DEGREE PROJECT IN VEHICLE ENGINEERING, FIRST LEVEL, 15.0 CREDITS
STOCKHOLM, SWEDEN 2019
Noise reduction in air ducts
Jimmy Söderberg and Lucas Westin
Abstract
The bachelor thesis report in the field of sound and vibrations aims to see the behaviour of sound reduction within low frequencies and see the tipping point for breakout noise, leakage of sound through the material. The focus is on whether or not there is saturation in noise reduction, coming from theory, of 6 dB per meter of silencer below 250 Hz.1The silencer used in this project is the CLA-A-250 from Swegon. Measurements are made in the MWL laboratory at KTH. The project arose from the company LN Akustikmiljö as they experienced different behaviour during projects of their own. As can be seen in results, the saturation of 6 dB per meter of silencer does not apply and is not a correct assumption. Regarding the breakout noise, it is clear that as long as the duct structure and its outlet are in separate rooms it is not a problem. This conclusion is safe to make according to standards and the margins considered in the calculations.2The project served as great insight in real life problems and applications of an acoustician.
Keywords
Noise, Noise reduction, air duct, breakout, bachelor thesis
Abstract
Denna rapport för kandidatexamensarbete inom ljud och vibrationer riktar sig åt att undersöka beteendet av ljudreduktion för låga frekvenser och se när brytpunkten för så kallat breakout noise uppstår. Fokus ligger kring huruvida det finns en mättnad i ljudreduktion, enligt teori, på 6 dB per meter av ljuddämpare under 250 Hz.1 Ljuddämparen som används i projektet är CLA-A-250 från Swegon. Projektet kom till liv från företaget LN Akustikmiljö då de upplevde ett annat dämpningsbeteende än just det. Mätningarna för projektet tog plats i MWL-laboratoriet på KTH. Som kan åskådas i resultatet visar det sig att antagendet om 6 dB dämpning per meter av ljuddämpare är felaktigt. Gällande breakout noise så är det tydligt att så länge kanalstrukturen och dess utlopp är i skilda rum så är det inte ett problem. Dessa slutsater är säkra att göra på grund av användandet av standarder och marginaler som togs hänsyn till i beräkningar och mätningar.2 Projektet var en bra insikt i hur en period i yrkeslivet kan se ut för en akustiker.
Nyckelord
Ljud, Buller, ventilationskanal, Ljuddämpare, kandidatexamensarbete
Authors
Jimmy Söderberg Lucas Westin
KTH Royal Institute of Technology
Place for Project
LN Akustikmiljö Stockholm, Sweden
Examiner
Hans Bodén
Marcus Wallenberg Laboratory KTH Royal Institute of Technology
Supervisors
Leping Feng
Marcus Wallenberg Laboratory KTH Royal Institute of Technology Deniz Hadzalic
LN Akustikmiljö Stockholm, Sweden
Nomenclature
f Frequency of a signal
c Speed of sound in room temperature, 343 (m/s) λ Wavelength of a signal
p Sound pressure Lp Sound pressure level DIL Insertion loss
S Cross-sectional area
Abbreviations
KTH Kungliga Tekniska Högskolan
MWL Marcus Wallenberg Laboratory
Contents
1 Introduction 1
1.1 Background . . . . 1
1.2 Problem . . . . 3
1.3 Purpose . . . . 3
1.4 Goal . . . . 3
1.5 Methodology . . . . 3
1.6 Limitations . . . . 3
2 Theoretical Background 5 3 Method 7 3.1 Theory . . . . 7
3.2 Approach . . . . 7
4 The Work 10 5 Result 14 5.1 Outlet sound . . . . 15
5.2 Breakout noise . . . . 18
6 Assessment 19 6.1 Conclusion . . . . 19
6.2 Discussion . . . . 19
6.3 Final Words . . . . 20
References 21
1 Introduction
The bachelor thesis in the field of sound and vibrations is comprised of a degree project over a full semester.3 This report has its focus on noise reduction in a ventilation duct.
Noise is defined as unwanted sound, a sound that entails a feeling of uneasiness.4
1.1 Background
For this report it is important for the reader to understand basic concepts within the field of sound and vibration. The reader is understood to know what a decibel is, be familiar with logarithmic equations, common physical relationships and some terminology. Examples of this are
λ = c
f [m] (1)
Lp = 10· log p˜2
p2ref [dB] (2)
The knowledge background of the students comes mostly from the fundamental course in sound and vibrations.5This project came to life as one of three available at LN Akustik &
Miljö. The company and, in this case, the client felt that they could use this project both for valuable results as well as building a relation with the students and KTH. A preliminary schedule of the project and its activities can be found in appendix A.
The project, as mentioned earlier, came to life as one of three available projects from LN Akustikmiljö. They have had several projects in studios and came to wonder how the reduction of noise works in the low frequency area with connected silencers, whether there is a pattern or solid thumb rules of how to go about. A full description can be found in the appendix A, see figure A.2. Their issue was that producers of silencers only provided information of how much one silencer reduces and no complete information of how they work together.
The silencer used for this project is the CLA-A-250 from Swegon, see specifications in figure D.1 in appendix D.6
According to theory there is a saturation of sound reduction of 6 dB below 250 Hz for ducts with a diameter of 200 mm.1
Figure 1.1: Saturation for 200 mm ducts
1.2 Problem
In air ducts there can be unwanted noise from air blowing through, typically because of the velocity of the air flow, the air pressure or leakage through the duct. The noise produced can be taken care of with silencing components. The task is to examine how much of noise reduction that can be done per meter of silencer in the duct. The second key feature is the breakout noise. This type of noise is defined as transmission of sound through the walls of the duct which in lay-man terms is leakage through the material.1
1.3 Purpose
The objective is to see how noise reduction works in low frequencies when connecting several silencers and to see where the breaking point is for the breakout noise.
1.4 Goal
The bachelor thesis comes with valuable lessons because of the goals set to be acheived.
They consist of several points. Being able to plan, execute and present the project is one of them.3Another important aspect is that the students should be able to identify, analyze and discuss work ethical problems.
1.5 Methodology
Structuring the setup is key for valuable data. The plan was to make reference measurements of sound pressure in the reverberation room and in the anechoic chamber, with and without silencers in the duct. In that way sound pressures achieved from the duct could be compared with the silencers installed to easily calculate insertion loss and breakout noise levels.
1.6 Limitations
A project is defined as a specific, finite activity that produces an observable and measurable result under certain preset requirements.7Time is the most obvious limitation
as it is often set beforehand. In this case the time frame was from February to the end of May. The location for the measurements which was in MWL, is often very busy and bookings can be tricky, as was experienced. Rescheduling of the measurements did put unnecessary pressure on the students and limited the preparations for them. The project had no relevant economical restraints and as for equipment, speaker and silencers, it was brought by the client. Laboratory equipment for the measurements was given by KTH.
2 Theoretical Background
This project can be related to the muffler project that was carried out in the second year of engineering studies as the course, sound and vibration, took place. The setup was similar.
The task then was to reduce the noise coming out of an exhaust pipe. This was performed in theory in a program, and dimensions of the pipe were calculated after reaching a conclusion of what types of silencer pipes to use, e.g a Helmholtz or an expansion chamber.
The exhaust pipe was later constructed and tested to see if the result matched the theory.
This was very applicable to the thesis and made it easier to understand the given problem as they reminded of one another. The most useful part was testing it in the laboratory, getting familiar with the process and the setup required for the measurements.
The main focus for the second year project was the transmission loss, which is defined as the quotient of incident and transmitted sound power and looks like the following.5
DT L = 10· log (W¯i
W¯t )
[dB] (3)
and, for sudden changes in cross sectional area, looks like this
DT L = 10· log(S1+ S2)2
4S1S2 [dB] (4)
with S1and S2 being the cross sectional areas from the smaller section pipe to the larger.
Figure 2.1: Principle model of an expansion chamber
For the bachelor project, equation 3 applies and is the key component. Aside from that, calculations made regarding sound pressure levels and summation of them come from the early part of the course and look like the following.5
Lp = 10· log p˜2
p2ref [dB] (5)
and the addition rule along the frequency axis
Lp,tot = 10· log
∑N n=1
10Lpn/10 (6)
3 Method
3.1 Theory
In order to do measurements of the sound from the duct, the real scenario had to be simplified so standing waves and other interference would not disturb the measurements.
It was simplified so plane waves are the dominating source of sound travelling through the channel.
In order for plane waves to be dominant firstly the loudspeaker had to be at a certain distance away from the test subject.5 The second requirement was that the cut-on frequency was high enough. According to SS-EN ISO 7235:2009, the cut-on frequency could be obtained through the following equation.8
fCd = 0, 59· c
d (7)
Where c is the speed of sound and d is the diameter of the channel. That frequency was put into equation 1 to decide the wavelength. The calculated wavelength put into equation 8 resulted in the required distance from the sound source to assume plane waves.
r > λ/3 (8)
The cut-on frequency was calculated to 809, 48 Hz. This indicates that below this frequency it is safe to say that plane waves are the dominating source. According to SS- EN ISO 11691:2009 it is suggested that a transition duct should be the length of two times its own diameter.2This is shown in figure 3.1. In this case that distance was much bigger than the calculated distance from equation 8. This would imply that, even if the sound source would be a point source, plane waves would still be dominant.
3.2 Approach
The most appropriate solution to measure the sound reduction was to place a loudspeaker connected to the duct with silencers in the anechoic chamber and then have the outlet
Figure 3.1: Speaker setup
in the reverberation chamber. In the reverberation chamber, a microphone placed on a rotating stand was used in order to get both room average and time average during the measurement. An example of this setup can be seen in figure 3.2, which is case 2A.
Figure 3.2: Noise reduction setup (2 silencers)
Measurements from every setup was compared to the reference case with no silencers, which basically means one straight pipe.
The plan for breakout noise was similar to the one for measuring noise reduction. The only difference was that the silencers was placed in the reverberation room together with the microphone. With only the loudspeaker left in the anechoic chamber the sound from the loudspeaker that was not going through the duct was isolated from the breakout noise of the silencers. The total length of the duct was kept the same in order for the results to be comparable. Figure 3.3 shows an example of the breakout measurement setup, which is case 5.
Figure 3.3: breakout noise setup (1 silencer)
In total, 11 different cases was made to get both different cases for the noise reduction measurement and the breakout noise measurement. All cases are shown in appendix B.
4 The Work
First all preparations was made to ensure that all measurements was made as equal as possible. This was done by calibrating all microphones and then placing them out on a suitable place. The background noise was measured so it could be considered in the result. After that the reference case was built, which consisted of one straight duct from the loudspeaker in the anechoic chamber, to the outlet in the reverberation room. Figure 4.1 shows how the reference case for noise reduction was planned to be built in the anechoic room. Later on, case 1 which consisted of one silencer was built and measured. The same was made for all cases up until case 4, which is the last case where all silencers are in the anechoic room.
Figure 4.1: Sound reduction setup (reference)
Figure 4.2 shows case 4 with four silencers mounted to the duct in the anechoic chamber.
Figure 4.2: Sound reduction setup (Case 4)
To ensure that the duct was leveled between each case, a spirit-level was used and the height adjusters which can be sen in figure 4.2 was adjusted so the whole construction would be leveled. The load speaker was placed in the white box together with the noise generator. The noise generator was set at a fixed value so it would generate noise at the same level between every measurement. Figure 4.3 shows how the noise generator was set. As can be seen in the figure, pink noise was used.
Figure 4.3: Noise generator settings
Pink noise is a type of random signal where the intensity is inversely proportional to the frequency of the signal.9,10
Figure 4.4 shows the outlet of the duct during sound reduction measurement, and also how the rotating microphone was placed in the reverb chamber.
Figure 4.4: Sound reduction setup (Outlet)
More photos from the measurements can be found in appendix C
5 Result
To simplify the reading of the result a table of all cases is presented below.
Table 5.1: Table of all cases
Case Nr. of silencers Measurment type
Case 0 0 (Reference) Outlet sound
Case 1 1 Outlet sound
Case 2A 2 (Placed next to) Outlet sound
Case 2B 2 (Placed far away) Outlet sound
Case 3 3 Outlet sound
Case 4 4 Outlet sound
Case 5 1 (With stopper) Breakout noise
Case 6 2 (With stopper) Breakout noise
Case 7 3 (With stopper) Breakout noise
Case 8 0 (No stopper) Breakout noise
Case 9 0 (With stopper) Breakout noise
The total sound pressure level of the background noise was calculated through summation along the frequency axis, see equation 9.
Lp,tot = 10· log
∑N n=1
10Lpn/10 (9)
The data that was put into this equation was the measured sound pressure level from 10Hz to 10 kHz in 13-octave bands.
5.1 Outlet sound
Figure 5.1: Raw data of outlet sound
Figure 5.2 shows the total sound pressure level from case 0 to case 4.
Figure 5.2: Result of outlet sound
The insertion loss was calculated through the sound pressure level as a difference between case 0 and each respective case from 1 through 4. In figure 5.3 the insertion loss was calculated for every case, after a summation has been done for all frequencies through equation 9.
Figure 5.3: Insertion loss of noise reduction
Table 5.2: Table of DIL[dB] from different cases
Case 31.5 Hz 63 Hz 125 Hz 250 Hz 500 Hz
CLA-A-250 (1m) —– 6 11 15 20
Case 1 11.8 6 9.9 17.8 22.7
Case 2A 14.6 11.5 18.5 33.1 42.5
Case 2B 18.2 13.2 22.3 32.8 44.5
Case 3 18.4 16.6 27.5 43.8 49.4
Case 4 32.4 23.2 35.7 46.1 49.9
Results compared to Swegons data on the CLA-A silencer. Data for the silencer CLA-A- 250 was retrieved from Swegon.6See figure D.1 in appendix D.
In the figures 5.4 and 5.5 data from 10 to 315 Hz was used in order to take a better look on the behavior for lower frequencies.
Figure 5.4: Outlet sound from 10 to 315 [Hz]
Figure 5.5: Insertion loss from 10 to 315 [Hz]
5.2 Breakout noise
In figure 5.6 the raw data from the measurement is presented.
Figure 5.6: Raw data of breakout noise
Figure 5.7 shows the total sound pressure level from 10 to 10 000 Hz for every case, where the total sound pressure was calculated through equation 9.
Figure 5.7: Result of breakout noise
6 Assessment
This section will present the conclusions from the results and see to explain possible sources of error and include a part of reflection with personal input and end with final words.
6.1 Conclusion
The main objective was to see the behaviour of sound reduction for low frequencies per meter of silencer used. As can be seen in figure 5.4 the curves for each case follow the same trend of noise reduction, that is reaching the highest sound pressure level at around 60 Hz and quickly drop after that. As per the question regarding the saturation of 6 dB per meter of silencer, we can draw the conclusion that it does not apply as from case 1 to case 2 there is a difference in DT Lof 8− 9 dB depending on silencer placement. This can be seen in figure 5.5. In detail, we can see for the 250 Hz octave band we can that from case 2A and 2B to case 1 there is a difference in DT Lof roughly 15 dB.
Regarding the breakout noise we can say that if the duct structure is setup in the same room as the outlet there is a tipping point of breakout noise already with 2 silencing components.
If this is worked around, that is having the outlet in a separate room from the duct structure, the breakout noise is easily avoided.
The use of standards and the way the setup was constructed along with the margins used in the calculations drastically reduces the influence of the error sources from the measurements.
6.2 Discussion
After evaluating the results we came up with that it would be great to have more measurements to compare between, especially when comparing the breakout noise with the noise from the outlet. Since the rooms we measured in was not the same dimensions it was not possible to rebuild all cases in both rooms. Therefore we only have three cases that are actually comparable with each other when looking for the tipping point between outlet noise and breakout noise. But if we look at the application of this system, it is
best to keep the silencers in a separate room so breakout noise will not be a problem for overhearing.
Since we followed the given standards as much as possible it is safe to assume that we do not have large errors, but we know for sure that we have some smaller errors. For example, the straight ventilation pipes were not cut at the same lengths, and they were not the same length as the silencers. We had a margin of error around 10 cm on the total length of the pipe between different cases. Compared to the total length that margin of error is relatively small.
Another margin of error is that the measurements were done in one reverberation room only. So our results is specific for this room but, since the reverberation room is built according to standards, the results should be usable for general cases. A comparison between another reverberation room would clarify how much this affects the results.
6.3 Final Words
This project was a great experience for the two of us. We feel that we did real work for real life applications that gave us a fun view into the life of an acoustician. We experienced pleasant exchanges with the people at LN Akustikmiljö, Simon and Deniz. We would like to aim a special thanks to our mentor for the project, Leping Feng, who helped us with the setup for the project and the measurements and to the silent hero, Ismail Malikov from LN Akustikmiljö, for preparing all the material used in the laboratory.
References
[1] Nyman H. and Danielsson S. Ljuddimensionering av ventilationssystem.
Byggforskningsrådet, Mars 1998. ISBN 91-540-5815-5.
[2] Acoustics - Measurement of insertion loss of ducted silencers without flow - Laboratory survey method, 2009. SS-EN ISO 11691:2009.
[3] Kungliga Tekniska Högskolan SCI Aeronautical & Vehicle Engineering. Bachelor thesis, 2017. URL https://www.kth.se/sci/institutioner/ave/edu/
soundvib/bachelor-1.69574. Last visited 2019-04-03.
[4] Andersson J. Akustik och Buller. Ingenjörsförlaget AB, 1974. ISBN 91-7284-090-0.
[5] Wallin HP, Carlsson U, Åbom M, Bodén H, and Glav R. Ljud och Vibrationer.
Marcus Wallenberg Laboratoriet för Ljud- och Vibrationsforskning, July 2014. ISBN 91-7170-434-5.
[6] Swegon. CLA kompaktljuddämpare för cirkulära kanaler, 2018. URL http://
www1.swegon.com/Global/PDFs/Acoustics/_sv/CLA-A-B.pdf. Last visited 2019-05-18.
[7] My Management Guide. What is a project?, (n.d.). URL https://www.
mymanagementguide.com/basics/what-is-a-project. Last visited 2019-04- 04.
[8] Acoustics - Laboratory measurement procedures for ducted silencers and air- terminal units - Insertion loss, flow noise and total pressure loss, 2009. SS-EN ISO 7235:2009.
[9] Wikipedia. Pink noise, 2019. URL https://en.wikipedia.org/wiki/Pink_
noise. Last visited 2019-05-02.
[10] Rouse M. Pink noise, 2006. URL https://whatis.techtarget.com/
definition/pink-noise. Last visited 2019-05-02.
Appendix - Contents
A Project description 23
B Measurement cases 25
C MWL measurement photos 31
D Silencer data 33
A Project description
Figure A.1: GANTT chart
Figure A.2: Description of the project from the client
B Measurement cases
All planed cases for measurement.
Figure B.1: Case 0 (reference)
Figure B.2: Case 1 (1 silencer)
.
Figure B.3: Case 2A (2 silencers)
Figure B.4: Case 2B (2 silencers)
Figure B.5: Case 3 (3 silencers)
Figure B.6: Case 4 (4 silencers)
Figure B.7: Case 5 (1 silencer)
Figure B.8: Case 6 (2 silencers)
Figure B.9: Case 7 (3 silencers)
Figure B.10: Case 8 (No stopper)
Figure B.11: Case 9 (With stopper)
.
C MWL measurement photos
Figure C.1: Case 0 (reference)
Figure C.2: Case 1 (1 silencer)
Figure C.3: Case 3 (3 silencers)
D Silencer data
CLA-A silencer data picked from Swegon.6
Figure D.1: Data for CLA-A silencer from Swegon
