The Organ

AdamSmith · February 28, 2018

AdamSmith · February 28, 2018

AdamSmith · February 28, 2018

AdamSmith · February 28, 2018

AdamSmith · February 28, 2018

AdamSmith · February 28, 2018

Switching gears... (? )

https://en.wikipedia.org/wiki/Duke_Chapel

Physics 136 / Music 126 Duke University Fall 2012 Handout 19

Acoustics and Duke Chapel

The original drawings for Duke Chapel were delivered to the University in 1930 by the Philadelphia firm of Horace Trumbauer, the same architects who designed Baldwin and Page auditoriums. The principal designer was Julian Abele, hired by Trumbauer at age 17, when he already was a graduate both of Philadelphia's Institute for Colored Youth and the University of Pennsylvania. Trumbauer supported Abele's studies at the Ecole des Beaux-Arts in Paris and appointed him chief designer of the firm soon after his return. There is no record of Abele ever having visited Durham. Much of the firm's business involved the design of mansions for wealthy clients and, while Abele remained on the payroll, the firm closed its offices for eight years after the stock market crash of 1929. Architecturally, Duke Chapel represents faithful adherence to gothic design principles. The basic fabric of the building is, just as it appears, weight-bearing stone.

That acoustical considerations also figured in the initial design of Duke Chapel is evident in such details as the large cavities originally provided in the wooden canopy over the pulpit (seen in the lower right corner of the photograph below of the Æolian organ) and in the stone wall between the choir and the Memorial Chapel -- both designed to house loudspeakers.

The principal axis of the cruciform building -- through nave and choir -- is 300 feet long, the width including side aisles is about 65 feet. The inner vaulting is 40 feet wide and reaches 75 feet above the floor of the nave. The original design included seating for approximately 2200 in the nave and transepts and 150 in the choir. It is instructive to approach the room in several different ways acoustically:

(1) Consider the Chapel to be a gigantic Helmholtz resonator. If, with its doors open, the narthex entrance were thought of as defining an oscillating mass of air, the cross section of that mass might be roughly ten square meters and its length ten meters. The main volume of air enclosed by the Chapel, about 3 x 10⁴ cubic meters, then would serve as a spring providing a restoring force to the mass of air in the narthex. This would lead to a resonant frequency of about 0.1 Hz. The resonance might be observed by a dedicated investigator prepared to spend some considerable time with a window fan at the open doors of the Chapel, reversing the fan's direction every five seconds or so. [Helmholtz resonances at frequencies far below the audible range can have practical consequences. Steady winds blowing past the mouths of caves can drive such resonances. Visitors lost in such caves sometimes try to find their way out by following a breeze, only to discover eventually that the breeze periodically reverses direction.]

(2) It is also possible to take a Modal Analysis view of the Chapel as an enormous pipe, closed at both ends. The lowest frequency mode then would correspond to a standing wave along the principal nave-choir axis: Thus the room will have a set of normal modes spaced no more than 2 Hz apart (harmonics of this 2 Hz "fundamental"). Since the lowest audible frequencies are at least three octaves above this "fundamental" there are plenty of room modes available to provide a smooth response, even for low pitched musical tones. The lowest mode along the transept axis (L = 112 feet) has a frequency of about 5 Hz.

(3) Ray Analysis. If one gazes along virtually any path that a ray of sound might take within the Chapel, he or she will observe surfaces that scatter sound diffusively over a wide range of frequencies. Gothic architecture typically subdivides massive stone elements into multiple convex surfaces, down to the scale of the span of a person's hand. Thus a massive pillar, for instance, will diffusely scatter sounds over the entire range of audible wavelengths in air -- from less than an inch to tens of feet. Thus reflected sound within the Chapel will not convey an image of its source, and the reverberant sound amplitude at any given frequency will have an equilibrium value that is the same everywhere in the room. This, in turn, means that the Chapel is ideally suited for . . .

(4) Sabine Analysis. The predominance of diffuse scattering at all frequencies in the Chapel ensures the validity of this approach, which assumes a steady-state or equilibrium condition in which the reverberant sound at each frequency is uniformly distributed throughout the room. In its present state, the Chapel's volume is approximately 1.1 x 10⁶ cubic feet and its mid-frequency reverberation time is said to approach seven seconds. That would indicate the mid-frequency total absorption as little as square feet.

The Chapel's reverberation time was not always so long. Originally it was a mere 2.5 to 3 seconds. What appears to be stone between the ribs of the vaulting and on the flat surfaces of the upper walls is in fact a material called Akoustolith, manufactured by the R. Guastavino Co. in New Jersey and designed to be acoustically absorbent. In one-inch thicknesses, this porous cast material originally had octave band absorption coefficients (band center frequencies from 125 Hz to 4 kHz) of .09, .17, .46, .77, .77, and .56, while five-inch layers had coefficients of .43, .92, .91, .88, .86, and .74. It was used extensively in the Chapel to maximize speech intelligibility for sermons. (It was also popular at the time for limiting the noise levels in monumental banks, libraries, and government buildings.) Akoustolith is only about one third as dense as the stone it is designed to resemble. The resulting saving in weight in the vaulting of Duke Chapel is one reason the building's buttresses can be so small as to arouse suspicion in some visitors that they must contain structural steel. [Another reason the buttresses can be so slender is that local building codes required that the exterior roof be supported by steel trusses rather than any traditional gothic arrangement. The only other place structural steel has been used in the Chapel is within the wood columns supporting the rear gallery and its heavy organ case.]

In the late 1960s, when D. A. Flentrop was approached about building a large pipe organ for the Chapel, he insisted that the room's acoustics be made consistent with its appearance. The consulting firm of Bolt, Beranek, and Newman was hired to identify a substance that could be applied to the Akoustolith to seal its pores (thus reducing its sound absorption) without making its appearance objectionable (shiny, for instance). A beeswax-like material was applied in two stages to much of the Akoustolith in the room, with dramatic acoustical results. [Evidence of trial applications of such materials can still be seen just inside the doorway at the top of the stairs to the crypt.]

We turn now to a consideration of some of the consequences of having such a long reverberation time:

(1) Sound absorption by the air itself becomes a significant factor. With a very long reverberation time, sound travels so far back and forth through the air in the room that its absorption by the air, normally insignificant in comparison with absorption by room surfaces, becomes considerable. In metric units, where at a frequency of 1 kHz the air absorption coefficient square meters per 100 cubic meters of air, assuming a 50 percent relative humidity. Thus the additional absorption by the air itself in Duke Chapel is about 100 square meters, the equivalent of the the sound that would escape through a ten by ten meter hole in a wall!

(2) Modest continuing inputs of sound power can result in surprisingly high levels of noise. A quiet conversation in a far corner of the Chapel will produce a constant level of indistinct noise throughout the room. The threshold for detecting the softest musical sounds can be raised significantly in such a circumstance, and the overall dynamic range available for music accordingly reduced. [The threshold for pain -- defining the upper limit of the available dynamic range -- will remain unchanged.] [Such an effect in the Malabar Caves was featured in E. M. Forster's novel A Passage to India. Forster's account emphasized that, whatever the nature of the sounds created by humans in the cave -- angry shouts, resigned sighs, desperate screams -- the reverberant response was the same, an undefinable sustained sound he likened to the meditation syllable "Om". For Forster this served, in part, as a metaphor for the doomed efforts of the colonial British to effect rapid changes in Indian society.]

(3) A long reverberation time not only imposes a slow decay of sound rather than a sudden silence at the conclusion of a musical note, it also provides a built-in crescendo of reverberant sound for any note that is sustained for several seconds. The temporal structure of music performed in the Chapel -- rhythm, phrasing, etc. -- must be conveyed by direct sound alone -- sound traveling directly from source to listener plus reflected sound arriving at the listener's ear within the association time (35 to 50 milliseconds) of that earliest arrival. At times this direct sound may have to compete with reverberant sound for the listener's perception and/or attention. In typical music performance situations, the distinction between direct and reverberant sound is a subtle one, with the reverberant sound sharing and conveying much of the temporal structure of the music. A room like the Chapel offers an excellent opportunity to observe and understand the distinction, because in this case the two play very different roles and the distinction between them is not at all subtle. In such a room the ratio discussed in the next section varies from seat to seat and has real practical implications for listeners and performers.

(4) Consider the ratio of direct to reverberant sound intensity. As noted above, the near-ideal conditions in the Chapel for the diffuse scattering of sound means that the reverberant sound intensity will be the same anywhere in the room. Specifically, at a given frequency, where is the total sound absorption, and is the intensity of reverberant sound for a continuous input of sound power W.

The direct sound intensity on the other hand, will depend on the listener's distance r from the sound source:. [It is assumed that there is no barrier between source and listener. The equality holds when the source radiates sound equally in all directions.] We calculated above that square feet for the Chapel. Then:, where r is in feet. Thus in the "worst case" (i.e. if there are no nearby reflecting surfaces concentrating the direct sound toward the audience) the direct and reverberant sound intensities will be equal at a distance feet from the source. Put another way, in that case the reverberant intensity anywhere in the room will equal the direct intensity at 50 feet. Now consider a situation in which there is a flat reflecting surface right behind the sound source, so that all the direct sound is concentrated into only half of all the available directions. will be doubled, and will be multiplied by a factor of, becoming about 70 feet. Exactly what ratio of direct to reverberant sound intensity is optimal will depend on personal taste and the nature of the music being performed. Notice that placing a pipe organ at the rear of the nave lets it speak directly along the main axis of the room. Placing it on a gallery well above the floor of the nave both (a) allows direct rays of sound to reach the maximum number of seats without grazing over the heads of intervening listeners, and (b) avoids having the rearmost seats too close to the sound source. Placing the pipes in a massive wooden case concentrates the direct sound toward the listeners. Each of these factors helps achieve desirable ratios of direct to reverberant sound over a significant fraction of the listeners' seats. Notice that, as the size of the audience increases, so too do and the distance r at which any given optimum ratio is achieved, so the best seats at an organ recital will be further from the organ the larger the audience.

(5) Long reverberation times present a special set of problems. All of that Akoustolith was installed originally to ensure acceptable levels of speech intelligibility in the Chapel. In addition, an unobtrusive location was provided for a reinforcing loudspeaker in the canopy above the pulpit from which sermons were to be delivered. When the reverberation time was increased, speech intelligibility was reduced significantly and more sophisticated electronic measures were needed. Providing all reinforcing sound from a single location above the head of the person speaking has several advantages, including the reinforcement's coming from an appropriate direction and arriving only slightly later than the sound coming directly from the speaker's mouth. With a long reverberation time, however, such a single powerful loudspeaker will fuel the reverberant intensity as well as the direct. To avoid increasing the reverberant intensity (the source of the problem to begin with) it is better in such cases to resort to a large number of loudspeakers located quite close to the listeners (to minimize the additional sound power added to the room), and designed to be highly directional (projecting sound only toward the listeners and not toward any reflective surfaces that would feed the reverberation). A nearly vertical array of small loudspeakers utilizes linear superposition to concentrate sound radiation in a plane perpendicular to the axis of the array. The sounds from all the loudspeakers in the array combine constructively at points on that plane, while the interference becomes increasingly destructive as distance from the plane is increased. An additional advantage of such arrays is that they are relatively unobtrusive visually when mounted on gothic columns. In order to make the reinforcing sound acoustically as well as visually unobtrusive, the signal to each array can be delayed just enough to make its sound reach its listeners slightly after the true direct sound reaches them. Such a "phased delay" arrangement was installed in the Chapel when the reverberation was increased, initially using a magnetic tape loop with a single recording head and multiple playback heads to introduce appropriate delays. Subsequent upgrades of the Chapel sound system have achieved the delays using solid state electronics. A switch allows the phased delay feature to be defeated when, for instance, the person speaking is doing so from the rear of the Chapel.

Another class of problems associated with long reverberation times involves parts of the choral repertoire. Imagine a choral work with intricate rhythms, a rapid tempo, and lyrics that must be conveyed distinctly to an audience. The Chapel's acoustics pose special difficulties for the performance of such works. On occasion, such works have benefitted from positioning the singers on a platform set up on the main axis of the Chapel, with stage shell sections (borrowed from Page Auditorium) providing a reflecting surface behind the chorus. The reasoning behind such an arrangement is analogous to that discussed above in connection with locating an organ on a gallery at the rear of the nave. An advantage of a choir's singing from the choir stalls, on the other hand, is that the reflecting walls behind them on both sides help the singers hear each other well. Such choir stalls in great European gothic churches typically are made of thick, solid, acoustically reflective wood. The paneling on the walls behind the Duke Chapel stalls, however, is relatively thin and flexible and thus absorbs considerable sound at low frequencies.

In selecting instrumental as well as choral works for performance in the Chapel it is important to keep in mind such consequences of a long reverberation time as the impossibility of sudden dramatic silences (there are limits, even, on sudden dramatic reductions in dynamic) and the danger of inner voices being lost in a general reverberant blur. [In a smaller room, not only is the overall amount of reverberant sound less, but there also are brief transient disturbances at the beginning of each note, as the reverberant sound energy is distributed among the more widely spaced available normal modes of the room. Such disturbances can make inner voices easier to recognize.] Some ranks of organ pipes are designed to include brief attack noises ("chiff") that make the movement of inner voices much easier to hear in highly reverberant conditions. Sometimes an adjustment in tempo can help.

Finally, musicians performing in the Chapel have to deal with the fact that people sitting in various locations will experience quite different balances between direct and reverberant sound.

(6) Long reverberation times also can present some special opportunities. The Chapel is a truly great place to perform a substantial part of the organ repertoire and some types of vocal music (liturgical plain chant, for instance). It is a rare resource, a room made truly optimal for the performance of certain types of music, rather than being compromized for all uses in an effort to achieve a "multipurpose" room.

http://webhome.phy.duke.edu/~dtl/136126/36hj_chl.html

AdamSmith · February 28, 2018

Beyond that, alert, creative, and insightful performers have found ways to utilize the Chapel's acoustical properties in sometimes surprising but artful ways. Orchestral climaxes have benefitted from the additional crescendo supplied by the buildup of reverberant sound during a sustained chord. Sometimes a choice of just the right tempo has allowed some of the characteristic times of the room to support a passage rather than confound it (the half-second return time for sound dispatched the length of the nave and choir, the similar time related to the length of the transepts, the reverberation time, etc.). A memorable Chapel recital for solo violin featured works chosen to exploit the possibility of creating sustained reverberant chords from arpeggios and then using them as self-accompaniment. Occasionally what a composer intended to be a routine abrupt change from one long chord to another is enhanced by the added dimension of its reverberant sound instead undergoing a gradual transition over several seconds.

In section (4) above we discussed how a listener might exercise some control over the ratio of direct to reverberant intensity in choosing a seat for, say, an organ recital. Another type of control of that ratio is routinely exercised by organists in adjusting the lengths of each note within repeated patterns. Consider a chord involving three notes. If the chord is sustained long enough, the reverberant sound intensity will grow to some maximum value and then remain constant so long as the chord is held. If, instead, the three notes are used alternately in a repeated sequence (1-2-3-1-2-3, etc.) played in a legato style, the maximum reverberant intensity will be only one third as great, since each note now is being sounded only one third of the time. If the repeated figure changes to, say, 3-2-3-1-3-2-3-1, etc., the maximum reverberant intensity of note 3 will be twice that of note 1 or note 2, since it is "on" twice as much of the time. Such a "duty cycle" analysis of some of the Preludes and Fugues of J. S. Bach, for instance, reveals stately sequences of different pitches succeeding each other in predominating the reverberant sound in a room like Duke Chapel, an effect that also would have occurred in many of the churches for which Bach composed but cannot occur in many of the rooms in which his music is played today. In addition to the composer's control over the direct-reverberant sound ratio, the performer can substantially reduce the reverberant contributions from notes in repeated patterns by electing to separate the notes in a marcato or staccato style, varying the duration of each note without changing the tempo.

As a final aspect of our acoustical examination of Duke Chapel, we should comment at least briefly on several musical instruments that are permanent parts of the building.

(1) A fifty bell carillon cast by the Taylor company in England. The bells are fixed, their clappers moved by cables from a mechanical clavier in a tower room just below the bells. The University Carillonneur is Special Collections Librarian J. Samuel Hammond. As you probably have noticed, he plays the carillon each weekday afternoon at five o'clock and after Chapel services on Sundays. Students who would like to learn more about the carillon probably can make an appointment to meet Mr. Hammond at the tower elevator door before one of his performances and go up to the clavier with him.

(2) A 120 rank pipe organ with electropneumatic action, largely contained within stone pipe chambers on either side of the choir. One of the last large instruments built by the Æolian company of Chicago in 1932 (the company's Opus 1785), this organ was installed when the Chapel was first built and originally included an antiphonal division at the rear of the nave (removed for installation of the Flentrop there). When the antiphonal was used alone there was approximately a quarter-second delay between the organist's pressing a key and hearing the result. Fully renovated and sightly expanded in 2009-2010, the instrument has four manuals and more than 6700 speaking pipes. With few exceptions, the façade pipes visible on the choir and transept walls do not speak, but visually screen relatively small rectangular openings between the pipe chambers and the choir and transepts. Pipes served by two of the windchests are enclosed in boxes, with venetian shutters that can be opened and closed by pedals at the console to achieve gradual changes in dynamics, balance, and/or timbre. This organ is tuned to an equal tempered scale. Among its lowest pitched ranks is a pedal 32' Bombarde. A variety of manual pistons and toe stud controls allow the organist to make rapid predetermined changes in the combination of stops that are drawn, and a variety of couplers are provided within and among the manuals and pedal clavier, including couplers with one or two octave pitch offsets in both directions. This instrument is used extensively during religious services, for solo works as well as to accompany choir and congregational singing. It is employed frequently in large orchestral and choral works and in organ recitals.

(3) A 66 rank mechanical action pipe organ completed in 1976 by the organ shop of Dirk A. Flentrop in Zaandam, Holland. Most of its 5000 or so pipes are housed in a mahogany case only about 4 1/2 feet deep, elevated on a gallery at the rear of the nave. Most are consistent with those found in north European organs of the early 18th centruy. The lowest pitched ranks are 16'. There also are three ranks of horizontal reeds like those found in great Spanish organs of the 18th century. The lowest of the four manuals controls the Rugwerk behind the organist's bench, using long stickers and horizontal trackers. The remaining three manuals are connected by vertical trackers and rollers to pallet valves in the Bovenwerk, Hoofdwerk, and Echo windchests in classicWerkprinzip fashion. No pipes are in shuttered boxes. Windchests for the Pedaal pipes are at the base of towers on either side of the main case. The stop action is a traditional mechanical slider arrangement. Some early 18th century organs had limited reservoirs of bellows-regulated wind. This would cause the wind pressure to drop momentarily when sudden large demands were made (i.e., when a full chord was sounded with many stops drawn). Since such an effect would have been anticipated by some composers, this organ allows performers the option of reducing the size of its wind reservoir in order to obtain that effect.

In earlier centuries pipe organs were the technological marvels of their day. Some incorporated spectacular features, such as kettledrum playing cherubs, moving statuary, and various other "bells and whistles" powered by the organ's wind supply. A modest blower and a light for the music rack are the only electrical devices used by this organ. The tremulant motors that can produce periodic fluctuations in wind pressure, for instance, are wind powered, as are this organ's one "bell" and one "whistle" --- a tinkling Cimbelster and a birdsong-like Rossignol using a combination of pipes inverted in a container of water. The couplers are all mechanical and there are no preset devices for moving multiple stops at once.

Flentrop specified this organ's tuning as "Lambert Chaumont 1695," named for R. P. Lambert Chaumont, the curate of the parish of St. Germain in the small Belgian town of Huy, whose only surviving musical work is a book of pieces for organ published in 1695. A handy set of instructions for tuning a harpsichord was printed at the end of that book, which included works written in all eight church modes. Careful examination of those instructions reveals that they involve tuning a sequence of perfect fifths that, if each fifth were exact, would result in a Pythagorean scale. The text, however, admonishes the reader to narrow each fifth by "tant soit peu" ("ever so little") so that the thirds in major triads specified for tests throughout the sequence are "good". This would appear to be an exact, reproducible prescription for a tuning only if the thirds were made exact, which would make "Lambert Chaumont 1695" precisely the same as the quarter comma meantone tuning published in 1523 by Pietro Aron.

This organ is used in a liturgical context to perform solo works and to accompany processions and other hymns. A select team of local artists play daily demonstration recitals on it. It has attracted an impressive list of recitalists from throughout the world.

(4) A 21-rank two-manual mechanical action instrument built by the firm of John Brombaugh in Eugene, Oregon, to support performance of works from the 16th and 17th centuries. Installed during the summer of 1997 in a "swallow's nest" gallery in the Memorial Chapel, the new organ operates with a low wind pressure and has strict quarter comma meantone tuning. Two extra pipes per octave have been provided in each rank to support playing in a wider range of modes and keys. Levers allow substitution of the sets of alternate pipes, rather than having extra or "split" keys on the keyboards. There are no separate ranks of pipes for the pedals, only couplers between the pedal clavier and the lower manual. The sound of this organ is based on a chorus of several sets of pipes modeled after those of 16th century Tuscan instruments. While the whole organ uses slider chests, these stops are activated by levers fashioned to resemble controls for the spring chests found in most Italian instruments of the period, and a pedal is provided to engage them together as a group. Added to the lower manual's windchest are several additional ranks that extend the instrument's flexibility beyond a purely Italian sound, and a shorter upper manual provides four additional ranks faithful to north German designs of the same period. The stop action for these added ranks uses the knobs found in most slider chest organs.

For photographs and more information on the Æolian, Flentrop, and Brombaugh organs, see the Chapel's excellent web site.

(5) A tiny Flentrop positive organ that is easily portable, along with a second even smaller case containing its electric blower. This instrument is used primarily to provide continuo for small ensemble performances.

The Chapel organs are maintained by Curator of Organs and Harpsichords John Santoianni. Among those performing regularly on these instruments are Associate University Organist and Chapel Organist David Arcus, and University Organist Robert Parkins.