Epidauros ampitheater in Greece (Image from Wikimedia Commons)


Room Acoustics 101: Achieving exemplary Greek theater acoustics in modern spaces

To achieve the idyllic Greek theater atmosphere, proper acoustic design and quality construction technique must be employed

  • April 8, 2020

  • Written and illustrated by Martin C. Gallan

  • Introduction by Denny Mata

As the onlsaught of the novel coronavirus pandemic continues, Filipinos are mandated to stay at home for another 15 days as the national government and the frontliners strive to flatten the curve. While essential businesses are still open, some businesses have resorted to working remotely. With this setup, both employers and employees, and service providers and clients, can only discuss business matters via online meetings. Here’s where proper room acoustics come in handy.

The following is a peek at room acoustics, and how you can recreate it in your space.


“If it is good, it was probably made in the past.”

Transport yourself back to the time of the ancient Greeks, where the highest technical standards were seamlessly fused with the aesthetic. On a hillside, just outside the busy side of town, you enter a place that is not enclosed with walls. A cool, soft breeze from the nearby woods greets you. You notice that the seats are arranged in a semicircular pattern. You make an effort to seat yourself because there is a substantial rise from one row to the next one behind it. You find yourself in an acoustically near-perfect theater.

You and the rest of the audience will hear predominantly direct sound, that is, the sound that comes from the orator, singer or musical instrument. Any annoying sound distracting you from hearing the performer is absent. The only acoustic problem would be for the sound to reach each and every one in the theater at the same time. There is no external noise. This would not even be considered since the whole town is with you watching the performance. There are no planes, jeepneys, tricycles or factories to generate noise that would disturb the show. Idyllic, isn’t it?

BluPrint Room Acoustics
A pure tone sound wave. Sound is caused by vibrating materials. The source creates pressure in the medium (such as air) and the air molecules form waves of pressure and rarefaction

Fast forward to the present. Modern times do not allow such venues to exist in densely populated areas. You now find yourself in a room completely enclosed. To recreate the ancient Grecian experience, a theater, concert hall or auditorium must be totally sealed. Aside from the usual aircraft passing overhead and acoustically unregulated motor vehicles outside the place, mechanical noise from fans and air-con units located in the building along with other activities must be shielded from the room if this is to be a pleasing venue to listen to music, a speaker, or theatrical performance.

Most of us have experienced being inside a room enclosed by walls with the ceiling and floor made from hard materials. The simple act of walking across the room generates sound that is reflected back and forth between these surfaces, until all the sound from one’s shoes are absorbed and converted into heat. If we were to clap continuously in the room, the sound will rise to a maximum steady state value (loudest volume); if we all stopped at once, the sound will not stop as abruptly but will progressively decrease until it is no longer audible.

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When one walks in between two full-length mirrors, one sees many images of oneself. It is a function of light waves striking the highly reflective surface of the mirrors. Sound too is reflected from any surface with a little of it converted into heat. The conversion into heat of any reflection is proportional to the intensity of the sound and a quantity called absorption coefficient. The steady clapping earlier will cause the sound level of the room to increase, until the room’s absorption rate of the sound energy is equal to the clapping. If all the surfaces in the room were as efficient in reflecting as mirrors, the sound level will increase as long as there is clapping in the room.

When we rest our hands and stop the clapping, the intensity steadily drops because of the absorption of energy by the various surfaces. The sound that continues after the source has stopped generating is called reverberation. Acoustically, the time it takes for sound to die away to a millionth part of its original intensity, i.e., through -60dB, is called its reverberation time (RT).

BluPrint Room Acoustics
A sound wave reflecting off a solid surface. Reflection: Sound is typically reflected from a hard and smooth surface. It follows the rules of optics. Imagine light bouncing off a mirror

Reverberation time is dependent in part on the distance sound must travel between the surfaces it will reflect from. This is why structures with large rooms such as churches have longer reverberation times as opposed to smaller rooms like a classroom. Not all forms of reverberation are bad. Sometimes it is expected, as when there is a live musical performance. Choirs need it to sound glorious. Pitchy singers need it to cover up inconsistent notes. Its effect is most noticeable while singing in a heavily tiled bathroom. Speech, however, requires less reverberation to be understood. Try listening to a lecture amplified by a karaoke with its reverb on full blast. Those who dabble in acoustics soon realize that it is never quite that simple.

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Sound isolation

A quiet place is a sign of refinement.

In order to achieve that idyllic Greek theater atmosphere, we should first prevent sound from coming in and out of the room. Proper acoustic design and quality construction technique should ensure that the sound energy will not be transmitted. If a homogeneous material is chosen for the wall, sound will have difficulty passing through it. If you increase the mass per unit area of the wall, this will make it more difficult for sound to pass through. Every time you double the mass, your transmission loss will improve by -6dB. Structural concerns will be your primary limiting factor in the acoustic design.

BluPrint Room Acoustics
The start of a standing wave. A Standing Wave happens when a parallel wall creates a mirror image of the original sound which ultimately develops into a stationary pressure pattern in the room. This will result in some areas of the room getting louder or softer depending where one stands

In practice, adding multiple layers of different materials, both solid and absorptive, increases isolation. These various materials, having different acoustic impedance, will create multiple reflections between each layer. If you add an absorptive material in between your walls, this will convert the waves into heat, further eliminating the transmission of sound. A properly designed system of walls, ceiling and floor will give you an excellent soundproof box for a room.

Controlling reverberation

Unfortunately, having a venue with good acoustic barriers is not enough. Any sound generated from within will definitely reflect many times from all the hard surfaces. Unlike the Greek theater that had no walls, sound generated within this box has nowhere to go. What you will have is an awfully reverberant room. Many have reacted violently to this before, but it is true—spending money to soundproof a room will not give you an acoustically correct room. This paradox is a frequently misunderstood concept of acoustics in the construction industry.

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We have to work on the interior acoustics. When we start adding furnishings such as carpets, curtains, soft chairs and including people, we call these sound absorbers.

Other solid architectural elements such as columns, moldings, and tables break the sound waves. They are called diffusers. The use, mix and match of these help tune the room to perform for the purpose it will be used for.

BluPrint Room Acoustics
Diffusion of sound. Diffusion occurs when a sound wave bounces off a convex or uneven surface. This may be used as a tool to spread sound evenly in a room. But too much diffusion is also undesirable

The absorptive and diffusive qualities of objects vary with frequency, high frequencies having smaller waveforms. Think tiny ripples on a pond. Low frequencies are larger and full of energy. Think large waves hitting the shore on a stormy day. Small interior objects will normally affect higher frequencies only, while larger, lower frequencies will just go around the diffuser. Meanwhile, an absorber will be more effective the thicker it gets. The position of the latter will greatly affect its performance.

Coloration

Not all venues sound alike—this is due to coloration. This is when you hear that certain frequencies are more pronounced in the reverberation. Coloration lends a distinct acoustic signature or trait to each room. As stated earlier they can enhance, reinforce a direct sound, be destructive, or distort the original sound. These are often caused by hard parallel walls allowing many reflections of certain frequencies to bounce off from its surface. The other less dominant frequencies die out.

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In a larger room, an echo may be detected. An echo occurs when there is little reverberation but a distinct sound is perceived between the original and the repetition of it. Often these two terms (echo and reverberation) are confused with each other. Knowing the difference is one indicator of the level of understanding one possesses in the science of room acoustics.

It is interesting to note that a study of concert venues through the ages will show that some notable compositions were made with the characteristics of the venue in mind. They took into mind how certain instruments would develop inside the hall. That is why some musical performances are not as emotionally moving in some venues as compared to a specific concert hall.

BluPrint Design
A sound wave interacting with a partition. In controlling sound one must first know what to do with it. Either reflect/spread it, or absorb it thus eliminating the sound and preventing transmission

That is why Gregorian chants sound so good in gothic churches with their tall vaulted ceilings, and complex tunes by Mozart sound better in smaller, less reverberant, highly ornate (thus diffusive) baroque rooms. Eventually, architects learned from these venues. As these venues evolved, architectural innovations were introduced. Sadly, these would often have disastrous results.

It was only in the turn of the 20th century that a physicist from Harvard University pioneered and founded the field of architectural acoustics. Wallace Sabine was called to investigate an auditorium in Harvard University, and from his experience, he developed an interest for acoustics. We are merely scratching the surface of the study of room acoustics.

There is so much to learn, let alone master.B ender

This first appeared in BluPrint Volume 2 2012. Edits were made for BluPrint online.
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