PHYS341/2024/Project24
Open Space Acoustics
Project by Katie Stoddart
The layout of a room can drastically alter the sound produced within. For example, an empty room with regular walls and a tile/hardwood floor will have an echo. This is because the sound waves are not absorbed by these hard surfaces or anything else, and thus are left to bounce around the room. A furnished room, or a room with carpet flooring and/or soundproofing will not have this echo. The objects filling the room and/or soft, absorbent surfaces will stop the bouncing and reverberation of the sound waves. Sound and echo can be controlled in a number of ways, which will depend on the context in which this is being done.
Sound waves
Sound waves are how we visualize the disturbances that sound makes on the air/space that it occurs in. When particles vibrate within a medium, such as air, these vibrations are perceived by our ears as sound. Layers of air molecules being displaced one at a time results in traveling sound waves, which we are able to record and visualize.
As sound waves travel, they go through regions of high pressure where the particles are close together (known as compressions) and low pressure where the particles are more spread apart (known as rarefactions).
When sound waves meet our ear drums, they cause the eardrum to vibrate. The vibrations travel past the bones in the middle ear (called ossicles) to the cochlea in the inner ear, which contains tiny hair cells that convert the vibrations into electrical signals. These get sent to the brain, resulting in us hearing sounds.
Sound waves are characterized by frequency, amplitude, wavelength and wave speed. Frequency refers to the number of cycles of high pressure and low pressure that occur per unit of time, as well as determines the pitch of a sound. A high frequency corresponds to a high pitch and subsequently a low frequency corresponds to a low pitch.
Amplitude means the loudness of a sound. A greater amplitude comes from a greater amount of particle displacement during a wave vibration, which creates a louder sound.
Wavelength simply refers to the distance between two compressions or rarefactions. Higher frequency waves have shorter wavelengths, while lower frequency waves have longer wavelengths.
The wave speed is dependent on the medium that it is traveling through. Sound travels at about 343 meters per second in air at room temperature, but this speed can change in other conditions.
To sum it up, sound waves are produced by vibrations which propagate through mediums as variations in pressure. These vibrations enter our ears where they are converted into electrical signals that are ultimately interpreted by the brain.
Echoes
An echo occurs when sound waves are reflected off of a surface back to the listener’s ears. When sound waves hit a surface and bounce back, they reach the listener’s ears after a short delay, causing us to perceive a sound which is distinct from the original sound. To understand echoes we must look at factors such as the speed of sound (as discussed above), the distance to the reflecting surface, as well as the nature of the surface itself.
When a sound wave encounters any solid object, a portion of this wave is absorbed and the rest is reflected back into the air. The reflected sound travels back to the listener, and the time that it takes for this reflected sound to return to the ear is what creates the echo.
Hard surfaces create clearer echoes. Less of the sound is absorbed, leaving more to be reflected. Large, open spaces will also create stronger echoes because the reflecting surfaces are further away from the listener. The longer it takes for the sound to be reflected off of the surface, the clearer the delay will be from the original sound, resulting in an obvious echo.
In considering room acoustics in terms of music, an architect will consider the echo in a room such as a concert hall or a recording studio and minimize this appropriately to make for optimal sound quality depending on the intended purpose and effect for the listener.
Room acoustics
In the image of the sound waves above, two sounds are compared between a note sung in a regularly furnished room, and the same note sung in a completely empty room (also pictured above). The echo in the empty room creates the appearance of a higher amplitude, though the amplitude of the echo itself would be lesser than that of the original sound. However, in a relatively small room, it is difficult to isolate and measure the echo on its own.
The echo produced in an open space will not have an effect on the frequency of a sound, and therefore will not change the wave length either. It will also not change the wave speed. Despite this, the sound produced in a large open space may not be perceived exactly as intended, therefore the extra reverberation of sound waves that cause echoes can be minimized. This is done in many contexts, especially in the music world. This may be done through either sound absorption or soundproofing.
In a regular room, things such as furniture or carpet will stop an echo because these objects absorb the sound waves. In spaces such as most classrooms or rooms in a house, for instance, this is often all that is really needed. However, in a space such as a concert hall, perhaps specific materials such as foam or sponge will be used for sound absorption to balance sounds and enhance the clarity.
Other situations may call for total soundproofing, such as recording music in a studio. This will keep all of the sound in the recording studio as well as make sure that external noises are not interfering with the recording. This is done with heavier materials, such as mass loaded vinyl as opposed to something softer that will not completely block sound from entering or exiting a room.
Ultimately, the objects that are used to control sound are situational. A completely open and uncontrolled space is far from ideal in most musical contexts, but could also be used depending on the desired effect.
References
Zola, Andrew. “What Is a Sound Wave, and What Do Sound Waves Move or Travel Through?” WhatIs, June 7, 2022. https://www.techtarget.com/whatis/definition/sound-wave#:~:text=Compression%20occurs%20when%20particles%20move,tines%20move%20back%20and%20forth.
B, Phil. “How Echoes Work - What Causes an Echo to Occur?” SoundAssured, October 1, 2021. https://www.soundassured.com/blogs/blog/how-echoes-work-what-causes-echoes-to-occur
“Soundproofing vs. Sound Absorption: What’s the Difference.” Soundproof Cow, February 23, 2024. https://www.soundproofcow.com/soundproofing-101/soundproofing-vs-sound-absorption/#:~:text=Sealing%20gaps%20in%20a%20room,that%20travels%20through%20a%20structure.