Acoustics

Acoustics

Name

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:

Question 1: Part B

 dBA for the following spectra:

32 Hz	64 Hz	125 Hz	250 Hz	500 Hz	1 kHz	2 kHz	4 kHz	8 kHz
30	30	30	45	20	30	20	25	25
35	53	60	30	35	30	50	36	35
65	83	90	75	43	35	30	45	55
10	10	10	10	10	10	10	10	10

 NR for the following spectra:

32 Hz	64 Hz	125 Hz	250 Hz	500 Hz	1 kHz	2 kHz	4 kHz	8 kHz
30	30	25	35	30	45	20	35	30
30	50	65	30	35	30	50	36	35
65	83	90	75	43	35	25	40	50
10	10	10	10	10	10	10	10	10

Question 2: Part A

The composite sound reduction of two elements can be used using the weighted sound reduction index formula given below:

Where;

D = Level difference

S = Area of the test element through which the noise is transmitted

A = Total sound absorption by the element

Therefore, the composite sound reduction of the two elements can be determined by using the above formulae which can be used to the absorption factors of the two elements given by []. With the absorption factors of the two elements and the average sound pressure of the source and receiving area, the composite sound reduction can be determined for the two elements.

Question 2: Part B  

Using the explanation given above it is possible to show that the 10 dB rule works, i.e. “There is no benefit in increasing the sound reduction of the larger element more that 10 dB over the smaller element which has a lesser sound reduction.”

Let’s now consider the parameters of the two elements;

Element 1: Area = 9 m² and R_w = 40 dB

Element 2: Area = 2 m² and R_w = 30 dB

The composite sound reduction equation provided above can be used to proof that the 10 dB rule works, by computing the absorption factors for each element, which in the case of these two elements can be calculated from the given details of the two elements. The calculations for absorption factors for the two elements are shown below:

Element 1

Element 2

The above results for the absorption factors of both elements proves the 10 dB rule since their values are close despite the difference of 10 dB between the two elements.

Question 3: Part A     

Sound Reduction is given by the formula shown below:

• Glass/windows

• Brick wall

• Spandrel panels

Question 3: Part B

The plot is shown below:

Question 4

Acoustics and sustainability are the two key considerations when designing sustainable or green buildings mainly because both are positively related in terms of the selected specifications, where those of one aspect of a building i.e. acoustics are dependent of the specifications of the building’s sustainability. This is highly attributable to the fact that, acoustics which are majorly concerned with sound insulation have become a very important factor when designing sustainable buildings as our environments continue to become noisier. The reason why acoustic has become a highly influential factor is because sound insulation has a significant impact on the extents of embodied energy in a particular building, which makes it essential to clearly understand the effect of sound insulation on embodied energy levels right from the beginning of the designing of sustainable buildings.

The close connection between acoustic (sound insulation) and sustainability is how the former influences the latter on basis of embodied energy, especially because embodied energy in a particular building can be effectively reduced by re-using an existing building through refurbishments that enables acoustic specifications are fulfilled in a manner that is very cost effective. This is in most cases achieved through timely up front acoustic testing of an existing building to understand its performance prior to refurbishment. Hence, upon establishing the acoustic performance of an existing building as well as understanding its restrictions and limitations, the sustainability of the building can be significantly improved by alleviating these restrictions or limitations.

When designing sustainable buildings, acoustics have become a very significant consideration because it determines the type of materials used in the construction of the sustainable building which is an indication of how the two components i.e. sustainability and acoustics are interrelated. Therefore, early acoustic testing is in most cases carried out as an effective an method to make sure that designs of sustainable buildings are sufficient, and that there is high quality construction in which sustainability is given priority. As a result, the architects usually make decisions of the construction materials to be used in a sustainable building either lightweight or heavyweight materials based of the intended quality of acoustics (sound insulation) as well as its subsequent sustainability. For instance, high/heavy weight materials are in most cases favored for enhanced acoustics; however lightweight materials such as timber often offer better or equal acoustic performance. This means that sustainability of buildings can be improved by using lightweight construction due to their less embodied energy compared to heavyweight construction materials apart from achieving higher levels of acoustic performance do not lead to desired sustainability.

Therefore, as it is theoretically know, a ±6 dB change in acoustic performance or sound insulation is equivalent to a doubling or halving of mass of construction materials for a particular building. This implies that, from the perspective of sustainability when acoustic parameters are over specified there will be an eventual significant impact on the created waste. This indicates that acoustic performance considerations or specifications or parameters in a particular building have a direct impact on its sustainability, hence necessitating the need to always make sure that a balance has been achieved between acoustic and sustainability at the design level of a building.

Furthermore, there are various products that are used for the enhancement of the acoustic of a vented façade. For instance, glass fiber reinforced concrete (GFRC) acoustic panels are one of the products often used in constructing vented facades for enhanced acoustic performance because they not only lead to sound insulation, but also improves sustainability of the building. Mineral wool is the other product used within partitions of vented facades for the purpose of acoustic dampening within stud walls, and it results to significant enhancement in acoustic performance in addition to being sustainable and cost effective method.

Question 5

A copy of the Excel spreadsheet is attached below:

When the absorptive panels are spread throughout the room more sound will be absorbed because they will be in proximity to the source of sound leading to significant sound reduction; however when they are placed in one big area there will be less sound reduction since they will be overlying each other.

Question 6

Where;

L_w = Sound power level 10-12 w(dB)

Lp = Sound pressure level 20 mPa (dB)

D= distance from fan in meters

 dB

D = 25 meters

References

Fahy, F. and Gardonio, P. (2007), Sound and Structural Vibration: Radiation, Transmission and Response, (2^nd edition). London: Academic Press.

Lohse, D., Schmitz, B. and Versluis, M. (2001), “Snapping shrimp make flashing bubbles”. Nature , Vol. 413 Issue: 6855, pp. 477–478.

Pohlmann, K. (2010), Principles of Digital Audio, (6^th edition). New York, NY: McGraw Hill Professional. p. 336-339. 

Raichel, D. R. (2006), The Science and Applications of Acoustics, (2^nd edition). London: Springer.

Wilson, C. E. (2006), Noise Control, (Revised edition). Malabar, FL: Krieger Publishing Company.