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Acoustics and Sound Insulation: Concepts and Applications

Question:

A1 Sketch a graph to show how the sound reduction performance of a simple partition varies with frequency. Identify four characteristic zones.

A2 Concert halls often have solid panels located around the stage and above the audience. Explain the purpose of these panels and describe the benefits that these provide in terms of audience and performer experience.

(a) Explain the difference between Rw and R’w.

(b) Why might a partition not achieve the same performance once installed, as is measured in a laboratory test?

(a) Explain what occurs at the room radius.

(b) A room of dimensions 8 m x 5 m x 3 m high has an average absorption coefficient of 0.3 at 500 Hz. Calculate the room radius for an omni-directional source.

A video conference room has a small area of acoustically absorbent ceiling treatment, with a carpeted floor and plasterboard walls. One side of the room is on the building façade and has windows. There is a glazed partition directly opposite the room’s windows. Comments have been made that the room is ‘echoey’ and it is difficult to understand speech when the video conference system is used. Explain what might be causing the problems and offer suggestions of possible design solutions.

A contractor has been asked to convert an existing building into individual flats and is required to enhance the timber floors to meet the Building Regulation Approved Document Part E requirements. What sound transmission types will they need to consider and what design solutions are available to improve the acoustic performance for each?

BA 2020 Q © The Institute of Acoustics, Registered Charity Registration No 267026 Page 2 of 3 A7 Explain what the noise and vibration risks are for a new building  built directly adjacent to a railway station. Suggest design solutions that can be adopted to mitigate the risks identified.

The natural frequency of a machine on vibration isolators is 8 Hz.

(a) Assuming the damping ratio is zero, what is the transmissibility when the machine is operating at 900 rpm.

(b) If the damping ratio is 0.1, calculate the transmissibility at 1500 rpm.

Part (B) Answer any two of the following questions

Each question is worth 30 marks

B1 You are designing a 200-seat lecture theatre.

(a) Describe the main factors affecting speech intelligibility. [8 marks]

(b) Explain how you would select and arrange absorbing and reflecting room finishes and the seating to optimize the acoustic conditions within the theatre. [8 marks]

(c) Briefly explain an appropriate way of arranging the loudspeakers of a speech  reinforcement system and when such a system might be needed [6 marks]

(d) For the absorptive finishes the architect is proposing to use perforated timber panels  which have an open area of 25% and are 6 mm thick. They are mounted on 50 mm thick timber battens which form a cavity behind the panels and mineral wool fills the cavity (density 33 kg/m3  ). In each case, if all other factors remain the same,  describe what effect the following would have on the variation in absorption with frequency of the panels:

(i) Reducing the open area to 10%,

(ii) Increasing the perforated timber panel thickness to 15 mm,

(iii) Increasing the thickness of the timber battens and void to 120 mm,

(iv) Increasing the density of the mineral wool to 100 kg/m3

B2 Five measures of acoustical performance are set out below. In each case describe what the measure means, how it is used and a range of typical values for a situation or building component of your choice.

(a) DnT,w [6 marks]

(b) NR [6 marks]

(c) Dnew [6 marks]

(d) DLw [6 marks]

(e) aw [6 marks]

BA 2020 Q © The Institute of Acoustics, Registered Charity Registration No 267026 Page 3 of 3 B3 An air handling unit, which provides ventilation to the bedrooms, is located in a plant room adjacent to one of the bedrooms in a hotel.

(a) List five transmission routes that need to be considered to control the plant noise  into the bedrooms.

(b) The ventilation grill opening into the bedroom has a sound power of 58 dB LwA due to the ventilation fan. Assume that Q = 2, the room dimensions are 3 m x 4 m x 2.2 m  (high) and the reverberation time in the room is 0.5 s. Predict the noise level at the  bed location, which is 3 m from the ventilation grille and calculate the insertion loss  required for a system attenuator to achieve 25 dBA at the bed location. [9 marks]

(c) Explain why moving the grille further from the bed would not reduce the noise level  at the bed location? [3 marks]

(d) Describe two types of silencer which can be installed in ventilation ductwork and  explain which is likely to be more suitable for this situation. [4 marks]

(e) When the plant is operating with the correct attenuator installed, the measured noise level at the bed location is 35 dB LAeq,T. List three possible explanations and briefly describe how they could be investigated on site. [9 marks]

(a)Each of bedrooms in a new block of flats occupies an area in the building’s façade which is 3 m wide and 2.5 m high and incorporates a 1 m wide and 1.8 m high  window. Calculate the composite Rw of the façade when the Rw of the window is 29 dB and that of the remaining façade area is 45 dB. [4 marks]

(b) The rooms are to be ventilated with a simple unattenuated slot vent of area 0.005 m2 and during warm weather the windows can be opened to an area of 0.5 m2

Calculate the composite sound reduction with the slot vent fully open and secondly with the window open. [5 marks]

(c) The proposed glazing consists of two panes of 6 mm glass separated by 16 mm to form a double-glazed unit. Describe three modifications to the proposed glazing which would improve its sound insulation and briefly explain why the sound insulation is improved in each case. [9 marks]

(d) With the aid of annotated sketches, describe three wall types from Approved Document E (ADE) which could be used to form the separating walls between the flats and explain how each component contributes to the overall sound insulation performance

 

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