Balliol College Oxford

The quality of sound that is consciously heard in a space is what traditional “acoustics” is all about, particularly if that sound is music. There are schools, theories, numerous research projects and multiple textbooks devoted to the topic of producing the prefect acoustic environment for listening to various types of music. I am not even going to touch the surface of this topic – if you’re interested in designing a concert hall or a high-end recording studio you’ll need to become familiar with concepts like “early lateral reflections”, “room modes” and much more. And you’ll be using modelling tools that are more sophisticated (and MUCH more expensive) than SoundSoup.

What I’d like to focus on is a single property that is useful for designing the sound in ANY space – reverberation. This is the degree to which sound bounces around the space before it is finally absorbed through interaction with the materials on the walls, ceiling and floor.

In a typical room, if you are more than a few metres from the sound source then most of the sound you are hearing has bounced off at least one surface before reaching you – so reverberation is important for your experience of the sound.

The simplest, and most useful, measure of reverberation is the Reverberation Time (RT). This is normally expressed as the time it takes for sound in a room to reduce by 60 decibels after it has been turned off – although actual standards define it in a slightly different way that depends on the slope of the sound reduction.  Typical RT values vary from less than 0.3 seconds (which might represent a recording studio or a space with lots of thick carpet and plush furnishings) to over 2 seconds (which might represent a cathedral).

Rooms come in all shapes and sizes, and the RT doesn’t always express everything about reverberation in the room. However, typical rooms are pretty close to a particularly simple model, called an “ergodic” or “Sabine” room, and for these rooms the RT:

  • is the same everywhere in the room; and
  • depends only on the volume of the room and the total amount of sound-absorbing material in the room (not on where the absorbing material is placed).

When modelling reverberation, SoundSoup assumes that the room is Sabine. The RT depends on the frequency of the sound, because absorbing materials are better at absorbing some sound frequencies than others. Usually the absorption is better (and so the RT is lower) at high frequencies.

The important thing to know is that reverberation affects the way sound is perceived in ANY room, not just when listening to music. Here are a few guidelines.

If the room has a low RT (less than about 0.5 seconds):

  • the sound will be experienced as “soft” and “comfortable”;
  • the room will feel small;
  • people will naturally speak more quietly;
  • speech will usually be understandable at distances of 4m or more.

If the room has a high RT (greater than 1 second):

  • the sound will be experienced as “jangly” or “buzzy”;
  • the room will feel larger;
  • people will naturally speak more loudly;
  • speech will only be understandable at short distances.

For completely different reasons, concert halls for unamplified music also need a high reverberation time, and recording studios need a low one. However, as I said above, I’m not going into design details for those spaces.

As an example, a café designed for patrons to come in from a busy street, order a takeaway espresso and leave might be designed with a high RT, to match the “buzzy” atmosphere. On the other hand, a café where patrons sit in an armchair, pull out their laptop and work on their next novel over a few skim lattes might be designed with a low RT, to promote a peaceful, relaxed atmosphere.

SoundSoup allows you to play with different levels of reverberation, by adding absorbing materials to your room, and listen to the result. You can add different materials to the walls, floor and/or ceiling – you’ll find that the level of reverberation depends on the type of material and the area over which you can spread it. You can also add room contents, like people, chairs and furniture, which also reduce the reverberation. SoundSoup-Free shows you a reverberation time (at a frequency of 500 Hz, probably the most useful frequency for voice sounds) while SoundSoup-Pro shows you the RT at all sound frequencies as well as more details of the absorption properties of your materials.

If your sound design incorporates the three fundamentals:

then you’ll be well on the way to taking charge of your acoustic environment.

Stirfry Software demonstrated acoustic modelling using SoundSoup at the DesignBUILD Expo on 2-4 May 2018, which was held at the Melbourne Convention and Exhibition Centre. We had lots of interest from architects, designers, builders and acoustic material suppliers – who especially appreciated our Fantale lollies!

 

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.

By Jean-Pierre Dalbéra from Paris, France [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons

As I described in the previous post in this series, soundscapes have numerous components, which are the sounds from individual noise sources (cars, people, trains, a dishwasher – whatever). When your brain processes the sound, some of these components will stand out in your conscious awareness, while the rest are not processed consciously, but form part of the unconscious “backdrop” of your experience. This mental background affects mood, task performance and other things, even though you are not consciously aware of its presence.

The sounds that DO reach your conscious awareness affect you in a very different way – they may convey information (speech), they may transmit aesthetic values (music), or they may cause annoyance (an aircraft overhead).

How your brain decides which components of the soundscape will become conscious and which will not is something that neuroscientists have yet to unravel completely. However, from experience acousticians have developed a few rules of thumb. Sound that does NOT become conscious – that is, sound that forms part of the mental “background”, is generally:

• fairly constant over time;
• broadband – that is, it sounds more like a “shhhh” or a rumble than a “la” on a particular note;
• relatively low in sound level (loudness); and
• lacking information content – for example, there are no intelligible words.

Typical examples of sounds that become part of the mental background are distant traffic noise and noise from an air-conditioner.

The first job in designing a soundscape is to design the background sounds – what are they, and how loud are they? This should also be the first job in modelling a soundscape in SoundSoup. Ignoring background sound results in spaces (and models) that sound “empty” and unreal. It also means that other sounds will be more audible and potentially more annoying than they should be.

What should the background sound be? One source is mechanical systems within a building, typically noise from the air-conditioning system. In SoundSoup this would be modelled by an interior sound – “Air-con vent noise”, under “Mechanical”; “Air-conditioning”. Other mechanical sounds can be reasonably simulated by the “Fridge” sound, under “Miscellaneous”, “Interior Sounds”.

In some cases you may also want to use “piped” music as a background sound. There are two options for this in SoundSoup under “Music”, but beware, they need to be used in addition to other background sounds or they will become conscious.

Another common background sound is traffic noise, generally heard through a window, wall and/or door. This is an external sound, and in SoundSoup, among sounds that can be “outside a wall” there are a number of samples for “Traffic”. The loudness of the traffic noise inside the room depends, of course, on what type of wall, window and/or door construction you use.

So how loud should the background sound be? You can find help in an Australian Standard – AS/NZS 2107:2000 “Acoustics—Recommended design sound levels and reverberation times for building interiors”. This sets out a range of recommended BACKGROUND sound levels, in decibels (dBA) for many types of space. They represent typical background sound levels in those spaces – outside that range, either above or below, and the space will sound odd.

I can’t post a copy of that Standard here because it’s copyright, but you can buy a copy here, and many libraries have copies.

So, in either designing or modelling a soundscape, the first step is to decide on the dBA level of background sound that you should have, using AS/NZS 2107, and decide on what sources will produce that sound and how they will produce it.

Remember:
• We’re talking about BACKGROUND sounds here – fairly constant, fairly quiet sounds that you will not be conscious of. These do not include sounds from people, buses or anything else that varies in time or has information content;
• The target dBA level from AS/NZS 2107 applies to the TOTAL of all the BACKGROUND sound sources. You can calculate this total in SoundSoup-Pro.

Once you have an appropriate level of background sound in your space, the next step in soundscape design is to control intruding sources. That is the subject of the next post.

“Grand Central Station” by Neo_II is licensed under CC BY 2.0

You know when a certain place just doesn’t sound right. You might say it’s “too noisy” or it’s “echoey”, or maybe it’s “weirdly quiet”. Sometimes a certain sound is part of what makes the place feel good – think of surf sounds in a beachside bungalow, or sounds from a playground in a nearby café.  Or bad – aircraft noise in your living room; footsteps from the apartment above.

A “soundscape” is the set of sounds that are present in an environment, and how they relate to each other. I’d like to define two types of soundscape:

  • The physical soundscape – all the individual sounds that reach your ears, loud or soft; either directly, or via reflections, or after passing through walls, windows, etc.
  • The mental soundscape – the physical soundscape after it’s been processed by your brain, which makes some sounds stand out as a focus of your attention (for good or bad) while others are relegated to a “background” status, and other sounds are completely inaudible.

When you enter an environment such as a room, or a park, the soundscape that you perceive (your mental soundscape) has been shown to affect your level of stress and your task performance in that environment. More subtly, it has a “halo” effect that changes the way you perceive visual and other aspects of the environment, and affects your overall judgement – is this comfortable; do I wish to stay here?

Therefore, good architectural practice includes designing not just the visual aspects of a space but also the soundscape. Effectively, we design the physical soundscape to try to produce a positive mental soundscape for people using the space.

Given the myriad of different meanings and associations that we give to different sounds (what music makes you feel relaxed? alert? distracted?), it might seem that engineering a positive mental soundscape for everyone is beyond anyone’s ability. Nevertheless there are a few basic guidelines that acousticians follow (whether they know it or not) when designing soundscapes. They are in three key areas:

In subsequent posts I’ll discuss each of these aspects and how you can use sound modelling in SoundSoup to help ou understand how changes in the physical soundscape will affect your mental soundscape.

To simulate acoustic reverberation in a room, SoundSoup needs to take a “dry” sound recording (with no reverberation) and add the effect of reflections from the walls, ceiling and floor of the room, as well as reflections of the reflections; reflections of the reflections of the … etc. Each of these must be added to the “dry” sound with a delay that depends on how far the sound has travelled; and the strength of the reflection should depend on the sound absorption of the surface(s) that it was reflected from. Accurate modelling will need to include many thousands of these reflections.

There are computer models that will (in simplistic terms) follow the paths of thousands of sound “rays” as they are reflected from each surface in the room, calculate the time delay for each, apply a filter to simulate the effect of sound absorption by each surface, then add them to the “dry” sound. As you can imagine, this takes significant time, even for a very fast computer. It also requires precise knowledge of the position and sound absorption of everything in the room, so these models typically require a detailed computer model of the room before they can do their work.

SoundSoup uses a simper, but still relatively accurate, process. The “dry” sound is fed into ten digital “pipes” with different lengths, but whose average length represents the average distance between reflections in the room.  At the end of each pipe is a filter which represents the average sound absorption by surfaces in the room. The sound that comes out of each pipe is then added to the “dry” sound, but also to the input of each pipe, so that each reflection is reflected again and again.

To calculate the average length between reflections, all we need to know is the room volume V and the total surface area A. In any room, the average reflection length is just 4V/S. Isn’t geometry wonderful?

Technically, that process gives an exactly accurate result if the room is a “Sabine” room, which is a mathematical idealisation of a real room. However, for most rooms the difference between SoundSoup’s result, which can be heard in real time, and the result of a more exact model which takes much longer to set up and run (and is much more expensive) is hard to detect. Don’t use SoundSoup to design a critical space like a concert hall or recording studio, but for school classrooms, offices, hospital spaces, foyers and all kinds of “ordinary” spaces, the sound simulation is very close to what you will hear when the space is built.

The attached article describes SoundSoup’s modelling process in much more detail.

Article in Acoustics Australia