Calculating Noise Levels Inside A Room: Assignment

Assignment Aim

The assignment is concerned with calculating the noise levels inside a rooms. This involves repetitive calculations and it is recommended that you use a spreadsheet in order to undertake these calculations. You should also look to incorporate the data from the CDROM in order to help your calculations. However,please not that it is vital that the logic of your technical workings can be easily followed, therefore it should be accompanied by a brief and clear explanation of what you have done.

In Question 2 of this assignment, you will be asked to make use of the general knowledge which you have gained in the Acoustics part of the module to advise on some aspects regarding the acoustics design. You should present your discussion in the form of a short executive report, including diagrams where appropriate. In the context of this assignment, the people who will consider the findings of your report have no other background data and are not experts in environmental design. You will therefore need to write your answer as a carefully structured explanation which leads up to each of the key technical points. Do not forget to acknowledge the source of information provided in the report where appropriate.

It is intended that this assignment should take around eight hours to complete. If you have done the spreadsheet examples in the text, the calculations should not take more than four hours. Planning and writing the report (Question 2) should take up to four hours.

A new crematorium is proposed for the town of Eastmouth on the South Coast of England. A shortage of land has dictated that a site only 45 m from a residential housing estate is to be used. The aim of Question 1 in this assignment is to establish the expected sound pressure levels within the chapel in the crematorium.The aim of Question 2 is to investigate and discuss whether noise from the crematorium will be of concern to residents in the housing estate and to recommend any remedial action if necessary.

Inside the new crematorium, the noise level in the chapel is considered to be a critical factor. Accordingly, the reverberant sound pressure level in the chapel is to be limited to NR35. There are two significant noise sources in the chapel: (i) a ventilation system driven by a mixed flow fan; and (ii) a cremator furnace in an adjacent plant room (called here the “furnace room”). The ventilation system for the chapel is shown in Fig 1 (see end of assignment). In addition to serving the chapel, the ventilation system also serves a waiting room and an office. There are three bends in the system; each bend is a mitre bend that is unlined. Each outlet from the ventilation system terminates in the centre of the ceiling of the room into which it supplies air.

Using the data supplied below
a) Establish the reverberant sound pressure level in the chapel due to the ventilation system acting on its own (i.e. with the furnace installation turned off)
b) Establish the reverberant sound pressure level in the chapel with the ventilation system working and the furnace installation turned on (i.e. you need to account for the noise from the furnace room when estimating a final value for the sound pressure level in the chapel).
c) Establish whether the sound pressure level in the chapel meets the NR35 design requirement. Discuss any issues or problems with the design; is any further action required, if so, what action would you take? Note that you have two
(identical) acoustic attenuators at your disposal – these attenuators may be placed within the ducting system to attenuate the sound power emitted by the fan. The characteristics of the attenuator are listed below.

You have been retained by the local authority to prepare a report on any potential noise problems at the Eastmouth Crematorium. In the report you should examine all potential sources of noise problems, as well as discussing any remedial action that may be necessary. You should also discuss any general utility advice suitable for the ventilation installation.

A new crematorium is proposed for the town of East mouth on the South Coast of England. A shortage of land has dictated that a site only 45 m from a residential housing estate is to be used. The aim of the first portion of this project assignment is to establish the expected sound pressure levels within the chapel in the crematorium. Secondly, the second portion of the project will be to  investigate and discuss whether noise from the crematorium will be of concern to residents in the housing estate and to recommend any remedial action if necessaryInside the new crematorium, the noise level in the chapel is considered to be a critical factor. Accordingly, the reverberant sound pressure level in the chapel is to be limited to NR35. There are two significant noise sources in the chapel:

• A ventilation system driven by a mixed flow fan; and
• A cremator furnace in an adjacent plant room (called here the “furnace room”).

The ventilation system for the chapel is shown in Fig 1 (see end of assignment). In addition to serving the chapel, the ventilation system also serves a waiting room and an office. There are three bends in the system; each bend is a mitre bend that is unlined. Each outlet from the ventilation system terminates in the centre of the ceiling of the room into which it supplies air.

In this case, we shall trace the noise transmission path right from the source through various channels, corners and bends until the designated point. Then, we shall do algebraic deduction to have net noise pressure level as the final result. Where acoustic material characteristic value is not provided then we shall assume some values for the same. In the meantime, this question excludes consideration for the furnace as it is turned off hence no calculation is performed from its noise contribution: Firstly to start calculating the ventilation system SPL in the chapel, we have to assume that this source -Fan-is a Point Source, and has a mixed flow function type by referring to the information quoted from the assignment paper.

1. Background noise sources are not included since they are negligibly small and may not really affect the final value to be obtained; it is about 2.5 dB for the background noise.
2. We will only use the fan SWL values provided. It is not necessary to calculate the fan’s actual SWL as the fan’s characteristics values are not sufficient

Therefore total attenuation will be derived from these individual components of the ventilation system:

(i) The straight ducting runs

(ii) The bends (there are about 3 bends in the system)

(iii) At the contractions and expansions of the ducting

(iv) The branching effects in the ducting (there are about 3)

(v) The 4 terminations at the centre of the designated room units.

(vi) The miscellaneous attenuation due to materials that are cannot clearly be seen but they do have an impact on the noise level

Problem Description

(v) The atmospheric portion of the system as we also need to consider the surrounding air attenuation level

Notably, however, we once again emphasize that:

• The system under consideration, if keenly checked, one realizes that there is only one exit leading out of the chapel hence additional outlet attenuations may be ignored accordingly.
• Since the straight lining duct is 550mm, there will be no point in considering the effects of the lining unless the ducting length was equal or above 850mm.
• Fan does not serve path A-B which is actually opposite to the flow hence we neglect the attenuations due in this path.

   This is the given information and from it, based on the identified paths, we determine the various attenuation levels for each path as follows:

Octave band                                             centre frequency (HZ)	125	250	500	1K	2K	4K
SWL of fan (dB)	94	99	93	85	74	68

Firstly, attenuation for paths BE and EJ are determined as provided in the layout sketch. From now henceforth BE is designated primary path and EJ is the secondary path. Notably, some of these sound characteristics are deduced from the standard tables provided in the excel files:

Now, considering the attenuation due to bends and note that we are only considering 2 bends (are identical) as the other is not in the active path under consideration:Overall bends attenuation

Octave band                                             centre frequency (HZ)	No. of bends	125	250	500	1K	2K	4K
Mitre bend unlined                             width (300-400) mm	1	0	2	8	5	3	3
Overall Bends Attenuation	2	0	4	16	10	6	6

Next, we consider attenuation due to branching hence:Attenuation due to Branching

Octave band                                             centre frequency (HZ)	125	250	500	1K	2K	4K
Branch C-D  (350x350)	2.1	2.1	2.1	2.1	2.1	2.1
Branch E-J   (400x400)	2.6	2.6	2.6	2.6	2.6	2.6
Overall attenuation due to 2 branches	4.7	4.7	4.7	4.7	4.7	4.7

There is also contribution from the terminating points hence:Attenuation due to grille

Octave band                                             centre frequency (HZ)	Grill Area	125	250	500	1K	2K	4K
Area of terminal grille D	0.1225 m²	5.5	2.5	0	0	0	0
Area of terminal grille J	0.16 m²	5.5	2.5	0	0	0	0

Therefore, we can now easily determine the overall attenuations and the net noise pressure level as illustrated in the table :Ventilation SWL

Octave band                                             centre frequency (HZ)	125	250	500	1K	2K	4K
SWL of fan (dB)	94	99	93	85	74	68
Straight duct attenuation	22.44	15.06	9.52	7.12	5.28	5.28
attenuation due to 2 branches	4.7	4.7	4.7	4.7	4.7	4.7
Bend attenuations	0	4	16	10	6	6
total grille attenuation	11	5	0	0	0	0
final ventilation SWL(dB)	55.86	70.24	62.78	63.18	58.02	52.02

Now, let us consider the chapel, where absorption coefficient is determined:

We calculate the average Absorption Coefficient and is given by:

Average Absorption Coefficient = (?1.S1+ ?2.S2+…. + ?n.Sn) / ∑Sn

?: Material Absorption Coefficient.

S: Area of a specified surface

Octave band centre frequency		125	250	500	1K	2K	4K
Floor	120	0.020	0.040	0.050	0.050	0.100	0.050
Ceiling	120	0.150	0.350	0.650	0.800	0.750	0.700
2 walls (15x4)m :Plaster on solid backing -25% glazing -6mm glass	90	0.03	0.03	0.02	0.03	0.04	0.05
	30	0.1	0.07	0.04	0.03	0.02	0.02
1 wall (8x4)m	32	0.310	0.330	0.140	0.100	0.100	0.120
(8x4)m	32	0.050	0.040	0.020	0.040	0.050	0.050
25 SEATS  upholstered	25	0.450	0.600	0.730	0.800	0.750	0.640
Occupants	20	0.330	0.400	0.440	0.450	0.450	0.450
Average Absorption coefficient		0.131	0.193	0.280	0.328	0.327	0.296

Next, we also need to calculate the chapel room constant (Rc) hence:

Rc= ?.s/(1-  ? ) 

Now, we calculate the SPL within the chapel with the fan and ventilation considered only while the furnace is switched off and this is given by:

Reverb SPL =SWL+10*log [ (Q/4πr²)+(4/Rc) ]   

But directivity Index which is related to the direct component is neglected hence:

Rev SPL =SWL+10*log (4/Rc)   

Finding the matching NR to this SPL :

NR: Critical Noise; SPL=α+(β.NR)     α,β :constants

These constants are retrieved from the excel sheets attached in the assignment:

Table 9: Constants α and β values

The NR value obtained finally, as can be seen is far above the minimum requirements of NR 35

1. Establishing the reverberant sound pressure level in the chapel with the ventilation system working and the furnace installation turned on (i.e. we need to account for the noise from the furnace room when estimating a final value for the sound pressure level in the chapel).

The same procedure is implemented as part a but now including contribution from the furnace:

Firstly, calculating Average Absorption Coefficient, it results in:

Octave band  centre frequency (HZ)		125	250		500		1K	2K	4K
48	Concrete	0.02	0.02		0.02		0.04	0.05	0.05
48	Concrete	0.02	0.02		0.02		0.04	0.05	0.05
7.2	Glazing (15%)	0.1		0.07		0.04	0.03	0.02	0.02
40.8	Brickwork painted (85%)	0.05		0.04		0.02	0.04	0.05	0.05
4.8	Glazing (15%)	0.1		0.07		0.04	0.03	0.02	0.02
27.2	Brickwork painted (85%)	0.05		0.04		0.02	0.04	0.05	0.05
32	Brickwork painted	0.05		0.04		0.02	0.04	0.05	0.05
Average Absorption Coefficient		0.047		0.043		0.033	0.0531	0.0612	0.0596

Next, we calculate furnace room constant (Rc). 

Therefore, the SPL inside the chapel with the effects of the furnace -switch on is determined and the results tabulated:

Recall that we exclude consideration of partition since our termination point does not go beyond the chapel room:

Ventilation System in Chapel

Now get to find the Average Sound Reduction Index (R) of the Partition between the rooms.

Rav=10*log(1/τ1)

Rav: Average Sound Reduction.

Τ: Overall Transmission coefficient.

In turn T=(A1*10^(-R1/10)+A2*10^(-R2/10)) / Total Partition Area

R1: Transmission coefficient of the plastered single leaf brick partition.

R2: Transmission coefficient of the partition’s door.

Partition transmission coefficient	125	250	500	1K	2K	4K
plastered single leaf brick m2	44	43	49	57	66	70
Door	17	21	26	29	31	34
overall transmission  coefficient	0.0199	0.007	0.002	0.001	0.0008	0.0004
Average Reduction Index	16.991	20.97	25.97	28.99	30.99	33.99

 SPL chapel= SPL furnace - Rav+10*log (S) – 10*log (Rc) chapel

Reverberant spl due to furnace room-without partition effect-	81.898	82.30	85.40	85.34	62.69	52.815
Average Reduction Index	16.9913	20.972	25.978	28.993	30.999	33.998
Rc	63.819	101.5	164.5	206.9	206.2	178.3
Reverberant SPL in chapel due to furnace noise	47.110	42.01	37.86	33.39	8.593	-3.663

 Now finally to calculate the SPL in the chapel with the effects of the furnace -switch on- along with the ventilation system using the equation:

 SPL total = 10*log(10^(spl fan/10)+10^(spl furnace/10))

Table 15: Total SPL due to ducting and furnace

Reverberant SPL in chapel due to furnace noise [dB]	47.110	42.019	37.866	33.399	8.593	-3.663
ventilation spl	43.831	56.197	46.637	46.041	40.897	35.528
total spl(duct+furnace)	48.783	56.360	47.178	46.272	40.899	35.529

Finding the matching NR to this SPL :

NR: Critical Noise.

SPL=α+(β.NR

α,β :constants

Table 16: NR value

Again NR is above the nominal NR , so just to go and find solutions as per the given data and materials , let us try the silencers and calculate their effects

1. Establish whether the sound pressure level in the chapel meets the NR35 design requirement. Discuss any issues or problems with the design; is any further action required, if so, what action would you take? Note that you have two (identical) acoustic attenuators at your disposal – these attenuators may be placed within the ducting system to attenuate the sound power emitted by the fan. The characteristics of the attenuator are listed below.

The concrete allows for more reverberant sound pressure to be generated. A soft absorbing material needs to be aligned against the inner part of the walls. There should also be sound erection barriers such that noise coming from where the fan is located is minimized. Besides, noise from outside should be minimized by having the erection barriers taller so that the dispersion of sound wave would occur and therefore reduce its intensity.

Now, we will integrate the silencers in our calculation and check the effects on the net sound reduction levels hence firstly, we begin with the first silencer:

Ventilation SPL	43.831	56.197	46.637	46.042	40.896	35.528
Attenuator	12	25	38	39	23	16
SWL with 1 silencer	31.831	31.197	8.637	7.041	17.896	19.528
Furnace SPL	47.110	42.019	37.866	33.399	8.593	-3.663
Total with 1 silencer	47.118	42.364	37.871	33.409	18.379	19.549
NR Octave band	28.871	32.650	33.954	33.409	21.556	25.023

We can clearly see that the NR value obtained in this case is slightly below the minimum required which is NR 35 hence that is okay however, we need to check with two silencers since there are noise sources which are hidden and yet they also contribute (and were not considered in the system).

Hence we check the effect with 2 silencers in place:

Ventilation SPL	43.831	56.197	46.637	46.042	40.896	35.528
Attenuator	24	50	76	78	46	32
SWL with 2 silencer	19.831	6.197	-29.36	-31.95	-5.10	3.528
Furnace SPL	47.110	42.019	37.866	33.399	8.593	-3.663
Total with 2 silencer	47.118	42.020	37.866	33.399	8.775	4.287
NR Octave Band	28.871	32.280	33.948	33.399	12.093	10.134

We can see, however, from the table, that the system seems to be responding well with only one silencer. Besides, it will not make any economic sense to have two silencers in place yet one can still achieve the reduction level that two could do.You have been retained by the local authority to prepare a report on any potential noise problems at the East mouth Crematorium. In the report you should examine all potential sources of noise problems, as well as discussing any remedial action that may be necessary. You should also discuss any general utility advice suitable for the ventilation installation.

Calculating Sound Pressure Levels

Area of terminal grille A = 0.2500 m2.

Area of terminal grille D = 0.1225 m2.  

Area of terminal grille G = 0.1925 m2.

Area of terminal grille J = 0.1600 m2.und Attenuator (or Silencer)
Octave band centre frequency (Hz):	125	250	500	1k	2k	4k
Attenuation delivered (dB)	12	25	38	39	23	16

Dimensions: 15 x 8 x 4 m high.

Floor: Wood blocks

Ceiling: Proprietary mineral wool fibre ceiling tiles on battens.

Walls: 15 x 4 m walls - plaster on solid backing - 25 % glazing (6 mm glass).

8 x 4 m rear wall - 12 mm wood panels on battens.

8 x 4 m partition wall between chapel and furnace – 360mm thick plastered single leaf brick.

1 x 1 m door for coffin reception located in the partition

wall – solid hardwood, normal cracks. (ignore this door for calculations using ROOM spreadsheets).

Seats: 25 (upholstered).

Occupants: 20

The dimensions of the partition wall between the chapel and the furnace are 8 metres by 4 metres high and there is a small door 1.0 m2 for reception of coffins from the chapel.

Dimensions: 6 x 8 x 4 m high.

Floor: concrete.

Ceiling: concrete.

Partition wall : 360mm brickwork painted (no plaster).

Other walls: brickwork painted (no plaster) - 15 % glazing (6 mm glass).

Seats: 3 (upholstered).

Occupants: 1

Noise is basically unwanted or unwarranted noise that is produced when vibrating surfaces or particles are excited. For example, a guitar will produce sound when the strings are plucked. Likewise, in building and construction industry, there are sources of sound during construction and when the building is occupied. The latter is considered in this case such that various machineries such as pumps, blowers, fans among other produce lots of noise when in operation. This is a great concern to the occupants’ wellbeing and comfort. Therefore, the section hereinafter dwells on the possible causes of noise in such situation and the mitigation factors are discussed. Besides, we explore the findings as a result of the walk down that we did within the crematorium building.

Noise in buildings, as mentioned earlier, can be produced from the building services machineries such as the HVAC system, pumps and fans among others. Additionally, there is a portion of noise that penetrates the building from outside. This can be due to the passing-by cars, passengers and aero planes. Besides, if building is located in a very windy place, then noise from vibrating window panes due to wind blowing towards it can also be a source of noise in buildings. Lastly, activities of occupants within the building can also be a potential source of noise problems.

Attenuation Levels

Notably, we performed a walk down on the entire crematorium building to get, from firsthand, the noise problems that have been theorized before. A number of acoustic tests were performed using sound level mitre-analyzer; the building had to be vacated so that the real test results could be arrived at. Hereinafter, we present our findings along with the possible remedial actions that need to be applied in order to limit the noise levels in the building in future:

The major sources of noise in the building are the ventilation system where the mechanical mixed flow fan drives the system and a furnace located in the adjacent room to the chapel. Admittedly, currently, the fan operates almost 24 hours per day during summer season.

It was also found out that when furnace is switched off, the NR (Noise Level) still becomes maximized due to the ventilation sound pressure level (SPL).

Table 19: NR actual values

The average NR is still above the maximum NR requirement of NR 35

However, when we introduced one silencer, the following were the results

When we turned the furnace on and the silencer isolated, the following results were obtained:

Reverberant SPL in chapel due to furnace noise [dB]	47.110	42.019	37.866	33.399	8.593	-3.663
ventilation spl	43.831	56.197	46.637	46.041	40.897	35.528
total spl(duct+furnace)	48.783	56.360	47.178	46.272	40.899	35.529
NR octave band	30.785	47.69	43.51	46.27	43.743	40.61

 Once again we see that this is way above the maximum required NR hence some adjustments in the system have to be made. But how will that be implemented? Here is the solution:

We reintroduce the silencer in the system and that will yield the following results

ventilation spl	43.831	56.197	46.637	46.042	40.896	35.528
attenuator	12	25	38	39	23	16
Swl with 1 silencer	31.831	31.197	8.637	7.041	17.896	19.528
furnace spl	47.110	42.019	37.866	33.399	8.593	-3.663
total with 1 silencer	47.118	42.364	37.871	33.409	18.379	19.549
NROctave band	28.871	32.650	33.954	33.409	21.556	25.023

Now we are somehow safe than before as far as the maximum required NR is concerned, that is: NR 35; but even this attenuation performance can further be improved by doing some design changes as mentioned in the next paragraphs.

There are a number of actions that have been used to mitigate the problem of noise in buildings. Basically, they are normally grouped into three categories: (i) Reduction at the source (ii) Reduction during transmission (iii) Reduction at the destination

In this case, both administrative and engineering solutions are applied to reduce the noise levels at the source. In engineering solutions, the machinery can be encased in a soundproofing barrier so that less noise penetrates the material by absorbing most of it. Secondly, there are technologically innovations that can completely cancel the noise, although these are expensive to implement; however, it works in a manner such that there is an opposing sound wave being introduced into the sound pressure field and due to interference the sound wave is deadened. Notably, if noise reduction is efficiently done at the source then there will be effective reduction in noise levels. However, because of the transitive nature of sound, for instance, the ducting of HVAC system components, there is need to also institute measures to reduce the noise during transmission.

Report on Noise Problems at Eastmouth Crematorium

The cheapest method is to use the super sound absorptive material that is often lined inside the ducts. However, this can prove quite expensive hence design of HVAC ducting must also consider the noise reduction strategies. Besides, design engineers nowadays isolate the noisy rooms from the less noisy ones. For example, in  a production shop where machines are being used; the offices can be isolated from the main factory line; however, in some cases, the offices and the shop floor are normally demarcated using an erection barrier than can potentially disperse sound as it crosses over hence reducing its noise level.

It is imperative that the HVAC design engineer must liaise with the acoustics technician to establish the requirements as far as general acoustics is concerned. There are techniques at the HVAC design stage, that if implemented can be a boost to the noise reduction efforts without necessarily incurring costs of redesign to include acoustics. Additionally, operations of this noise-producing machinery should be checked such that they are switched on only when necessary. Besides, isolate very noise machineries from less noisy areas to limit on the impacts.

It has been established through numerous acoustic tests in such scenarios that another cause for the noise from these machineries is poor maintenance. The vibrating parts of the machinery normally occur due to loosening of the parts. It is crucial for the maintenance technician to perform regular checkups on the machinery to ensure that the machinery is in proper operating condition. Vibrating parts normally produce a lot of noise. Besides, the fan needs to be controlled in an automatic fashion where it will be demand-driven so that during peak times, the fans operates at maximum capacity while its performance can easily be limited during low demand situations during the day and night.

This has proven to be inexpensive way to boost the attenuation performance in such buildings. The walls, the furniture and even the carpets need to take part in attenuating the noise levels in the building. In fact they have proven to be effective in absorption of background noise sources hence leading to better acoustic performance. The chapel room could still be experiencing reverberations challenges; however, the sound absorbing erection walls can limit this deficiency and ensure excellent performance.  

Conclusion 

This report has briefly looked at the noise problems; their causes and proposed solutions. Basically, engineering controls and administrative actions must be in harmony to ensure maximum noise attenuation is achieved. However, from the foregoing, we recommend that the silencers should be removed as they do not necessarily take part in active attenuation. In fact, as mentioned earlier, if the absorptive lining within the ducting system can be improved, then a great portion of the noise will be attenuated during transmission such that it will substantively be reduced at the terminating point within the chapel room.