Chapter 7, Partitions, Enclosures and Barriers Video Solutions, Noise Control: From Concept to Application

Problem 1

(a) Using Figure 7.12, estimate and tabulate the sound Transmission Loss in $\mathrm{dB}$ for each $1 / 3$-octave band from $400 \mathrm{~Hz}$ to $4000 \mathrm{~Hz}$, for a double wall made up of one mild steel panel, $1 \mathrm{~mm}$ thick, and one gypsum board panel $16 \mathrm{~mm}$ thick, fixed to $100 \mathrm{~mm}$ deep studs, placed $600 \mathrm{~mm}$ apart, with a $50 \mathrm{~mm}$ thick blanket of acoustic material in the cavity. Assume that the panels are line supported. Use a loss factor of 0.1 for the gypsum board and 0.02 for the steel panel.
(b) Calculate the sound Transmission Loss for the $500 \mathrm{~Hz}$ OCTAVE band.
(c) If the wall were fitted with a door having a sound Transmission Loss of $20 \mathrm{~dB}$ in the $500 \mathrm{~Hz}$ 1/3-octave band, calculate the overall $500 \mathrm{~Hz}, 1 / 3$-octave band Transmission Loss of the wall and door combined if the wall area (including the door) is $10 \mathrm{~m}^2$ and the door area is $2 \mathrm{~m}^2$.

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03:35

Problem 2

The sound Transmission Loss of the wall of an enclosure containing electric motors (of total power $80 \mathrm{~kW}$ ) and hydraulic pumps is $30 \mathrm{~dB}$ in the $500 \mathrm{~Hz}$ 1/3-octave band. Your task is to design a cooling system that cools the motors without increasing the noise external to the enclosure and resulting in a maximum temperature rise in the enclosure of $5^{\circ} \mathrm{C}$. Calculate the required Insertion Loss of the mufflers directing air into and out of the enclosure, calculate the cooling air flow rate and indicate where the cooling fan should be installed in relation to the air inlet and exhaust ducts.

Dading Chen

Numerade Educator

Problem 3

For Additional Problem 5, in Chapter 4, calculate the attenuation in the $500 \mathrm{~Hz} \mathrm{1} / 3$ octave band due to a $4 \mathrm{~m}$ high barrier, placed along the edge of the freeway and 2.5 $\mathrm{m}$ from the sound source. You may assume that there are no losses due to ground reflection on the freeway side of the barrier and that the reflection loss, $A_{r f}$, on the community side of the barrier is $3 \mathrm{~dB}$. You may ignore any other excess attenuation effects.

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02:00

Problem 4

A panel of area $10 \mathrm{~m}^2$ is placed between two reverberant test rooms and broadband noise is introduced into one room (the source room). Average sound pressure levels are measured in both rooms and the difference between rooms is $31 \mathrm{~dB}$ in the $500 \mathrm{~Hz}$ $1 / 3$-octave band. The reverberation time in the receiver room in the $500 \mathrm{~Hz} 1 / 3$-octave band is 2.1 seconds and the dimensions of the receiver room are $6 \mathrm{~m} \times 4 \mathrm{~m} \times 3 \mathrm{~m}$ high. Calculate the sound Transmission Loss of the panel in the $500 \mathrm{~Hz} \mathrm{1} / 3$-octave band.

Ankit Gupta

Numerade Educator

Problem 5

(a) For Additional Problem 15 in Chapter 6, calculate the sound power transmitted through the window if the normal incidence Transmission Loss of the window is $20 \mathrm{~dB}$ in the $500 \mathrm{~Hz} 1 / 3$-octave band and all the sound energy were contained in that band.
(b) What would be the sound pressure level at a distance of $1 \mathrm{~m}$ from the window (normal to the centre of the window), assuming that the enclosure is mounted outdoors on a concrete slab that extends $6 \mathrm{~m}$ from the enclosure walls? State any assumptions you make.

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Problem 6

An outdoor sound source is causing a nuisance to a residence located a distance of 10 $\mathrm{m}$ from the source. It is proposed to place a $30 \mathrm{~m}$ long and $4 \mathrm{~m}$ high barrier between the sound source and the residence, at a distance of $2 \mathrm{~m}$ from the sound source. The sound source is $1.5 \mathrm{~m}$ above the ground and it is mounted on a concrete slab. You may assume that $100 \%$ of any sound incident on the ground on the residence side of the barrier is absorbed by the ground so there are no reflected waves on that side of the barrier.
(a) Calculate the noise reduction that you could expect from this barrier for the $500 \mathrm{~Hz}$ octave band at a residence location $1.5 \mathrm{~m}$ above the ground. Assume that in the absence of the barrier, the ground-reflected wave is reflected from the concrete slab. State any assumptions you make.
(b) If the sound source were a line of traffic and you could assume that sound refracted around the ends of the barrier was negligible, what noise reduction would you expect for the same configuration as part (a).

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Problem 7

The sound pressure level generated in the $1000 \mathrm{~Hz}$ octave band by cars driving along a straight section of highway is $55 \mathrm{~dB}$ re $20 \mu \mathrm{Pa}$ ( $L_{\text {eq }}$ ) at an observer location in the community that is $41 \mathrm{~m}$ from the closest point on the nearest edge of the road. It is proposed to erect a $3 \mathrm{~m}$ high noise barrier along the road, $3 \mathrm{~m}$ from the edge of the road. The ground between the road and observer is grass-covered and no other obstacles between the observer and road exist. Calculate the expected sound pressure level at the observer location after erection of the barrier. Use a source height of 0.5 $\mathrm{m}$ for the line of traffic and assume it is centred $1 \mathrm{~m}$ in from the edge of the road. Assume an observer height of $1.5 \mathrm{~m}$. Also assume that when the barrier is in place, the ground-reflected paths can be ignored and only the single path over the barrier top need be considered. However, the ground-reflected path must be included when the barrier is absent. State any other assumptions that you make. [Hint: use the plane wave reflection coefficient method to estimate the ground effect].

Victor Salazar

Numerade Educator