HISTORY OF ACOUSTICS

Introduction

It is unfortunate that in the world of architecture, sound and the architectural implications regarding sound have taken a back seat to form, order, and space.  Acoustics is seldom emphasized in architectural schools and the field of acoustics has become the responsibility of the acoustician, not the architect.  Why is this so?  One cannot deny that sound is an important aspect of any space, room, or building, however, there is little to be found regarding the architectural implications linked with the study of sound.  Hopefully, by investigating how sound works, the developments in the history of sound, and historical examples of the different types of architecture created for sound propagation (theater, opera house, concert hall), one will be able to understand the importance of sound in architecture.

Before delving into a certain topic, it is always wise to embark on a definition of the word.  However, upon inspection, the words ‘acoustics’ and ‘sound’ are not to be found in the Penguin Dictionary of Architecture and Landscape Architecture.  As it can be seen, although acoustics is undeniably an aspect of building design, one has to look to a more general source.  According to the Merriam-Webster Dictionary, ‘acoustics’ is defined as follows:

Acoustics n sing or pl 1: the science dealing with sound  2: the qualities in a room that make it easy or hard for a person in it to hear distinctly[i]

As it can be seen, although the word ‘room’ is mentioned, this definition does little to illustrate the complexity and pool of knowledge associated with acoustics and the application of acoustics in design.  As a student of architecture, one finds that the emphasis on sound is greatly shadowed by the realms of structure, form and concept.  Even its close partner, light, has been more understood.  To begin this discussion on sound and its role in architecture, let the question of how sound works be answered.

How sound works

Like light, sound propagation depends on a source and a receiver.  However, since sound bends around corners and the ear is only barely directional, the mystery of sound lies in the complexity of ear.  At the center of the ear contains a converter that transforms vibrations conducted from the ear into digital nerve pulses, which are sent to the brain.  Because the ear is so complicated, it is impossible to make any concrete judgments regarding sound from the point of view of the receiver.  A lot of what we know is regarding the source of the object.  Sound is generated in most cases by a vibrating object.  A sound wave consists of a pressure fluctuation that is alternately positive and negative relative to atmospheric pressure.  The passage of sound wave causes air particles to move backwards and forwards parallel to the direction or motion of the wave.  At 20 degrees Celsius, the speed of sound is 345 m/s or 1125 ft/s.  A sound wave consists of three properties: amplitude, frequency and wavelength.

The amplitude is defined by the magnitude of the pressure fluctuation.  The ear can detect a ratio of a one million to one difference therefore the ear’s response to sound is not linear.  This is the basis of the Decibel sound measurement system, which is logarithmic and tries to match the characteristics of the ear.  The smallest perceptible level of change by the ear is 1dB (although in noise situations, it is only considered worthwhile if a 5dB reduction is achieved).  As a standard reference, normal conversational speech is about 50 dB, where extreme climaxes in concert halls reach a Decibel level of 100 dB. 

Frequency can be defined as the number of cycles per second (measured in hertz) of a vibrating surface.  In the case of musical instruments, the lowest frequency amongst all of its frequencies determines the pitch.  The ear can perceive frequencies between 20 Hz and 20,000 Hz (upper limit decreases with age).

The wavelength of a sound wave is the distance between adjacent pressure maxima/minima.  This factor is quite useful in calculating the frequency of a sound since the following equation is true:

Speed of sound = frequency * wavelength

Applying more to architecture, another importance aspect of sound is resonance.  A room can be treated as complex a resonant system.  If sound is produced in a room, certain frequencies are enhanced.  This is dependant on the dimensions of the room.  In auditoria, however, the number of resonant frequencies in the room is so high, resonance usually not observed.  The term ‘resonant’ is also used for rooms with high reverberation (where sound decays slowly) but one must not confuse the two meanings.  Resonance itself does not intrude in room acoustics, except for certain frequency-specific absorbers.

Sound propagation is also important.  Essentially, when sound travels long distances outside, it is influenced by wind and temperature but even at scale of Greek classical theatres (distance 50m) there is no obvious instance of effects influencing acoustics, however, in an enclosed space, direct sound decreases same way as outside but indirect becomes a factor.  In auditoria, the major concern is the reflection of sound.  “In a large concert hall a listener receives about 8000 reflections of a short sound within one second of the direct sound.”[ii]  The reflection of light and sound are almost identical with the exception that sound requires a greater surface to reflect than light.  However, it is not as simple as mere geometry, reflection from a finite surface depends on relationship between the size of the reflector and the wavelength.  Once the frequency of a sound wave is lowered, it is reflects less and less along geometrical a path.  Although this is so, designers have some control though sound reflection.  An acoustic mirror consists of a large, planar (convex panels tend to dispense sound and concave reflectors focus sound which creates problems and are avoided), massive surface, which is usually concrete or plastered masonry and is used to direct sound.  Sound reflected by one surface will continue to be reflected until its energy is removed by absorption.  There are three aspects of sound reflection that differ from the regular reflection pattern: diffraction, diffusion, and absorption.  Diffraction is when in low frequencies, the wavelength of sound is large compared with size of obstacle and bending of the sound wave occurs.  Diffusion occurs when sound hits an irregularly profiled surface with projections of a depth between 0.3m and 0.6m.  This aspect is important to auditorium design.  Lastly, absorption is when “sound energy is dissipated in a porous material owing to the friction involved in movement of air particles in the pores”[iii] for example, when sound hits fabrics, curtains, carpets, fiberglass, and acoustic open cell foam.  For auditoria, the major absorbing surfaces are the clothed audience and performers.  As for additional absorption interventions, there are three types used: porous absorption, panel absorption, and the Hemholtz resonator (little used since complicated).

The sound itself can also be categorized.  Direct sound is the first thing that audience members hear and it comes as a straight line from the source.  The early reflections come from the sidewalls and ceiling.  Since they travel farther, they arrive later and are not as loud as direct sound.  It must be noted that an echo is only used to describe a reflection heard as a discrete event.  Reverberant sound, on the other hand, includes echoes and is essentially a late sound occurring after 100ms.  Since sound decays in a linear fashion, the duration of sound is described by reverberation time.

However, despite this understanding and categorizing, one fact remains, many spaces have acoustical defects that prevent the smooth operation of an event that requires specific sound qualities.  These defects usually fall into three categories: echoes (when a reflection is heard as a discrete event), flutter echoes (occurring when there are short path lengths, iterated many times as in 2 close parallel walls) and background noise, often from the exterior, intervening spaces or ventilation.  It is up to the architect to deal with such matters or anticipate such defects in a design.

Alas, this introduction to the workings of sound has only scratched the surface of what is known about sound.  It is important to realize that sound has not always been understood as fully as it can be seen by looking at the history of the study of sound.

The Study of Sound

            A question that often arises in the minds of architects designing spaces for sound propagation is: “What form should one use to enhance to the utmost the brilliance, harmony and depth of sound?”[iv]  Unfortunately, there is little out there to help a designer answer such a question.  According to Barron, “as an applied science, room acoustics is less than one century old.”[v]

General study of sound goes back to ancient Greece.  It is known that the ancient Greeks studied stretched strings and that harmony depended on a simple arithmetic ratio.  As well, Aristotle (325 BC) looked studied the production and reception of sound and applied these empirical findings to design outdoor theatres.  The design elements that shaped sound quality were concluded to be good line of sight to stage, the use of a reflecting wall, and low ambient noise.  Unfortunately, compared to later scholars who studied sound, little literary evidence remains of the contributions of the ancient Greeks.  It would not be until Roman Empire that one would find the earliest written record on sound and architecture.  Marcus Vitruvius Pollio (1^st c. BC), as illustrated in his work, the “De architectura,” based his geometric designs for theatres on an understanding of acoustics.  However, this is not say that he was completely on the ball regarding sound propagation.  For example, he believed that sound curved upwards rather than just horizontally.  Knowing this, he justified raked seating as not on visual but to optimize the ‘ascending voice.’  Despite this, the fan shaped plan and arena plan became highly developed in classical times.  It seems that the classical age presented an appealing approach through its use of certain ratios of proportion, arguing that since ears appreciate harmony and harmony is based on simple proportions buildings should likewise be built in favorable proportions.  However, this approach is false since the dimensions of a space also relevant.

Despite some experiments by scientists such as Boyle and Newton during the 17^th century, it would not be until the 18^th century that any serious study of sound presented itself.  At this time, the auditoria as a building type was established in Dumont’s “Parallele de plans des plus belles salles de spectacles”.  Written in 1774, it compares plans of several theatres and opera houses.  Unfortunately Dumont’s own designs with their vast concave domed ceilings can now be see to be an acoustician’s nightmare.  The ‘Age of Reason’ also fathered the first attempts at relating auditorium form to acoustical behavior with Patte’s 1782 proposal of elliptical plans being the best plans followed by Saunders’ 1790 proposal of circular plans.  It can be seen that despite these developments, sound remained highly misunderstood, since “both plan forms would now be considered dangerous due to focusing by concave surfaces”.[vi]

In the next century, the 19^th century, “the problems [associated with the design of large halls] attracted growing attention, and it is obvious that they grew with increasing size of auditoria”.[vii]  In 1834, R. B. Reid said ‘”any difficulty in the communication of sound in large rooms arises generally from the interruption of sound produced by a prolonged reverberation”. [viii]  Around the same time, Langhans (in Germany) published a book where he shows that he correctly appreciated how early reflections enhance intelligibility which is very close to understanding the independent role of reverberation.  Later in the century, Scott Russell (1838) calculated optimum floor profile for good vision and hearing (it must be noted, that Russell equates sound with vision, in his conclusion).  Yet, despite such advances, Charles Garnier, the architect of the Paris Opera complains in 1880:

It is not my fault that acoustics and I can never come to an understanding… after 15 years labor, I found myself hardly in advance of where I stood on the first day… I had read diligently – nowhere did I find a positive rule of action to guide me; on the contrary nothing but contradictory statements.[ix]

It seems that “concerning useful advice for the architects, the situation remained woeful regarding suggestions for suitable room form and size for good acoustics.”[x]  Barron presents 2 reasons for lack of progress.  Firstly, the fundamental relationship of sound is statistic rather than simply linear.  The linear approach only accounts for a single wave and this must be repeated thousands of times for complete picture.  Secondly, measurement of sound is difficult.  The adaptability of the ear keeps sound from being purely analytical.  The advent of the microphone and electricity were a breakthrough to the study of sound. Eventually, research in how sound behavior in architectural spaces was prompted by a growing need for public assembly spaces and development of orchestral music.  In 1812, architect Charles Bullfinch hired to mitigate poor acoustics in Hall of Representatives (U.S.).  He tried using stretched fabric to absorb sound, which did not solve problem.  Later, Robert Mills raised the floor by 4 feet, successfully shifting the focal point of the dome below the listening plane.  Joseph Henry, during this time, identified the role of room volume, shape, and surface absorption in the behavior of sound waves.  However, the real breakthrough in acoustics as directly applied to architecture was the work of W. C. Sabine.  As an assistant professor at Harvard University (Physics Department) he previously worked with optics and electricity.  His investigation into sound was precipitated by his being asked by the President of the University to find a solution to appalling acoustics in lecture room of Fogg Art Museum.  His studies showed that it was due to extensive reverberation.  Stemming from his research, Sabine developed a technique for measuring decay time of residual sound after organ pipe went off by using only ear and stopwatch and discovered that reverberation time was proportionate to the reciprocal of the amount of absorption.  This led to the “Sabine reverberation equation.”  “Sabine found the quantitative approach, not only to informal room acoustics but also to noise transition through buildings.”[xi]  His findings explained transient behavior of acoustic energy within enclosure and approximate exponential decay of sound field and created a tradition that concentrated practical room acoustics on the calculation of reverberation time and the measurements of this parameter.  With this increase in the possibility of predicting reverberation time (RT) on architectural drawings of a hall and an increased knowledge of the absorbing properties of materials architects could have more control over the sound of a design and shy away from relying only on trial and error.  With speech halls it was relatively easy to estimate definite RTs, but for concert halls it was much more subjective and no consensus could be reached.

On the heels of Sabine came the 20^th century.  The 1930s produced two influential books on sound: “Planning for Good Acoustics” by Bagenal and Wood (1931) and “Architectural Acoustics” by Knudson (1932).  However, despite the developments of the 19^th and 20^th centuries, the design of acoustical spaces remains “hit and miss.”  Reverberation time proved important but by itself, it did not guarantee good acoustics – auditorium form and size recognized as also significant.  Search for other significant measurable quantity began in 1950s.  There existed many unanswered questions concerning ear’s processing of sound.  Many subjective experiments with subjects listening to controlled acoustical conditions were initiated.  It was at this time that auditorium acoustics entered experimental psychology.  Much like Sabine changed the 19^th century study of sound, Beranek did so for the 20^th century.  His book “Music, Acoustics, and Architecture” (1962) was the first book that concentrated predominately on acoustics as opposed to study of existing auditoria.  It was also the first book to attempt to answer misconceptions.  As Barron put it,

Acoustics of concert halls, theatres and opera houses is frequently referred to as an inexact science, or worse, an area of myths… the success of old halls is often considered to be due to secret ingredients.  The suggestion, for instance, that concert halls mature with age is still prevalent, as if they behaved like good claret.[xii]

In today’s information age, there is an increase in the use of acoustical scale computer models to help develop ray trace diagrams and monitor RTs and wave patterns.  There are 3 possible directions for the study of sound – investigation of properties (of instruments and voices), response to the wave nature of sound (understand how it travels and reflects), and the use of the ear as a guiding principle – and every day there are more and more developments.  Architects and acousticians are realizing that each concern by itself is inadequate to obtain a holistic understanding of sound.  The study of sound now concentrates on both measurements of the physical acoustical characteristics of a space as well as on listening tests at public performances (subjective characteristics).  Lastly, “most accessible resource is existing auditoria.  In an area as complex as three-dimensional acoustics design, science is forced to rely on precedents”[xiii]

Auditoria – a Theatre, an Opera House, a Concert Hall

As an architectural form, auditoria have existed for centuries.  One of the most important sources for information on sound through the ages has been past auditoria.  The term ‘auditoria’ generally describes spaces for sound propagation.  It is rational to choose this particular type of architectural space for discussion in a paper on acoustics due to the importance of sound in such spaces.  There are four primary types of auditoria: the theatre, the opera house, the concert hall, and the multi-purpose hall – listed chronologically by their development.  An example of each of the first three types will be discussed: the Greek theatre at Epidauros, the Auditorium in Chicago, and the Berlin Philharmonic. 

Theatres – Classical – Greek Theatre at Epidauros

The Greek theatre originated from the desire to have a holy site for choral dances performed at the Dionysian festivals.  This eventually evolved to become drama and at this time, the theatre was developed to house these events.  Perhaps the most well preserved specimen of Greek auditoria is the theatre at Epidauros (Figure 1).  Pausanias considered it, in the 2^nd century AD, as the most perfect of Greek temples.  Located in the Peleponnese and dating from 350 BC, it was designed by the ancient Greek architect, Polyclitus.  Embedded in a hollow of 55 meters, the furthest seat lies 70 meters from the front of the stage.  Since this is too far for an audience member to observe the facial expression of the actors, the performers wore masks, which doubled as voice amplification devices.  The appeal of the theater lies in its symmetry and is structured as a fan spanning a 210-degree angle as seen in Figure 2.  In 1973, Shankland measured word articulation in several classical theatres, including Epidauros.  According to his findings, with the speaker at the center of the orchestra and the listener at the back of the theatre on the axis of symmetry, there was 72% word articulation, which is quite spectacular considering the size of the theatre.

Opera House – The Auditorium, Chicago

Of the many opera houses to choose from, the Auditorium Theatre in Chicago was perhaps one of the more interesting examples (Figure 3).  The Theatre itself was one component of the Auditorium building, designed by architects Danklar Adler and Louis Sullivan.  During the late 1800s, the culture seemed to be waning in Chicago and businessman Ferdinand Peck decided to cure this by erecting in the city a ‘shrine of culture.’  He envisioned a building, which would multi-purpose and financially self-sustainable.  The Auditorium Building became a theatre within a skyscraper that also housed a hotel, bar, offices, and a weather station/observatory on the top.  At 95 feet, the Building became the tallest in Chicago when constructed and the Auditorium Theatre the largest permanent theatre at that time.  Considered to be America’s first modern opera house, the Theatre was designed as a square with staggered seats (to ensure sight lines were clear).   With seating for over 4200 people, it contained 3 tiers, a main balcony and 2 galleries, all of which were lavishly decorated (Figure 4).  While Sullivan was primarily in charge of the decoration, the acoustical qualities of the space fell to Adler.  “Adler… had already won a brilliant reputation as a technical innovator and (on strength of his Central Music Hall) as an expert in acoustics.[xiv]”  Adler based the steep rake of the audience seating on the principle of the ‘isocoustic curve.’  The ceiling was determined by acoustical considerations as well.  As well, the arches in front of the stage were not structural but hid ventilation and air ducts, diminishing background noise.  Adler’s calculations involved absorption, reflection, and reverberation.  It appears that despite the infancy of acoustical theory, the Auditorium Theatre was miraculously successful thanks to Alder’s knowledge of sound propagation.  All together, theatre and building, the whole endeavor cost $3, 200,000, a huge sum by 19^th century standards.

Concert Hall – Berlin Philharmonic

As compared to the other types of auditoria, the concept of a concert hall is much younger than that of a theatre or an opera house.  The concert hall developed from smaller recital halls, which were conventionally rectangular spaces.  Scharoun’s Berlin Philharmonic (1956-1963), however, is probably the most interesting specimen within this genre (Figure 5).  The design presents a very wide stage with relatively shallow audience.  Sharoun wished to break down the barrier between actors and audience and even out the quality of seating.  According to Jones, “in his concert halls these two considerations become even more pronounced and one could go so far as to call the Philharmonic the first socialist concert hall.” [xv]  The shape of the hall itself is quite unorthodox (Figure 6).  At first glance it appears sculptural but the design is purely functional.  As a matter of fact, it seems to work better than most rectangular concert halls.  In conventional halls, the audience was placed as a single mass facing the orchestra, passively outnumbering it.  Space was linear and the back seats were very far away from the stage.  The Philharmonic placed the audience around the orchestra, taking advantage of the fact that sound radiates outwards in all directions.  This allowed a compact layout and a more enclosed unified space.  Despite accommodation for 2200 people, the Hall is only 60 meters in length, with the furthest most seat only 32 meters away from the stage.  The seating is also divided into ‘terraces,’ which break the audience into units of comparable size to the orchestra.  This gives rise to a convenient circulation strategy since each ‘terrace’ has its own separate exit to the foyer.  As well, in using a bowl-shaped section, the audience is spread vertically, taking advantage of omidirectional sound radiation and gives a clear view of the orchestra.  The Philharmonic also avoids echoes by avoiding parallel walls and any horizontal or vertical surfaces.  The ceiling is designed to avoid echoes by using a convex surface to diffuse sound instead of a flat one.  The ‘terraces’ themselves are sloped as well.  As a result of such design interventions, “orchestral performances in the Philharmonic sound rich, warm, and natural without loss of detail; the music seems to fill the hall and arrive from all directions.[xvi]”

Conclusions

As it can be seen, scientists and architects have both come a long way in the world of acoustics.  In today’s design world, there is increasing interest in experimenting with shape and materials, however, despite this emphasis on a scientific approach, the fact remains that there is a subjective aspect to sound.  For today’s architect, perhaps the most useful tool in designing acoustical spaces is the study of precedents such as the Greek theatre, the Auditorium Theatre and the Berlin Philharmonic.  Each existing theatre, opera house, concert hall or multi-purpose hall has something to offer either in its success as a sound structure or its failure.  In the end, one fact remains true: sound will always be part of architecture, whether one is designing a living room, a ballroom, or a concert hall, and it is the responsibility of the architect to be knowledgeable of this subject.