Lindab Academy Acoustic Basic Training

Transcription

1 Lindab Academy Acoustic Basic Training Tuomas Veijalainen, Product Manager, Acoustic

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 Wavelength l: Distance between two pressure maxima. Frequency f: number of pressure variations per second Speed of sound c relates frequency and wavelength pressure λ atmospheric pressure c [m/s] = f λ distance 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 f Frequency [Hz] λ Wavelength [m]

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 neighbour 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 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 0,00002 Normal hearing level 50 0,006 Quiet conversation Scream; 1 m 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 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. Frequency [Hz] A- weighting [db] B- weighting [db] C-weighting [db] 63-26,2-9,3-0, ,1-4,2-0, ,6-1, ,2-0, ,2-0,1-0, ,0-0,7-0, ,1-2,9-3, C-filter 1,2 1-0,8-0, ,2-0,8-1,1-3,2-3 -9,6 A-filter -16,1-26,2 63 Hz 125 Hz 250 Hz 500 Hz 1000 Hz 2000 Hz 4000 Hz 8000 Hz Example of db(a) value calculation. Frequency Tot Sound db A-weighting A-weighted db(a)

10 0104 lindab we simplify construction 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] NR 80 NR 70 NR 60 NR 50 NR 40 NR 30 NR 20 NR 10 Sound level [db] NC 80 NC 70 NC 60 NC 50 NC 40 NC 30 NC 20 NC Frequency [Hz] Frequency [Hz]

11 Sound Power L w 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).

12 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.

13 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

14 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)

15 Sound power of air-device Lw = 33 db(a) Sound pressure Lp10m 2 Sabine in room = 29 db(a)

16 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.

17 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

18 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.

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

20 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

21 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 calculations and design. It s just for additional assurance.

22 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 Typical sound reduction in duct splits is 1 4 db

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

24 Lindab Acoustic Laboratory lindab we simplify construction

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

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

27 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 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

28 Attenuation Materials Mineral wools slabs for rectangular silencer: stone or glass wool Always covered with surface to prevent fibres coming out Lindtec; thick surface which can be cleaned by rotating nylon whisks, vacuum cleaner or even by wet cloth Traditional glass fibre surface; think surface to prevent from fibres. Can be cleaned by rotating nylon whisks or vacuum cleaner. Lamella mat for circular silencers Stone or glass wool Wool always covered with perforated inner tube and polyester fabric called Polduk Special element for LRCA Stone wool element covered by Lindtec surface

29 Attenuation materials Polyester (Acutec): - Coated plate or moulded element - No risk of fibres - Washable - can be cleaned by rotating nylon whisks, vacuum cleaner or even by wet cloth

30 Round Sound Attenuator Traditional attenuator with many benefits and low price Can be equipped with baffle or central pod style attenuation element. Small pressure loss The Principle is based on sound absorption to attenuation material Used on ducts or with axial fan SLXU glass wool SLU stone wool SLBU stone wool SLGPU, glass wool Wool protected with perforated plate / stretch metal inner tube + Polduk

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, cross talk and residential AHU LRCA special attenuation element with Lindtec surface LRCB, stone wool mat + perforated plate / stretch metal inner tube + Polduk LRBCB, stone wool mat + perforated plate + Polduk

32 Sound Attenuation Comparison Value can be added for the customer to compare different products. LRCA Hz 125 Hz 250 Hz 500 Hz SLU Hz 125 Hz 250 Hz 500 Hz x SLU Hz 125 Hz 250 Hz 500 Hz

33 Round Bend Silencers Small pressure loss The Principle is based on sound absorption to attenuation material Sizes Ø mm 50 or 100 mm insulation Used on ducts BSLU, stone wool with perforated plate + Polduk surface

34 Rectangular Sound Attenuator Effective sound attenuation Sizes according to customer s needs 400x300 mm 2400x2400 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, on ducts or cross talk SLRS, DLD, attenuation material Lindtec TUNE-S stone wool protected with glass fibre surface TUNE-PS stone wool + Melinex inter liner Splitters SLRA, TUNE-A, TUNE-PA Low build model LRLB, attenuation material Lindtec. Used as duct silencer, cross talk silencer and VAV silencer Other available models, DLDY, MINKA and DACKA

35 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 Attenuation material Lindtec

36 Flexible Sound Attenuator Aluminium or plastic outer duct Aluminium inner duct Easy to install Absorption + sound break out through casing FSA, SLFA, SLFU, AKUCOM

37 Exhaust Air Terminal Devices For big air volumes SLKNU combination of silencer and bellmouth EXAD, combination of DIRU damper, silencer and takeoff cone Conic inlet with mesh / wire guard (option in EXAD)

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

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 (openable etc.)

44 Sound calculations by DIMsilencer Available on - Downloads Free of charge. Plug in with Magicad Youtube DIMsilencer in MagiCad

45 Sound Calculations by DIMsilencer lindab we simplify construction

46 Sound calculation by lindqst Free of charge

47 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.

48 Fan connection lindab we simplify construction

49 Low Pressure Loss = Low Noise Level Less turbulence = less pressure loss less noise

50 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

51 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.

52 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

53 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)

54 On the following pages these symbols will be used: = Logarithmic addition = Logarithmic subtraction = Logarithmic multiplication = Logarithmic division lindab we simplify construction Addition Subtraction Difference added to the highest db value [db] L1 L2 Ln L tot = 10log ( ) Difference between the values [db] Subtraction from higher value [db] Differense between values [db] Ltot L1 L = 10log (10 10 ) Sound Level increase [db] Multiplication = L Ltot Number of Sound Sources (n) 10log( n) Sound Level decrease [db] Division Ltot 1 10 Number of Sound Sources (n) = L 10log( n)

55 Multiplication Multiplication Sound Level increase [db] Number of Sound Sources (n)

56 QUESTIONS? lindab we simplify construction

57 THANK YOU! lindab we simplify construction