How seat cushions changed the understanding of acoustics

Epidaurus

For millennia, scientists were trying to figure out how building acoustics behave. Around 350 BC, there was a Greek sculptor and architect, Polykleitos the Younger, who had the assignment of designing a 6.000 seat amphitheatre in Epidaurus, Greece, which was built in two phases, one during the 4th century BC, and the second in the mid-2nd century.

This was a massive project with 14.000 seats that was divided into two sections: the lower rows were for high ranking members of society and the furthest rows were for everyone else. But how could the spectators in the furthest rows hear the lecture from the stage, which was 60 meters below?

Back in 2007, two Belgian researchers from the Georgia Institute of Technology were able to reveal a part of its acoustic mystery. It was shown that there is a link to the corrugated shape and rocky material of the seats that serve as a filter for the sound coming from the orchestra^[1]. The theatre was built with specific shapes and dimensions that are governed by the mathematical principles influenced by Pythagorean philosophy.

Today it is one of the best-conserved theatres of its kind in the world.

An interesting question I've always ask myself is,  “Did the architect Polykleitos the Younger know about the theatre’s acoustic capabilities or was it just a coincidence?” Well, the word "acoustics" originates from the Greek word ἀκουστικός (akoustikos), meaning "of or for hearing, ready to hear"^[2]. So, is Polykleitus the Younger the founder of the science of acoustics? Believe it or not, for that we had to wait another 2.300 years.

Music composed for the performing space

Between Polykletius the Younger and 1895, there were a lot of scientists working on the theory of sound, soundwaves, resonators and frequencies. Architectural acoustics or more specifically, room acoustics, was not really top of mind for scientists of the time.

It was really the composers that recognized the importance of the acoustics in a room or space, which lead them to compose music tailored to the room where it was performed and not the other way around. For example “Toccata and Fugue in D Minor”[3], written by Johann Sebastian Bach, is a musical composition played on an organ, which is ideal for a cathedral with an average reverberation time of 5 seconds. On the other hand, it was Mozart who composed music to be played in highly furnished chambers. Many of the operas he wrote are performed best in rooms with a reverberation time of 1,00- 1,30 seconds [4].

So why didn’t they start designing rooms and concert halls with optimal room acoustics? The science of acoustics was still considered a mysterious combination of many different and undefinable factors. But that all changed at the end of the 19th century.

Introducing Wallace Sabine

It was in the year 1895 when a young American assistant professor of physics at Harvard University, named Wallace Clement Sabine, was asked to solve a difficult problem in the Fogg lecture hall of Harvard's recently constructed Fogg Art Museum. The problem with the university's lecture hall was that it was excessively reverberant. Sabine, who had never received his PhD and had no particular knowledge of sound, got the assignment to improve the infamously bad acoustics in the hall. It was generally considered impossible, but Sabine set himself the task of determining what made the Fogg lecture hall so different from other acoustically acceptable spaces.

Sabine noted that when anyone spoke in the Fogg lecture hall, the sound of their voice remained audible for 5.5 seconds, even at normal conversational levels. You can imagine how difficult this made it to understand what the speaker was saying.

Sabine was triggered by the fact that on the campus of Harvard University, there was another lecture hall with hardly any complaints about the acoustics - the Sanders Theatre. It had the same acoustically complex shape as the Fogg lecture hall. Its capacity was three times higher, and it had 700 more seats. However, there was one significant difference; the seats in the Sanders Theatre had cushions.

Seat cushions

Instead of improving the acoustics he focussed his investigation on the sound absorbing properties of the cushions. It wasn’t an easy task because so many variables had to be taken into consideration. Together with his assistants, he moved the seat cushions from the Sanders Theatre into the Fogg lecture hall and noticed that the sound conditions of the hall where changing. He took many different measurements using only an organ pipe and a stopwatch, listening how long it took for the sound to decay. Sabine carried out most of his experiments in the middle of the night, so that he could control for background noise, which would interfere with his measurements. At some point he even found that the outcome of the measurements was different because of the different clothes he was wearing. For the remaining tests, he wore the same outfit.

Over a three year period, Sabine took thousands of measurements using hundreds of seat cushions. He also did measurements in the other halls of Harvard’s University and used other materials like rugs.

I have found it at last!

With so much data collected, Sabine was looking at the relationship between acoustic quality, hall size and the number of absorption surfaces, but he didn't know how to put all the pieces of the puzzle together. Then suddenly in a clear moment, he said, “I have found it at last!” ^[5].

What Sabine found was that when he plotted the quantity of Sanders Theatre seat cushions (x) versus the corresponding reverberation time for a room (y), the resulting graph was a rectangular hyperbola, a standard mathematical curve characterized by the equation xy = k, where k is the constant^[5]. He realized that his discovery of the hyperbolic relationship was a breakthrough for his understanding of reverberation.

The father of room acoustics

Sound, being energy, once produced in a confined space, will continue until it is either transmitted by a boundary wall or is transformed into some other kind of energy, generally heat. The process of decay is called: absorption[6].

This disturbance from absorption may be regarded as a process of multiple reflections from the walls, the ceiling and from the floor. First from one and then another, losing some of its strength at each reflection, until ultimately, inaudible. This is called reverberation[6].

He defined the reverberation time as “the number of seconds it takes for the reverberant sound energy to decay 60dB from the original sound signal”.

The end result of Sabine's investigation was the equation, T = 0.161 V/A

T              = reverberation time (seconds)

0.161       = hyperbolic constant

V              = volume of the room (cubic meters)

A              = equivalent absorption surface (square meters)

This became the Sabine Formula which is still used for Architectural Acoustics today. Eventually Sabine became the dean of Harvard’s Graduate School of Applied Science from 1906 until 1915. In 1919, at 50 years old, Sabine died in the hospital from complications following surgery.

Since his death Architectural Acoustics has accelerated until where we are today, and Sabine is widely recognized as the father of Room Acoustics.

Room Acoustics Myths

Now that we know more about the science behind room acoustics we can reveal a few myths.

Myth: You can hear a coin drop from the back rows of the Epidaurus Theatre.

In 2017 researchers from the University of Technology in Eindhoven mapped the acoustic qualities of the Epidaurus theatre. The results of the research showed that the sound quality of the theatre is good, but not as impressive as many travel guides claim. The sound of a coin being dropped can only be recognised as a coin being dropped halfway up the rows of seats [7].

Myth: Room Acoustics is only important in performance-orientated large spaces

First we have to ask ourselves “Why are acoustics important?”. Acoustics are commonly associated with concert halls, recording studios and lecture halls. In these venues acoustics are a critical aspect, but good room acoustics also improves people’s well-being, it increases productivity at work, it enhances learning at school and it reduces recovery time in hospitals. I think most people can relate to uncomfortably loud spaces like restaurants, spinning classes or even your own living room. So it’s essential to look at improving the acoustic environment in every room where we live, sleep, work, heal and spend our free time.

Myth: Just add carpets and curtains for great sound in your home.

Improving room acoustics at home can mean a more pleasant environment and more enjoyment when you want to watch movies on television, listen to music on the radio or simply have a dinner with friends or family. But for example, adding a carpet and curtains in your living room is not a complete acoustic solution. This is because of their limited surface area and mostly low sound absorbing properties when compared to an acoustic ceiling solution. We have multiple high sound absorbing acoustic ceiling designs for living rooms to provide optimial acoustics.

The complexity of Room Acoustics

The science of room acoustics is very complex. You can fill in some numbers in an online Room Acoustics Calculator, but that is just the start. It requires analysis of the room dimensions, its shape and ceiling height. What is the design of the space? How many people are occupying it and what’s kinds of activity are happening there? 

Sound absorbing wall panels in combination with a fully covering suspended ceiling are often necessary to meet the acoustic requirements. Put this together with the science of Architectural Acoustics and it’s getting even more complicated.

I wonder if Sabine knew what he started when he was moving around all those cushions…

In order to achieve great acoustics and make people feel better, different solutions can be applied and installed. At Rockfon, we are experts in creating spaces that sound beautiful to everyone.