News 09-12 (No.264)
Issued : December 25, 2009
[ Japanese Version ]
Curtains and Room Acoustics
by Fumiaki Sakamaki
Curtains serve a wide variety of purposes, from blocking out light, providing heat insulation or absorbing sound to solving an architectural need to partition a space or an interior design goal to achieve a certain decor. The fabric chosen for use in a hall varies depending on the purpose the curtains will serve.
In room acoustical designs, curtains play an important role as an adjustable sound absorbing element. Because curtains can be relatively easily installed and adjusted to various open and closed positions, we use sound absorbing curtains to create a mechanism for varying the sound absorption characteristic of a room interior. In this article, I will discuss the significant attributes of curtains used to absorb sound and examples of room interiors in which we have made effective use of curtains.
<< Sound Absorbing Properties of Curtains >>
When deciding to incorporate curtains in a room acoustical design for the purpose of adding a sound absorbing element, we must, of course, consider the sound absorbing properties of the curtain. Some vendors now market curtains with product names that hint at the manufacturer's claims about the curtain's sound absorbing properties. To begin with, sound absorbing curtains must be made of heavyweight fabric with airflow permeability.
Fig. 1: Test results for a sound absorbing curtain
The accompanying graph (Fig. 1) shows an example of the sound absorbing properties of a curtain that can be considered a "sound absorbing curtain." Each of the three lines represents the variation in sound absorption results depending on the depth of air space behind the curtains. Fortunately, curtains are not meant to be adhered to window or wall surfaces. They hang with some air space between the curtain and the window or wall they cover. If the curtain material has airflow permeability, then just as a porous material with air space behind it exhibits good middle and high-frequency sound absorption characteristics, so does the curtain.
In addition, the sound absorption characteristics at low frequency vary depending on the depth of the air space behind the curtain. The data shown in the graph represents the sound absorption characteristics for an installation of curtains that drape with folds and with the curtain fabric having so-called 100% fullness, meaning that the fabric measures twice the covered area. The fullness of the curtain is significant to obtaining the results shown in the graph. If the curtain hangs flat and does not drape with folds, then the sound absorbing characteristics shown in the graph will not be obtained and the sound absorption coefficient of the curtain will be smaller.
The properties of the curtain's fabric (its airflow permeability and its texture or nap), the weight of the curtain, and its fullness all contribute to the sound absorption coefficient of the curtain. Determining the specifications of a curtain and how it will be installed are important decisions. Curtains used in everyday applications, such as a home or office, typically lack the airflow permeability and the appropriate texture and, therefore, would not be suited to use as a sound absorbing curtain.
<< Curtains that Adjust the Reverberation Time in Small Rooms >>
Photo 1: Curtains used along the walls of a room
(Studio1, Hita City's Cultural Center, Oita Pref.)
In rehearsal and practice rooms, curtains may be used as the sound absorbing mechanism to enable adjustment of the reverberation characteristic to meet a range of uses and users' preferences. When designing a room to have curtains for this purpose, we design the room to have a rather long reverberation time and enable users of the room to shorten the reverberation time by drawing the curtains. On some projects where the requirements do not include providing a variable reverberation time feature, we install curtain rails so that the project owner can easily add curtains at a later time.
<< Curtains that Adjust the Reverberation Time in Halls >>
Depending on the programming objectives of a hall project, it is not unusual for us to include the adoption of variable acoustics mechanism in our room acoustical design. The variable acoustics we adopt for a particular project may involve means of adapting major elements of the hall's architectural design , that is, sound reflecting and sound absorbing panels are switched with motorized mechanism. Or, the acoustical design may adopt the comparatively simple approach of adding curtains.
When a room acoustical design incorporates the use of curtains to create a variable acoustics, and the design includes a manually operated mechanism to open and close the curtains, this design has the benefit of being economical to implement. To achieve the desired reverberation time differential between when the hall will be configured with the curtains open and when they are closed, our design work includes carefully selecting the appropriate curtain fabric, determining the dimensions of the coverage area and the percent fullness of the curtains, specifying a sufficient allowance of air space behind the curtains and designing the enclosure where the curtains will be stored when not in use.
Photo 2: Curtains used to vary sound
reverberation time in a hall (Music Hall,
Keiyo Bank Culture Plaza, Chiba Pref.)
In addition to the above considerations, if the hall will be used for non-amplified drama performances, the room acoustical design must obtain with excellent speech intelligibility in the hall. The very early reflections from the walls of a hall contribute significantly to achieving excellent speech intelligibility and, therefore, these walls must be avoided as locations for the curtains.
In multipurpose halls, the stage is configured with a stage curtain during lectures, ceremonies and other non-music events. The stage curtain serves the purpose of hiding the stage wings from the audience's view and may also hide stage lighting fixtures and other stage equipment. At least equally important from the acoustical perspective is the role played by the stage curtain in dampening the hall's reverberation time and thereby adapting the hall's acoustics to the needs of non-music programs. When the configurations of multipurpose halls are converted from stages with orchestra shell to stages with stage curtains, the reverberation time differentials are measured in the range of 0.3 to 0.4 seconds. This degree of reverberation time variability enables a hall to successfully serve as the venue for many genres of performances and a wide variety of event programming.
<< Examples of Curtains Installed in Concert Halls >>
Photo 3: Curtains at the back of a concert hall
stage (Harmony Hall Fukui, Fukui Pref.)
At concert halls such as MUZA Kawasaki Symphony Hall and Harmony Hall Fukui (shown in Photo 3), the programming objectives include primarily classical music performances and other events secondary, particularly those that require speech intelligibility. To make the acoustical environments of the stage areas of these halls adaptable to the non-music events, the halls' acoustical designs include sound-absorbing curtains that hang along parts of the stage, the source location of the music during concerts. During lectures and ceremonial events, the curtains in these halls are set in the unfurled position, so that they dampen the hall's reverberation and achieve excellent speech intelligibility.
The reverberation time differentials in these halls between when the curtains are in their open and closed positions is in the range of 0.4 to 0.5 seconds (at 500 Hz). This represents a substantial variation in these halls' reverberation times.
<< Curtains that Adjust the On-stage Acoustics >>
Ensemble Hall Murata, the small hall of Kyoto Concert Hall, which opened in October, 1995, has a stage rear wall that is covered with 50% perforated aluminum panels. Behind the perforated aluminum we installed curtains that can be unfurled. This design serves as a mechanism to decrease the amount of sound reflections from the stage rear wall. Rather than affecting the reverberation time of the hall, the significant impact of this design during performances is a change in what the musicians hear on stage.
Performing groups may try and choose the curtains opened or closed during their rehearsals, either maximizing or decreasing the amount of sound reflections from the stages' rear walls. And so, the flexible and easily adjusted use of curtains in the stages' designs allows the musicians to achieve their desired stage acoustics.
In summary, sound absorbing curtains can be useful in a variety of room acoustical designs. If we keep in mind that sound absorbing curtains offer minimal sound absorption properties with regard to low frequency sounds, room acoustical designs and architectural designs alike may find sound absorbing curtains to be the design element that best meets the needs of certain project objectives.
The Effects of Diffusion on Room Acoustics - 3
by Dr. Minoru Nagata, Founder of Nagata Acoustics
On March 27, 2009, the Architectural Institute of Japan and the Acoustical Society of Japan held a joint symposium on the topic of "Recent Research on the Diffusion in the Room Acoustics". The Acoustical Society of Japan published the content of the symposium as a special feature in Volume 65, Issue 11 of the society's journal. Nagata Acoustics' Dr. Keiji Oguchi presented a paper at the journal in which he spoke about the practical application of acoustical diffusion to the room acoustical designs of projects and shared collected data on this topic.
In this third and final newsletter article about the effects of sound diffusion on room acoustics, I also will approach the topic from the perspective of how our knowledge about acoustical diffusion informs our current room acoustical design work.
1. Acoustical Diffusion and Key Room Acoustical Design Parameters
Current room acoustical design work focuses strong attention on not only a room's reverberation time, but also on the temporal structure of early reflections and their spatial distribution. Therefore, I think it appropriate to review the relationship of diffusion and its acoustical effects on each of the key parameters of reverberation and the early reflections.
Surely, when we listen to music in a hall, we empirically sense that the effects of acoustical diffusion are necessary to a hall's having the desired acoustics. However, at present, the acoustical discipline has still not developed clear design guidelines regarding acoustical diffusion. The effects of acoustical diffusion in a room first become perceivable when hearing music/sound. Therefore, in my opinion, progress in understanding the role of acoustical diffusion relates to our understanding of the essential nature of the interaction between human beings and the sounds we hear.
2. Is the Diffusion-or Scattering-of Early Sound Reflections Necessary?
For early reflections to effectively reach a listener, such as a person sitting in an audience seat, the sound reflecting surface should be relatively close. Strictly speaking, the difference between the direct distance from the sound source to the listener and the distance from the sound source to the sound reflecting surface and, from there, to the listener, should be no greater than 30 m. (98 ft). For large scale concert halls with high ceilings, these conditions are difficult to achieve. Room acoustical designs for large scale halls must devise strategies such as hanging overhead reflection panels and obtaining early reflections from the wall at the balcony fronts. A major objective of room acoustical design for a large scale hall involves solving the need for sound-reflecting surfaces that will produce effective early reflections capable of reaching the audience seating. Patrons desire acoustics intimacy, and early reflections contribute significantly to creating this kind of acoustics.
Fig. 1 the Bonn Beethovenhalle
the sound-scattering elements
fixed in the latticed side walls
Several decades ago, the evenly diffused sound fields as a means to design the ideal concert hall came into vogue for a period of time. Acoustical diffusion was prioritized at the cost of other objectives, and halls designed according to this principle had protruding, sound- scattering surfaces throughout their interiors, on the ceiling surfaces as well as on the sidewalls. The most representative example of this design approach is the Bonn Beethovenhalle, shown in Fig. 1. I only visited this hall once, but I draw my conclusions about this approach from its disappearance as a methodology for the acoustical design of newer halls. In large scale halls, the surfaces that produce early sound reflections do not need to be designed to scatter the reflections. I am of the opinion that placing sound scattering elements everywhere in a hall can dilute the acoustical effects of early sound reflections.
3. Using Subtle Texture Variations and Furrows to Reduce Acoustic Glare
In the preceding paragraphs, I did not yet mention the appropriate materials for sound reflecting surfaces. The use of some materials should be avoided or approached with caution and mitigating strategies. Specifically, glass and marble walls, as well as smooth board surfaces are a cause for concern. These kinds of surfaces abound in cafes and restaurants, with the result that conversations, laughter at one table and the clatter of dishes carry across the room and grates on the ears.
The well-known acoustician Dr. Leo Beranek calls the noise from these kinds of surfaces "acoustical glare". While I understand that these kinds of smooth surfaces may appeal to architects, they are not desirable materials to use in spaces intended for the performance of music. These kinds of materials may be acceptable in locations far from the audience seating, for example, on ceilings. But, if they are used for the surfaces of walls near the audience seating, they create acoustics that are too harsh and strident. To mitigate the negative acoustical effects of marble, glass or smooth panels, the rough unfinished side of marble can be used instead of the polished side, glass can be combined with an overlay of netting, and texture or furrows can be added to smooth board surfaces.
4. Preventing Echoes
Two kinds of echo phenomena can occur in halls: flutter echoes and long path echoes. The presence of either of these echoes will negatively impact a hall's acoustics and it is imperative that acoustical room designs include a proactive strategy to prevent echoes. The strategy may include sound absorption measures and setting sound reflecting surfaces at specific angles. Acoustical diffusion elements that disperse or scatter the sound waves can also be an effective strategy to attain this objective.
5. Diffusion and Spatial Impressions
In my experience, the great cathedrals of Europe provide the ultimate sense of being immersed in the acoustics of a space, an auditory sensation encountered rarely in a country like Japan, which does not have an architectural legacy of large stone cathedrals. In the English language, the acoustical discipline refers to the auditory sensation produced by cathedral acoustics as the sense of "envelopment". When I was in Germany some 40 years ago, at a time now remembered as the dawn of concert hall acoustics research, I heard the term "Schwimmbadeffeckt"-literally, "swimming pool effect"-used. In a swimming pool, a person immerses totally, and the pressure of the water can be felt by every part of the body. In my opinion, the German "Schwimmbadeffeckt" aptly describes the auditory experience heard in the diffuse sound field conditions of a European cathedral. Recent advances in room acoustics research include progress in understanding this immersion-like auditory effect and these advances shed new light on the relationship between acoustical diffusion and room acoustical parameters.
These recent researches are inspired by our hearing mechanism that the lateral reflection arrives our left and right ears with some differences in phase and strength. The difference is expressed as an index value named the interaural cross-correlation coefficient(IACC).
Kobe University's Prof. Masayuki Morimoto has studied spatial impressions for many years and his published research helps us understand the relationship between diffusion and how we hear sound. To summarize his key points:
- Spatial impression is comprised of two components. One component is Auditory Source Width (ASW), which is the perceived size of the sound image. The other component of spatial impression is Listener Envelopment (LEV), which is the sense of being enveloped or immersed in the sound.
- The ASW value is not affected by the spatial distribution of sound reflections or by acoustical diffusivity. The value can be determined based on the degree of interaural correlation. That is, the greater the difference between the signals heard in the left and right ears, the wider the sound image.
- The LEV value is affected by spatial distribution, which means that it is affected by the acoustical diffusion conditions in a room. Also, the value cannot be determined based on the degree of interaural correlation. The better the acoustical diffusion, the greater the sense of envelopment.
- The room elements included in a room design for the purpose of creating early reflections do not need to have acoustical diffusion properties. However, polished materials such as marble and glass should be avoided as the materials included in a room design for the purpose of producing very early sound reflections. If the use of materials such as marble and glass cannot be avoided, their surfaces should be given a rough finish, textural variations or furrows.
- For sound-reflecting surfaces that are not involved in producing early sound reflections, the surfaces should be designed with sound scattering elements to increase the diffusivity of reverberation and promote the sense of envelopment experienced by listeners.
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.
Tel: +1-310-231-7818, Fax: +1-310-231-7816
75, avenue Parmentier
75011 Paris, France
Tel: +33 (0)1 40 21 44 25, Fax: +33 (0)1 40 21 24 00
[ Japanese Version ]