1. Lindab Group We simplify construction
Lindab Acoustics
Tuomas Veijalainen
Product Manager Acoustic
Mikkeli, Finland

2. Agenda Acoustic theory What are sound and noise?
A-weighting dB – dB(A)
Sound pressure – sound power
Cross talk
Sound insulation – sound attenuation
Sound sources in ventilation
Sound attenuators
Testing of silencers
Materials
Lindab silencers
Sound measurements
Sound calculations by DIMsilencer
Questions and comments

3. Sound
Sound is a variation of pressure. It travels forward in zones of negative and positive pressure. It is of longitudinal wave motion i.e.. vibration.
Movement of sound requires a certain material; air, water, concrete, wood etc.
Sonic speed 344 m/s (at sea level, +20 °C)

4. Sound The simplest vibration is sine wave (pure tone)
Frequency is the number of occurrences per time unit
The Unit is Hertz (HZ = 1/s)

5. Wave Length - Frequency
0104
Wave Length - Frequency
pressure
c [m/s] = f · l l
distance
Wavelength l: Distance between two pressure maxima.
Frequency f: number of pressure variations pr second
Speed of sound c relates frequency and wavelength
atmospheric pressure
It follows from this that: low frequency sound has a long wavelength. Therefore low frequency sound is difficult to attenuate.
Ventilations systems are normally assessed in eight frequency bands, from 63 to 8000 Hz. These are called octave bands

6. Noise
What is noise? That sounds like a pretty simple question, but noise is a perceived value. You may think of noise as anything you can hear, but we will define noise as any unwanted sound. Sound becomes noise when it
Is too loud: the sound is uncomfortable or makes speech difficult to understand
Is duct rumble created by a fan
Contains unwanted pure tones (e.g., a whine, whistle, or hum)
Contains unwanted information or is distracting (e.g., an adjacent telephone conversation or undesirable music)
Is unexpected (e.g., the sound of breaking glass}
Is uncontrolled (e.g., a neighbor’s lawn mower)
Happens at the wrong time (e.g., a door slamming in the middle of the night)
Is unpleasant (e.g., a dripping faucet)

7. Sound pressure level Lp dB
Noise – Lp (SPL)
These small pressure waves send a signal to the brain, through the ear, which recognises it as sound
Unit: decibel (dB)
Sound pressure level Lp dB
Sound Pressure p Pa
Sound Source
0,00002
Normal hearing level 50
0,006
Quiet conversation
100 2
Scream; 1 m
140
200
Jet engine

8. Noise – Lp (SPL)
Pressure waves cause the eardrum and microphone of the sound level meter to react in the same way and the meter will register the sound pressure level.
When noise is in a certain place e.g. room, yard etc. We talk about Sound Pressure Level.

9. Weighting Curves
A-weighting compensates the human’s sense of hearing and it is most commonly used.
Ventilation noise is normally A-weighted
B-weighting (e.g. Cars)
C-weighting compensates almost un-weighted sound (used mainly frequency range Hz)
D- weighting is standard for aircraft noise
Frequency [Hz]
A-weighting [dB]
B-weighting [dB]
C-weighting [dB] 63
-26,2
-9,3
-0,8
125
-16,1
-4,2
-0,2
250
-8,6
-1,3
500
-3,2
-0,3
1000
2000
1,2
-0,1
4000
1,0
-0,7
8000
-1,1
-2,9
-3,0

10. Sound weighting [dB(A)] & [dB(C)]
Often requirements for a system is stated as a single value in stead of in the 8 octave bands. This is done by adding a filter value to each octave band and add the octave bands logarithmically.
The human ear can perceive frequencies from Hz. However a sound pressure of for instance 30 dB at 63 Hz is perceived as less annoying than 30 dB at 1000 Hz. This is the reason that dB(A) filters are used. There are large negative filter values in the low frequencies as shown in the graph.
Sometimes there are stricter requirements in the low frequencies. In these cases the C-filter is used.
Frequecy
[Hz]
A-weighting [dB]
B-weighting [dB]
C-weighting [dB] 63
-26,2
-9,3
-0,8
125
-16,1
-4,2
-0,2
250
-8,6
-1,3
500
-3,2
-0,3
1000
2000
1,2
-0,1
4000
1,0
-0,7
8000
-1,1
-2,9
-3,0
Example of dB(A) value calculation.

11. Sound Weighting ( NR & NC - curves)
0104
Sound Weighting ( NR & NC - curves)
Both NR and NC value are found in a similar way.
The sound level is drawn in the NR/NC diagram. The highest NR/NC value is read from the curves.
NR 68 (found at 500 Hz)
NC 69 (found at 500 Hz)
Sound level [dB]
Sound level [dB]
Frequency [Hz]
frequency [Hz]

12. Sound Power Lw
Noise levels for sound technical calculations must be given as sound power levels.
In sound calculations we use the Sound Power Levels (Lw) from the fan manufacturer.
Sound power is measured inside the duct. Compare to sound pressure level which is measured outside the duct (room or in a open space).

13. Sound Power of a Fan, Lw
Sound power of a fan is dependent on the type of fan.
Noise is caused mainly by two ways; mechanical noise and air flow noise.

14. Sound Power of a Fan, Lw
Specifying sound power by exact calculations is not possible. It must always be measured.
Corrections for external influences must be corrected after the measurement & calculation

15. Sound Power of a Fan, Lw
Sound power levels must be declared so that their are divided to different frequencies per octave band is precisely specified.

16. Sabine room attenuation
Many producers gives sound pressure levels for air-devices in Sabine 10 m² room
Sound power level is in this case +4 dB(A)

17. Sound Attenuation – Sound Isolation
Sound attenuation; e.g. Reduce of sound power level caused by a fan or regulation damper etc.
Sound isolation; e.g. prevention of noise travelling from one room to the other.

18. Sound Movement Between Rooms - Cross Talk
Through walls
Via holes in the walls / floors etc.
Via water and heating pipes
Via ventilation ducts
Round duct is always better than rectangular if we want to avoid sound travelling via ducts

19. Attenuation Between Rooms
Sometimes we must prevent noise travelling from one room to the next. The most effective way of achieving this is through the use of silencers.

20. Sound sources of Ventilation systems;
Air handling unit (fan)
Grilles
Roof hoods
Valves & diffusers
Dampers
Air flow
Compressors and condensers
Attenuators

21. Sound Moving Inside The Duct
Sound is progressing in duct same way as flowing air.
Airflow direction is not relevant ; sound will travel in either direction

22. Sound Attenuation of Duct System
In sheet metal ducts, and concrete ducts with smooth surface, the sound attenuation is very low only dB / m.
That attenuation is usually ignored in measurement and design. It’s just for additional assurance.

23. Distribution of Sound Power
Distribution of sound power is according to areas of duct branch
Lw’ = Lw-10 lg (A/A’) dB
For example, if the duct splits into a branch this results in a reduction of 3 dB in sound power level. i.e. if the ducts reduces from 500 mm2 to 2 pieces 250 mm2 the result is a constant reduction of 3 dB.
Lw’ = Lw-10 lg 2 dB = Lw – 3 dB

24. Sound Attenuation
Attenuators are used for reducing the sound emissions of fans, dampers and other sources.

25. Lindab Acoustic Own test laboratory
IT-solutions, calculation tools DIMsilencer, LindQST etc.
Technical support
Effective production
Wide product range
Short delivery times

26. Lindab Acoustic Laboratory

27. Silencer Production Grevie and Boliden, Sweden
Karlovarska. Czech Republic
Haderslev, Denmark
Jyväskylä, Finland
Tallinn, Estonia

28. The Principle of Attenuation
When noise passes through the attenuator, it is absorbed by attenuation material (sound is converted into heat)
Attenuation material is normally some soft material like mineral wool or polyester (Acutec)
The level of attenuation depends on the model of attenuator and attenuation material.
The silencer model will be selected according to the need of attenuation in the space

29. Attenuation Materials
Mineral wool:
Mineral wool - plate or mat
Rock wool or glass wool
Always covered with surface
and sometimes also with perforated plate
Polyester (Acutec):
Coated plate or moulded element

30. Round Sound Attenuator
Traditional attenuator with many benefits and low price
Can be equipped with baffle or ”central pod” style attenuation element.
Mineral wool or Acutec polyester
Small pressure loss
The Principle is based on sound absorption to attenuation material
Used on ducts or with axial fan
SLXU, SLU, SLCU, SLBU, SLGPU, PVA, PVAP, PVD, PVDP, TAO, AKS

31. Rectangular Silencer with Round Connections
Effective attenuation
Can be installed in lower ceiling voids.
Small pressure loss
The Principle is based on sound absorption to attenuation material
Used on ducts and residential AHU
LRCA, LRCB, LRBCB, KVAP / KVDP

32. Sound Attenuation – Comparison
Value can be added for the customer to compare different products.
LRCA
63 Hz
125 Hz
250 Hz
500 Hz 9 10 19 36
SLGU 4 11 21 32
2 x SLU 12 30 50

33. Round Bend Silencers Small pressure loss
The Principle is based on sound absorption to attenuation material
Sizes Ø125… 315 mm
50 or 100 mm insulation
Used on ducts
BSLU and BSLCU

34. Flexible Sound Attenuator
Aluminium or plastic outer duct
Aluminium inner duct
Easy to install
Absorption + sound go thru duct
SLFA, SLFU, AKUCOM, FSA, TAO-S FLEX

35. Exhaust Air Terminal Devices
For big air volumes
SLKNU combination of silencer and take-off cone
EXAD, combination of DIRU damper, silencer and take-off cone
Conic inlet with mesh / wire guard (option in EXAD)

36. Rectangular Sound Attenuator
Effective sound attenuation
Sizes according to customer’s needs 400x300 mm…
Various baffle and split alternatives
The air flow must be noticed
The Principle is based on sound absorption to attenuation material and End Reflection i.e. Sound waves cancel each other out
Used with AHU or on ducts
SLRS, TUNE-S, DLD, MINKA, DACKA, TAP
Splitters SLRA, TUNE-A
Low build models LRLB and LRLS 150x150… 1000x400 mm

37. Rectangular Bend silencers
Effective sound attenuation
Sizes according to customer’s needs 400x300 mm…
Various baffle and split alternatives
The air flow must be noticed
The Principle is based on sound absorption to attenuation material and End Reflection i.e. Sound waves cancel each other out
Used with AHU or on ducts
SLRB, BDLD

38. Reactive Attenuator
Measurement must be performed separately in each case.
Principle is based on Reflection i.e. Sound waves cancel each other out
Used when specific attenuation to achieve a certain frequency

39. Industrial projects and products
Calculation as standard products
Special materials (stainless steel, aluminium, painted steel etc.
Material thickness 1,2 … 5,0 mm
Flange connections
Used especially to lower the noise to surroundings
CE PVA-P

40. Industrial projects and products
Attenuation baffles
Calculating as whole silencers

41. Industrial projects and products
Bend silencers

42. Industrial projects and products
Round attenuator

43. Sound calculations by DIMsilencer
Required information for sound calculations:
Air volume l/s, m3/s or m3/h
Maximum pressure loss Pa
Sound power levels by Hz
Required attenuation, sound power or sound pressure level dB(A)
Lay out (fan size, space for silencer)
Material of the silencer
Galvanized steel, EN1.4404, EN1.4301, S235 with epoxy paint
Thickness of the steel (seamed or welded structure)
Material of insulation
Should we use mineral wool or polyester fibre
Operation temperature
Other requirements if any

44. Sound Calculations by DIMsilencer

45. Sound Calculations by DIMsilencer
Available in Downloads
Free of charge.

46. Be Aware for These
Many sound sources(two equal source of noise causes 3 dB increase in noise level)
Using different equipment than what was in the original drawings
Moving of sound via structures & services of the building
Regulation using wrong methods. First adjusting fan to right air volume then balancing airflow by regulation dampers, valves or diffusers.
Too small ducts  too high velocity / pressure
Wrong selection of diffusers
Wrong connection between fan and duct system and / or wrong kind of bend on rectangular duct system.

47. Fan connection

48. Low Pressure Loss = Low Noise Level
Less turbulence = less pressure loss  less noise

49. Sound Measurements
Only use meters calibrated in accordance to local standards
Usually A-weight in ventilation
On the meter select the time setting F (Fast) for more accurate result

50. Sound Measurements
First measuring background sound. NB! Maybe different between rooms
Measuring mainly in the middle of the room. 1,2…1,5 meters above floor level.
In big rooms several measurements required. Take the average from measuring places.

51. Typical Mistakes In Sound Measuring
Discreet background noise. Difference between ventilation noise and background noise must be at least 10 dB.
Noisy building site
Measuring too close to the diffuser
Measuring inside the duct or fan unit
Doors are open to the other rooms
Microphone too close to the device

52. Examples from Summation of Sound
Background noise is 31 db(A) and measured noise is 35 dB(A)  noise from ventilation = 33 dB(A)
Background noise is 25 dB(A) and measured noise is 35 dB(A)  noise from ventilation = 35 dB(A)

53. On the following pages these symbols will be used:
0505
On the following pages these symbols will be used:
= Logarithmic addition
= Logarithmic subtraction
= Logarithmic multiplication
= Logarithmic division

54. Multiplication

55. QUESTIONS?

56. Thank you! Good luck for studies and have a nice spring!
Photo: Pasi Koukkula