Acoustic Guidelines

IT IS AT LEAST TEN TIMES MORE EXPENSIVE TO CORRECT AN ACOUSTIC PROBLEM THAN TO PREVENT IT.

All Nuaire products are tested in accordance with the UK, European and Internal industry standards for performance and sound levels.

These laboratory test standards are designed to provide a consistent test method and which to a limited extent that simulate the ways that a fan may be used in a variety of installations.

Nuaire generally quote the noise levels separately for the unit inlet, outlet and casing radiated (or breakout) sources, to enable the system designer to properly evaluate the product in the application.

The noise level measured on site, for example during the commissioning process, is the sound pressure level – and this cannot be directly compared with the sound power levels quoted in our fan selection tools (Nuaire catalogue and Fan Selector).

Nuaire typically also quote a free field dBA level at 3m, and this figure, whilst useful for comparison with other products, has a very specific definition – and cannot be used for acoustic design purposes.

The dBA @ 3m figure quoted will never correspond to a DBA measurement taken at 3m from a product in a real building.

Acoustics is a complicated subject and must be treated with caution as part of the services design process. Our sales engineers and technical support staff will often be able to offer advice on appropriate product types for an application, but it is often necessary to refer to acoustic specialists for a definitive design solution.

Nuaire have put together the following bullet points for your information and assistance:-

SYSTEM DESIGN – GENERAL AND COMMON SENSE ISSUES

  • Position the fan for minimum noise impact
  • Position grilles and diffusers for minimum noise impact
  • Design the system for low pressure drop and smooth air flow = low velocity ducting and grilles etc.
  • Where appropriate, use barriers and enclosures of a suitable specification – ensuring that they are properly installed (eg ceiling tiles and bulkheads). Special acoustic grade materials are generally available, and the first principle is to add mass to the system – eg layers of plasterboard.
  • Use appropriately specified attenuators and Anti-Vibration mounts – again - properly installed and not bridged to the support structure.
  • Flexible or lightweight supporting structures, or slow running fans, can require higher levels of mechanical isolation, and this can only be achieved with high deflection spring type mounts.
  • Flexible Connectors are often a particular source of breakout noise – if they must be used in sensitive areas, then an acoustic grade should be specified.
  • Single skinned rectangular attenuators can also be vulnerable to breakout at the fan connection – consider this carefully in your specification. Ideally, use attenuators that have been specified with breakout protection, and fit the flexible connectors at inlet attenuator entry and outlet attenuator exit. Any deviation from a straight inlet/outlet condition, whether by crumpled flexible connectors, bends, or offset transformation pieces, will lead to an increase in the quoted induct noise levels.
  • Specify the correct duty with minimal allowances - and commission the systems properly.

As a very rough “rule of thumb”, due to the combined effects of an acoustic room property known as “directivity” and a distance correction for a real room situation, it is possible that the dBA level measured in a room with a ceiling void mounted fan above it, can be increases of 8-9 dBA up to 14 dBA higher than the Free Field figure.

In general terms, a specification of NR 35 or 40 dBA represents a very acceptable level for most people in an office environment.

A requirement to achieve levels of NR 30 or 35 dBA or less in a working space, can be considered as challenging, and may require specialist acoustic assistance. At the very least, with such a specification, the acoustic requirements should be a major design consideration.

Designated “Quiet” areas and bedrooms will need to operate at these levels and preferably lower.

A final note: It is at least ten times more expensive to correct an acoustic problem than prevent it.