30 June 2013

Is Oriel Chambers the first curtain wall ever?

Oriel Chambers, 14 Water Street Liverpool. Peter Ellis 1864
Back to oldies. In October 2012 I was invited to an architectural seminar in Liverpool, organized by Tile of Spain, to discuss about ceramics in facades. Another speaker was Maurits Van der Staay, an associate in Renzo Piano Building Workshop, who presented their terracotta façade on Central St Giles London. The morning after the venue Maurits and I re-visited two architectural jewels in Liverpool: Oriel Chambers in 14th Water Street (1864) and a similar office building in 16th Cook Street (1866). This post will remind us about these two buildings and their now forgotten architect; and in particular how they deserve credit as some of the first examples of a lightweight glazed façade – a real curtain wall – installed in a multistory building. Please take note of the dates again, 1864 and 1866: that means only thirteen years after the Crystal Palace and thirty years before the design of pioneering Chicago School facades as the Reliance or the Fair Store. Real oldies, right?

16 Cook Street Liverpool. Peter Ellis 1866.
(Image from Maurits van der Staay 2012)
The Wikipedia entry for curtain wall has two images of Oriel Chambers and 16 Cook Street, and it refers to them as two of the world's first buildings to include a curtain wall façade. By this they mean a fairly glazed multi-storey building set in an urban context; not a warehouse, a dockyard or the gable of a large train station. Let’s reckon from the start the impossibility of determining what the real ‘first’ curtain wall was, since the development of any building system is an evolutionary process moving through several directions simultaneously. But it’s time to give Peter Ellis, the architect of Oriel Chambers and 16 Cook Street, the merit he deserves as one of the pioneers in curtain walling, regardless the fact that his influence was small, if any.

The available literature on these two buildings is scarce. It is clear that their author was Peter Ellis (1804-1884), a local architect and surveyor about whose life and work there are strong shadows. These are the only two buildings clearly attributed to him; we don’t know if Ellis signed any other project in Liverpool or anywhere else. There are no records of his previous and later activities: he may have been a surveyor, a civil engineer, or a developer before designing Oriel Chambers and 16 Cook Street; and apparently he went back to surveying or to other businesses after finishing the second building in 1866. The reason seems to be found in the fierce critics with which the two buildings were received at the time, but we will come back later to that.

Oriel Chambers among its neighbours facing Water Street
Oriels and stone: it's not all glass!
Both buildings have been designed with the same purpose: office space for rent in the shape of chambers. A chamber is what the Americans would call suite, that is, a small office valid for any purpose; whether legal, financial or commercial in general. Each chamber was supposed to be rented for a private business with a small number of users, between two and ten generally. There is little space for sumptuosity, and above all no plan space to be lost in halls or extra money to be devoted to architectural features on the façades.

The Oriel Chambers building
It seems that Peter Ellis won the commission for the first building (Oriel Chambers) as a result of a competition between local architects organized by the developer (an unknown T.A., as the golden initials on top of the main façade declare). The reasons for selecting Ellis’ scheme – from a developer’s perspective - are clear by having a look at the plans and sections. The entrance to the building is located out of the main façade axis because it fronts the corridor, which is located at the axis of the inner part of the building, much longer than the Water Street façade. In other words, internal space efficiency is given much more importance than architectural expression.

Oriel Chambers: Ground floor plan and elevation to Covent Garden St
Oriel Chambers: Elevation to Water St, section and elevation to the inner courtyard
The building from the outside seems to have three main floors but in fact there are five, all with a large amount of natural light and free available space. The central corridor separates the plan in two lettable areas per floor: one side opening to Covent Garden Street and the other side opening to a narrow internal courtyard. Covent Garden Street was not too wide even by 19th century Liverpool standards, so not much light would be expected from the street. But the bit facing the courtyard would be much darker than the other, resulting in a potential loss in let revenues. So a scheme providing lots of light to both sides of the building (external and courtyard’s façade) must have sounded appealing to the owner. And the was right in his selection: the building is still a lettable space today and it houses the same kind of small firms (barristers among others) with apparent success almost 150 years after it opened.

Oriel Chambers: proposed occupation in a typical office floor.
The courtyard is the narrow strip located above to the centre-right.

Prior Bolton's oriel window at St Bartholomew
the Great church, London c. 1500.
It's interesting to note that the church
was being restored during the 1860s...
Oriel Chambers was named this way because of the oriel (projecting bay) windows that cover in vertical stripes the two facades. The short façade, seven stripes of windows with the non-symmetrical main entrance, opens to Water Street with a southeast orientation. Water Street was one of the important arteries of the city at the mid-nineteenth century, connecting the City Hall with the Mersey river docks. The lateral northeast façade to Covent Garden Street was by far the largest, extending originally along 20 window strips, divided in five sectors of four vertical bands each. The back half of the building was damaged by German bombs drop over Liverpool during the Second World War, so that now only the first 12 strips are original, and the rest of the volume has been rebuilt in a different style by James & Bywaters in 1959. In fact, the wartime damage enabled the original construction to be fully appreciated regarding its architectural and engineering significance.

Oriels facing Water St.
The original structure is a combination of cast iron H-shaped columns forming a grid frame with cast iron inverted T-section beams (girders), spanning along the short direction of the building. The shape of the girders comes from other sources; I have verified the columns section on site and they remain as originally. Lateral stiffness is provided by transversal brick walls with chimneys that interrupt the long volume every four stripes. The span between frames is small and it coincides with the windows’ module. This means that the external and internal facades (opening to a large but narrow courtyard) become free of any structural stiffening requirement.

Oriels facing Covent Garden St. The slim stone column is cladding a cast iron H-shape stanchion.

And here is where Peter Ellis started making his magic. The external façades facing Water St and Covent Garden St are covered with oriels: bow windows with an overhang bottom support. Oriels facing Covent Garden St are wider than those facing Water St (see image below); which seems a good idea since the former receive less natural light. The column line between oriels is externally clad with a thin section of stone pieces, reminding slender Gothic columns. These lines don’t end up in arches though, but instead finish rather abruptly – one would say in an abstract way – when they reach the top of the building line. For us the visual result isn’t striking: we are used to building facades resembling rectangular grids. But for Ellis’ contemporaries this façade must have been a clear break in relation to what was considered ‘proper urban architecture’.

Oriel width comparison: left to Water St.; right to Covent Garden St.
The lightness achieved through the oriels is really high by today's standards, and it must have seemed extraordinary to Ellis' contemporaries. The first two images below are inner views of office space overlooking Covent Garden St, and the last one is an image of a narrower window located at the ground floor, with views to Water St.

View of the oriels from inside. Façade overlooking Covent Garden St.
Same as above
View from an oriel towards Water St.
Air ventilation is obtained via one side-hung window at every oriel. The height of the window is 1/2 the height of the oriel, so they can be read as vertical sliding openings, which they are not. The sash opens to the inside and the sash retainer is visible from the outside. See details here below.

Side hung window, opening to the inside. Notice the bottom hinge.
Sash retainer located outside the window. The sash can be locked in three positions.

The (hidden) curtail wall in Oriel Chambers
Now, glazed as it looks, how could this be the first curtain wall in a multi-storey building in history? This is not the case if you focus on the external facades only: the amount of glass is noticeable but what we see is a continuum of large projecting glass elements in a rectangular grid of stone blocks cladding an iron frame.

There is at least a previous building in Glasgow that could claim precedent, the Gardner’s Warehouse in 36 Jamaica Street (see below), built in 1856 by John Baird using a structural system patented by R. McConnell, iron founder. The Gardner’s façade reminds the Crystal Palace, built in London in 1851, in a more direct way than Peter Ellis’ one. The building in Glasgow was a warehouse after all, ours is an office building located in prime location in Liverpool.

Gardner's Warehouse. 36 Jamaica St, Glasgow. John Baird 1856.
The real secret of Oriel Chambers is hidden behind; at it becomes visible only when you are given access to the inner courtyard. My colleague Maurits and I were lucky to arrive to the building on a workday morning at 9am, when clerks were entering their offices. Looking like an architect has advantages if you want to access a private property, and we were soon taken to the courtyard through the solicitor’s firm occupying the main floor.

There came the surprise: a receding, all-glass façade with a shape of protruding elements between columns seemed to absorb any little ray of light arriving to the courtyard. Again, an architectural solution that seems contemporary to us but absolutely new at the time of its design. One may say that the oriel glass boxes at the front are the ‘culturized’ version of this completely ‘form follows function’ glazed solution at the back.

Left: view of Oriel Chambers inner courtyard. Right: vertical section of the curtain wall opening to the courtyard. Notice the cast iron vertical stanchion and how the curtain wall moves out of it every floor down.
Look at the vertical section above right and you will get it: each floor recedes a bit over the one below to allow for more light coming to the bottom. The sloped rooftop piece above the windows at each level is made of wired glass to obtain direct solar radiation. A counter-sloped panel acts as bottom parapet, and it looks like a thin piece of timber with an external bituminous layer. The iron H-shaped columns are not at the receding façade line but in a vertical axis independent from it. The glazed wall acts as a thin, lightweight layer gently cladding a structure, not taking any load but its own, with a shape that bows to light and brings it in without losing a bit. That's a mature curtain wall in concept.

Since there are very few images of this extraordinary piece or architecture in the Web, I am adding here below a selection of the pictures I took during our early morning visit.

General view of the courtyard. All structural members of the curtain wall are in timber.
The top and vertical members are in glass; the bottom one is a timber panel.
Detail of the curtain wall, floors one to three (4th floor is flat vertical)
Contrast between the curtain wall and the receding structural wall to the left.
Corner of the curtain wall at the edge of the courtyard. The narrow strip is located opposite to Water St.
Corner of the curtain wall looking up. Note the receding structural wall in the centre.
Connection between the curtain wall and the receding brick wall. The building to the left is the bombarded wing that was re-built after the war.
Detail of the above. The sloped glass on top is very visible.
View of the curtain wall to the courtyard from inside. See the sloped glass on top.

This is clearly a proto-20th century office building curtain wall, thirty years older than those of the Chicago School but going far beyond them and connecting directly with Gropius’ Fagus Factory in Alfeld – which was to be built fifty years later! Now is when you grasp the importance of this hidden place.

16 Cook Street – more news to come
After the successful visit to Oriels Chambers Maurits and I walked to Cook Street, located less than ten minutes away, also at the city centre. 16 Cook Street is another rental office building, smaller than the previous one, that Peter Ellis finished in 1866. I have not found any information about the owner. Was it the same developer from Water Street or a different one? Was this building the result of a competition or a direct commission? Judging by its smaller dimensions and the use of very similar architectural features I tend to think that this was a direct commission, for a client who knew well what he wanted.

The building plan is an L-shape (as Oriel Chambers) but much smaller in size and with only a main façade. The rear and lateral walls open to a courtyard that was as narrow as the former.

16 Cook Street, top of front view
Contrast with contemporary neightbour facade
Again, you can find some references in today's architectural literature to the main façade but very few to the rear one. The façade to Cook St is perfectly symmetrical. The play between glass and stone appears again, but here there are no oriels: glass remains flat between slender stone-clad columns. The whole can be read as an abstract gothic- or Venetian-like window: a central, three-strips bay ends in an arch at the top and is flanked by two smaller vertical bays, also ending in smaller arches. The building, as that in Water St, has five floors, but here all floors are fairly the same height and express themselves similarly to the façade. Verticality together with light-catching seems to be the theme for Peter Ellis here.

The entrance to the building is located at the left corner, with the shop entrance conveniently symmetrical at the right end. The entrance hall is a slender corridor connecting the street with a spiral staircase that opens to the back courtyard, clad almost entirely in glass with the thinnest of cast iron mullions.

16 Cook St: back elevation (left) and spiral stairs from the courtyard (right)
Spiral stair from inside
The stair and its cladding are supported from both sides at every floor, leaving the impression that the whole is floating without any column. Clever but not so difficult considering its tiny dimensions. This leaves the rest of the plan available for one or two offices per floor, with plenty of light entering through the street and/or the rear courtyard windows.

The façade as seen from ABW Architects office
Maurits and I were lucky once again. The second floor is at present occupied by ABW Architects, a firm formed in 2008 by two partners, Simon Almond and Andrew Brown, working across the northwest UK. We were given access to their studio and could have a chat and take pictures. The atmosphere inside was great – lots of light but definitely a small space. Old Peter Ellis was clever enough to conceal the limited available space playing with a continuous volume that seems much larger from the street.

Drawing from inside the office (from ABW Architects webpage)

And again a curtain wall surprise was waiting for us there. It was not visible in the front façade, glazed as it is. Only when you access the office floors you perceive the small courtyard and the way the building opens to it at the back in search of light.

Back curtain wall as seen from the stairs
Here the façade to the courtyard is not receding back as it moves up, probably because of the lack of space. But we find again the lightweight, protruding curtain wall in three planes: vertical and sloped with glass, counter sloped with a timber panel. And this time the curtain wall ends in a transparent corner at the very back of the building.

We saw two H-shaped columns completely independent from the wall. One of them shows how the curtain wall is attached to the cast iron structure using an iron strip in tension (see bottom image left).

Inner cast iron column at the back office and curtain wall connection
Corner glass around cast iron column
The other column at the back corner is even more striking, because the glass wall completely clads the column from the outside without touching the structure. We have seen this detail many times in modern curtain wall architecture, but in 1866?

The contact of the curtain wall horizontal stripes with the vertical glazed cylindrical staircase takes place in a clean way. It could be a Dutch architectural detail from the 1930s. An amazing solution but concealed from everyone’s view – as much today as when it was built.

Details of the curtain wall around the corner cast iron column

Reaction to the two Ellis’ buildings
Seen from today’s perspective Oriel Chambers and 16 Cook Street may be seen as a precedent and even a paradigm of the Modern Movement - but it was not one immediately appreciated to say the least. Oriel Chambers was seen, for the local Porcupine, as ‘hard, liney, and meagre’. The strongest critic by far came from the London architectural periodical The Building News in February 7, 1868, signed by a ‘our own correspondent’. The critic pompously dismissed Oriel Chambers out of hand:

This is a kind of greenhouse architecture run mad; consisting of a series of vertical bays running completely from top to bottom of the building (…) rising from the plinth without any basis, said shafts being flanked by a very large coarse “nail head” ornament. (…) The style, in short, might be described as “lunar Gothic;” and no one who has not seen it would believe, we think, that such a thing could, in the present day, be erected in cold blood by any person calling himself a member of the architectural profession.

In a similar vein, The Builder stated:

The plainest brick warehouse in town is infinitely superior as a building to that meager agglomeration of protruding plate-glass bubbles in Water Street termed Oriel Chambers. Did we not see this vast abortion (which would be depressing were it not ludicrous) with our own eyes; we should have doubted the possibility of its existence. Where and in what are their beauties supposed to lie?

As late as 1921, Charles Reilly, head of the Liverpool School of Architecture, called it the ‘oddest building in Liverpool, at once so logical and so disagreeable … as a cellular habitation for the human insect is a distinct asset to the town’.

Oriel Chambers' glass windows protected during the Second World War
It is clear now that those radical cast iron frames that Glasgow, Manchester, and Liverpool produced among others in the 1860s and 70s, led after the 1870s to a slow falling-away from industrial innovation and to a shift back to London-supported historicist decorations. Probably this was the origin not only to the decline of the North but also to British near-absence from the Modern Movement up to very late in the twentieth century.

The positive reaction produced by the two Ellis' buildings, although little, was not completely inexistent in 20th century British architecture. As Brian Hutton wrote in Architectural Review in 2008:

Perhaps Oriel displeased locals because it abstracted from a Gothic model in a city that remained mostly Classical. (…) And indeed, to eyes now less Modern than Post-Modern, what may strike from the Oriel is less a paradigm of rationality than something both more abstract and more wilful. So that when, in the 1960s, James Stirling drew from Oriel in his Leicester Engineering Laboratory, his model was neither its chamfered details nor even its functionalism, but the geometric glass cascade of its atrium walls. 

James Stirling, although born in Glasgow, grew up in Liverpool and studied architecture there.

Adam Caruso (from Caruso St John) is a contemporary architect with a strong personal link to the two Ellis buildings. He wrote in 2010

I’m not so interested in the Ellis buildings being examples of a proto-modernism, a part of that inexorable linear progression from the Crystal Palace to European inter-war modernism. I think that’s a convenient post-rationalisation perpetuated by modernist historians. I am more interested in the Ellis buildings in the context of the cast-iron offices and warehouses that were being built in the mid-19th century in Liverpool and Glasgow, like the Gardner’s Warehouse in Glasgow by John Baird in 1856. These buildings had cast-iron structures and facades and had all but eliminated most of the elements of what would have previously constituted a “correct” urban facade. I am particularly interested in why Peter Ellis chose to clad his cast-iron structures in stone,organised according to a Gothic language (…). He was developing an expression for his building that was in addition to, and was autonomous of, their technology.

Peter Ellis, John Root and curtain walls
Can we spot an influence of Peter Ellis’ two proto-modern curtain walls in any later period of architecture? As Adam Caruso mentioned above, it is almost impossible to trace any linear progression from the Crystal Palace to the glazed boxes of the 1930s passing through Peter Ellis. A potential link might be the application of glass and iron frame technology to the front of urban buildings, almost for the first time in history. But many years had to pass before large glass plates and iron / steel frames could come back to the front.

An interesting side influence, though not too obvious, has been established between our two buildings and the architectural training of John Wellborn Root (1850 – 1891), who would become the partner in the Chicagoan firm Burnham & Root, one of the founders of the Chicago School around the 1880s and 90s. Root was born in Georgia and raised in Atlanta with his parents. In 1864, when Atlanta fell to the Union during the American Civil War, Root’s father managed to send him with two brothers on a steamer to Liverpool, where John’s father had shipping business contacts. While in Liverpool, Root studied at a school in Claremont for three years (from 14 to 17 years old), and he even passed the exams for entering Oxford. But in 1867 he returned to the US to study architecture at New York University.

It can only be a speculation, but a teen-aged John Root might have seen and remembered the brand new Oriel Chambers in Liverpool, together with the spiral staircase of 16 Cook St, the latter finished just months before he sailed back to America. Now fast forward to the 10 floor-high Rookery building in Chicago, built in 1888, one of Root’s masterpieces. Can you see a vague influence from Oriel at the top corner stone pinnacles?

The Rookery Building Chicago, 1888. Burnham and Root architects.
There are no oriel windows here, except maybe the gentle curvature of the central bay of windows up to the sixth floor...

The Rookery building Chicago, 1888. Spiral staircase at the inner atrium
Perhaps the clearest reminiscence, once again, does not take place at the front but in the courtyard above the glazed atrium. Here yes, the spiral staircase in the centre is in a similar vein to Ellis’ model in Cook Street, and the window-to-wall ratio of the inner façades reminds that of the two courtyards back in Liverpool.

Engraved plate at the door of Oriel Chambers

What happened with Peter Ellis after his two office buildings were finished (and received with derision in Liverpool and London)? We have no idea: he seems to have come back to surveying or to civil engineering, but there are no traces of his activities at all. There are no buildings signed by Ellis after 1866, so it seems quite obvious that the sharp criticism ended with his short architectural career. The last news is that of his death in 1884, at the long age of 80 years. His obituary appeared in the Liverpool Daily Post in October 21.

It is an irony to see that Peter Ellis is remembered (in a stone engraved plate by the door to Oriel Chambers, see above) as a 'pioneer in the use of prefabricated structural units in cast iron'. This, being true, is unfair to his evident contribution as a forefather of curtain walling, clearly his largest achievement and the one by which he is and will be remembered. Sometimes two buildings are enough to have your name written in history.

30 June 2012

Acoustic properties of glass: not so simple

At Arup, working with different specialists creates many opportunities to learn from each other. Sometimes one forgets that engineers who know everything about dark matters as climatic loads in glass or intricacies of structural silicone may not have a clue about the acoustic performance of a window.

That's why these questions keep coming to my desk (remember I'm an incurable generalist in the façades world): what effect does glass thickness have in the acoustics of a double glass unit? Or what matters more in the acoustical performance of insulated glass: the thickness in a monolithic pane, the effect of lamination or the dimension of the cavity? Here you will find some graphical answers to these questions. As usual a number of hidden surprises will come out from the data mining.

Let us start by reviewing two concepts that are paramount to measuring glass performance against noise: loudness (in particular sound pressure level, the decibels thing) and frequency (the Hertz, not related to car rental)

1/ Loudness: sound intensity, sound pressure and sound pressure level

From physics to applied acoustics in buildings. No pain, promised. Loudness is an intuitive concept: a loud noise usually has a larger pressure variation and a weak one has a smaller pressure variation. Depending on what we are looking for - the cause, the effect or the perception of noise - we use different variables and units:

  • Sound intensity refers to the cause of noise (not of our concern, only of interest for acusticians). It measures energy flow at the source, so its unit is W/m2.
  • Sound pressure refers to the effect of noise as a wave impacting any given surface, that is, noise as energy being transfered through air. Not of our concern either, more for physicists. Its unit is the Pascal or N/m2 (1Pa = 1N/m2). 
  • Sound pressure level or SPL (here comes the fun) refers to the perception of noise in humans as it can be "read" by our ears. So SPL is what matters to us, poor construction buddies. For ease of numbering SPL is measured in decibels (dB). A dB is a dimensionless unit used to express logarithmically the ratio of a value (the measured sound pressure) to a reference value (the lower threshold of hearing). Decibels are used since sound pressure level expressed in Pa would be too wide. 0 dB (the lower threshold of audition for humans) equals 0,00002 Pa; whilst 140 dB (the upper human threshold or threshold of pain) equals 200 Pa. This is a range of 140 against 10 million. But logarithms are not "natural" to understand, so some examples will be of help.
Sound intensity, sound pressure and sound pressure level are obviously related, but they measure different things and they should not be confused. The table below, taken from the very useful Sengpiel audio webpage provides some tips for getting it right, at least conceptually:

SPL variation (left column) related to sound pressure (field quantity) and sound intensity (energy quantity)

Lessons from the table above:
  • A raise in sound pressure level (SPL) of 3 dB equals an increase in sound pressure (field quantity) of 1.414 times, and (everything else being equal) it comes as a result of doubling the sound intensity (the source of sound).
  • A reduction in sound pressure level measured inside a room of 10 dB equals a reduction in sound pressure of 3.16 times, and it comes as a result of dividing the sound intensity (noise generated on the outside) by ten.
A typical opaque façade (not glass) can have a sound reduction index (a reduction of SPL) of around 40 dB. This means that if the SPL measured at the street is 70 dB, inside the façade one would perceive only 30 dB. Up to here, just arithmetic. 

Now, if the sound reduction index of the façade could be raised from 40 to 43 dB, the perceived noise coming from the street would equal that of reducing the source of noise by half. Even more, if the façade could be acoustically improved so that its sound reduction index raised from 40 to 50 dB (difficult but it can be done), the perceived noise coming from the street would equal that of reducing the source of noise (sound intensity) by ten: ten times less cars in the street, ten times less people celebrating the victory of their football team outside.

Expected sound pressure levels for different noises and their equivalent sound pressure and sound intensity. Source: Sengpiel Audio.

We got the point: sound pressure level measured in dB (sometimes indicated as dB-SPL) is critical for architectural physics - a small variation can make a lot of difference. But loudness (sound expressed as pressure variation) is not the only story. Noise - what we want to avoid inside our buildings - is the mixture of sounds of different "quality", some are bass, some are treble. Is our façade or our glass pane capable of stopping each of these "noise qualities" in the same percentage? Could an envelope act as a barrier for bass and a filter for treble? What do bass and treble have to do with noise?

2/ Frequency of sound

Sound is the quickly varying pressure wave travelling through a medium. When sound travels through air, the atmospheric pressure varies periodically (it kind of vibrates). The number of pressure variations per second is called the frequency of sound, and it is measured in Hertz (Hz) which is defined as the number of cycles per second.

Graphic representations of a sound wave. (A) Air at equilibrium, in the absence of a sound wave; (B) compressions and rarefactions that constitute a sound wave; (C) transverse representation of the wave, showing amplitude (A) and wavelength (λ). Source: Encyclopaedia Britannica.

The higher the frequency, the more high-pitched a sound is perceived. Sounds produced by drums have much lower frequencies than those produced by a whistle.

The unit of frequency is the Hertz (Hz). For a sound vibration to be audible to human beings the object must vibrate between 20 and 20,000 times per second. In other words the audible sound has a frequency of between 20 and 20,000 Hz.

High-pitched sounds (treble) have a frequency much greater than bass sounds. The treble frequency ranges between 2,000 and 4,000 Hz while the bass range from 125 to 250 Hz. 

Above: measure of loudness (wave height). The higher the louder.
Below: measure of frequency (wave length). Bass sound has long waves, treble has short waves.

Bad news: frequency and loudness are interrelated in the human ear. The range of 20 Hz to 20,000 Hz is called the audible frequency range - we know this already. But the sounds we hear are a mixture of various frequencies, and we don't perceive all of them with the same clarity. Let's see what the implication of this is.

The entire audible frequency range can be divided into 8 or 24 frequency bands known as octave bands or 1/3 octave bands respectively for analysis. An octave band is the band of frequencies in which the upper limit of the band is twice the frequency of the lower limit. Any particular sound or noise can be represented as a number of 8 (or 24) sound pressure levels in the frequency bands, as illustrated by the diagram below.
A real sound shown as a combination of different sound presure levels, one per each of the 24 frequency bands. Column width: 1/3 octave band (24 in total). Column height: SLP at each frecuency band, measured in dB.

The response of the human ear to sound is dependent on the frequency of the sound. The human ear has its peak response around 2,500 to 3,000 Hz and has a relatively low response at low frequencies. Hence, the single sound pressure level obtained by simply adding the contribution from all 1/3 octave bands together will not correlate well with the non-linear frequency response of the human ear.

This has led to the concept of weighting scales. The following diagram shows the "A-weighting" scale:

Reduction of SPL (in dB) at frequencies below and above 2000 to 3000 Hz to reflect the frequency response of the human ear.

In the "A-weighting" scale, the sound pressure levels for the lower frequency bands and high frequency bands are reduced by certain amounts before they are being combined together to give one single sound pressure level value. This value is designated as dB(A). The dB(A) is often used as it reflects more accurately the frequency response of the human ear. 

Other, less used weighting scales, are dB(B) and dB(C). The decibel C filter is practically linear over several octaves and is suitable for subjective measurements at very high sound pressure levels. The decibel B filter is between C and A. The three filters are compared below:

Noise filtering at different octaves of frequency applying decibel filter scales A, B or C.

That was enough for theory. Let us now see how all this affects the performance of glass as a real acoustic barrier.

The four hand-sketched graphs shown here below are all taken from the first edition of a great book called "Detailing for acoustics", written by Peter Lord and Duncan Templeton. There are three editions by now and I highly recommend buying one if you are an architect interested in acoustic issues applied to buildings.

3/ Glass thickness effect

The sound attenuation of any material depends on its mass, stiffness and damping characteristics. With a single glass pane the only effective way to increase its performance is to increase the thickness, because stiffness and damping cannot be changed. The sound transmission loss for a single glass pane, measured over a range of frequencies, varies depending on glass thickness.

Thicker glass tends to provide greater sound reduction even though it may actually transmit more sound at specific frequencies. Every glass pane thickness has a weak frequency value; that is, a frequency for which that glass is less 'noise absorbent' than for the others. That value is known as critical frequency. See the graphic below:

Sound reduction (in dB) measured at different frequency bands for glass panes of different thickness. Source: Detailing for Acoustics, Lord and Templeton.

A 4 mm-thick glass is rather transparent (poor attenuation measured in dB) for high frequencies at the range of 3500 Hz; 6 mm-thick glass is poor for frequencies around 2000 Hz; and 10 mm-thick glass performs bad at 1300 Hz. The higher the mass the less of a problem critical frequency appears to be: 25 mm-thick glass has no weak point as it can be noted from the graph above.

An insulating glass unit built with two panes of the same thickness experiences the issue of critical frequency: it is said that the two panes vibrate (resonate) together at that frequency, thus reducing the glass overall acoustic performance.

For this reason we recommend using different thickness in a double glass unit. A 6-12-4 mm glass will absorb more sound at high frequencies of 2000 Hz (claxon noise) than a 6-12-6 mm glass, in spite of having less mass. On the other hand, at lower frequencies between 125 and 250 Hz (traffic noise) this is not the case: a 6-12-6 mm glass reduces sound more effectively than a 6-12-4 mm glass. At low frequencies sound attenuation is directly proportional to mass.

4/ Laminated vs. monolithic glass

A laminated glass will attenuate sound transmission more than a monolithic glass of the same mass. See the graph below:

Sound absortion of monolithic (solid) glass compared to laminated glass with the same mass. Source: Detailing for Acoustics, Lord and Templeton.

A laminated glass of 2+2 mm reduces sound at high frequencies considerably more than a monolithic glass 4 mm-thick (that's 8 to 10 dB of additional attenuation). Why? because the critical frequency effect disappears due to the sound damping provided by polyvinyl butyral (the soft interlayer used to permanently bond the glass panes together dissipates energy by vibration). The same applies to the 3+3 mm laminated against the monolithic 6 mm. In contrast, at low frequencies (traffic noise) the effect of butyral is less pronounced, although it is still positive (about 2 dB increase).

5/ Air cavity effect

Surprise: a standard double glazed unit does not reduce sound transmission much more than a monolithic glass. What matters is the thickness of the air space between glass panes, but only for really wide cavities.
Effect of air space width on the acoustic performance of double glazing. Source: Detailing for Acoustics, Lord and Templeton.
The acoustic attenuation of a 6-12-6 mm glass is generally superior to that of a monolithic 6mm-thick glass, but only by 2 or 3 dB, and still there may be low frequency bands where the DGU performs worse. Of course if we compare a 6 mm-monolithic with a double glazed 12-6-10 mm, the sound reduction is much better at the double glazed unit.

What really matters is the width of the air space, not the small one found at double glazing but the one of a double skin. The ideal cavity width to boost sound attenuation is 200 mm. For widths less than (or greater than) 200 mm the effect is less noticeable (although a wide air space will always perform better than a narrow one). A double glazing with 10 mm air space performs almost like a 20 mm airspace.

6/ Combined air cavity & glass thickness effect

The conclusion comes in the last graph: a combination of large thickness, different one between the two panes and wide air space distance (even better if we use laminated glass) provides the maximum noise attenuation. We can reach up to 45dB.
Combined effect of glass thickness and air space on the acoustic performance of double glazing. Source: Detailing for Acoustics, Lord and Templeton.

To achieve this with a conventional double glazing width (about 28-35mm only) we have to employ an acoustic interlayer or a sort of resin between two panes in a laminated glass combined within a DGU. These acoustic interlayers or resins dissipate sound waves much more than two or three PVB interlayers as in a typical laminated glass. Some brands of enhanced acoustical laminated products are:
SGG Stadip Silence effect as part of a double glazed unit. Other brands perform similarly. By the way, the scale below is not frequency but loudness (it measures dB). Taken from Saint Gobain Stadip Silence brochure.

What about the effect of using argon or krypton instead of air? In theory, a higher density gas in the space between panes should have a positive effect on acoustical performance. Comparison testing of standard symmetrical insulating units indicates though that common gases as argon have virtually no increased effect on sound attenuation ratings. While some improvement was noted at some frequencies, resonance effects actually became more pronounced.

7. Some useful values

Rw index: The Rw index or sound reduction index (expressed in decibels) measures, in just one number, the acoustic performance of a specific glass unit. The higher the Rw index, the better the level of acoustic insulation offered by that glass composition. The Rw index of ordinary double glazing is around 29 dB whereas a good acoustic interlayer offers an Rw index of around 50 dB.

Rw is a single figure rating for the airborne sound insulation of building elements (not just glass). It includes a weighting for the human ear and measures actual sound transmittance. Rw is measured in a laboratory, not on site (the site-measured equivalent value has the Egyptian denomination of DnT,W). The Rw value is merely an average simplifying mutual comparison of various building components. That can be confusing some times. Two glass units can have the same Rw index while one of them performs well at low frequencies and bad at high ones, and the other one performs just the opposite.

C and Ctr factors: To slightly avoid this issue two spectrum adjustment factors: C and Ctr, have been added to modulate the Rw average. For sound waves featuring high frequencies, the factor C is added to the Rw value. For lower frequencies, factor Ctr needs to be added. The acoustic behaviour of a building component is hence defined by three numbers: Rw (C, Ctr). A building component with the values Rw (C, Ctr) = 40 (-1, -4) provides an average insulation performance of 40 dB. For higher pitched sounds the sound insulation is lessened by 1 dB (39 dB) and for lower pitched sound sources it is lessened by 4 dB (36 dB).

The table below, extracted from Saint Gobain, helps showing how these three numbers apply to different laminated units with acoustic interlayers:

Sound reduction index values for several laminated glass units with acoustic interlayers. The thickness shown at the right column is the total one. 13 mm means 6 mm + 6 mm + 0.8 mm interlayer. Taken from Saint Gobain Stadip Silence brochure.

C takes into account medium and high frequency noise sources such as TV, music, loud conversations or aircraft noise a short distance away. Ctr takes into account medium and low frequency noise sources such as urban traffic noise or aircraft noise a long distance away.

Pink Noise:  Expressed in dB(A), this is an assessment of the sound insulating properties of a building material over specified standard frequencies, which represent general activity noise when equal levels of power are applied at each frequency. So, in pink noise each octave carries an equal amount of noise power. Funnily: the name arises from the pink appearance of visible light with this power spectrum.

Ra:  Ra is the abbreviation for the sound reduction index when the spectrum adaptation term C is applied to the single number weighted sound reduction index (Rw), using pink noise as a sound source.

Ra,tr:  Ra,tr is the abbreviation for the sound reduction index when the spectrum adaptation term Ctr is applied to the single number weighted sound reduction index (Rw) using pink noise as a sound source.

So far so good. Acoustic performance of glass should now be less of a dark matter for us. But this is not all: remember that detailing to achieve a proper air tightness between glass and frame will always be required! Loose gaskets can severely harm the best glass selection for acoustics...