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Building Services Acoustics Dos and Don’ts

Acoustic Terminology
Building Services Recommended NR Levels
Addition of Decibels
Calculating a dBA Level
Noise Reduction by Distance
Other Common Terminology
Dos and Don’ts
Building Services Noise Control Products
Cross Section through typical IAC Quiet-Duct Silencer
Forward and Reverse Flow
Acoustic Louvres
Sound Absorptive Panels
Typical Applications for Absorptive Panels
Anti Vibration Isolation
Floating Floors
Air Handling Units


Acoustic Terminology


Acoustic Terminology
Sound Sound is vibrational disturbance,exciting hearing mechanisms,transmitted in a predictable manner determined by the medium through which it propagates.(to be audible the disturbance must fall within the 20 to 20,000Hz range)
Noise Basically too much of the above.
Decibel (dB) Unit for measuring sound derived from the Latin deci meaning tenth and bel from early researcher Alex Bell. Ten times the logarithm (to the base of ten) of the ratio of two mean square values of sound pressure,voltage or current.
Spectrum The audible range of sound frequencies arranged in sequence according to cycles per second (Hz). In our work we use 8 octave bands 63 to 8000Hz.
Octave Band That part of the sound spectrum included between the limiting frequencies of an octave.(an octave has a frequency ration of 2 eg 63-125- 250 etc)
Frequency The number of complete cycles per second (Hz) which characterises any periodic phenomenon.
Sound Pressure Level (SPL) This is the measured or calculated sound pressure which exists at a distance from the sound source due to the sound energy (power) and stated in decibels (dB) either as an overall level or in octave bands .Is often quoted as an ‘A’ scale figure.
Sound Power Level (SPwL) Where the SPL is affected by conditions surrounding it ie presence of barriers,wind speed and direction or ground cover the SPwL is the noise energy AT THE SOURCE.
Ambient Noise The sound pressure levels associated with a given environment .Ambient noise is usually a composite of sounds from near and far sources none of which are particularly dominant.
Audible Frequency Range The range of sound frequencies normally heard by the human ear.
‘A’ Scale This is the scale derived to measure sound pressure level (SPL) .It is weighted electronically in the frequency range below 1000Hz to compensate for the reduced sensitivity of the human ear to low frequency sounds at low pressure levels.(There is also B and C scales rated lower but not generally used on building services work).
‘A’ Sound Level This is a measured or calculated SPL referenced to the A scale eg 85dBA sometimes used as a measure of loudness.(For example a fan 65dBA at 3 metres)
Noise Criteria (NC Level) A system of rating how noisy an occupied space is with readings given in dB in each frequency. The curves from these are not used in Europe now.
Noise Rating (NR ) This is the standard introduced by ISO in 1970 to which we now rate our noise requirements.This uses the 1000Hz band as a reference point and you will note for example NR55 is 55dB at 1K.
Breakout Noise Noise emitted through a surface eg Fan or AHU Casing
Free Field Sound from a source free from obsticles ie walls,buildings etc.
Flanking Noise bypassing measures to reduce it.
Reverberation Time (T ) Acoustical absorption of a room measured in seconds.
Self Noise Noise generated by an attenuator or louvre by air velocity.
Dynamic Insertion Loss Attenuator acoustic performance with airflow.
Static Insertion Loss Attenuator acoustic performance with no airflow.
Cross Talk Audible voice communication between compartments.Often a problem in ducts passing between rooms.



Building Services Recommended Noise Rating (NR) Levels


Noise rating curve Application
NR 25 Concert halls, broadcasting and recording studios, churches
NR 30 Private dwellings, hospitals, theatres, cinemas, conference rooms
NR 35 Libraries, museums, court rooms, schools, hospitals operating theatres and wards, flats, hotels, executive offices
NR 40 Halls, corridors, cloakrooms, restaurants, night clubs, offices, shops
NR 45 Department stores, supermarkets, canteens, general offices
NR 50 Typing pools, offices with business machines
NR 60 Light engineering works
NR 70 Foundries, heavy engineering works



Addition of Decibels

Adding 40 apples to 40 apples makes 80 apples.This is not the case with decibels as they are logarithmic.
40dB + 40dB actually gives you 43dB. The following gives you how to add them up

Sound Power Level Difference between two Sound Sources
(dB)    Added Decibel to the Highest Sound Power Level (dB)
0    3
1    2.5
2    2
3    2
4    1.5
5    1
6    1
7    1
8    0.5
9    0.5
10    0.5
> 10    0

(As most of our work involves allowances and approximations round half decibels up to one.)
This is useful when you have a number of plant items in a room and you need to know the likely noise level when all are operational.

Calculating a dBA Level
(from SPL in dBA in each freq.)
Hz  63  125 250  500  1K   2K   4K   8K
Measured SPL                          dBA  54   60   64   53    48   43   39   32
dBA Filter                                        26   16    9    3      0    -1    -1    1
Resulting SPL in dB                          28    44   55  50    48   44   40    31
Addition (see above)                            44         56           50          41
56                       51
(If the decibel figures given are not ‘A’ weighted you do not apply filter figures)

Noise Reduction by Distance

For every doubling of the distance from the noise source the sound pressure levels Lp will be reduced by 6 decibels.
This is only a rough guide as barriers ,weather,ground conditions etc have to be taken into account and calculation is necessary.


Other Common Terminology

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  • Breakout Noise :Noise emitted through a surface eg Fan or AHU Casing
  • Free Field : Sound from a source free from obsticles ie walls,buildings etc.
  • Flanking: Noise bypassing measures to reduce it.
  • Reverberation Time (T ): Acoustical absorption of a room measured in seconds.
  • Self Noise : Noise generated by an attenuator or louvre by air velocity.
  • Dynamic Insertion Loss: Attenuator acoustic performance with airflow.
  • Static Insertion Loss : Attenuator acoustic performance with no airflow.
  • Cross Talk:Audible voice communication between compartments.Often a problem in ducts passing between rooms.



Do’s and Don’t

Manufacturers present their acoustic data in different ways probably to confuse us into thinking their product is the quietest.For instance chillers and condensers are quoted as a single dBA figure at a set distance eg 1.5,5,10 metres or more.So do not think one supplier who quotes 50dBA is necessarily quieter than another who quotes 60dBA.The best supplier is the one who clearly gives their noise parameters and preferably SPwL in single figures across eight octave bands. Fan suppliers commonly quote  their noise levels as a single figure dBA level at 3 metres distance.They also tend to call it ‘in a free field’ .Fans are never fitted in a free field. This figure is also assumed to be breakout noise via the casing although some suppliers are a bit vague about this.So no account is taken with regards to the induct noise (inlet or outlet) in this figure. The best way to use the dBA at 3m figure is simply to compare one product with another .Remember if Supplier A moves xm3/s at yPa with a cased axial fan at 1400rpm with a 5.5Kw motor then Supplier B cannot use the their cased axial with same motor and dimensions and be quieter. Most fan suppliers present decent in duct sound power level information.Just check always that the figures across the spectrum are always dB and not dBA. It has been noted that at least one manufacturer quotes their noise data at speed controlled level.Ensure that you are happy that this is the speed you expect the system to run at. When possible avoid selecting 2 pole motors (2800rpm) for anything other than very small fans.Generally they are ‘screamers’ and often ,while we can deal with the in-duct noise, we are left with a breakout problem which can be difficult to resolve.That said, when you have high pressure drop systems,you need to develop high energy which equals higher noise so often a bigger fan at lower speed may be almost just as noisy as a high speed model. Bifurcated fans have their motors out of the airstream so keeping these quiet can be problematic.The motor needs air for cooling so duct wrapping or jacketing is not an option.Stand off barrier panels can be utilised but may only offer 7/10dB benefit.


Building Services Noise Control Products

Duct Silencers (or Attenuators)
Passive method of reducing noise from air handling plant by insertion into the airflow.These products are designed and tested and are supplied rectangular,circular,fibre free,cross talk,low frequency,green,stainless steel and Upvc.
Acoustic Louvres
For screening external plant such as chillers and condensers of air handling plant or as inlet /outlets for plant rooms.
Screens and Enclosures
Solid acoustic panels can be used to enclose or screen noisy plant often in conjuction with acoustic louvres.
Absorptive Panels
For reducing the distracting echo or reverberation inside buildings eg plant rooms.Can improve environmental and speech clarity .Often employed in school gymnasiums.
Vibration Isolation
Spring ,rubber or neoprene mountings or pads and inertia bases along with pipework flexible connections.
Floating Floors
Comprises two mass layers separated by air space and decoupled from one another with resilient mountings.
Air Handling Units
On floor units available that offer up to 15dB less noise than conventionally constructed units.

Cross section through typical IAC Quiet-Duct Silencer

1. Die-formed single-piece splitter constructed throughout
2. Shell-noise radiation minimised by double-skin or splitter construction in most models
3. Acoustic splitters designed for maximum attenuation at low frequencies, the toughest job.
4. Straight-through air passages designed for minimum pressure drop
5. Solid, rounded noses that increase noise reduction
6. Bell-mouth entrance and exit to minimise turbulence, pressure drop and Self-Noise
7. No protruding fastener heads to case turbulence or Self-Noise
8. Solid air-impingement surfaces and self-cleaning air passages to minimise dirt entrapment
9. Acoustic fill protected against erosion by perforated metal containments

Forward and Reverse Flow

In 1972, IAC developed silencer Dynamic Insertion Loss and Self-Noise ratings
both under  FORWARD FLOW (+) and REVERSE FLOW (-) conditions for Rectangular
and Cylindrical Duct Attenuators. Since attenuation values are generally higher in the
first five octave bands in the Reverse Flow (-) mode compared to the Forward Flow (+)
mode, more economical silencer selections can often be made on return air systems.
(Silencer selection help in appendix)
These phenomena are illustrated below.

Acoustic Louvres

High-performance acoustic louvres should be laboratory tested and rated. All  products
come with comprehensive acoustic performance data, offering a genuine solution to
noise reduction challenges. The main features and benefits of acoustic louvres are:
1. Rugged all-steel galvanised construction. Stainless steel, aluminium and other materials available.
2. Inert, vermin-proof, weather-rated, non-combustible acoustic fill.
3. Aerofoil shaped splitter blade (type R and LP) for maximum noise reduction with minimum pressure drop.
4. Perforated splitter underside for maximum sound absorption.
5. Weather stop inhibits rain/snow entry.
6. Available in a variety of depths, in both single and double banked versions.
7. Available in a variety of durable, attractive finishes, e.g. vinyl coated steel, polyester powder paint, mill finish aluminium, Syntha Pulvin, galvanized and stainless steel.
8. Modular sizes enable assembly of rectilinear louvre walls of almost any size.
9. Louvre blade orientation blocks horizontal line of site, enhancing both aesthetics and acoustic performance.
10. Detachable galvanised bird screen fitted as standard on rear of louvre. Insect screens also available

All plant rooms will require inlet and outlet louvres.While acoustic louvres may cost a little more than typical aluminium louvres the difference is worth having to give a bit more comfort. These louvres come 100,150,300mm deep as standard and can be doubled up to 600mm when required. On applications where screens or enclosures are necessary doors can be incorporated for access purposes.

Sound Absorptive Panels

Sound absorption panels have the following benefits:
Can reduce noise levels by up to 10dB
Control reverberation
Laboratory tested
Unobtrusive wall or ceiling mounting
Robust construction
Rapid installation
Attractive finishes easy to clean
Typical Applications for Absorptive Panels
Sound absorptive panels control noise and reduce reverberation inside buildings
and offer ideal solutions in a wide variety of environments including:
Boiler Rooms
Bottling Plants
Exhibition and Conference Centres
Factory Interiors
Theatres and Concert Halls
Water Pumping Stations

Anti-Vibration Isolation

With modern day design, equipment speeds have become faster, and concern with low RPM, while  theoretically correct, is becoming less and less important. At the same time, modern buildings are constructed using lighter structures than in the past. This means they require greater protection if noise and vibration are to be prevented from being structurally transmitted to critical areas. Anti-Vibration solutions that effectively combat such structure-borne noise and vibration is vital. Use from an extensive range of vibration isolation equipment including pads springs and hangers. Vibration Isolators and Anti-Vibration Mounts are ideal, for example, in creating floating floors
for gymnasiums, or performance spaces.

Floating Floors  (Three main types)

Panel Mount Systems are the standard type of floating floors. They incorporate neoprene, glass fibre, cork or spring isolators that provide an acoustic break and a fixed air gap between the ‘floating’ concrete floor slab and the structural building slab. Jack-Up Systems have a similar construction to the Panel Mount System with the addition
of adjustable isolators providing the ability to vary the air gap between the ‘floating’ concrete floor slab and the structural building slab. Acoustic Panel Systems incorporate acoustic panels in lieu of the concrete floor slab.
Using pre-fabricate modular panels removes the need for wet trades on site and enables the floor to be dismantled and re-located if required.

Air Handling Units

On-floor air handling units (AHUs) are up to 15dB quieter than conventional AHUs. They can incorporate axial, mixed flow and plug fans, and have highly sound-absorptive acoustic panelled housings with high-performance Noise-Lock® doors and access floors.

These AHUs are the best choice for a ‘new build’ and high-rise commercial buildings. In simple terms, our they provide quiet air handling. The airflow inside a mechanical equipment room is typically 17m³/s – with a noise rating of NR60. However, with a Quiet Flow Unit in place the sound levels in adjoining offices are typically between NR35 – NR38. This provides a quiet working space for employees. A flexible UK manufacturing and assembly plant will accommodate small or large production runs. AHU assembly can occur either in dedicated areas at the factory, adjacent to job sites or for overseas projects. Incorporate locally manufactured components and includes the delivery, offloading, positioning, installing and commissioning of most AHUs.Available also is the design and manufacture of space-saving and compact-footprint AHUs just for slotting into a restricted space. The Quiet Flow’s duty range is between 5 – 18m³/s.