7-5 Sound Within An Enclosure
What happens to sound intensity as you travel away from the source?
As an observer travels away from a sound source, both the sound intensity level and loudness decreases. In fact, as the distance from the source doubles, the intensity will decrease by a factor of four and the sound intensity level will decrease by 6db.
Why do some sounds appear more "intense" than others?
The first sound to reach the observer is the sound traveling directly from the source. Therefore, this sound will have the highest intensity. After that comes the sound that has been reflected off one surface, with slightly less intensity. Then comes the sound that has been reflected off two surfaces, three surfaces, etc.
What is reverberation?
Reverberation occurs when the sound reflects back to your ear in under 0.1 seconds after the original sound wave. There is no time delay noticed because the original sound wave is still in the listener's memory.
Watch the following animation to see the difference between reverberation and reflection: Reverberation vs Reflection
What is noise reduction?
Noise reduction is the process of removing all the "extra" noises that exist in an area. When listening to a concert, the audience expects to hear pure sounds without any conflicting external factors. This is why it's important to build a hall or auditorium with noise reduction in mind.
NR = 10 log (A2/A1)
where NR is the noise reduction and A is the absorption
How can you find the resonant frequency in an enclosure?
Have you ever noticed you seem to sing better in the shower?
This is because the hard and smooth surface of the shower stalls are highly reflective. This makes the reverberant sound levels higher and excites the area's natural vibrational modes. Therefore, your voice produces a rich, vibrant sound thanks to the three dimensional structure you are singing inside.
The resonant frequency is the frequency which allows a wave to oscillate with maximum amplitude.
fxyz = [(1/2)v] X [the square root of ( ((nx)(lx))^2 + ((ny)(ly))^2 + ((nz)(lz))^2 ) ]
where fxyz is the resonant frequency, v is the speed of sound (about 345 m/s), and nx/ ny/ nz are integers (01,2,3,4...)
Practice Problems:
1. A room with the dimensions 5 m X 5 m X 4 m has acoustic tile added to its ceiling. This causes the total absorption to increase from 7.10 sabians to 35.35 sabians. What is the noise reduction?
2. A shower in your home measures 0.74 m, 1.18 m, and 2.13 m. What is the lowest resonant frequency?
Continue: #REDIRECT 7-6 Quantitative Acoustical Parameters
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