The current knowledge, with regards to the acoustic performance of open windows, is limited. The table to the right provides typical data used by acoustic consultants when determining the sound reduction of a facade containing open windows. No information is provided with respect to the window type, the angle of sound to this window, the amount the window is open or the number of open windows in the facade.
Doubts on accuracy
This table shows that different standards provide information as dB, Rw and dBA values. These parameters are not interchangeable and are very different to each other, which leads MACH to have doubts in the accuracy of the information provided.
Does 5 dB Matter?
From the previous table, it can be seen that there is typically a 5 dB spread in the level of sound reduction provided by the different standards. By scrolling through the noise maps to the right, it can be seen that a 5 dB error in under estimating the performance of a façade, can result in a 20% reduction in the chances of using natural ventilation.
This factor has been determined by measuring the area available for natural ventilation based upon a vented façade achieving a sound reduction of 10 dBA and 15 dBA, within the noise maps.
Napier Work – Directivity
Napier University carried out acoustic tests to 14 different open windows, where five of these windows are particularly suited to venting schools and offices. The information presented provides a summary of the work carried out by Napier University.
The illustrations to the right show the directional sound reduction of the five window types. As shown, as the line of sight through the window is reduced, their acoustic performance is enhanced. As an example, the lowest measured sound reduction was 12 dB, and the highest 23 dB, the open area was maintained at 0.2m2.
Napier Work – Sound Reduction to Open Area
Analyzing the directivity curves in more detail, it can be seen that for every window type and for every sound angle, the rate at which the sound reduction reduces over an open area is different. In other words, if you double the size of the opening, the sound reduction for the different windows and at different opening angles is not constant.
MACH Acoustics has therefore derived the following equation, which can be used to characterize the acoustic performances of an open window, where X is the slope of the sound decay, A is the open area, D is the directivity component associated to the angle of incidence and C is a constant relating to a specific window type.
Napier Work – MACH's Open Window Model
Using the above equation, MACH Acoustics has built a free to use software App, aimed at helping designers to use Napier’s data, such to calculate the sound reduction of an open window.
To derive this model, MACH Acoustics has established the slope for every 1/3-octave level difference measured by Napier University. We have then arranged the data such to enable MACH to calculate the Rw levels of a specific window type, at a specific opening and at a given sound angle of incidence as those measured by Napier University.
This software can be found on MACH Acoustics website http://www.machacoustics.com/WindowDatabase.php.