting real sound in a scale model of the hall, we can automatically take into account the wave characteristics of sound, in contrast to tracing the straight propagation of sound in virtual field on the computer modeling.
Nagata Acoustics performs acoustical scale model testing during the last stage of a hall project design phase to scrutinize and confirm the room acoustical design's detailed specifications. In particular, this technique is a powerfully efficacious method for confirming what the acoustical conditions will be in a hall that has a unique shape not used in earlier halls. Specifically, our testing falls into two categories: (1) We test for undesirable echoes, find their root causes and test echo mitigation remedies; and (2) We measure the impulse response and compare the physical data we gather with data from existing halls.
<< Checking the Scale Model for Echoes >>
Loudspeakers for model test
Miniature dummy head Microphone
& Audience model
To mention that everything in the 1/10th-scale model has dimensions 1/10th the size of the actual concert hall is, of course, to state the obvious. The sounds used during scale model testing also have wavelengths that are 1/10th of the sound waves in an actual (full-size) concert hall. That is, compared with the sound in an actual concert hall, the scale-model sound must be frequencies that are 10 times higher.
Frequencies that are 10 times higher than the sound heard in a actual concert hall include ultrasonic sound-waves beyond the range audible to humans, so we must use special loudspeakers in the scale model. In the scale model testing for the Paris and Helsinki projects, we used two types of loudspeakers, both are capable of stable sound reproduction to 80 kHz, omnidirectional units and tweeters intended to represent instruments such as trumpets that have some directional characteristics.
We collected the sound impulse response by means of miniature dummy head microphones placed inside the scale model. By expanding the collected impulse responses' time duration to 10 times their original length, we were able to listen to the actual sound and determine if undesirable echoes are present or not. In the days before personal computers were as handy as they now are, in order to obtain the conversion of the test recording's speed, we relied on a bulky tape recording machine with a control capable of 10:1 speed changes. The narrow frequency range reproducible by the tape recorder and the insufficient output power of the sound source loudspeakers of the past, imposed limitations on the quality of sound achievable in scale-model testing. Nowadays, thanks to computers and digital signal processing technology, we have the capability to create good sound quality for scale-model testing.
Among the echoes we may detect during scale-model testing, some are single echoes that can even be anticipated on the display of impulse response. Compared with these easily identified echoes, the sounds that pose a challenge to interpret on the wave form are the clusters of small sound reflections that can sometimes be heard as an echo. By actually listening to the sound from the scale model, we can accurately evaluate if the sound is an echo or not. This is one of the situations in which the scale model demonstrates its power as a valuable testing tool.
If we determine the presence of an echo, we search for the location that causes it using a trial-and-error approach. That is, we temporarily apply a sound absorbing or sound scattering material at the location we suspect is the cause and test whether this change eliminates the echo. This may seem like a very low-tech approach; nevertheless it is a method that delivers sure results.
<< Measuring the Impulse Response >>
In actual (full-size) concert halls, there is a phenomenon known as air absorption. This phenomenon occurs when sound propagates in the air of both of the oxygen and water molecules existing. If the ambient medium in the scale model is air, air absorption specifically impacts the reverberation time of high frequency sound, making it shorter than it would be in the full-size hall.
Happily, a solution exists. If we replace the oxygen inside the 1/10th-scale model with a high concentration of nitrogen, we are able to simulate the air absorption that will occur in the full-size hall. When we conduct impulse response testing, we do so with a high concentration of nitrogen in the scale model.
After collecting the impulse responses, and calculating physical parameters from the impulse responses, such as the REC curves (reflection energy cumulative curve excluding direct sound) etc., we compare the data with data for existing halls to learn about the acoustics of the new hall. Based on these comparisons, we adjust the details of the new hall's design as necessary.
As for the two European projects, when the results of their scale model tests have been incorporated in their room acoustical designs, the time is ready to begin actual construction of these concert halls. We look forward to the next phase of these projects.
Sound Isolation Design - Part 3 of a Series:
Applying Sun Tzu's
"If You Know Your Opponent and You Know Yourself..."
by Chiaki Ishiwata
In Part 2 of this series, published in the May, 2006 issue of this newsletter, we focused on real-life examples of sound isolation and Japanese taiko drums. The stalwart sound of Japanese taiko drums, so appealing to people all over the world, represents the kind of very large sound volumes that are difficult to isolate from transferring to other rooms and spaces. Also, Japanese taiko drum playing is rhythmical and the technique for beating the drums includes a strong attack, two qualities that draw the attention of the human ear.
When strategizing and planning a sound isolation design, we consider the sound source (such as a taiko drum) and we also think about the sound environment that characterizes the space to be protected via the sound isolation design. For example, if the space is a concert hall, then the level of quietness expected differs from that of ordinary living space. In ordinary living space, we do not mind hearing rain falling outside or people in conversation, while in a concert hall, we should hear only the performance and no other sound. If the characteristics of the sound to be isolated and the desired quietness in the room benefiting from the isolation are the two sole variables used to determine the required sound isolation performance, then the sound isolation performance level needs to be remarkably high.
However, for example, if the space into which the sound transfers has internal sound sources generating a certain level of sound, such as in a factory, then even if there is sound transference from external sources, the already present sounds will drown out the transferred sound and, even if some of the external sound can be heard, it will not impede the activities performed in the building.
Sun Tzu's treatise, The Art of War, teaches: "If you know your opponent and you know yourself, then even should you fight 100 battles you will not fear the results." We can apply this teaching in our approach to sound isolation design, where the very first step we take is to "know our opponent and know ourselves." That is, we need to fully understand the target sound that we are trying to isolate, and we also need to fully understand the sound environment of the space that is being protected by the sound isolation design and the effects that the transmitted sound would have on the room. By having this complete understanding, we can plan an effective and economical sound isolation design and we will also be able to give input to the facility's design and programming development at an early stage of a project.
<< Knowing the Sound We Want to Isolate >>
Examples of sound transmission loss
of building materials
Sound isolation designs address the presence of many kinds of sounds. To plan the design, we need to know more than simply if the sound is big or of small volume. It is very important that we also learn the acoustical characteristics of the sound. We ask if the sound is high pitched or low pitched, if only certain frequencies are big, and if the volume varies or is constant, so that we can pinpoint the specific characteristics that the sound isolation design must address.
One reason that we need to be detailed and precise about the sound's characteristics is that the various construction-based sound isolation techniques deliver different sound isolation performances for different sound frequencies. For example, typical concrete walls provide small sound isolation for low frequency range, while they provide high sound isolation performance for high frequency range. Additionally, some building materials, such as single layer glass, exhibit a marked drop in sound isolation performance at specific frequencies. If the sound to be isolated should happen to match the frequency range for which the building material provides minimal sound isolation, then the sound transference becomes especially conspicuous.
Another reason that we need to know the specifics of the sound to be isolated is that the human ear reacts with different levels of sensitivity to different sound frequencies. Specifically, the human ear perceives low frequency sound less sharply than it hears high frequency sound. Also, humans are more sensitive to sound that varies in sound volume than to sound that exhibits a constant volume level and we notice rhythmical sound more easily than non-rhythmical sound.
Noise or sounds targeted to be isolated from a specified room or space fall into one of two large categories. The sound may come from a source entirely external to the facility. Vehicular and other transportation sounds may be the most common example of this kind of sound addressed by sound isolation designs. The other category of sound that sound isolation designs target are sounds produced within the same structure or facility as the room that the sound isolation design will protect. Instrumental music performance in a practice or rehearsal room exemplifies this category of sounds.
[Sounds from External]
In today's cities and towns, one would be hard to find a location where the sounds of vehicular traffic or other transportation are entirely absent. Vehicular traffic, trains, underground trains and monorails each generate sounds with a unique set of acoustical characteristics. Even if we consider just vehicular traffic, this grouping includes everything from automobiles to dump trucks and large trailer trucks to motorcycles. On some roads large trucks are not allowed, while on major transportation arteries the ratio of trucks to cars can be high. Each of these different vehicle types and road conditions generates different sound frequency characteristics.
Rail transportation likewise exhibits sound variations within its grouping. Japan's high-speed bullet trains (example of Shinkansen) and slower trains produce different sound characteristics. In addition, trains are the primary example of an external sound source that generates both sound and vibration. Since vibration generates noise, the potential impact of vibration on the building that is being isolated must be carefully considered and addressed.
Other than noise produced by transportation, the large equipment and machinery at factories frequently generate sound that needs to be isolated from another building. If a project's location is near a fire station, police station or hospital, the sirens of emergency fire, police and ambulance vehicles need to be considered in the isolation design planning. Depending on the project site location, children's voices from outdoor schoolyards and noise from nearby rooftop machinery may also need to be evaluated for their acoustical impact.
Comparison of a variety of sound sources
[ Sounds from Other Rooms in the Same Facility ]
The kinds of sounds that may be generated within a facility, and that may need to be isolated from other rooms or spaces, are limited only by the variety of rooms a building may have, how the rooms will be used and the equipment in the facility. That is to say, myriad sounds are generated and propagated in virtually any building. Typical examples of the kinds of rooms that generate sound addressed in sound isolation designs are: music practice rooms, gymnasiums, conference rooms, theaters, concert halls, workshops and nurseries. For each of these kinds of rooms, we need detailed information about how the space will be used to predict the characteristics of the sound that will be generated. A music practice room will generate different levels of sound and have different frequency characteristics if used for electric instruments and drums playing rock music compared with if the room will be used for acoustic instruments such as violin and piano.
The name given to specific rooms in a project's documentation does not serve as a reliable way to discern what the acoustical characteristics of sound generated in the room will be. The most desirable approach is to gain a thorough understanding, during the project design phase, of the intended use of each room in the facility.
In addition to the noise that may be generated in various rooms of a facility as a "side effect" of the intended activities being performed in the rooms, the building's HVAC and other operational, maintenance and machinery rooms may contain equipment that generates noise. Recently, in Japan, we see an increase in the installation of generator as well as machinery for large-scale parking garages that have noise levels necessitating sound isolation design attention. To develop the best sound isolation design, we need to obtain complete information regarding what equipment will be installed, how big the equipment's noise is, what are the characteristics of the noise frequencies generated and whether or not the machinery causes vibrations.
[ When Data is not Available ]
If data is not available for sound isolation design, on-site measurements for project's external sounds and measurements of sounds similar to the target internal sounds in the facility similar to the target project are needed.
<< Obtaining the Appropriate Level of Quietness >>
Once we know the nature and characteristics of the sound to be isolated, the question remains as to the extent that the sound must be isolated from a specific room or space. In order to determine the sound isolation objectives, we must know the room's requirements for quietness.
To determine the level of quietness a space needs, we primarily consider how the space or room will be used. Is the space a concert hall where people will listen to performances, a conference room for seminars and meetings, lodging for sleeping and relaxing or a space for active sports? We associate the appropriate level of quietness to the activities expected in the room.
For concert halls, the desired acoustics enable the audience to hear subtle sound nuances and, therefore, the quietness goal will be in the range of NC-15 to NC-20. By comparison, for typical conference rooms where people gather face-to-face for meetings, the level of quietness required need only be sufficient for conversation or discussion participants to hear and understand what other people in the room say. In this case, NC-30 to NC-35 would be a typical quietness goal.
The quieter a space becomes, the more one hears the unnecessary and sometimes undesirables sounds in the room. For example, multi-unit dwellings now use window frames that shut out street and traffic noise. As a result, occupants become more concerned about sound transference between adjacent units. In the June, 2006 issue of this newsletter, we introduced readers to the considerations now required in designing restroom facilities due to concerns about the noise that water generates when patrons use the sinks and toilets. This kind of problem will increase as higher levels of quietness become more prevalent.
One more aspect to remember about quietness is government regulation. In Japan, for example, if a particular piece of machinery is to be installed in a facility, a national, Prefectural or local law may stipulate the maximum noise level permitted at that location. These regulations must be checked and regulatory compliance must be reflected in the goals set for a project's sound isolation design.
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
2130 Sawtelle Blvd., Suite 307A,
Los Angeles, CA 90025, U.S.A.
Telephone: (310) 231-7818
Fax: (310) 231-7816
[ Japanese Version ]