WHEN SILENCE IS GOLDEN

Published:  01 October, 2018

John Tarling, fan coil unit, noise, sound power level, SWL, Sound path, Noise rating, NR, Dunham-Bush
John Tarling

Fan coil units are a popular choice for engineers when selecting a reliable and efficient source of heating and cooling for a building. They do not require a large central plant to function and with simple proven, controllable technology and installation flexibility, they meet the demands of most commercial applications.

One of the key factors influencing the choice of fan coil units for buildings, such as leisure centres, shopping malls, museums, art galleries and colleges, is noise. As the selection of fan coil units is governed not only by thermal or air volume flow rate requirements, but also by any constraints on the level of noise permitted in the room, the manufacturer is frequently required to select fan coil units based on the predicted noise levels in the room because of the fan coil unit application, however these are influenced not only by the fan coil unit but also by the room itself.

During the preliminary design stages, not all the room conditions may be known, but certain assumptions allow room noise levels to be approximated.

The human ear reacts to fluctuations in air pressure, which we hear as sound. The level of sound heard by the human ear (and measured by an acoustic meter) is known as the sound pressure level (SPL). The ear reacts logarithmically over a very broad range of sound levels and therefore sound pressure levels are quantified on a logarithmic scale relative to a reference sound pressure. They are expressed in decibels (dB) referenced to the pressure corresponding to the threshold of hearing for a typical human ear.

Noise is a key factor when selecting FCUs for retail and leisure projects

Sound pressure level (SPL) is given by:

SPL = 20 Log10 P / Pr where:

P = sound pressure being measured (Pa)

Pr = sound pressure reference i.e. limits of hearing (2 x 10-5 Pa)

In generating sound, the source will expend energy. The rate of transfer of acoustic energy from the source to the medium through which the sound is transmitted is known as the sound power level (SWL). In a similar manner to sound pressure levels, sound power levels are expressed on a logarithmic scale, in dB above the power corresponding to the threshold of hearing for a typical human ear.

Sound power level (SWL) is given by:

SWL = 10 Log10 W / Wr where:

W = sound power of source (W)

Wr = sound power reference (1 x 10-12 W)

In simple terms, the SWL is the rate at which acoustic energy is transferred to a room from a particular source and the SPL is a measure of the resulting sound at a point within the space. A useful analogy is to consider a heater. The output of a heater is measured as power in Watts (W) and the effect of the output of the heater is measured as temperature in degrees Celsius (°C).

Sound path

It is important to consider that sound can be emitted from different sources to the listener. Typically, inlet and case radiated sound is transmitted through the ceiling into the room space. In-duct sound is carried along the duct and through the supply air diffusers and radiates in the room space. It is also important to consider that sound is transmitted via more than one route in the space. The direct sound path is a straight path between source and listener. Reverberant sounds begin at the source, but sound is reflected off one or more surfaces before reaching the listener.

Room sound

The sound power levels of an FCU are typically specified as two separate sets of acoustic data, namely discharge in-duct sound power levels and inlet and case radiated sound power levels.

Also, the size, length and type of ductwork together with the quantity of bends incorporated into it affect the level of discharge in-duct sound reaching the supply air plenum. The acoustic characteristics of supply air plenum is affected by its size, number of duct connections and whether or not it is acoustically lined.

The fan coil unit: a popular choice of system

The type and size of the supply air grille affects the level of the radiated discharge sound radiated into the room. High air velocities through the grille or duct may result in noise regeneration at an observed point. To prevent the cumulative effect of all of the radiating sources of noise, the sources should be separated to distribute the noise evenly.

The level of inlet and case radiated sound transmitted into the room space is dependent upon the insertion loss of the ceiling.

The position of return path affects the inlet and case radiated sound passing into the room. The return path should be installed as far as possible from the inlet to the fan coil unit.

Sound pressure levels resulting from the direct and indirect component of sound transmitted from the ceiling and the grilles. The indirect or reverberant sound path is affected by both the surface area of the room space and the sound absorption quality of the room surfaces. In addition, the direct path is affected by the distance between the observer and the source.

Noise rating (NR) curves are the most commonly used method for assessing noise level in the UK. Each curve connects predefined sound pressure levels, which consider the response of the human ear at each of the octave bands. The noise level at a particular point in space is represented by the value of the NR curve whose constituent sound pressure levels are either less than or equal to those of the curve.

John Tarling is commercial manager for Dunham-Bush



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