Acoustic Enclosure Design – The Basics

Ever wondered how our team approach the design of an acoustic enclosure? Well, you’re in luck, as this is the theme of our first technical blog post.

Having done the design and install of numerous bespoke enclosures CSG Acoustics in conjunction with Acoustic Hardware Ltd get it right every time. So don’t rely on an acoustic consultant who has no understanding of the onsite constraints – give us a call and we’ll assist you each step of the way!

This technical guidance is considered ‘high-level’ design information and is brief, and should not be used to specify any item, be it AC unit, noise control or spatial requirement. CSG Acoustics should be contacted for a detailed design.

All images, designs and drawings are copyright of CSG Acoustics and Acoustic Hardware Ltd and should not be used without approval from either company..

Ever wondered how our team approach the design of an acoustic enclosure? Well, you’re in luck, as this is the theme of our first technical blog post.

All images, designs and drawings are copyright of CSG Acoustics and Acoustic Hardware Ltd and should not be used without approval from either company.

How Does a ‘typical’ Acoustic Enclosure Work ?
Airflow – how does this influence the design?
Pressure – how does this influence the design?
Acoustic Attenuation – how much attenuation can you expect?
Typical AC Configuration – and an example of each type of design

HOW DOES A ‘TYPICAL’ ACOUSTIC ENCLOSURE WORK?

Typically an acoustic enclosure works by attenuating the noise of the AC unit whilst maintaining suitable airflow and access requirements.

Figure 1 shows the simplest form of an acoustic enclosure, which includes acoustic louvres to the side and front to allow airflow in and out of the enclosure.

The method of attenuation selected depends on numerous elements including;

Spatial constraints – architectural requirements
AC Unit Airflow – typically ranging from 1m³/s – 7m³/s
Allowable Pressure – typically ranging from – 0 – 60 Pa
Required Acoustic Attenuation – usually based on planning requirement – typically between 5-20dB
AC Configuration – Top or Front
Access

Each of the above will be dealt with briefly in this design note.

Assuming all of the design considerations can be accommodated the unit(s) will still maintain the manufacturer’s efficiencies and with the correct coordination will also be covered under the manufacturer’s warranty.

AC AIRFLOW

How does this influence the design?

There is a great variance in the amount of airflow that the AC unit can circulate, typically between 1m³/s – 7m³/s. In simple terms, the greater the airflow, the more ‘open’ to air the acoustic enclosure needs to be, ultimately resulting in a bigger and more cumbersome acoustic enclosure.

The acoustic design engineer will look at the air face velocity, airway passage velocity and the passageway profile of the constriction to determine both the amount of regenerative noise and the pressures that are put on the AC unit.

PRESSURE

How does this influence the design?

The velocity of the airflow through the constrictions of the acoustic enclosure (usually acoustic louvres/attenuators) creates backpressure. The AC unit needs to have the capacity to overcome this pressure to be able to move the air efficiently. A greater level of acoustics often creates more constrictions, which in turn increases the pressure.

Generally, pressure allowances for AC units range from 0-20Pa for front-discharge units and between 20-60Pa for top-discharge.

It is often beneficial for the acoustic design engineer to request a change from front-discharge units to top-discharge units so that there is more flexibility to design an acoustic solution with higher pressures.

ACOUSTIC ATTENUATION

How much sound reduction can you expect?

Acoustics is complicated. The AC unit has a spectral content and the type of acoustic attenuation selected also has a frequency dependent performance. Therefore the amount of reduction is specific to the unit/installation.

Generally, it could be accepted that a louvred enclosure would give between 10dB and 15dB of overall attenuation, whilst an enclosure employing splitter banks could reach attenuation of up to 25dB.

For specific octave band performance values for various solutions please visit http://acoustichardware.co.uk/products.html.

TYPICAL SPATIAL PLANNING

The images below are an example of the typical dimensional requirements for noise control to be added to top discharge units. The images show access requirements, airflow space, ant-vibration mount allocation and connections and expansions.

Acoustic Enclosure Design – The Basics