Kitchen extract fan with duct-mounted attenuator

Kitchen extract fan

Posted: 14th October 2022

How to control noise from kitchen extract fans

Noise from kitchen extract fans can cause a nuisance to people living near restaurants and other licensed premises.  Planning permission for HVAC equipment will often include a noise condition to be achieved.  Even where there is no planning condition then noisy equipment could be considered a statutory nuisance and need to be controlled.  This guide discusses how to assess noise from kitchen extract fans and the best way to control noise and vibration.

What noise level is required for kitchen extract fans?

There is no single set noise level criterion for kitchen extract fans and other HVAC equipment.  Many Local Authorities use the method in BS 4142.  This compares noise from the fans against the background noise level.  Penalties are applied if there are adverse acoustic characteristics.  These would make the noise more noticeable, including if the noise is tonal, has a distinguishable ‘hum’ or ‘whine’, or is intermittent.

BS 4142 requires that the context of the development needs to be considered.  Typically, where the corrected noise from the fan (the rating level) doesn’t exceed the background sound level at the receptor, then it’s likely there is a low impact.

Some Local Authorities use the BS 4142 assessment outcome as written in the standard.  Other councils use the BS 4142 method but set their own criteria.  Typically, this would require the rating level to be 5dBA or 10dBA below the background noise.  Others have their own assessment method, while finally a few councils have set fixed numerical noise limits.

 

Background sound level survey

Almost all sites require a background sound level survey.  Noise levels are measured over the proposed operating times of the kitchen extract fan.  Usually the survey is carried out over a minimum 24-hour period.  Appropriate weather conditions are important to accurately assess the background sound level.  Results of the background noise survey will determine the maximum acceptable noise level for your kitchen extract fan.

 

 

Noise data for kitchen extract fans

Most kitchen extract fan manufacturers publish noise data from their fan.  However, it can be confusing as noise levels from different manufacturers isn’t always comparable.

 

Difference between sound power and sound pressure levels

Most manufacturers published fan data as sound power levels however some only give sound pressure levels.

The difference between sound power and sound pressure can be thought of as the difference between the power rating of a heater and the temperature in the room.  The Kilowatt rating of the heater doesn’t change.  However, the temperature varies depending on the size of the room and how far you are from the heater.  Similarly, the sound power level of a particular kitchen extract fan model is constant, while the sound pressure level varies depending on the measurement conditions.

For this reason, we always recommend using sound power levels as a basis for acoustic calculations wherever possible.

 

Inlet, discharge, and casing radiated sound power levels

Sound levels are usually slightly different on the suction side of the fan compared to the discharge side.  Noise levels radiating from the fan casing will be considerably lower than the noise levels in the ductwork.  Some manufacturers publish all the data, whereas other might only give a single level for their fans.

 

The effect of air volumes and pressure on sound levels of kitchen extract fans

Changes to the air volume or the pressure on the fan can have a big impact on the noise levels.  If a fan must work harder, then noise levels will increase.  Some fan manufacturers provide accurate noise data for the fan at the working duty.  Other companies only give noise levels at a specific duty.  This can be the maximum level from the fan, or at a ‘typical’ duty.  It is important to be aware if this is the case, as it could mean that noise levels from the fan at your site could be considerably higher (or lower) than the manufacturer’s data suggests.

 

 

Calculating noise from a kitchen extract fan to nearby receptors

Once you have sound power levels for the fan being installed, it is necessary to calculate what the noise level will be outside noise-sensitive receptors.

The impact of the ductwork system needs to be considered.  Generally, noise levels will reduce as they travel down the ductwork.  There are various ductwork system noise calculators available, including some good free ones available online.  If the airflow velocity in the ductwork is very high, then it can add to noise levels from the fan.  These free ones won’t normally include the effect of airflow generated noise in the duct and other more complex elements.  But, for most systems they will give a good indication of expected noise levels.  At ACA Acoustics our computer calculation models use the detailed calculation methods for ductwork systems given in CIBSE Guide B4.  This includes assessing airflow generated noise.

Having determined the noise level radiating from the kitchen extract fan exhaust it is then necessary to calculate noise levels at the receptor.  Noise levels will change depending on many factors, including the distance between the kitchen extract fan exhaust and the receptor, any acoustic screening such as from intervening buildings or walls, the angle from the exhaust to the receptor, and acoustic reflections from nearby buildings.  We use the detailed calculation method of ISO 9613-2 to calculate external noise transmission.

 

 

Design of noise control treatments

Calculated noise levels from the kitchen extract fan and other HVAC equipment at the residential receptor are then compared against the required noise criteria.  Noise control treatments will be required if there is any excess.

 

Duct-mounted attenuators

Noise from the exhaust terminal of a kitchen extract fan, or the inlet of a supply fan, is usually controlled using duct-mounted attenuators.  These can be circular or rectangular type and are sized to achieve the performance required.

 

Acoustic enclosures

If noise from the fan casing exceeds the criteria, then this is usually controlled by installing an acoustic enclosure around the fan.  Acoustic enclosures are usually formed using 50mm or 100mm thick acoustic panels, with Rockwool insulation core and steel casings.

Depending on the orientation of the fan and receptor, and level of noise reduction required, it is sometimes possible to install an acoustic screen, rather than a full enclosure.

 

Acoustic ductwork lagging

Noise breakout from the ductwork can sometimes be a problem.  This is often the case when a kitchen extract fan riser passes close to residential windows as it travels up the building.  To control noise breakout, it can sometimes be necessary to acoustically lag the ductwork – or even manufacturer the ductwork with double-skinned casings.

 

Vibration isolators

If a kitchen extract fan is structurally connected to noise-sensitive properties, there is potential for structure-borne noise or vibration to transmit into the dwellings.  In this instance kitchen extract fans would normally be installed on steel spring type vibration isolators.  It is also important to consider transmitting into the ductwork – both from the fan but also potentially due to airflow turbulence in the ductwork.  This may require flexible connectors between the fan and ductwork, and potentially installing the ductwork on acoustic hangers.

 

 

How ACA Acoustics can help

Using free calculation guides online and considering the best-practice noise control methods discussed above is a good starting point to assess noise from your kitchen extract fan.  However, the assessment is likely to require a background sound level survey which requires specialist monitoring equipment and expert knowledge to ensure the results are correct.  Furthermore, BS 4142 requires that the assessment is carried out by someone with a suitable level of competency.  Noise assessments carried out by someone without sufficient experience or qualifications are unlikely to be accepted by the Local Authority.

We have extensive experience in assessing the noise from kitchen extract fans and other types of mechanical services equipment.

To discuss your project in more detail with one of our experienced acoustic consultants, and to find out how we can assist in controlling noise from your kitchen extract fan, please call us at your local office number, or use the contact form.

Posted By:Rob Cant

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