Kvistgaard, August 16th 2002

This is a short walk-through on D-PAC's thoughts and experience and the background for further developments of products and technical papers.

Concept
What is a line-array? A line array is a line of speakers stacked in a vertical plane. In order to obtain correct coupling between the speakers that take care of the different frequencies, it is necessary to make sure that the distance between the centers of the radiating devices are no longer than half the wavelength away from each other in the crossover region. This is a wellknown principle and has been used for many years in Hi-fi speakers.

Advantages
One of the main advantages in Line-arrays are, that it should be possible to obtain a more "point-source" like reproduction and avoid lobing between adjecent transducers in the horizontal plane, and at the same time, have more control in the vertical plane, where it should be possible to "aim" towards the public. Another claimed advantage is the line-array effect, where the decreasing in sound preasure is only 3dB for every doubling of the distance, where normal speakers have a 6dB loss. Also - line-arrays are (or should be) easier to fly, than conventional systems.

Disadvantages
Funny enough - not many manufacturers of line-arrays point to the disadvantages of line-arrays, however, there are a few very important drawbacks to all line-array systems. One is a result of the line array effect which earlier where descriped as an advantage, however, as seen on the below diagram, this effect have a terrible result when frequency / distance are compared to soundpressure level (a 2 meter high array is projected)

Above, it is shown, that at 32 meters, there is a loss of 15dB in the frequencies from 5 to 16 kHz, however, frequencies below 250 Hz have a loss of 30dB or 15dB more than the HF. This creates a very problematic imbalance in the reproduction of sound. The line array effect are depending on the height of the array - this paper shows where the effect ends depending on the height. Another problem created by the line array effect, is the "over-shot", where a 2m high array, will have the decreased loss for 115 meters at 20kHz, and 58 meters at 10 kHz - perhaps the neighbours don't complain, however, using a 10 meter high array, 10 kHz will run out at 1.449 meters, creating problems in reverbant halls or nearby cities.
It is also necessary to create a special waveform for the line-array for the intended frequency range. This creates the need for purpose built transducers with special charateristics, not excactly making the system less expensive.

Will D-PAC make a line-array system?
We, at D-PAC, have looked very close at the line array concept, and for some uses it is an excellent choice. When dispersion in the vertical plane needs control due to reflective surfaces on floor or ceiling. Also, the horizontal dispersion aids the possiblity to make a stereo image, for the lucky ones in the area between the two lines. There still needs to be done a lot of in depth research and prototypes with actual field measurements, and not just lab modelling. It will take a year or two, and we will have to overcome many of the obstacles described above, before we present our solution.

We welcome your comments and suggestions - please e-mail to Anders Christensen.