Hearing Architectural Design:
Quinsan Ciao, Ph.D.
Department of Architecture
Virginia Tech., Blacksburg, VA 24061-0205
This paper with demonstration is devoted to revealing and establishing the relationship between space and sound through computational acoustic analysis, simulation and electronic synthesis of audible sound.
Based on science of acoustics and computing technology, acoustic effect of an architectural 3-D design can be analyzed and the resulted sound in space can be synthesized and predicted accordingly and being heard. Auralization refers to this process of acoustic analysis, sound synthesis and audio presentation of the result in the form of audible sound. Design alternatives can be experimented until satisfactory acoustic effect is achieved.
Traditionally, designers rely on some minimum and vague understanding or specialists’ experiences to predict and design for a desirable sound behavior in spaces. Most likely acoustic design and analysis are seen as a luxury remedy only affordable in large-scale theatres and concert halls. The recent available PC based auralization tools brought significance in both in terms of new knowledge towards the science and art of architectural acoustics and the methods and practice in the design process.
The examples demonstrated in the presentation will indicate that the auralization technology make it possible for the designers, consultants, end users or potential occupants to examine and evaluate the performance of different designs by hearing it directly before an informed decision can be made. The case studies also illustrated that the auralization is a powerful tool for general public with common building types to uncover everyday acoustic problems that have been constantly harming their well being and would otherwise be undetected.
During the last couple of decades, our cities and urban environments have been experiencing some major transformations: recent population migration back to city centers, small town revitalization and redevelopment of previous rundown industrial areas. One of the many adverse phenomena resulting from the vast scope of modern and post-modern technological progress is the escalating urban noise that is becoming more and more intolerable.
While modern industry and society produced acoustic and noise problems, fortunately the development of modern technologies is providing better tools that can help to address the problems when appropriately utilized. New acoustic computational technologies, including digital analysis and simulation plus multimedia synthesis and auralization, provide efficient means in analyzing and improving acoustic performance of architectural designs.
With such acoustic simulation and auralization tools, the architectural design process is largely altered. First of all, the acoustic performance evaluation becomes a key integral part of the design, as it should be. Acoustic performance is evaluated for design alternatives or ideas while the design is being conceived and produced. Therefore, the acoustic simulation and auralization directly influence the design process and result. Secondly, clients or end users become an active part of design force. With the simulation and auralization tools, clients (sometimes architects) without training and expertise in acoustic can evaluate the performance of designs through audible hearing experience.
Unfortunately, for a long time, acoustic simulation and auralization computation have been complex and expensive. Only large theaters and music halls can afford such rigorous analysis and evaluation. Most of architectural designs and projects proceeded without such luxury until construction is completed and occupants moved in. Acoustic problems are often discovered too late and are expensive to correct. Sometimes, problems are not discovered while harmful environments are endured.
With the benefit of the new numerical methods and rapid improvement low cost computers and software development, the acoustic analysis, simulation and auralization are much more accessible today. Sophisticated acoustic simulation and multimedia synthesis and auralization can be performed on almost any common personal computer. Such technology advances made it possible for common architectural designs and projects to afford much more rigorous acoustic evaluation and modification.
This paper presents two small scale projects that will demonstrate the effectiveness acoustic simulation and auralization in acoustic evaluation and modification of architectural projects. The first project is in a rundown industrial area of metropolitan Washington DC where an old warehouse is to be remodeled into a theater as the entire vicinity is becoming revitalized. This situation is typical for urban areas in the United States, especially inner city areas, due to the current trend of migrating back to the cities after a few decades of abandoning them. For this particular project, among all the complex acoustical issues, a major problem is the aircraft noise from the Reagan National Airport near by.
The second project is to transform a church assembly hall into a worship space for Sunday services, while the existing sanctuary room is no longer able to provide sufficient space for increased membership. Such a situation is typical for a church like this located in a small town of Southwest Virginia.
In these case projects, computer aided prediction and auralization results are presented, compared and discussed. Some field measurements and technical analysis are mentioned as a base for achieving meaningful understanding.
In the metropolitan Washington DC area near National Reagan Airport, a rundown industrial and low cost housing area is in the process of being revitalized into a new business and residential community space. The occurrence of this process is a small part of the nationwide movement in the United States where rundown urban and inner city spaces are being reclaimed and remodeled into new work and live spaces. Examples can be seen in cities all over the country. This case study presents a moderate remodeling project in Washington DC area where an old warehouse is to be remodeled into a community theater code named MetroStage.
MetroStage's site is very close to the flight path of jet planes’ taking off from and landing to the National Reagan Airport at Washington DC. The noise of aircraft traffic, especially the noise of jet plane taking off, is causing intolerable disturbance to the area. The noise at the ground level of the site is estimated around 95 dBA due to jet planes taking off. The noise of such level is causing serious discomfort. Under such noise, conversation with mouth-to-ear shouting can be barely understood, not to mention stage performance. Obviously, air born noise insulation is extremely important for the building constructions in the area, especially for a theater where undisturbed aural experience is one of the most important factors.
The existing warehouse building at the site has a corrugated metal wall. It provides very little reduction (~ 2 dB) to the interior space from the environmental noise in low frequency range, and some audible reduction (~ 10 dB) to ear pinching high pitch (> 3 kHz) noise.
For a small community theater as is the type of the MetroStage project, the noise level (as a measure of quietness) is typically desired to be below the standard NC-30 that specifies the noise level to be no more than 38 dBA. As shown in Figure 1, the existing environmental noise level is significantly (40 dB) higher than the NC-30 standard.
The insulation requirement, or required sound transmission reduction, for the wall structure of the remodeled building is nothing but the difference between the environmental noise and desired tolerable interior noise level. This required sound transmission reduction is computed by subtracting the NC-30 spectrum from the worst-case estimate of the environmental noise spectrum.
Figure 1. Environmental noise measurements are summarized as the worst-case estimate that is the maximum value of all noise measurements in each frequency band. The broadband exterior worst estimate is 82 dBA and that of interior is 80 dBA, while the NC-30 requires 40 dBA or lower.
There are many different alternatives of wall assembly that provides different level of noise insulation better than the that provided by the existing wall. In this case study, several alternatives are considered, including:
1. 12-inch concrete block wall
2. Steel stud (with RC-1 channels) wall filled with mineral fiber and cellulose mixed spray (Sound-Pruf™) between gypsum boards
3. Steel stud wall filled with mineral fiber blanket between gypsum boards
4. 2x4 stud wall filled with a natural material spray (ICYNENE™) between gypsum boards
5. 24-gauge corrugated metal with cellulose spray.
As the characteristic Sound Transmission Loss (STL) curves shown in Figure 2, the first two alternatives satisfy NC-30 requirement. The third alternative is fairly close, while the last two do not meet NC-30 requirements. For reference, the existing corrugated metal wall's performance is also shown in Figure 2.
The two assembly alternatives that can meet NC-30 requirement can provide a quite interior environment with almost no audible noise. Simulated of jet taking off noise and synthesized sound effects of these well insulated situations indeed reveal a fairly quite environment. Figure 3 demonstrates the comparison of interior residual noises insulated by different types of walls, with both sound wave plots and frequency spectrum plots. Figure 3(a) shows the sound wave plot and frequency spectrum of 10 seconds of field measured noise inside the existing building where little insulation is provided by the corrugated metal walls. Figure 3(b) shows the sound wave and spectrum plots of noise that would be in the building if the existing walls are replaced by the gypsum boards filled with Sounf-Pruf™ spray and steel studs. This result is generated by computer simulation. Figure 3 clearly shows the superior insulation result of the new wall material, a 40dBA further reduction.
Figure 2. Residual noise level in the interior space with different alternative wall assemblies.
(a) Measured interior residual noise with the existing bare corrugate metal wall.
(b) Simulated residual interior noise insulated by the wall of gypsum boards filled with mineral fiber and cellulose mixed spray (Sound-Pruf™) and steel stud with RC-1 channels.
Figure 3. Comparison of (a) noise inside the existing building measured during a jet plane taking off and (b) simulated noise inside the building with remodeled wall material during the same jet plane taking off. Synthesized interior noise for the interior after remodeling is almost not audible even under close examination, while the measured noise inside the existing build prohibits mouse-to-ear shouting difficult to understand.
Although slight difference is visible in sound wave and spectrum, no significant difference is audible between the result of the 12-inch concrete block wall and the Sound-Pruf™ spray filled steel stud wall with RC-1 channel mounted gypsum boards. They are both practically inaudible. The later, however, has the advantage of lower material and construction cost even though caution may be required during construction. The other three alternatives are not as effective as the first two. The residual noise left after these insulation walls is above NC-30 level and are audible while everything else is quite.
In addition to the wall insulation to eliminate audible noise from interior space of MetroStage, the interior design is carefully arranged with acoustic boards and other surface materials such that the synthesized auralization result is audibly satisfactory. In this remodel design process, although the design remains in drawing boards (and scaled models as shown), the sound effect is heard and evaluated. In an iterative process, the design is modified and its sound effect is heard and evaluated until a satisfactory result is achieved. The space design and the sound are created and recreated in parallel.
The subject of this project is a multipurpose assembly hall in a church located in the historical district of a university town in southwest Virginia. The church congregation has converted the hall into a worship space for Sunday services, as the original sanctuary hall is no longer providing sufficient space for the increased church membership. However, due to the poor acoustic design, the room messes up speeches with strong echoic sound and various type of noise. The echoic sound effect is illustrated in Figure 4 as sound pressure wave plots.
Evidently, the sound effect in the church room (shown in the second graph) is seriously distorted from the original sound (shown in the first graph). The distinct words shown by clearly separated blobs of the original sound wave are stretched into a fuzzy mumble shown as a continuous patch of the sound wave in the church room. The echoic effect clearly affects the intelligibility of the speech such that the words become difficult to recognize.
To understand the causes of the undesirable echoic effect, we need to examine the church room's design configuration carefully. The church room design configuration is shown in Figure 5. In this church room, the high slope ceiling creates a large volume of interior space and the tile floor and bare wood ceiling and wall surfaces provides little absorption, where both factors, according to Sabin's law, lead to large reverberation time that is responsible for the echoic effect. Both field measurements and CATT (an acoustic analysis and simulation software) analysis and simulation indicate that the reverberation time is close to 2 seconds. This level of reverberation time is not uncommon in church halls that lend the space a strong echo effect that would enhance the effect of music played by pipe organs. However, for normal speeches, the intelligibility can be adversely affected severely, although for some slow and simple sentences the echo effect might be dramatizing (e.g., the famous Martin Luther King’s speech of “I have a dream …”.) The members of the church consider the original sanctuary hall provides a well mixed, clarified sound, which is most comfortable for listening. The field measurement shows a reverberation time near 1.0 second for that room. Here we have clients who recognize the noise problem and demand to change because of their many years of preferred, good listening experiences.
(a) Recorded original female voice. Recording was taken in an anechoic chamber, equivalence of open space with no reflection and reverberation.
(b) Simulated reverberation effect in the existing church room, where the original sentence becomes blurred and not very intelligible. Besides amplifying the magnitude, the reverberation "elongates" the syllables and makes them indistinct and blurred together.
Figure 4. A simulation and auralization software program, CATT Acoustics, generated the audible sound effect of the church room. The simulated sound effect sounds quite similar to the field experience. A sound wave analysis software, GoldWave, generated the visual waveform of the different sound effects.
Further analysis of the church room's acoustic properties reveals that the long paths of sound reflected off the ceiling surfaces are indeed one of the most important contributors to large reverberation time. The paths of sound reflected off each ceiling surface accounts to 15% of the total reflection. It is the next highest percentage only lower than the 27% off the floor.
Figure 5. The existing church room design. A high slope ceiling and wide separated side walls create a large volume of interior space and the tile floor and plaster ceiling and wall surfaces provides little absorption, where both factors lead to large reverberation time that is responsible for “noisy and echoic” sound condition.
Considering the causes of the undesirable long reverberation time, the following basic correction schemes are analyzed and simulated:
1. Lower the ceiling with acoustic panels, reducing the lengths of sound reflection paths and reducing room volume, increasing absorption;
2. Carpeting the floor, increasing absorption.
The remodeled church room with considered schemes is shown in Figure 6. The considered design feature changes reduce the room volume and increases absorption, both contributes to lower reverberation time. From another point of view, with the lower ceiling, the sound reflection paths are shorter leading to lower reverberation time. CATT analysis indicates that the reverberation time (RT) after remodeling would be slightly less than 1 second. This level of reverberation time would provide enough amplification and enhancement to the voice’s volume and its richness, as has been confirmed by the community from their experience with the other “good” room in the church. Figure 7 illustrates the same speech’s sound effect in the remodeled church room, with sound wave plot and sound wave clips that can be heard on a multimedia enabled computer.
Although the considered schemes are shown to provide desired acoustic effect with proper reverberation time, the architectural feature changes (e.g., lowering the ceiling) may not be acceptable for other reasons such as visual and spiritual perceptions. Many other alternative correction schemes can be and are considered and analyzed, such as adding sound absorbing banners and suspended partial ceiling with adjustable treatment, adding reflection surfaces to enhance shorter path reflections (or early energy factor), etc. The acoustic design for this church space is actually a process of architectural design. To keep this paper abbreviate, we will not list all ideas and design alternatives. With the analysis, simulation and auralization tools, many alternative solutions can be evaluated rapidly without only relying on “ rules of thumb” or sometimes “guessing work”. It is also interesting to see that some commonly believed and practiced “guidelines” are not supported by the result of the simulated CATT runs.
Figure 6. The church multiple-purpose room after remodeling for better speech intelligibility. The ceiling is lowered with acoustic panel and the majority of floor area is covered with carpet.
(a) Simulated speech effect in the existing church room, where the original sentence becomes blurred and not very intelligible.
(b) Simulated speech effect in the remodeled church room. The sentence is well intelligible and enhanced in both volume and tone.
Figure 7. Effect of remodeled church room on sound of a speech, provided by CATT simulation and auralization.
More frequently for dense urban environment, tenants and occupants tolerate existing serious acoustic problem without realizing it until it’s eliminated, or without awareness of possibility for any changes. Due to architectural acoustics’ gradual vanishing as a basic design aspect in professional practices, many poorly designed spaces exist and are tolerated. Many of such problems, such as severe noise annoyance and poor intelligibility of speeches, are causing serious effects to the well being of inhabiting tenants. Although not necessarily realized, these noise and acoustic problems contribute directly to many of modern day common problems such as stress, anxiety, fatigue and even hearing loss.
Contrary to common myth, solutions to such problems do not have to involve huge costs. The two case studies shown in this paper demonstrate that moderate remodeling can significantly improve acoustic performance and noise reduction/elimination. Given modern computer based analysis, simulation and auralization tools, the searching and verification of cost effective solutions among various design alternatives become practically viable. The case studies illustrate this point well. Not only efficient, auralization tools make result verification and evaluation much more effective. Such tools make the intuitive and subjective perception and evaluation possible before costly constructions are implemented.
The awakening, or re-awakening, of acoustic performance and noise pollution control demands are occurring worldwide, such as America and many Asian and European countries. With the increased awareness and education, designers, builders, as well as owners and occupants should realize that architectural acoustics is not a luxurious and expensive remedy for upscale concert halls and theaters only. Serious commitments to better acoustic design for all urban buildings are urgent needs. The development of technology and our understanding today enable us to practically design buildings and spaces for better acoustics with much more confidence. Meantime, the task of creating a sound environment remains to be a mostly challenging and complex science and art. Such challenging tasks can certainly use all helps available from new technologies and tools.