acoustics model for scala fume cupboards by waldner · 2012-01-31 · acoustics model that allows...

Departmental Report of CMS Project Service EV - 11 – 30

Acoustics Model for SCALA Fume Cupboards by WALDNER

Calculation of Sound Power Levels 2nd Edition (language: english) This report includes 28 pages Customer WALDNER

Laboreinrichtungen GmbH & Co.KG Haidösch 1 88239 Wangen im Allgäu

Date September 2011 Report Dr. Hermann Leis Measurements Dr. Hermann Leis Order No. 5574993 Key words Acoustics, Control valve,

Modeling Search Nos. Distribution k All rights reserved LTG Aktiengesellschaft. Subject to technical alterations. No reproduction of this report, in whole or in part, without the written approval of LTG.

LTG Aktiengesellschaft

D-70435 Stuttgart Grenzstraße 7 D-70405 Stuttgart Postfach 40 05 25 (0711) 82 01-180 Fax (0711) 82 01-720

All rights reserved LTG Aktiengesellschaft. - 2 - September 2011

ev_10_30: Acoustics Model for Scala Fume Cupboards by Waldner

Contents

ABSTRACT .............................................................................................................................. 3

1 INTRODUCTION ........................................................................................................... 4

2 ACOUSTICS THEORY ..................................................................................................... 5

2.1 Fundamental Terms ..................................................................................................... 5 2.1.1 Frequency...................................................................................................................... 5 2.1.2 Sound Pressure Level Lp ................................................................................................ 5 2.1.3 Sound Power Level LW ................................................................................................... 6 2.1.4 The Difference between Sound Pressure Level Lp and Sound Power Level LW ............ 7

2.2 Noise Rating: A-weighting ............................................................................................ 7

2.3 Room Absorption in Typical Labs ................................................................................. 8

2.4 Sound Level Calculations ............................................................................................. 9

2.5 Measuring Technology ............................................................................................... 11 2.5.1 Sound Power Measurements in LTG Aktiengesellschaft’s Reverberation Room ....... 11 2.5.2 Pressure Measurement ............................................................................................... 15 2.5.3 Flow Rate Measurement ............................................................................................. 15

3 ACOUSTICS MODEL .................................................................................................... 15

3.1 Model Structure ........................................................................................................ 15 3.1.1 Sound Transmission Paths .......................................................................................... 16 3.1.2 Sound Sources in SCALA Fume Cupboards ................................................................. 16 3.1.3 Airborne Sound Absorption ........................................................................................ 17 3.1.4 Structure-borne Sound ............................................................................................... 18

3.2 Investigation Findings ................................................................................................ 19 3.2.1 Sound Sources ............................................................................................................. 19 3.2.2 Structure-borne Sound of Control Valve and Entire Fume Cupboard ........................ 21 3.2.3 Absorption................................................................................................................... 21 3.2.4 Calculation Model Summary ....................................................................................... 22

3.3 Verification of Calculated Results and Accuracy of Model ........................................... 23 3.3.1 Standard Fume Cupboards ......................................................................................... 23 3.3.2 Low-ceiling Fume Cupboards ...................................................................................... 24 3.3.3 Accuracy of Model ...................................................................................................... 24

4 PLANNING RECOMMENDATIONS ............................................................................... 25



Abstract

In a joint development project by WALDNER Laboreinrichtungen GmbH & Co.KG and LTG Aktiengesellschaft, a mathematical-empirical model was designed to calculate the sound power levels of Waldner SCALA fume cupboards of various sizes. This model is based on an third-octave band sound power level model for the control valve used. Together with the absorption phenomena encountered inside the fume cupboard, the sound power level emitted by the fume cupboard may be calculated as sum level and in octave band resolution.

This model may also be used to provide the planner with acoustic data for SCALA fume cupboards.

The project was successfully completed in individual stages, each building on the preceding one(s). For application, a Microsoft EXCEL based program was elaborated and made available.

LTG Aktiengesellschaft

Per pro. Dr. Hans Werner Roth On behalf Dr. Hermann Leis



1 Introduction

The present report is an abstract of the project titled “Design of an Acoustics Model for SCALA Fume Cupboards made by WALDNER Laboreinrichtungen.“ The issue of acoustics in labs is becoming more and more important. One reason might be the use of computers for analysis and assessment of tests making labs the sole workplace of staff and researchers. Another, that labs are still insufficiently designed based on acoustics criteria. Therefore, fume cupboards, including the duct system to which they are connected, play an important role since they represent a continuous and significant sound source during operation.

Waldner Laboreinrichtungen has made it their objective to make sound power data available for their SCALA fume cupboards in order to help planners of extract air systems as well as lab designers in their decision making process based on acoustics.

Since determination of the sound power levels for all types and sizes of SCALA fume cupboards would be time consuming and expensive, LTG Aktiengesellschaft was ordered to develop an acoustics model that allows for calculation of sound power levels of a variety of fume cupboards at various operating points.

The development project was split in 4 stages:

1. Review of a proposed simplified empirical acoustics model to be developed to allow for calculation of the sound power of a variety of fume cupboard types and sizes. For initial evidence, already existing acoustic measurements using Waldner fume cupboards were referred to.

2. Detailed metrological analysis and assessment

– of control valve, baffle including air vane, sash, and Secuflow system as sound sources

– of structure-borne sound shares from control valve and entire fume cupboard

– of attenuation due to installation of the control valve in the fume cupboard, the air vane and baffle as well as the sash

using a bench-mounted fume cupboard including rear wall installation, grid length 1500 (RWI 1500) with Airflow Controller and Secuflow system.

3. Verification of physical phenomena in a bench-mounted fume cupboard with rear wall installation, grid length 2100 (RWI 2100) with Airflow Controller and Secuflow system with view to transferability to other model sizes. This is based on a calculation model determining the sound pressure levels of the control valve depending on duct pressure and exhaust air flow rate.

4. In stage 4, further measurements were performed to ensure transferability to fume cupboards with side installations. In addition, parameters for the calculation model of control valve DN 315 were determined. Finally, the model shall be turned into an EXCEL program enabling Waldner to calculate the sound power levels of varying fume cupboard types and sizes for different operating points. This report explains the model structure and the application range. For better understanding of relations between sound power level, sound pressure level, and



room acoustics, some acoustic basics are outlined in Chapter 2. The report concludes with certain general notes on acoustic planning of extract air systems for fume cupboards and labs.

2 Acoustics Theory

2.1 Fundamental Terms

2.1.1 Frequency

Frequency f is the number of sonic vibrations per second measured in Hertz (Hz). With a vibration time of T applies:

fT 1

=

f 20 50 100 200 500 1000 2000 5000 10000 Hz

-----------Air conditioning-------------

2.1.2 Sound Pressure Level Lp

Sound pressure is the root mean squared value of sound change pressure in the propagation of a sound wave

dtpT

pT

eff21

∫=

Acoustics calculations use the term „level“ which means the logarithm of the quotient of two physical parameters, such as intensities or sound pressures:

0

10IIlgL =

in dB (decibel)

Sound pressure level Lp

The sound pressure level Lp refers to the hearing threshold po:

p0 = 2 10 -5 Pa (referring to 1000 Hz)

02

0

2

2010pplg

pplgL ==



Table 1: Several sounds and their typical sound pressures and sound pressure levels

Sound Root mean square value of sound pressure

in Pascal

Sound pressure level

in dB

Whispering leaves 0.0002 20

Conversation in low tone 0.002 40

Loud calling at a distance of 1m 0.2 80

Air hammer at a distance of 1m 2 100

Pain threshold 20 120

The sound pressure level is depending on the location and may be measured directly using a microphone!

2.1.3 Sound Power Level LW

Determination of the sound power P:

The surface integral over the sound intensity I is the sound power passing through the surface S:

P =S

→ I d→s

The total output of a sound source is obtained from the surface integral over any closed far-field surface S surrounding the source entirely:

∫=S

ges dSIP

Expressed in level it may be derived as follows:

From: 0

10PPlgLW =

and the reference sound power P0 = 10-12 Watts and S0 = 1m² applies:

with P and P0 plugged in:

010

010

020

010

2

010

SSlgpLWL

SSlg

pplg

SSlg

pplgWL

+=

+=+

=

P0 =p0

2 S0Z



2.1.4 The Difference between Sound Pressure Level Lp and Sound Power Level LW

• A sound source (here: fume hood) is clearly defined by the sound power level. Stating the sound power levels is part of a lab fume hood’s technical specifications for a defined operating state.

• If levels are given in "dB" please check whether they mean the sound pressure or the sound power!

• The sound power level is larger than the sound pressure level if the reference surface is larger than 1 m2.

• If sound pressure levels are given with reference to the sound source (here: lab fume hood) the measuring distance is important! The level decreases with the distance to the source and it is also affected by the sound absorption ability and the reflections of the surrounding room.

• The term 10 lg S / S0 only applies to free field. Inside of a room it is replaced by room absorption L. Inside rooms the following applies: LW = LP + L in [dB]

2.2 Noise Rating: A-weighting

In 1968, DIN phon rating based on ISO was replaced by DIN 45633 based on

A-weighting LA in dB (A)

B-weighting LB in dB (B)

C-weighting LC in dB (C)

In air conditioning, the A-weighting is common use. C-weighting is used e.g. to assess aviation noise.

A-frequency-weighting considers the sensitivity of our hearing with view to certain frequencies. The sum level LA is a measure describing the “loudness“ of a noise. The LA-value often conceals low-frequency duct noises caused by resonance or insufficient vibration isolation or insufficiently large sound absorbers behind the fans. It says nothing about the annoyance. Noises with small-band level maxima, so-called tonal shares, must be assessed based on their annoyance.

For flow noises of diffusers, dampers, ducts due to turbulence and flow separation, A-weighting is a good standard of comparison and a useful computable value as “individual parameter”. The dependency on the flow speed or air flow rate can be clearly depicted.



Table 2: Frequency-dependent correction of A-weighting

Octave band/

Third octave Band

fm A-correction 50 63 80

-30.2 -26.2 -22.5

100 125 160

-19.1 -16.1 -13.4

200 250 315

-10.9 -8.6 -6.6

400 500 630

-4.8 -3.2 -1.9

800 1 000 1 250

-0.8 0.0 +0.6

1 600 2 000 2 500

+1.0 +1.2 +1.3

3 150 4,000 5 000

+1.2 +1.0 +0.5

6 300 8 000 10 000

-0.1 -1.1 -2.5

2.3 Room Absorption in Typical Labs

Room absorption depends on the sound absorption and the reflections of the room (here the lab) in which the sound source (here the fume hood) is located. The higher the sound absorption (described by the reverberation time) the greater the room absorption.

Factors of influence for sound absorption and consequently the room absorption, in that case of the lab, are:

• Nature of the ceiling (e.g. acoustic, suspended, solid – in this sequence with decreasing sound absorption capacity)

• Nature of the floor (e.g. carpet, PVC, tiling => in this sequence with decreasing sound absorption capacity)

• Vertically enclosing surfaces such as walls, doors, windows (e.g. drywall, wood, concrete, glass => in this sequence with decreasing sound absorption capacity)

A-weighting Curve



Based on the room volume V and the reverberation time T the equivalent absorption area A may be calculated in Sabine characterizing the sound absorption capacity of the room. The reverberation time is the time that passes until an event has achieved a 60 dB sound reduction. Just like the absorption capacity of the room it is frequency-dependent.

For a lab (reverberation time T = 2 secs) with a floor area of 7.5 m x 7.5 m and clearance of 3 m the equivalent absorption area A is as follows:

SabinemTVA ²75.13

235.75.7163.0163.0 =

⋅⋅==

Based on VDI 2081 Book 1, Chapter 3.1 Table 2 a chemistry lab working room can be expected to have a reverberation time T of 2.0 seconds.

The room absorption for the diffuse sound field may be rough calculated in sufficient distance to the sound source (and thus only depending on the equivalent absorption area). It applies:

dBSabinemA

L 5²75.13

4lg104lg10 ≈

−=

−=∆

2.4 Sound Level Calculations

Sum-up of Sound Levels

Sound levels L require logarithmical sum-ups:

++== ∑ ...1010lg1010lg10 101010

21 LL

n

L

SumL

Example of a level sum-up of L1 = 60 dB and L2 = 56 dB via logarithmical level sum-up:

dBLSum 5.611010lg10 1056

1060

=

+=

Sum-up of sound sources of the same intensity

LnnnLLL

n

L

Sum +=+=

⋅== ∑ lg1010lg10lg1010lg1010lg10 101010

Example of calculation for sound level changes if 3 sound sources of the same intensity with L = 60 dB are turned off:



• 5 sources: 60 dB + 10 lg 5 = 67.0 dB => 3 sources turned off = 2 sources operating: 60 dB + 10 lg 2 = 63.0 dB => - 4 dB sound reduction

• 50 sources: 60 dB + 10 lg 50 = 77. 0 dB => 3 sources turned off = 47 sources operating: 60 dB + 10 lg 47 = 76.7 dB => - 0.3 dB sound reduction

To lower the level of 50 sound sources by 3dB 25 sound sources would have to be turned off!

Subtraction of Sound Sources

Subtraction of sound sources is performed in analogy to their sum-up. Via subtraction, possibly existing background noises during measurements may be subtracted.

−=

+

1010 101010HHM LL

M lgL

LM = Sound level of the machine or the investigated sound source

LM+H = Measured total sound level of the machine including background noises

LH = Background noise only

0,0

5,0

10,0

15,0

20,0

25,0

0 20 40 60 80 100

Leve

l inc

reas

e

Number of sound sources

Level increase with sound sources of the same intensity



2.5 Measuring Technology

2.5.1 Sound Power Measurements in LTG Aktiengesellschaft’s Reverberation Room

Measurements in LTG’s reverberation room are performed based on EN ISO 3741:1999 “Determination of sound power levels of noise sources using sound pressure -- Precision methods for reverberation rooms (class 1)“. The following excerpt from the “LTG Test Center“ documentation provides an overview of technical key data.

Reverberation Room

A reverberation room serves to determine the sound power level of air technology devices. Contrary to a sound absorbing room where measurements are realized in a closed area encasing the sound source, the reverberation room provides a uniform, i.e. diffuse sound field with a sound pressure level that is converted into sound power levels depending on the frequency using room correction factors.

Great attention has been paid to a low quietness level, i.e. to a good sound insulation of the reverberation room walls and as little a low-frequency vibration pickup as possible via the foundation. The internal pressure loadable concrete body (differential pressures at 5000 Pa, dynamic) is supported by 2.5 Hz harmonized spring packages placed on a concrete frame firmly cast with four supports. Two supports each are connected through a foundation strip isolated from the building floor.

The weighted sound reduction index Rw of 59 dB is the result of the following measures:

• 30 cm reinforced concrete walls

• Openings equipped with two-leaf, tight-closing, antidrumming armor-plated doors

• Twin-shell sound absorbers connected to the reverberation room

• Very high attenuation of the air outlet sound absorber

With these measures quietness levels of 18 and 22 dB(A) may be achieved at daytime.

The relatively smooth curve shape of reverberation time over frequency (third-octave) shown in Figure 1 indicates that the sound field is relatively well diffused. This is achieved by sound-reflecting walls and diffusers on the ceiling and in front of the walls with an oblique-angled arrangement to one another. In addition, two low-frequency absorbers were installed for room correction.

Registration data of the levels from six microphones distributed in the room are sufficient to meet DIN EN ISO 3741, Precision methods for reverberation rooms. Standard deviations above the room’s limit frequency of 110 Hz are significantly below the DIN limits (refer to Table 3). In the range of 40-80 Hz the results are sufficiently good. For specific measurements, the number of measuring points and consequently the low-frequency precision may be increased.



As can be taken from the basic scheme, the reverberation room has been provided with a total of five measurement sections including sound absorbers and centrifugal fans. Flow rate and fan pressure increase may be adjusted either via dampers or via speed (frequency converter).

The flow rate in the range of 0-22000 m3/h is determined via (Table 4)

• one nozzle measuring chamber (large section of measurement)

• three orifice measuring sections with quick-change devices and tube diameters of 100, 225, 315 mm.

Using the large measurement section, decision whether to extract air from the reverberation room, blow into it or run the system with recirculating air is made by opening or closing the dampers. By changing the flexible connecting lines of the two smaller fans each orifice measurement section may be used for either supply or return air.

Due to duct sound absorbers the background noise level emitted by the fans into the reverberation room is negligibly small versus the level to be measured. It varies in the range of 200-36000 m3/h between 18 and 50 dB(A).

The measurement is performed based on comparison method with the use of a standard sound source. As standard sound source a free-wheeling blade of a drum rotor is used with a sound power frequency spectrum measured and officially certified by the Federal Physical-Technical Institute. After each test set-up or alteration the reverberation room correction, i.e. the



conversion from sound pressure to sound power level is measured anew and saved electronically.

During the measuring procedure the microphones are read in sequence one by one and the time average values of each position are determined logarithmically and converted into sound power levels. The resulting octave and third-octave spectra (charts), the octave spectrum (bar chart), and the linear and A-weighted sum levels are saved and printed.

Table 3: Reverberation room standard deviation (Sh): Comparison of actual standard deviation (Sh) with the required standard deviation (SS) according to DIN 45635 Part 2. The calculated lower limit frequency of the reverberation room is 110 Hz.



Figure 1: Reverberation times

Table 4: Measurement Section Data

Measurement section

Flow rate max.

Large measurement section with 3 nozzles

Nozzle D=150 2 000 m³/h

Nozzle D=315 10 000 m³/h

Nozzle D=500 22 000 m³/h

Orifice measurement section D=315

Orifice D=250 5 000 m³/h


Orifice D=100 600 m³/h







Orifice D=45 120m³/h



Table 5: Reverberation Room Data

Volume 254 m³

Total weight 2010 kN

Springs on structure-borne sound insulating boards

14

Resonance frequency 2.5 Hz

Weighted sound reduction index of the reverberation room wall

59 dB

2.5.2 Pressure Measurement

The initial operating pressure or differential pressure of the fume hood in a defined operating state of the investigated fume hood equals the difference between static duct pressure in [Pa] in direction of air flow behind the exhaust air main duct and the ambient pressure. Measurement of this differential pressure is performed using a calibrated SI-special Instruments GmbH micromanometer.

2.5.3 Flow Rate Measurement

Investigation of the acoustic characteristics of air technology devices requires to determine the exact air flow rate. For this purpose, LTG uses a variety of orifice and nozzle measuring sections. Via fan, an air stream is either supplied into or sucked out of the reverberation room. Sections of measurement of varying diameters permit a wide range of controllable and measurable air flow rates. Using a standard orifice based on DIN1952 and VDI/VDE 2040 allows to determine the air flow rate from the measured differential pressure via orifice if density and temperature of the air are known factors. The flow rate may be controlled via dampers or fan speed using a frequency converter.

3 Acoustics Model

3.1 Model Structure

The acoustics model is based on the analysis of all relevant fume cupboard sound sources, their sound paths, and the encountered attenuation effects.

The sound sources are characterized by a variety of frequency ranges that also depend on the flow rate. Attenuation effects are equally frequency-dependent. Therefore, a „one-number parameter“ such as e.g. A-weighted sum sound power level is unavailable for this acoustics model. For that reason, the sound sources as well as the attenuation phenomena are measured and analyzed in third-octave band resolution. The thus derived mathematical models allow to calculate the sound power level emitted by the fume cupboard into the room in third-octave band resolution. From the octave band resolution and based on the A-weighted curves in combination with the rules of sound level sum-up, the sound power level in octave band resolution and the A-weighted sum level (in dB(A)) may be calculated.



3.1.1 Sound Transmission Paths

In general, there are two paths the sound may use to exit the fume cupboard and enter into the room. On the one hand, sound is airborne from the spot it originates into the room. Attenuation of sound plays an important role along the way. On the other hand, there is the so-called structure-borne sound which means the sound transmitted via the casing into the room (see Figure 2). One reason for structure-borne sound is the excitation of the casing caused by airborne sound inside the cupboard. In that case, the wall of this casing acts as sound insulation of the airborne sound. Another reason for structure-borne sound may be direct connections of vibrating components (such as the damper blade of the control valve) to the casing whereas the casing may enhance the vibration. Such sound transmissions may be reduced by mechanically decoupling the vibrating component from the casing.

Figure 2: Sound transmission of a fume cupboard

3.1.2 Sound Sources in SCALA Fume Cupboards

In the SCALA fume cupboards, noise may be produced wherever air flows through gaps or alongside edges. This is called “flow noise”. The most significant sound source is the control valve used to produce a pressure loss so that the fume cupboard, which is connected to a duct system, may be operated with suitable flow rates. This causes pressure losses of 150 to 300 Pa which result in noises typical for valves. In the flow cross sections, in the air vane and baffle areas and at the closed sash, additional flow noises are generated.

Another sound source in the SCALA fume cupboards represents the optional Secuflow system. In general, the motor of the fan can be considered a mechanical oscillator which may produce structure-borne sound. The fan and the airflow up to the linear diffusers are further sound sources with typical spectra of fan and flow noises.

Structure-borne sound

Airborne sound



Figure 3 shows the sound sources investigated and analyzed in this project.

Figure 3: Investigated sound sources of the SCALA fume cupboard

3.1.3 Airborne Sound Absorption

When traveling from its source to the room, the sound is absorbed. Significant impact on this absorption have the geometrical form of the duct and the size of the air gap through which the air is directed. Since resonance may be produced in closed spaces such as the one behind the baffle or in the fume cupboard itself, it is important to analyze and assess the attenuation phenomena in third-octave band resolution.

Comparing the sound levels of the stand-alone control valve and the one installed in the fume cupboard results in a reduction of the sound level since the path from the control valve to the room leads through the fume cupboard which muffles the sound. In the acoustics model the following attenuations have been investigated and considered (see Figure 4):

• installation of the control valve

• baffle including air vane

• closed sash

During investigation to design a model, the difference each of sound power level without and with the investigated component as a attenuator was defined:

• free control valve – installed control valve

Secuflow - system

Sash

Control valve

Baffle with

air vane



• extraction without air vane and baffle – extraction with air vane and baffle

• sash open – closed

Figure 4: Absorption phenomena investigated and considered in the model

3.1.4 Structure-borne Sound

To be able to assess the structure-borne sound and its root causes, measurements were performed with both the non-installed control valve and the entire fume cupboard. Since the control valve represents the most important sound source it may be analyzed whether or not the structure-borne sound originates directly from the valve or is enhanced by transmission to the cupboard casing.

For the measurement, the control valve and the entire fume cupboard were left in the reverberation room while absorbing the airborne sound via inline silencer to a degree that its sound power level would be insignificant compared to the level of the structure-borne sound.

Baffle and air vane Sash

Installation of control valve



3.2 Investigation Findings

3.2.1 Sound Sources

Control Valve

The control valve turned out to be the fume cupboards’ major sound source. Therefore, it is of the utmost importance for the calculation model to be able to calculate as precisely as possible the sound power levels of the control valve for the operating points of interest. For that purpose, the characteristic sound diagram has been determined through measurement for relevant operating ranges for the control valves DN 250 and DN 315 in third-octave band resolution. Based on these data, a mathematical model has been developed to allow for calculation of sound power levels for any third-octave band depending on duct pressure and flow rate. These sound power data in third-octave band resolution form the basis for the acoustics model of the entire fume cupboard. Figure 5 and Figure 6 show a maximum deviation between measurement and calculation of +- 2dB in the sum level.

Table 6: Range of measurement and application for the control valves’ characteristic sound diagram

Valve Duct

pressure Flow rate

DN 250 100 - 250 Pa 200 - 1100 m³/h

DN 315 100 - 250 Pa 300 - 2000 m³/h

Figure 5: Measured values of A-weighted sound power level for control valve DN 315 and

500 1000 1500 2000 2500

45

50

55

60

65

70

75

Measured value calculated value

Soun

d po

wer l

evel

LW

A [d

b(A)

]

Flow rate [m³/h]

Control valve DN 315: Measured vs. calculated values



Figure 6: Difference of measured values and A-weighted sum sound power levels calculated from third-octave band pressure levels for control valve DN 315.

Extract Manifold Without Control Valve

In the 4th stage of the project, a calculation model in third-octave band resolution was developed for the two extract manifolds without control valve. The accuracy with a deviation of only ± 0.5 dB from measurements can be considered excellent. This can mainly be attributed to the fact that without the valve no impact on the airflow is possible and the sound displays the typical spectrum of flow noises.

Air Vane and Baffle

Contact with the air vane and baffle generates flow noises on the edges and gaps. Measurements of the bench-mounted fume cupboard with rear wall installation and control valve DN 250 resulted in clearly measurable sound power levels from 600 m³/h. However, levels are still significantly lower than those of the structure-borne sound from the fume cupboard with control valve. Therefore, when operating the fume cupboard with control valve, the sound produced by the air vane and baffle can be neglected.

Sash

The sash’s sound power level is only insignificantly higher than the background noise in the reverberation room which means that the sound produced by the sash is virtually imperceptible and can be neglected. This noiselessness is, among others, the result of the sash’s aerodynamic optimization.

-2,0

-1,5

-1,0

-0,5

0,0

0,5

1,0

1,5

2,0

0 500 1000 1500 2000 2500 3000

Diff

eren

ce o

f A

-wei

ghte

d so

und

pow

er le

vels

[dB]

Flow rate [m³/h]

Control valve DN 315:Difference measurement - sum level calculated from third-octave band power level

p=100...250 Pa; V=300...2000 m³/h

A-weighted sum level

Upper limit for airflow



Secuflow System

The sound of the Secuflow system is not produced in the main air system but in the substructure and discharge slots in the profiles which are also located in close proximity to the sash’s gap. This is why during measurement no sound absorption was discovered by the baffle, the air vane and the sash. Analysis of the measurements resulted in the finding that the Secuflow system can be considered a completely independent sound source. According to sum-up rules for sound levels, this specific sound source may simply be added to the sound level of the fume cupboard without Secuflow system.

3.2.2 Structure-borne Sound of Control Valve and Entire Fume Cupboard

The share of structure-borne sound of the control valves was measured independently by virtually completely muffling the airborne sound using sound absorbers. It turned out that the structure-borne sound emitted by the control valve may be neglected compared to the airborne sound entering the fume cupboard by air.

In view of this insignificant structure-borne sound of the installed control valve, the question rose whether or not any transmission to the fume cupboard takes place at all and whether the structure-borne sound of the fume cupboard will increase due to excitation of the fume cupboard casing. Measurements have determined here, too, that the structure-borne sound of the entire fume cupboard can be neglected compared to the airborne sound.

3.2.3 Absorption

As mentioned before, absorption phenomena depend on the geometry of the air volume and of the opening surface into the corresponding room. Since there may be resonance phenomena, it is important to perform the analysis in third-octave band resolution.

Installation of the Control Valve

Absorption due to installation of the control valve into the fume cupboard depends on the type of valve used, DN 250 or DN 315. With frequencies below 500 Hz, the attenuation with the larger valve is larger. Attenuation also varies with different model sizes and frequencies between 125 and 400 Hz. The reason for this probably lies in resonance phenomena caused by the geometry of the control valve with the extract manifold in connection with the fume cupboard.

Air vane + Baffle

Attenuation of the air vane in connection with the baffle changes in the range of 100 Hz and 500 Hz depending on the model size and control valve. The difference between the bench-mounted fume cupboard with rear wall installation (RWI) and the bench-mounted fume cupboard with side installation (SI) regarding attenuation is minimal.

Reasons for variations are probably again certain resonance phenomena resulting from the space behind the air vane and baffle. This space changes with varying model sizes and is also affected by the size of the control valve.

Sash

Attenuation by the closed sash depends neither on the model size nor on the size of the control valve nor on whether the fume cupboard is of type RWI or SI. The main attenuation effect is



ased on the fact that the flow exit extent into the room is getting smaller when the sash is closed. Other resonance phenomena in the fume cupboard’s airspace which could lead to variations in attenuation in certain frequency ranges were not determined during the measurements.

Consideration in the Model

When installed to the fume cupboard, the two control valves have a typical, frequency-dependent attenuation behavior which is considered in the calculation model.

The attenuation behavior of the air vane and baffle depends on the fume cupboard’s size. Therefore, for the model, the attenuation of all the sizes would have to be determined. Since this would mean considerable effort, the mean attenuations of the investigated sizes were used for each third-octave band for the calculation model. This is reasonable since variations occur in a limited frequency range only and do not play a very important role for determination of A-weighted sum sound power levels. This compromise makes it possible to extrapolate the sound power levels to other model sizes.

Since the differences in attenuation behavior between types RWI and SI were insignificant, the calculation model does not differ between the two types.

The attenuation behavior of the sash is more or less the same with any investigated fume cupboard variant. Therefore, in the calculation model the mean of measured attenuations was taken.

3.2.4 Calculation Model Summary

Following measurements and analyses using the fume cupboards RWI 1500, RWI 2100, and SI 2100 in combination with the control valves DN 250 and DN 315, the calculation model was designed as follows:

Sound source:

• Calculation of sound power levels of the control valve in third-octave band resolution, depending on exhaust air flow, differential pressure and control valve size

Deducted are the attenuations for

• installation of the valve

• air vane + baffle

• sash (if closed)

Secuflow system (if existent)

• is added to the sound power level as an independent sound source.

The structure-borne sound plays no role and is neglected.

Generation of sound at the baffle, air vane, and sash during operation with control valve is of minor importance and, therefore, neglected.

Fume Cupboards Type FAZ

The fume cupboard types without control valve (type FAZ) are characterized by lower sound power levels since the control valve as a major sound source is non-existent. In that case, the



noise on the baffle and air vane is decisive for the sound level of the fume cupboard. Consequently, the calculation model cannot be used to calculate sound power levels for these cupboards without control valve.

Since the generation of sound has only been investigated for one air vane and baffle, integration of these sound sources into the calculation model would require further investigations using a variety of sizes and rear walls.

3.3 Verification of Calculated Results and Accuracy of Model

Application of the acoustics model is restricted to SCALA fume cupboards. In order to verify the calculated results, measured data on which this model is based and measurements dating back to the year 2009 were referred to.

Since the calculation model for SCALA fume cupboards provided practical data, it was investigated whether this model could also be used for low-ceiling fume cupboards. For that purpose, measurements were performed using a fume cupboard of type NTA 1800.

3.3.1 Standard Fume Cupboards

When comparing calculated data with the measurements, the calculated A-weighted sum levels are no more than 1.5 dB too high or 2.3 dB too low.

Regarding the third-octave band pressure levels being relevant for the sum level (100 Hz - 2500 Hz), the calculated data are up to 3.9 dB too high and no more than 5.1 dB too low. These relatively large deviations are found in the frequency range of 100 Hz and 400 Hz where attenuation figures are subject to major variations based on the geometry. In the entire third-octave spectrum, the calculated levels are by no more than 6.1 dB too high or 5.9 dB too low. An example for comparison of measurements versus calculated third-octave levels is shown in figure 7.

For the octave bands which comprise frequency bands of 3 thirds each, the calculated levels in the 250 Hz - 4000 Hz range are up to 3.2 dB too high and no more than 3.1 dB too low. Regarding the entire octave spectrum, the calculated levels are no more than 5.4 dB too high and 5.8 dB too low, respectively.

In 6 cases, calculation results could be compared to actual measurements from the year 2009. In the A-weighted overall sound power level, deviations are below ± 1 dB. When comparing the octave levels, the calculated values are up to 5 dB louder or 3 dB quieter. These deviations occur with low or high frequencies which contribute only insignificantly to the sum level and are, thus, tolerable.



Figure 7: Example of comparison measured versus calculated values

3.3.2 Low-ceiling Fume Cupboards

Comparing calculated versus measured values, the calculated A-weighted sum levels are up to 1.5 dB too high or 2.0 dB too low, respectively. Thus, deviations are of the same dimension as in the standard fume cupboards.

In almost all third-octave bands, the deviations found in the NTA 1800 are no bigger than those encountered with the standard fume cupboards. An exception is the third-octave band with the 160 Hz center frequency. Here, the calculated values may be up to 8.5 dB too low. This is probably due to resonance phenomena in the airspace behind the baffle and air vane in the fume cupboard.

In the octave spectrum, three third-octave bands are combined for each frequency band which usually makes the deviations from measurement smaller. In the range relevant for the sum level, calculated values are no more than 4.2 dB too high or 3.6 dB too low, respectively. If you look at the entirety of octave bands, the calculated figure may be up to 6.5 dB too small (with air flow rates of 200 m³/h and low duct pressures of 100 Pa).

3.3.3 Accuracy of Model

Comparisons provided in the preceding paragraphs suggest that the A-weighted sum level for standard fume cupboards may be calculated with an accuracy of ± 2.5 dB using the acoustics model. The same applies to low-ceiling fume cupboards.

0,0

10,0

20,0

30,0

40,0

50,0

60,0

50 63 80 100 125 160 200 250 315 400 500 630 800 1000 1250 1600 2000 2500 3150 4000 5000 6300 8000 10000

A-w

eigh

ted

sou

nd p

ower

leve

l [d

B(A

)]

Third-octave band center frequency [Hz]

Comparison Measurement vs. Calculation for RWI 2100 with control valve DN 250V= 570 m³/h

100

175

250

100 Measurement

175 Measurement

250 Measurement



In the third-octave bands relevant for sum level (100 Hz - 2500 Hz) the accuracy is +5.2 dB to -4.0 dB. For low-ceiling fume cupboards, this tolerance will increase in the 160 Hz band range to +8.5 dB.

In the octave bands relevant for sum level (250 Hz - 4000 Hz) the accuracy is +3.6 dB to -3.6 dB. For low-ceiling fume cupboards this tolerance will increase to +4.5 dB.

4 Planning Recommendations

For acoustic planning of lab extract air systems, the special features in the lab area are to be considered. Based on VDI 2081 Sheet 1, sound pressure levels may not exceed 52 dB(A) in chemical labs.

The sound pressure level inside the room depends on existing sound sources and room absorption.

Room Absorption

According to VDI 2081, the reverberation time of labs is 2 seconds which means a room absorption of about 5 dB for standard labs (55 m², 3.3m high). Reason for this low absorption are the sound-reflecting walls, floors, and ceilings. Suspended, acoustically effective ceilings which may reduce the reverberation time and muffle sound are often missing.

Sound Sources

When acoustically planning and assessing a lab ventilation system, all sound sources inside the room must be considered (see Figure 9):

• Lab fume cupboards

• Air diffusers

• Local exhaust ventilation

• Chemicals cupboards

• Supply air systems

• Extract air systems

Since the extract air system usually does not include sound absorbers, pressure and flow rate controllers play a significant role as the sound of these control valves travels into the room via fume cupboards and air diffusers. The only sound absorption achieved is through redirection, branches or diameter jumps and may be calculated based on VDI 2081. In lab fume cupboards, the sound absorption of the control valve is insignificant since the sound travels only a short distance from the valve to the lab and no sound absorbers can be installed.

Another sound source in labs which should not be neglected is the sound emission via air duct surfaces in both the supply and the extract air system. It will have to be considered that flow noise from control valves travels in both directions, upstream and downstream. This means that even air ducts located before the control valve do, in fact, emit sound. Furthermore, air ducts are often installed directly into the labs without the use of suspended ceilings so that there is no sound absorption from such suspended ceilings.



Figure 8: Example of a lab equipment including some of the most important, acoustically relevant air technology components

Recommendations

In the air supply system, the air is still unloaded and directed via standard air diffusers into the room. With these conditions, sound absorbers may easily be installed and the propagation into the lab of airborne sound originating from control valves may be minimized. The installation location may be selected in a way to also minimize the sound emission via supply air duct surfaces. In combination with low air velocities in the duct system (2-4 m/s) the air supply system may be designed to produce significantly less noise than the extract air system.

The main sound sources in the extract air system are control valves (flow rate controllers, duct pressure controllers) installed in the air ducts or fume cupboards. The most important factor is that the control valves of the fume cupboards are installed directly in or behind the fume cupboards and that no sound absorption may be installed. The higher the pressure loss the control valves must generate to keep up the flow rate or duct pressure, the louder the control valves. Therefore, it must be considered during planning that the extract air system is properly dimensioned and provided with a line pressure control that allows for operation of the fume cupboards at minimum required duct pressure.

Since the pressure control valves in the extract air duct generate sound as well, not only the duct pressure should be controlled but so should the overall pressure of the installation via targeted speed control of the fan. Under certain conditions, proper dimensioning of the duct system and the use of a pressure-controlled fan may even be sufficient to do without line pressure control (see Figure 10).



If the pressure control also considers the control valve settings, the fan speed may be perfectly adapted. In general, the speed is lowered to a degree that one control valve opening is maximal. With that setting in the line only the minimum required air volume is conveyed. All other valves will be closed to a necessary degree. This avoids that due to the fan speed unnecessary pressure reserves have to be made available.

Another possibility to reduce the sound level is to perfectly position the control valve inside the duct line. If the control valve is positioned close to the air diffuser, the distance for airborne sound from the valve to the lab is short while the absorption is poor. From an acoustics point of view, a system should be designed in a way to avoid any excessive number of control valves, install the valves as far as possible away from the diffusers, and generate only so much pressure loss as absolutely necessary.



Figure 9: Example of an optimized duct system with pressure-controlled fan

acoustics model for scala fume cupboards by waldner · 2012-01-31 · acoustics model that allows...

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