Photographed by Adrian Pingstone in July 2004 and released to the public domain

In the previous post I described the role of background sound in a soundscape. In this post we concentrate on intruding sound – sound that comes from outside the space we are designing. This is almost always NOT sound that you particularly want to focus on – rarely do we want to be consciously aware of sound from outside the space, if that can be avoided.

However there are two levels of “unawareness”, and in designing a soundscape we need to understand which of these is required.

  • INAUDIBILITY means we can’t hear a sound even if we concentrate hard on trying to hear it. Technically the sound is said to be “energetically masked”.
  • NOT A FOCUS OF ATTENTION means we can hear a sound if we concentrate, but unless we do, it just becomes one of the many unconscious sound streams that exist in our brain but don’t interrupt our conscious awareness. Technically the sound is said to be “attentionally masked”.

Scientists know quite a lot about energetic masking – it depends on the relationship between the “target” sound and the “masking” or background sound. The louder the background, the easier it is for a sound to be energetically masked. The actual relationship is quite complicated and depends on frequency and time characteristics of both the target and the background. However, acousticians have a simple “rule of thumb” that works for most typical environmental noises:

A sound will be inaudible (energetically masked) if its dBA noise level is at least 10 dB BELOW the dBA noise level of the background sound.

Attentional masking is more complicated and depends on subtle things such as the meaning of the sound for the listener. For example, at a party you may be completely unaware of the sound of someone’s voice on the other side of the room, but if at some point they mention your name, you can immediately “tune in” and become consciously aware of their voice. Despite these complications, acousticians have a “rule of thumb” for attentional masking well, although it applies ONLY to sounds that are NOT “tonal” (single-note) and NOT particularly meaningful. (The screech of a car tyre, for example, would be both tonal and meaningful.) For these “nondescript” kinds of environmental sounds, the rule is:

A sound will not be a focus of attention (attentionally masked) if its dBA noise level is no more than 5 dB ABOVE the dBA noise level of the background sound.

So we have a rough scale of awareness that depends on the noise level of the background sound, like this:

In designing the soundscape we need to think about all the possible intruding sounds – people or other sounds in other rooms; trains or construction noise from outside; the list might be quite long – and consider whether each of them needs to be inaudible or just not a focus of attention. It’s then a matter of designing walls, glazing and all the other building elements to achieve that goal. In this post I won’t go into the details of how that’s done – it’s a topic large enough for several posts by itself. The focus of this post is on setting the goals.

Achieving inaudibility for all intruding sounds is generally impossibly difficult and expensive, and also unnecessary. It’s usually enough to make sure that intruding sounds don’t become a focus of attention. However there are some cases – for example voices from a private office – where your goal should be complete inaudibility. It’s very important to consider each sound carefully and understand how it will influence the total soundscape.

SoundSoup can help here.  Once you have modelled the background sound you can add sounds outside the room and define the type of walls, windows and doors that separate the outside sound from the room. In SoundSoup-Pro you can see the dBA noise level of each intruding sound, and compare it with the background noise level. When you push “Play” you can experience what the soundscape would be like. This lets you understand the actual requirements for each intruding sound so that you achieve a soundscape that is right for your room and your design.

Once both the background sound and the intruding sound in our space are appropriate and provide the right “backdrop”, it’s time to think about the quality of sounds produced within the space, which are usually the sounds that we will hear and attend to consciously. This is discussed in the next post.

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