Acoustic privacy and screens – What are the technical details?

How do you improve acoustic privacy in open offices? This might be the most important question for anyone designing or managing open office spaces as speech noise is the primary contributor to distractions for office workers [1]. The answer is that the right acoustic features must be in place, from the very base build of the room to the right interior acoustic design. And one thing is for sure – acoustic screens are a must if you want to ensure adequate acoustic privacy in open spaces.

We recently posted a very easy introductory post on how to improve speech privacy in offices with acoustic screens. If you are new to acoustic design with screens, we recommend that you start there, but if you are looking for a more in-depth look at the technical details of how acoustic screens really work; this is the post for you.

How to improve acoustic privacy in open offices with screens

In this post, we will provide you with a thorough review of the technical aspects that you should be aware of if you want to ensure the highest effectiveness of your acoustic design.

The basics – Acoustic screens in outdoor conditions

So how do you improve acoustic privacy in open offices with screens? The main purpose of a screen is to act as an acoustic barrier between the sound source and the receiver, thereby reducing the sound level at the receiver position. Outdoors, without the influence of surrounding reflecting objects and a screen width much larger than its height, screen efficiency is determined mainly by the height of the screen and the location of the sound source and the receiver. The reduction of the sound level at the receiving point when the screen is inserted is called ‘screen sound attenuation’ or ‘insertion loss’. Screen attenuation is a common way of quantifying the efficiency of a screen.

For a screen to be efficient, the direct path from the sound source to the receiver should be blocked by the screen. Generally, a screen should be located close to the sound source or close to the receiver. An intermediate location is less efficient. This is illustrated in figures 1 and 2.

Diagram of screen close or far from sound source
Figure 1. A) Source and receiver located 3 m from screen, B) source located 1 m from screen, and receiver located 5 m from the screen. Screen height is 1.4 m and source and receiver heights are 1.2 m.

Installaing screens close to both sound source and reciever would be the most efficient solution, but attenuation effectiveness will increase per screen installed between source and receiver in an open office.

Attenuation as a function of frequency
Figure 2. Calculated screen attenuation for cases A and B in figure 1 [3]. Changing receiver and source positions for case B will give the same result. The difference in screen attenuation expressed as A-weighted sound pressure level is 1 dB.
The effect of different screen heights for case B in figure 1 is shown in figure 3 below:

Attenuation of different screen heights as a function of frequency
Figure 3. Calculated screen attenuation for different screen heights [3]. The configuration according to case B in figure 1.
The calculations in this post refer to outdoor conditions and the sound that reaches the receiver position is the sound that is diffracted (bends) over the screen edge. This assumes that the sound transmission through the screen can be disregarded and that the width of the screen is sufficient to prevent sound from being diffracted from the side edges. Since the diffracted sound normally sets a limit for the sound insulation demands, it is, from a practical point of view, sufficient that the screen has a solid core.

If the screen height and width are similar, the efficiency of the screen will be reduced due to the diffracted sound around the side edges of the screen.

If two or more screens are placed in conjunction with an opening in between, this will affect the screen attenuation. An opening between the screen and the floor will also influence the screen attenuation, with the extent depending on the size of the opening. These effects will be illustrated on the basis of a reflecting screen with a height of 1.8 meters and a width of 4.8 meters [3].

The effect of an opening between two screen elements is illustrated in figures 4 and 5.

Diagram of open gab betweens acoustic screens
Figure 4. A vertical slot between two screen elements. It is supposed that the width of the screen elements is large enough for the influence of diffraction from side edges to be neglected.
Attenuation of screens as a function of gab width
Figure 5. Measured screen attenuation as a function of the width of an opening between two screen elements [3]. The decrease in screen attenuation expressed as A-weighted sound pressure level in relation to the case in which there is no opening is: –1,9 dB for d=0.006 m and -3.9 dB for d=0.002 m.
The results for the case in which there is an opening between floor and screen are shown in figures 6 and 7.

Diagram of gab between acoustic screen and floor
Figure 6. Screen with an opening between floor and screen.
Attenuation of acoustic screen with different heights of gabs between screen an floor as a function of frequency
Measured screen attenuation as a function of the distance between floor and screen [3]. The decrease in screen attenuation expressed as A-weighted sound pressure level in relation to the case in which there is no opening is: -3.2 dB for hs=0.02 m, -4.7 dB for hs=0.05 m, -12.7 dB for hs=0.30 m
In a reverberant room – i.e. a room with a small amount of absorbent material – the insertion loss is generally much smaller than outdoors. The reason is that all reflections from the room surfaces negatively decrease the shielding of the screen. Introducing an absorbent ceiling, wall panels, or other types of sound-absorbing material will be beneficial to the shielding effect of the screen.

In open-plan offices, the ceilings are usually covered with a highly absorbent suspended ceiling. If the workstations are located far from walls or if absorbing wall panels are attached to the walls, the acoustic environment will be similar to the outdoor conditions and the effects of screens can be fully utilized.

Empty office with acoustic screens

Absorbent and non-absorbent screens

Generally, it is beneficial if both sides of the screens are covered with thick porous absorbers. In the case where several work stations are located in a row with screens in between, the absorbent material will prevent repeated reflections between screens and thus reduce the sound level between screens.

The effect of non-absorbent screens is illustrated for the open-plan office that is shown in figures 10 and 11. The sound distribution curves corresponding to path 1 in figure 11 are measured over work stations without dividing screens. Along path 2 there were two non-absorbent (hard) screens in the way. The corresponding sound distribution curves are shown in figures 12 and 13. In figure 13, two phenomena appear. Firstly, by comparing the sound distribution curves for different frequencies, it can clearly be seen that the high frequencies are more affected by the screens than the low frequencies. Secondly, between the screens, the sound pressure level does not continue to decline. Due to the multiple reflections between the non-absorbent screens the sound level is more or less constant between the screens. If instead the screens were to be covered with absorbent material, the sound pressure level would continue to decline at distances further from the sound source.

Open office with hard screens
Figure 10. Open office with hard screens
Plan drawing of office and measurement paths
Figure 11. Sketch of open-plan office with measurement paths 1 and 2.
Graph: Sound propagation curves in measurement direction path1
Figure 12. Sound propagation curves in measurement direction path1.
Graph: Sound propagation curves in measurement direction path 2 (with screens).
Figure 13. Sound propagation curves in measurement direction path 2 (with screens).

 

A special thank you to Helene Sallenhag & Erling Nilsson for their support and previous work with screens that oriented this post.


 

Read more about the different types of absorbers and their influence on acoustic privacy in offices in this expert interview about a very comprehensive study on this very subject:

Study interview: The effect of absorbers and screens on the decay of speech in offices

Sources:

https://softdb.com/_files/_sound_masking_division/Productivity_Acoustics_David_M_Sykes.pdf  

 

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