Concert Halls By: Chantelle

Sound

What is a sound wave?

If you were to ask the dictionary what a sound wave was it would say “A wave of compression and

rarefaction, by which sound is propagated in an elastic medium such as air,” (Name, date). But

what does that mean? To break it down, like any other wave (light, etc.) sound waves can be

reflected, refracted, diffracted, and exhibit interference. The same way that an ocean creates waves

that hit the shore; all sound that you hear travels through the air, or a “medium”, and hits your ear

drum. The air represents the ocean, the sound represents the wave the ocean creates on the surface

of the water, and your ear represents the shore. There are sets of compressions and rarefactions that

are simply when the particles are closer compacted, and further apart as shown below in the

diagram.

http://www.mediacollege.com/audio/images/loudspeaker-waveform.gif

How Sound Moves and is Heard

Sound moves in a wave-like form as shown in the image above. This type of wave is known as a

“Longitudinal Wave”. Sound is created as a vibration of matter, and due to this vibration the

particles of air surrounding the wave begin to vibrate at the same frequency. The matter that

vibrates can be solid, liquid or gas, but most of the sound you hear is the vibrating of the particles in

the Earth’s atmosphere. Frequency is measured in Hz, and one way to explain it would be how

many cycles of the wave are happening per second. Almost everyone in the world has heard a

radio, if you looked at the stations on a radio they would be measured in MHz (Mega Hz), for

example, 92.5 MHz. That is the frequency that is coming from that station, and if you’re in distance

your radio will pick it up if you’re tuned in to that frequency. Sound, however, can also move in a

“Transverse Wave” if it is acting through a solid.

Human ears can hear frequencies (or vibrations) between 20 and 20,000 Hz. If the frequency is

above 20,000 Hz it’s ultrasonic, and below 20 Hz it’s infrasonic (sonic being sound). To put this

into perspective, a human voice can range from 60 Hz to 13,000 Hz. In order for sound to be

recognized it must go through to reach the thin membrane of the ear drum where it continues to

travel and is processed by your brain.

Reflection

The reflection of sound waves is used by many animals such as bats or dolphins, and in most cases

it’s to identify objects or send messages to other animals. Most of the sounds sent by these animals

are beyond the human’s capacity of hearing and are sent at frequencies of way above 20,000 Hz.

Bats use sound waves as their sense of sight, and can catch a mosquito at amazing accuracy in

complete darkness using high frequencies of sound.

http://askabiologist.asu.edu/sites/default/files/resources/articles/bats/ecolocation_types.jpg

The image on the last page represents just exactly how reflection can be used. If high frequencies

of sound are sent out, then the amount of time it takes for the sound wave to bounce back at the

same frequency can help determine how far an object is away from something, someone or an

animal.

Refraction (Change in wave speed, moving from one medium to another)

Refraction most often happens in the study of optics. When light is traveling through one medium,

and reaches another medium it not only changes its direction by angle but changes its speed as well.

Sound waves, just like light, can be refracted as well however it happens under different

circumstances. Sound wave speed can gradually change over a given distance, and depends on the

temperature of the air. Therefore, the change in wave speed is generally due to the change in

temperature of the air. The speed of a sound wave is slower in cooler temperatures, and faster in

warmer temperatures. Since temperature decreases with height, a speed of a sound wave will also

decrease with height, meaning waves will bend as they travel. The bending of these waves with

create a “shadow zone” as seen in the picture below.

http://www.ec.gc.ca/foudre-lightning/4EFD3A52-8917-4C3D-BE38-94B088DE4E3E/X-2011041515523140615.jpg

In the Shadow zone the sound will not be heard, whether the observer could see the object creating

the noise or not. This shadow zone has been created by the refraction of sound waves upward due

to temperature.

Diffraction

Diffraction is when a sound wave bends around small obstacles, or the spreading of waves around

small openings. The reason that sound can be heard around corners or through openings is due to

diffraction. Diffraction often is pronounced with long wavelengths, therefore lower frequencies of

sound can be heard around corners easier than high frequency sounds. An example would be

lightning, because the closer you are the more distinct the noise is because the strike is a higher

frequency. The farther from the strike you are the more the sound will be heard as a low rumble,

because the lower frequency can travel farther. The image below depicts this information with the

example of a marching band walking down a street.

http://hyperphysics.phy-astr.gsu.edu/hbase/sound/diffrac.html

Interference

The same with ocean waves, sound waves can collide and interact with each other. Two different

results can come out of two sound waves interacting. Constructive interference is when the

compressions and rarefactions of two sound waves coincide and they reinforce each other. A

destructive interference is when the compressions of one interact with the rarefactions of the other,

often making a resultant wave that can either weaken or cancel each other out.

http://www.kirksville.k12.mo.us/khs/teacher_web/alternative/constructive_destructive_wave.GIF

Soundproofing

The uses of soundproofing are to either block out noise coming in to a room or going out. It is

impossible to block out sound completely, but a huge reduction can be done. Reducing sound

comes from making a room that has no holes. A lot of sound can get out from just a small crack.

Some materials used are foam, room dividers, temporary walls, or curtains. In order to make sure

there are no gaps, an air-tight barrier is created inside or outside the room, but not tight enough

where oxygen cannot get in. Certain materials also have the ability to absorb sound such as, dense

foam, and these materials are placed on the inside of the room. The absorption will reduce the

echo-effect that can come about when sound cannot escape. Certain areas are looked after more

than others as well, such as doors. Once a room is soundproofed, it’s easy to tell if it was done

right, because there should be a huge reduction of sound from start to finish when standing outside

of the room.

Concert Halls

http://static.worldarchitecturenews.com/news_images/1217_2_1000%20Buro%20Perth%20Concert%20Hall%202.jpg

How they Work

Concert halls are designed to enhance a listener’s experience in sound and music. An enclosed

space allows sound to bounce off the walls and ceiling of the room to create what is called a

reverberation (the delay of sound after a note has stopped being played) effect. In the design

process, an acoustic engineer looks at many acoustic factors such as the background noise level, the

amount of reverberation, the amount of side arriving from the side, timing and toning, and more.

Many “treatments” are done to existing rooms in order to improve their sound quality and

experience.

http://www.acoustics.salford.ac.uk/acoustics_info/concert_hall_acoustics/images/absorber_reflector_diffuser.jpg

Absorbers

If any room, is too reverberant then certain materials are used to absorb some of the sound. As

stated before certain materials can be used to absorb sound, for example absorbent ceiling tiles

might be used. However, in a concert hall absorbent materials are not used because there is a limit

to the amount of sound an artist or orchestra could make. Therefore, taking away any of the sound

they make would only hurt the sound quality for the audience.

Large Flat Surfaces

Flat surfaces are just like reflecting light on a mirror, the sound will hit the wall and reflect back.

The flat surface does very little to the sound, so it will still be heard the same to the human ear, it’ll

just be at a different angle. When the sound reflects the sound energy is conserved and

concentrated in the direction of reflection with equal angles of incidence and reflection.

http://images.books24x7.com/bookimages/id_15219/p30001d6bg2360001vpp_thm.jpg

This allows for the sound to travel throughout the hall, with very little damage being done to the

sound heard by the audience. However, frequencies and pitches can be emphasized or

deemphasized depending on the reflection, creating was is known as a “coloration of sound”.

Diffusers

A diffuser disperses the sound made by a singer or instrument both temporarily and spatially. The

frequencies and pitches reflected of the diffuser will have very little colouration of sound, allowing

the noise from the singer or instrument to reflect and be heard as is, rather than certain frequencies

and pitches emphasized or deemphasized. Any non-flat, ridged surface will have the ability to

diffuse. The reason diffusers are used rather than absorbers, is because when there is an echo effect,

if you absorbed you would get rid of the echo, but you would also take away from the acoustic

energy. A diffuser would be able to disperse the sound which would also take away the echo, but

would not steal any of the acoustic energy created by the performer(s). In other terms, a diffuser is

almost like reflecting light in a fun house, with multiple mirrors for the light to go, and the light

ends up all around the room.

http://ffden-2.phys.uaf.edu/211_fall2002.web.dir/quinton_harris/diffusion.jpg

Resonance

Resonance literally means “resound”. Resonance occurs when an object is vibrating at the natural

frequency of another object and forces the second object to vibrate as well. An example of

resonance is in a rod. As a hand slides along the rod, vibration is created between the rod and the

hand. The air column inside the rod is then forced to vibrate at its own natural frequency to match

the hand and rod’s vibration.

Resonance can be felt inside a concert hall. When there is a lot of resonance you can feel the beats

and sounds vibrating through your chest. This is the act of resonance, when the sound creates

vibrations around you, you yourself will vibrate at a natural frequency.

Bigger the Better?

http://upload.wikimedia.org/wikipedia/commons/7/79/RogersCentre_Toronto_Sept1-05.jpg

http://farm6.staticflickr.com/5174/5436131464_195ef25117_z.jpg

Have you ever thought about how a big concert hall such as the Toronto-famous Rogers Centre’s

experience would be versus sitting in the small Massey Hall? The size of a room changes many

aspects of a concert halls’ acoustics. The reverberation and resonances of the rooms are what is

changed most of all. Although reflection, absorption, and diffusers will help the sound quality in a

hall; the main thing to decide whether is a good hall or not is the ability to hear the “direct sound”.

The direct sound is the sound that hits the ear drums without being reflected. This sound has the

most quality, and will give the most effect to any listener. But in larger halls, it’s harder to hear the

direct sound, such as the Rogers Centre, which can hold up to 55,000 people! Extra speakers are

usually added to reinforce direct sound in large concert halls, but the experience will never be quite

as good as a smaller hall, with a much more ‘livelier’ direct sound.

Let’s take it Outdoors!

http://c308991.r91.cf1.rackcdn.com/SiteFiles/Venues/6484/d17432f6-67df-4ca9-a406-093e9676b8a0.jpg

http://farm8.staticflickr.com/7087/7283226558_99b4f28759_z.jpg

From the readings before, it’s a safe guess that outdoor halls are not the best experience for

audiences that are looking for good sound. Being outside takes away all of the walls that will trap

the sound waves, and leaves only the direct sound to be heard. Most outdoor concert halls include a

few walls that extend from the stage, such as that of the Molson Canadian Amphitheatre in

downtown Toronto. However, the seats that extend from this wall, including a lawn that some of

these types of concert halls have, there is extra work needed in order to keep the direct sound. Extra

speakers, just like in the larger halls, are often used in order to create an illusion that the audience

outside of the hall, is still in it.

Works Cited

Cox, Trevor J., and Peter D'Antonio. Engineering Art: Science of Concert Hall Acoustics. N.p.,

2003. Web. 14 Jan. 2013.

<http://www.acoustics.salford.ac.uk/res/cox/interdisciplinary/inter_science_reviews.pdf>.

"Diffraction of Sound." Hyper Physics. N.p., 2013. Web. 03 Jan. 2013. <http://hyperphysics.phy-

astr.gsu.edu/hbase/sound/diffrac.html>.

Harris, Tom. "How Hearing Works." How Stuff Works. N.p., 2013. Web. 03 Jan. 2013.

<http://science.howstuffworks.com/life/human-biology/hearing.htm>.

Kivi, Rose. "How Does Sound Proofing Work?" EHow. Demand Media, 17 Dec. 2008. Web. 04

Jan. 2013. <http://www.ehow.com/how-does_4683237_sound-proofing-work.html>.

"Resonance." Resonance. The Physics Classroom, 2012. Web. 10 Jan. 2013.

<http://www.physicsclassroom.com/Class/sound/u11l5a.cfm>.

Russel, Daniel. "Refraction of Sound Waves." Refraction of Sound Waves. N.p., n.d. Web. 02 Jan.

2013. <http://www.acs.psu.edu/drussell/Demos/refract/refract.html>.

Shetty, Satya. "How Does Sound Travel?" How Does Sound Travel ? Preserve Articles, 2012. Web.

02 Jan. 2013. <http://www.preservearticles.com/201012261704/how-does-sound-

travel.html>.