Roland Hemming looks at the ways you can direct audio only to those who want to hear it.
Think of a loudspeaker as if it’s a light. It shines a cone of sound at the audience. The further away the audience is, the bigger the cone and more people can be covered from a single point. But this additional coverage is provided at the expense of volume level.
Such an approach forms the basics of normal audio coverage but sometimes you want to target just a few people and not have others sitting close by to hear anything at all. A good example of this ‘directional’ audio can be found in museums and visitor attractions. A series of exhibits each has its unique audio source. You don’t want audio from different exhibits to interfere with each other.
A few technologies are available that can deliver very tightly focussed beams of audio to perhaps one or two people. As with all things they also come with compromises.
One technique is to use a parabolic loudspeaker. This is a semi-circular dome that has a loudspeaker pointing upwards into it. The dome then reflects the audio in a tight beam in the opposite direction, just like a satellite dish. It’s a very effective approach.
One thing you need to consider is that you really need to stand underneath, directly in the ‘sweet spot’ to hear anything. One person is best and it won’t work for small groups huddled together. You also have to locate the parabola – which can be difficult to coordinate – with other things in the ceiling. Using a conventional loudspeaker it will sound good but it isn’t something you would want to make particularly loud.
Another option is to use electrostatic loudspeakers which use a flat panel that vibrates to create a single wave front. The beam is the size of the panel so it can be made to any reasonable size. One such product is a ‘sound shower’ where a small group can stand underneath and all hear clearly. This tight beam of audio has amazing properties and I’ve even fired the audio across a car park and people have heard clearly. Step one side of the beam and you can’t hear anything. It’s a very strange effect.
The problem is that the technology does not provide wonderful audio quality. It’s perfect for speech and light music but nothing else, so you need to choose your content carefully.
Another approach to the exhibit problem is to use some conventional loudspeakers that are narrowly positioned. If you can design their position close to the listeners and there are not too many reflective surfaces, this approach can deliver good quality sound without interference to others. With some compromises on the design of the exhibit to allow for optimum loudspeaker positioning, you can provide good quality audio more cheaply than using a specialist solution. It all depends on how you can compromise.
Local audio
Another local audio solution is to use an inductive driver. This is a driver that is bolted or coupled firmly to a solid surface such as an MDF panel or even a window. The flat surface becomes the loudspeaker and has coined the phrase ‘whispering window’. Depending on the design and material used, these vibrating surfaces do not project sound very far, making them a great localised solution.
All these techniques can be used for digital signage where audio support is being increasingly introduced.
The other side of directional audio approach, is the need to fill large spaces from as few locations as possible. Remember our loudspeaker pointing at the audience from a long distance away? This is fine as long as it’s still just covering the audience, but not if it is dispersing the sound so widely that some of its energy is bouncing off the walls and ceiling. This will make the sound much less intelligible and maybe cause problems such as noise pollution elsewhere.
Aside from using well-designed conventional loudspeakers with narrow dispersion, there is a technology that has been the buzzword of the audio industry for the last 20 years – the line array (a column loudspeaker).
As with most things, this technology isn’t actually new. The principles were developed in the 1920s and 30s, but the technology wasn’t available to exploit it. Column loudspeakers, as they were then, earned a bad reputation for poor quality sound.
You can buy line array columns or make them into columns by putting line array elements together. This is done for most new concert systems.
Very simply a line array uses a number of loudspeaker drivers that are designed in such a way that the sound they project sideways is cancelled out by adjacent drivers. This means that only the sound that projects forward is heard. The sideways sound exists, but once you are a small distance away, it is eliminated.
The other principle with line array is that if you are close, you only hear the output of one or two drivers. The further away you are, you hear the combined output of more drivers. This means you get a more consistent volume whatever distance you are away. Line arrays ‘break the rule’ when you double the distance you get a quarter of the volume.
The longer your column, with more drivers, the more sideways cancellation you’ll achieve, making your loudspeaker more directional. It’s illogical that more loudspeaker drivers are required to cover a narrower field but that’s physics for you.
Three types of line array
Passive columns can be driven from a single conventional amplifier and are designed to have a particular coverage angle, just like any loudspeaker. The cancellation between the multiple drivers is pre-set and can’t be changed. These are fine for simple spaces and are lower cost.
Active columns have their amplifiers built-in. Each individual driver will have its own small amplifier. By using built-in digital signal processing and some complicated maths, you can tune these loudspeakers to disperse and bend the sound exactly as required. Just because an active column is pointing directly at you doesn’t mean its sound is.
It could have been programmed to point the sound downwards or to the left. When you first hear this in action it can feel quite eerie as it goes against everything you expect a loudspeaker to do but it’s very effective at controlling sound to point to particular places – at a cost.
An array made up of elements. These are extensively used for large concert systems but there are now many ‘mini’ line arrays used for all sorts of spaces, even down to conference rooms. These line arrays are made up of individual elements, which look like loudspeakers, but you cannot think of them as individual loudspeakers. You have to think of them as one virtual loudspeaker.
By using software and programming in the characteristics of the room, you then configure the angle of each element to give you the dispersion that you require.
There are other more unusual uses of line array technology where panels rather than columns are created and the different drivers on the panel interact and cancel each other to produce audio in one particular direction.
There is a lot of debate where some people say that line arrays sound better than normal loudspeakers. This simply isn’t the case. The quality of a sound system depends on what is most appropriate for the room.
Summary
Directional audio is a complex subject. You can’t just throw money or complicated technology at the problem. Real care has to be put into the area you are trying to cover, the acoustics and the content you wish to play.
You cannot expect to provide night-club level audio for one group of people in a space and for others not to hear it. The more work you put into creating better acoustic treatment for the space, so there are fewer reflective surfaces, and collaborating with the design team on better placement of loudspeakers will reap rewards for the experience of the listener.
Source: http://www.avinteractive.com
