Concert Hall Acoustics (Sound Science)

By: Annie Hu


If you can remember back to a concert you’ve attended, then try to imagine the concert hall in your head. Maybe a huge, grandiose stage comes to mind, a symphony or band sitting atop the raised platform in a beautiful, massive venue where the seats are completely filled by the audience. But all of its grandeur and dazzling appearances also serve a functional purpose. The purpose of a concert hall is to project sound coming from the stage out into the audience in the most appealing way possible.


Before we explain how concert halls do this, we first need to understand how sound works. Sound travels in waves made of vibrations of air molecules. Essentially, sound moves air molecules, much in the way waves in the ocean move water. As the sound travels air molecules bounce against each other, creating a pressure wave. A pressure wave occurs when air particles are pushed closer together in some places, but are farther apart in others. It is a wave, because this area of high pressure moves as the particles bump into each other when they move back and forth. Particle displacement occurs parallel to the direction the wave travels. During the course of the wave, these oscillations result in compressions (where the particles become densely packed with each other) and rarefactions (an area with relative scarcity of particles). Overall, the wave continues in a forward motion as particles bounce into each other. The areas of compression and rarefaction are what make the high and low points of the wave.


Scientists can play around with how we interpret sound waves by changing certain aspects of the wave. The main properties of a sound wave that can be changed to affect what we hear include the pitch (the frequency, or wave speed), timbre (wave shape), and the amplitude (or height of the wave). They also can change the conditions those waves travel through. An example is changing the medium through which sound waves travel; think of hearing a sound in air versus when you’re underwater. This same idea, that the way sound is heard can be manipulated by changing the conditions around it, is what powers acoustics or “the science of sound.”


Being able to hear sound in the first place takes a series of complex processes that turn sound waves into electrical signals. These electrical signals are what the brain perceives as sound. This process begins when sound waves (changes in air pressure!) travel through the outer ear into the ear canal, where they encounter the eardrum. The sound waves then cause the eardrum to vibrate as the air molecules bounce against it. These vibrations travel further to the middle ear and encounter 3 tiny bones called the malleus, incus, and stapes, which amplify the sound before it is sent through the cochlea, a snail shell-like structure. This structure contains a fluid and is lined with tiny hairs. The fluid in this structure creates an actual ripple wave for the sound to travel on. Hair cells move from the waves (like kelp on the sea floor moving with the tides), and this movement is enough to open up chemical channels near the hairs. These channels release chemicals and create an electronic signal for the brain to interpret.


Image Credit: Wikimedia Commons @ Inductiveload

But a lot happens to sound before it reaches our ears, so there are many ways sound can be changed or manipulated to help a concert sound its best. It is important to remember that sound bounces off of objects as it moves away from a source. That is one of the characteristics of sound that concert hall acoustic designers need to account for. Cloth and soft materials have a tendency to absorb sound waves and “deaden” the room by cutting out some of the reverb (or echo of sound). Sometimes this is needed to “darken” the sound of the performers if the music is such that it naturally has a lot of reverb, such as trumpet music and other loud instruments. That’s why concert venues are designed differently for symphony orchestras versus bands, as the sounds need to be handled differently to get the best outcome.


Another issue to contend with is the fact that concert halls are big, and the sound needs to carry all the way to the back of the audience. If you’ve ever seen the various pyramids and shapes that decoratively adorn the ceiling of a concert hall, not only do those provide an aesthetic effect, but they are essential to directing the sound to the audience. Their shape reflects the sound waves towards the back of the hall by “guiding” the waves as they bounce off.


But sometimes the sound is reflected very quickly after the actual sound is made, and bounces off the surface nearly immediately. Essentially, the waves encounter an object directly after being created. This has been found to make the original sound a little bit richer to the ear. Thus, having these sound reflecting aids can help enrich original sounds. But having the reflection take place too late will start blurring sounds and causing too much interference between signals, or “muddiness,” so there needs to be a balance in how much reflection occurs.


Image Credit: By KieranMaher at English Wikibooks - Transferred from en.wikibooks to Commons., Public Domain, https://commons.wikimedia.org/w/index.php?curid=61798745

It’s interesting to see how sound, so fundamental to daily life, is handled by the human ear. And more so, how it can be manipulated for particular purposes. Concert halls, in all of their grandeur, are probably the most notable examples of sound engineering at work. The decorations and elaborate designs of these halls are more than just aesthetically pleasing, but they also play a part in helping to direct, carry, and enrich the sound you’d hear from an orchestra, a big jazz band, or your favorite singer. Hopefully you learned a lot about the very interesting topics of how sound is heard and ways sound can be altered!


Citations:

https://link.springer.com/chapter/10.1007/978-1-4939-0755-7_1

https://www.nidcd.nih.gov/health/how-do-we-hear#:~:text=Sound%20waves%20enter%20the%20outer,malleus%2C%20incus%2C%20and%20stapes.

http://www.planetoftunes.com/synthesis/3-ways-to-alter-sound.html#.YDyIORNKi3J

https://www.acs.psu.edu/drussell/Demos/waves/wavemotion.html

https://www.washingtonpost.com/archive/1997/10/08/concert-hall-acoustics-can-make-music-or-break-music/a659d80c-1d16-4421-90b0-89bc4b377e2c/


Images:

Inductive Load, CC BY-SA 2.5 <https://creativecommons.org/licenses/by-sa/2.5 >, via Wikimedia Commons, File:Anatomy of Human Ear with Cochlear Frequency Mapping.svg - Wikimedia Commons

https://commons.wikimedia.org/w/index.php?curid=61798745


What Did You Learn?

Questions:

1. How do sound waves travel?


Sound waves originate from a source, and proceed to travel outwards from that source. As sound waves travel, they push air particles, which bounce against each other in a longitudinal wave. The particles oscillate in parallel to the direction of the wave, and cause areas of compression and rarefactions. Compression is where the particles bunch together, and rarefactions are areas where particles are pushed apart. Overall the wave continues in one direction while compressing air particles.


2. What properties of sound waves must be accounted for in designing a concert hall?


Acoustic hall designers must remember that sound bounces off of objects. The echoes can be manipulated by changing the materials the sound bounces off of. Certain changes can help the sound carry better and sound better, but there must also be caution because too much echo causes too much interference. To address the way sounds bounce off objects, and also the type of music the hall is designed for, the designers may find ways to deaden the echoes or to encourage reverberation. Sound can be deadened by materials like cloth which absorb the waves and cut off reverberation. The opposite can occur if the designer decides to create objects for the sound waves to bounce off, and to direct them towards the audience.


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