Acoustics

Main Assignment Questions (90% of unit weighting)
AR50345
The objective of the course work is to demonstrate an understanding of the acoustic
principles discussed during the unit week. This can be done in the form of mathematical
analysis, an essay style answer or as bullet points. Marks against each section are
indicated as are word lengths, as appropriate.
Q1 a) Please outline your understanding of the following acoustic
terms, (using 50 to 100 words each). Credit will be given for
each key requirement or unique feature included. Additionally,
please indicate where each of these terms are used in the field
of acoustics:
(10 marks)
 Free field
 Diffused field
 dB
 dBA
 NR
 W-weighting
 Rw
 R
0
w
(apparent sound reduction)
 Dw
 Dn
T
w
b) Please provide both a hand calculation and excel spread sheet
calculating the following.
(15 marks)
dBA for the following spectra:
32Hz 64Hz 125Hz 250Hz 500Hz 1kHz 2kHz 4kHz 8kHz
30 30 30 – – – – – — – – 30 35 36 50 36 35
65 83 90 75 43 35 – – — 10 10 10 10 10 10 10 -NR:
32Hz 64Hz 125Hz 250Hz 500Hz 1kHz 2kHz 4kHz 8kHz
30 30 30 – – – – – — – – 30 35 36 50 36 35
65 83 90 75 43 35 – – — 10 10 10 10 10 10 10 -Rw
(spread sheet only) for the following spectra:
125Hz 250Hz 500Hz 1kHz 2kHz
16 24 33 42 43
39 44 56 67 74
38 37 48 64 78
34 39 51 60 68
2 of 4 AR50345
Q2 a) Please explain, or undertake mathematically, how one determines
the composite sound reduction of the following two elements. The
area of element 1 should be taken as 9m
2
with an Rw
of 40dB.
The second element has an area of 2m
2
and an R
w
of 30dB.
(5 marks)
b) Use the above explanation to show how the 10dBrule works, i.e.
“There is no benefit in increasing the sound reduction of the
larger element more than 10 dBover the smaller element which
has a lesser sound reduction.”
(5 marks)
Q3 a) Calculate the sound reduction of a facade comprising of: (6 marks)
 10m
2
of glass/windows
 20m
2
of brickwork and a 5m
2
spandrel panel
The area of the room should be taken as 200m
2
. The
reverberation time in this room is equal to 0.7 seconds. Please
state the sound reduction used for each element making up the
facade and the source of this information.
b) Repeat this exercise but now include a plot of the combined
sound reduction when an open window is placed in the facade.
This window has an effective open area of 1m by 0.01m; this
window is then further opened in increments to a final effective
open area of 1mby 0.25m. The area of the window should be
taken out of the glazed element within the facade.
(4 marks)
Q4 Sustainability and Acoustics. (20 marks)
Acoustics and sustainability are not often referred to in tandem.
On the other hand, it has been shown that there are many links
between these two subjects. Please provide your thoughts with
respect to the links between acoustics and sustainable building
design.
Please also provide a review of products which are available to
enhance the acoustic performance of a vented facade. This
information can also be complemented with acoustics details
relating to cross ventilation. (500 words)
3 of 4 AR50345
Q5 Reverberation time calculation. (15 marks)
Compose an Excel spreadsheet that calculates the absorption
coefficient of the sound absorbing panels in the East Building
classroom from 125 Hz to 4000 Hz (octave bands). To do this you
should use the test data from the practical session for the empty
room (with tables and chairs removed). This data is as follows:
Octave Band Centre Frequency, Hz
125 250 500 1000 2000 4000
Measured Average RT Empty
(Seconds)
0.9 0.8 0.9 0.7 0.7 0.6
The room dimensions may be taken as 13.2W x 5.2D x 3.2H.
There are 65 absorptive panels each with an area of 0.45mx
0.6m. You will need to estimate the absorption coefficients for the
other surfaces (e.g. carpet, concrete soffit, glass facade, studwork
walls and doors and use these to calculate the absorption
coefficient of the panels. What effect would distributing the panels
throughout the room have compared with locating the panels in
one large area?
Q6 A building has two items of plant on the roof, (a kitchen extract fan
and a kitchen supply fan). The sound power levels of the units are
as follows:
(20 marks)
Frequency 63Hz 125Hz 250Hz 500Hz 1kHz 2kHz 4kHz
Kitchen Extract 84 85 83 80 78 76 72
Kitchen Supply 76 78 78 75 73 70 68
Calculate the A-weightedsound pressurelevel (dBA) at the
nearest house, which is approximately 25mfrom the plant. You
may assume that the kitchen extract fan is screened by a parapet
that just obscures the line of sight. The kitchen supply fan is
un-screened. You should clearly identify each step in your
calculation or submit a spreadsheet.

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