Session II

THE WHISPERING GALLERY: STATUARY HALL UNVEILED

Xholina NanoAmerican University

4400 Massachusetts Ave. NW, Washington, DC. 20016-8058, xn1023a@american.edu

Abstract — Completed in 1807, Statuary Hall is one of the most historic chambers in the U.S. Capitol. Also known as the Old Hall, the impressive two-story, semicircular room was the meeting hall for the U.S. House of Representatives for nearly 50 years. Modeling an ancient amphitheater, Statuary Hall was one of the earliest examples of Greek architecture in America. The Hall was damaged when British troops burned the Capitol in 1814 and was later rebuilt to its current state. The Hall presented acoustical obstacles that made conducting business difficult. The elliptical shape of the room and smooth curved wooden ceilings contributed to the interesting sound effects and large echoes. It has been stated many times that conversations of those standing in a far corner could be heard by others standing in the opposite corner. The science behind this sound effect can be explained by wave motion. Sound waves are reflected, refracted, and brought to a focus leaving the audience in the Hall experiencing a cacophony of sounds. A number of unsuccessful attempts to improve the acoustics in the Hall included hanging draperies and reversing the seating arrangement. This paper will explore the physics of sound waves and the architecture of Statuary Hall as it relates to acoustics.

Index Terms — Absorption, longitudinal waves, reflection, reverberation, sound waves.

INTRODUCTION

Acoustics can have a dramatic impact on the way sounds travel through a room. The shape of a room can facilitate the transfer of even the softest of sounds from one end to another. Bystanders in Statuary Hall experience an acoustical anomaly in which conversations are reflected along the surface of the elliptical ceiling and clearly heard on the opposite ends of the room. This effect has been present since it was built and played a large role in its future.

HISTORY

Statuary Hall has played a large role in the history of the Capitol and in shaping history for future generations. President Monroe appointed Charles Bulfinch as architect of the Capitol in 1818 to replace Benjamin Henry Latrobe. Bulfinch rebuilt Statuary Hall between 1815 and 1819, which was originally designed by Latrobe [1]. The Hall remains a symbol for the country and a national landmark. As the Hall of the House, it was home of many important events. In 1824 the Hall served as the Chamber where the

Marquis de Lafayette became the first foreign citizen to address Congress. Former Presidents James Madison, James Monroe, John Quincy Adams, Andrew Jackson, and Millard Fillmore all were inaugurated at the Hall [2]. John Quincy Adams has long been associated with the Hall and it was where the House of Representatives elected him President. After his presidency, Adams served as a Member in the Hall for almost two decades before suffering from a stroke in 1848. He died soon after the stroke and today there lies a brass plaque in remembrance of Adams marking where his desk was once located in the Hall (Figure 1). Many

FIGURE 1 [3]PLAQUE OF JOHN QUINCY ADAMS AT STATUARY HALL

historians believe that during session Adams would put his head down on his desk and would be able to hear the conversation of others standing on the other side of the Hall.

Today, Capitol Hill tour guides will point to the spot of Adam’s desk and proceed to demonstrate the Hall's acoustics and its advantages for eavesdropping. Skeptics, on the other hand, argue that Adams could not have possibly discovered the acoustical anomaly since the Hall’s interior design was different from its current form.

The poor acoustics in the Hall made it difficult to conduct business. The domed ceiling contributed to this, and draperies were hung in an attempt to muffle echoes.  After many failed attempts to reduce the echo effect, the only solution was to build an entirely new Hall, one in which debates could be easily understood [4]. In 1850, a new Hall was authorized to be built as an addition to the Capitol building. The House of Representatives was moved into its present location in the new House wing in 1857.

Once members of the House moved into their new chambers the fate of the Old Hall remained uncertain. One popular suggestion proposed to use the Hall as an art gallery.

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In 1864, Congress passed legislation sponsored by Representative Justin Morrill to invite each state to contribute statues of prominent citizens for permanent display in the room. The law led to the renaming of the Hall to the National Statuary Hall. By 1971 all 50 states had contributed at least one statue, and by 1990 all but five states had contributed two statues [5]. Artistic works intended for the Capitol extensions were put on exhibit; among these was a model for the Statue of Freedom, which rests atop the Capitol Dome. The legislation also provided for the replacement of the Hall’s floor, which was leveled and covered with the marble tile [6]. The floor modification, along with the replacement of the original painted wooden ceiling, eliminated some of the echoes that earlier plagued the room. Though the modifications were somewhat successful in reducing the excess noise in the Hall, there remained issues related to echoes in the Hall.

Today Statuary Hall is one of the most popular rooms in the Capitol. It is visited by thousands of tourists each day and continues to be used for ceremonial occasions. Special events held in the room include activities honoring foreign dignitaries and presidential luncheons. The Hall serves as an icon of American government and known throughout U.S. history for its attributes and flaws. Its architectural design plays a role in its unusually acoustic properties.

ARCHITECTURAL DESIGN

In Statuary Hall, the acoustic anomaly is most largely due to its architectural design. The Hall is an early example of Greek revival architecture in America. The shape and form was adopted from ancient Greek amphitheaters for the new legislative chamber, with a lantern in the ceiling to admit light during late sessions [7]. However, The Hall had one infamous flaw. As Members addressed the House, the sound of their voices echoed through the Hall. The high curved, smooth wooden ceiling prompted a cacophony of sounds.

FIGURE 2 [8]PRESENT DAY STATUARY HALL

Every noise reverberated several times throughout the room creating echoes, and conversations occurring across the room could be heard with alarming clarity, disrupting the conduct of business.

The great Hall experienced structural modifications throughout its history. While most wall surfaces were made out of painted plaster, the low gallery walls and pilasters were composed of sandstone. Around the room's perimeter stand colossal columns of variegated Breccia marble quarried along the Potomac River. The Corinthian capitals of white marble were carved in Carrara, Italy. The chamber floor is laid with black and white marble tiles; the black marble was purchased specifically for the chamber, while the white marble was scrap material from the Capitol extension project, which included the new House of Representatives [9]. Later additions made in 1901, including a fireproof cast ceiling and a marble floor, eliminated most echoes. However, some very interesting acoustical effects can still be demonstrated today and explained by the ways that sound travels through the Hall.

PHYSICS OF WAVES

In order to better understand the Hall’s special acoustical effects, it is necessary to understand the properties of sound waves. The world is full of examples demonstrating wave motion. One encounters waves of different types every day. This paper focuses specifically on the notion that sound is a wave while also acknowledging other types of waves. Sound waves will be explored in greater detail in a subsequent section. Besides sound waves, one experiences radio and TV waves, water waves, waves in microwave ovens, and even earthquake waves. In each case, some sort of vibratory motion causes a wave.

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FIGURE 3 [10]PAINTING OF THE OLD HOUSE

Types of Waves

A wave is a disturbance that travels through a medium by virtue of the elastic properties of that substance. A medium refers to a substance or material that carries the wave, such as air or water. Transverse and longitudinal waves are two types of waves. Transverse waves describe a motion where the direction of the pulse is at right angles to the direction of wave speed. Longitudinal waves are waves in which motion travels along the direction of the wave rather than right angles to it [11]. To help conceptualize the differences among types of waves it is best to use the example of the motion of waves within a rope and Slinky. When a rope is attached to a doorknob and moved up and down, a transverse wave similar to the second wave shown in Figure 4 is created. The disturbance caused by the up and down motion creates high and low points within the wavelength. The distance between the top of one crest to the top on the next is the length of the wavelength. Wavelength in a transverse wave is illustrated in the bottom of Figure 4. The crests are the highest point in the rope is and the trough is the lowest point below the equilibrium position [12]. There are also places along the rope where it is not displaced from its equilibrium position, referred to as nodes. Some examples of transverse waves include a violin string, or a string in a piano.

The top half of Figure 4 illustrates a Slinky being forced back and forth in the same horizontal direction. By pulling it out straight and then hitting the end, its motion will result in a pulse moving along it to the doorknob. The back and forth motion produce a pulse different from moving the rope up and down. The force of motion given disturbs the first coil; which pushes the second coil and displaces it from its equilibrium position. The coils continue to push or pull on each other, which then is displaced them from their equilibrium position. In Figure 4 the top image of the slinky illustrates areas of compression, areas of tightening, and rarefaction, regions where the coils are farther apart that usual. The result is a disturbance that moves down the Slinky and identified as a longitudinal wave. It is characteristic that the disturbance in is in the direction the wave is traveling in for longitudinal waves. This paper focuses on sound as the most important example of longitudinal waves.

Compressions and rarefactions are analogous to crests and troughs in transverse waves, as seen on top of Figure 4. The number of compressions, or rarefactions, that pass a given point per second is the frequency of the wave. A measured threshold of hearing at a specified frequency, expressed in decibels relative to specified standard of normal hearing. The frequencies audible to the human ear range from approximately 20 – 20,000 hertz (Hz) [13]. When the sound falls out of this range it is not audible and will not be detected by the human ear. The frequencies of waves are

dependent on how many oscillations a wave experiences per

second.

Figure 4 [14]PARTS OF LONGITUDINAL AND TRANSVERSE WAVES

Simple harmonic motion is a periodic motion in which the restoring force is proportional to the displacement. This motion repeats itself in time. A sine wave experiences simple harmonic motion in which a small section of it is repeated over and over. This concept helps explain how the period and frequency of a wave are derived. The period is the time it takes to make one oscillation. The disturbance of repetition is known as the wavelength of the wave, which is usually designated by a lowercase Greek lambda, λ [15]. The wavelength of a transverse wave can be measured from the maximum of one crest to the next, or from a minimum of a trough to the next; refer to the illustration on the bottom of Figure 5. The wavelength and frequency are inversely related, the higher the frequency, the shorter the wavelength. The distance from the equilibrium position to the maximum of a crest is called the amplitude of the wave. Another important property of the transverse, or sine, wave is the number of crests (or troughs) that pass a given point per second; this number is referred to as the frequency of the wave and is usually designated by ƒ. Frequency properties are important when discussing sound.

Sound

Sound is a wave created by a vibrating object that propagates through a medium from one location to another. The medium that transmits it is commonly air. A sound wave moves through air as a result of the motion of the air molecules. When one talks, vocal cords exert a force on the air molecules next to them. As a result, molecules are displaced from their equilibrium, or normal position. The air molecules, in turn, exert a force on the air molecules next to them. A push, or pull force, displaces neighboring air molecules. The disruption causes an acceleration or air

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molecules in an outward direction from the source. The push and pull motions continue all the way to the receiver [16].

Since the medium for sound waves in this example is air, the formula for the velocity of sound is measured to be 340m/s.

(1)

The significance of the wavelength formula is that it allows to solve for a missing value of frequency or wavelength since v is already known. The formula also shows the inverse relationship between wavelength and frequency and how it relates to sound waves.

EFFECTS OF SOUND

In the case of Statuary Hall, members of Congress who were holding session there in the 1800s were dissatisfied with acoustics of the structure. One explanation relates its structural design and the methods which this might influence the interactions with sound waves. When a sound wave is produced from a source, just as the conversations between members of Congress, it is scattered. The scattering effect distributes the sound wave almost like a ripple effect when dropping a stone in a body of water. The wave spreads out in all directions until it hits an obstacle.

Reflection of Sound

The ripple effect demonstrates the direction and motion of waves and can also be used to visualize the effects of sound reflection. If there is no obstacle in the path of the wave then the wave will continue to travel away from the source. When an object oscillates in water it produces constant waves that travel in ripples towards the edge of the water, as illustrated in Figure 5. When a sound wave encounters an obstacle, such as a wall, floor, or a ceiling, part of the wave is deflected from its original course. When sound waves encounter such an obstacle, it could be reflected or absorbed. If the source of the sound were in midair without any solid surfaces nearby, then it would be difficult to communicate at distances greater than a few meters [17]. For Example, members of Congress might hold their sessions outside instead of the Hall. It would be necessary to talk much louder outside in order to be heard by others, and it would make personal conversations not as audible to others. Usually members of Congress hold sessions inside where their feet are on the ground, there is a ceiling and there are walls all around. The sound reflected from hard surfaces then reinforces the sound, making it easier for Congressmen to hear at much greater distances.

If the source of sound is completely surrounded by hard walls as in a room, then the reflections may become

extremely troublesome, as the return of a sound wave from a surface is disruptive to meetings. Such is the case of Statuary Hall, where each syllable could be heard several times and in extreme cases the meaning of phrases was completely lost. In effect, the problem of architectural acoustics made it intolerable to carry Congress’ daily business. One of the keys to good architectural acoustics is the balance between getting enough reflections from the walls to keep the sound level up, while at the same time not producing blurring of the sounds because of excessive reverberation.

FIGURE 5 [18]THE RIPPLE EFFECT DEMONSTRATED IN WATER

Reverberation and Obstacles

The presence of walls creates an enclosed space around the sound wave source and changes the character of a sound field. The enclosed volume of air becomes a system that is excited with moving air molecules while the sound source is emitting. Enclosures, particularly those that are used as large halls, have linear dimensions that are significantly larger than the wavelength of sound waves, even for low frequencies. If the obstacle is very large compared with the wavelength, very seldom for sound, half of this scattered wave spreads out, and the other half is concentrated behind the obstacle. If the obstacle is very small compared with the wavelength, often for sound waves, then the entire scattered wave is sent out uniformly in all directions [19]. In these types of scattered wave the vibration dies away if it does not hit an obstacle.

With speech, characteristic vibrations arise in the air of the room at frequencies corresponding to the component frequencies of the source. When a wave strikes a body in its path, a scattered wave spreads out from the obstacle in all directions [20]. Reverberation is the persistence of sound after the source has stopped emitting, and is caused by multiple reflections of the sound within a closed space. It consists of multiple, blended sounds caused by reflections

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from walls, ceilings and other structures which do not absorb sound

As waves hit a hard, smooth surface, the waves are reflected. Obstacles affect the path of sound waves after its reflection. The wave that encounters the boundary, or incident wave, is reflected. The incident and reflected wave create a scissor like effect when they intersect, as illustrated in Figure 5. The intersection of sound waves along the medium constitutes a projection of the incident and reflected waves.

FIGURE 5 [21]SOUND BEING REFLECTED ON A BARRIER

The ear has the ability to receive sequences of sound impulses and sum their energy, provided that the interval between the impulses does not exceed minimum delay time. A person can often perceive a time delay between the production of a sound and the arrival of a reflection of that sound off a distant barrier. If the intervals are greater than the minimum delay, then there is a gap between the signals and the ear identifies the reflection as a distinct echo, which is particularly noticeable when the level of the reflection is comparable with the level of the basic signal [22]. The degree of sound amplification depends on the absorption of energy at the boundaries of the enclosure.

The level of sound amplification within Statuary Hall creates speech that sounds livelier and in other areas where the same conditions apply, music can become more full. A noticeably long reverberation renders speech less intelligible. The preferred reverberation time range for a space intended for speech is 1.0 second, or less. As the reverberation time becomes longer than that, it becomes increasingly difficult to understand speech. In this case, the sound of quality becomes lessened. When the reverb time is long enough, it not only masks the next syllable, but it can mask the next word. The loudness of sound, in certain conditions, is increased by virtue of the normal modes of vibration of the air contained in the enclosure. Contrary to experience in unconfined space, the increase in loudness is particularly noticeable by listeners fairly remote from the sound source. One method of reducing this loudness is to increase absorption in the Hall.

Absorption

Sound waves have unique effects on the surface of materials when a wave impinges on it. Different materials react differently to sound, depending on their structure. This allows a reduction in the amount of sound energy reflected. The introduction of an absorbent material into the surfaces of a room will reduce the sound pressure level, or physical intensity of the sound, in that room by not reflecting all of the sound energy striking the room's surfaces. The effect of absorption reduces the final sound level in the room. For example, nonporous materials yield slightly to the vibrations of sound waves, since they are never perfectly rigid. Porous materials also allow some air to penetrate below the surface, producing an additional effective motion of the surface. The reaction of different materials to sound is dependent on that material’s absorption coefficient. It is a measure of the sound-absorbing ability of a surface and defined as the fraction of incident sound energy absorbed. The values of the sound-absorption coefficient usually range from 0.01 for marble slate to almost 1.0 for long absorbing wedges often used in anechoic rooms. The reaction is expressed as a ratio between pressure at the surface and the normal velocity of the surface [23].

The absorption coefficient can be mathematically presented as follows:

(2)

Where: α is the sound absorption coefficient IR is the intensity of the reflected sound II is the intensity of the source sound

Absorption coefficient ( α ) for some common materials can be found in the table below:

TABLE I [24]THE SOUND ABSORPTION COEFFICIENT OF MATERIALS

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All materials in an enclosure will absorb and reflect sound. A fraction of the incident energy is absorbed and the balance is reflected. A perfectly hard surface will reflect back all of the energy. In the case of Statuary Hall, the hard materials it was made out of added to the reverberations and contributed to the line of echoes heard.

Echo, Echo

An echo is due to the reflection of sound waves. A reflected wave is perpendicular to the incident waves as a result of the law of reflection for sound waves, and illustrated in Figure 6. The law states that the incident angle will always be equal to the reflected angle when a wave is reflected from a barrier. A spherical surface, for example, does not reflect sound to a point, even though all waves head towards the same region [25]. Usually an echo is a strong reflected sound that is sufficiently delayed from the direct sound, the original sound wave. If a sound wave returns within 1-10 seconds, the human ear is incapable of distinguishing it from the original one. In cases which sound requires 1-20 seconds to reach the reflecting surface and the same time to return, it can be heard as a separate entity rather than as a continuation of the original sound [26]. The properties of reflection help to understand how echoes work.

Reflected sounds help create a sense of fullness in a room; however, it is not always desirable. Fortunately, high-frequency waves are easy to dampen because of their high level of activity. High-frequency sound waves tend to bounce around, similar to light waves, on reflective surfaces. Such surfaces, for instance mirrors or windows, add to the echoes. One solution is to cover them with blinds or curtains to dampen the effect. Dampening the effect was one of the methods employed by the staff at Statuary Hall. They hung draperies and added more materials that would assist in reducing the effects of reverberation and added more

absorbing material. Ultimately these efforts proved unsuccessful and a new House of Representatives was built. These acoustical effects were not only present in Statuary Hall, but also studied elsewhere.

Whispering-Gallery Effect

Nobel Prize winner and physicist, Lord Rayleigh explained the efficient propagation of sound waves around the inside dome of St. Paul’s Cathedral in London in the late 19 th

century, offering an explanation for the whispering-gallery effect. Between the speaker and listener multiple paths exist, each having a unique number of reflections off the surface of the dome. When the travel time of the sound is similar, constructive interference and amplification takes place [27]. Constructive interference describes a phenomenon in which the interference of two or more waves of equal frequency and phase produce a single amplitude. The amplitude will be equal to the sum of the amplitudes of the individual waves. An audible sound will travel around the inside of the dome with exceptionally low attenuation, or weaker in intensity form the original sound. The sound waves traveled around the gallery in a narrow belt guided by the circular walls. Therefore less reduction of sound intensity by various means such as air, humidity, and porous materials, is experienced. Whispering-gallery modes, numerous patterns of wave motion or vibration, depend only very slightly on its ability transmit sound. The explanation of the whispering-gallery effect is rooted in the principle of the succession of reflections, which is largely influenced by the Hall’s architecture.

SOUND FIELDS AND ARCHITECTURE

Sound reflected from walls generates a reverberant field that is time dependent. Reverberant field refers to the region in a room where the reflected sound dominates, as opposed to the region close to the noise source where the direct sound dominates. When the source suddenly ceases, a sound field persists for a certain time interval. The residual acoustic energy constitutes the reverberant field, or the sound that reaches a listener in a typical auditorium. The region in a room where the reflected sound dominates describes the effect of a reverberant field [28]. Two broad categories classify reverberant field: the direct (free filed) sound and indirect (reverberant) sound. The amount of acoustic energy reaching the listener’s ear by any single reflected path will be less than that of the direct sound. The differences in reflection are illustrated in Figure 6. The result is greater divergence, and all reflected sound undergoes an energy decreases due to absorption of even the most ideal reflectors.

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FIGURE 6 [29]REFLECTION OF SOUND IN AN AMPITHETER

Individual vibrations, which make up the reverberation, may decay at different rates. The dissimilar decay rates result from the fact that the sound absorptive properties of various areas of the surface forming the boundaries of the enclosure may be very different from one another [30]. As a result, the sound field becomes uneven. The response characteristic of an air space, like that of any other vibratory system, depends on the dimensions of Statuary Hall.

SUMMARY

The whispering-gallery effect at Statuary Hall is one that many physicists explore in depth. The idea that a whispering-gallery effect is produced at the great Hall is supported by physics concepts such as reflection, reverberation, echoes, and absorption. Ultimately, Statuary Hall is an example of poor acoustical design. The structure attributed to the intensified sound level and echoes made everyday business hard to conduct. Currently Congress members enjoy a new Hall with the proper acoustics. Statuary Hall remains as one of the most historic sites in the Nation’s Capital.

REFERENCES

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http://www.visitthecapitol.gov/Exhibitions/civilwar. [6] 23, Feb. 2012. http://uschscapitolhistory.uschs.org/tour/07.htm.[7] 25, Feb. 2012 http://www.capitalbay.com/links/LinksReadPrint. [8] 25, Feb. 2012 http://britton.disted.camosun.bc.ca/jbconics.htm.[9] 25, Feb. 2012

http://artandhistory.house.gov/art_artifacts/virtual_tours.[10] 23, Feb. 2012 http://www.thedcpost.com/?p=1601.[11] Hewitt, Paul G. Conceptual Physics. 10th Edition, New York;

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[15] Porges, G. Applied Acoustics. Edward Arnold Limited. 1977.[16] Ref. 4.[17] Stobart, H., & Kruth, P. Sound. United Kingdom at the

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Physics for the Acoustical Society of America. 1981.[20] Mankovsky, S.V., Acoustics of Studios and Auditoria. Focal

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http://learningsparkle.blogspot.com/2009/04/waves.html.[22] Ref. 10[23] Ref. 9 [24] 22, Feb. 2012 http://www.engineeringtoolbox.com/accoustic-

sound-absorption-d_68.html.[25] Rossing, M. The Science of Sound. 3rd edition.[26] The Columbia Electronic Encyclopedia. 2007, Columbia

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[30] Ref. 10.

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