LISCA Speaker System - Description. How do they perform? See our experiences Here.
LISCA stands for: LIne Source Ceiling Array and is an original concept developed by the late Mr. Vivian Capel of Bristol England and is described in detail by him below. There are a couple of books that describe the concept in detail. These are: 'Audio & Hi-Fi engineers pocket book' third edition ISBN 0-7506-2001-3 published by Newnes, and 'Public Address Loudspeaker Systems' ISBN 0-8593-4237-9 published by Babani. LISCA is a concealed line array (or series of arrays) of loudspeakers fitted to (or in) a ceiling which delivers greater speech clarity than traditional downwards facing ceiling speaker systems.
Line-source LISCA array - What is LISCA?.....Click a pic to enlarge
LISCA is a Line-Source Ceiling Array of public address loudspeakers and consists of two (or more) rows of units on angled baffles above the ceiling at an angle of 64° and is fitted across the whole width of the hall at 600mm intervals. The first array is located just off the edge of the platform, the second if required, about half way along the hall.
LISCA overcomes all the disadvantages of other systems and gives sound of unequalled coverage, clarity and naturalness. It can be installed with no structural alteration to a tiled suspended ceiling.
LISCA is not a new, untried system. It has been installed in many halls all over Great Britain from Scotland to the Channel Islands, Ireland to Norwich and has been in use for a number of years. From a few tens of people to audiences of a thousand and more, all benefit from the enhanced sound from LISCA systems.
What Are The Advantages?
Less Feedback The angle of the LISCA array reduces the sound directed back to the microphone, being some 3dB less than other systems, thus reducing feedback. It also minimises reflections from side and rear walls further reducing it. This also reduces hall reverberation and resonance.
Reduced Reverberation Reverberation and resonance produce a hollow, boomy effect that is not always noticeable, yet is always present to muddle and confuse speech syllables. The reduction of these is one factor contributing to LISCA's clear and distinct reproduction.
Appearance The arrays are possibly the most unobtrusive of any system. There are no visible housings as there are with most ordinary ceiling speakers. The units are mounted above the ceiling and all that is visible are surface grilles level with the rest of the ceiling.
Optimum Sound Levels Sound level is lower in the first rows where it is only required to reinforce natural sound from the platform. From there the level increases, then as it starts to diminish, the second array takes over to maintain an even volume to the back of the hall. The level is thus at an optimum everywhere with no blind spots. Apart from the intentional lower level at the front, sound levels are usually within 1dB over the rest of the hall, something not possible with any other system.
Greater Clarity This is its greatest advantage. The array behaves acoustically as a single-source from which the plane (flat-fronted) sound waves travel down the length of the hall like straight-fronted waves rolling along a flat beach. All other systems that use multiple ceiling speakers or columns, generate spherical wave-fronts which mutually interfere with each other like confused ripples expanding in a pond when several stones are thrown in at the same time.
LISCA produces in-phase sound everywhere, like a laser beam, whereas all other systems produce complex reinforcement and cancellation patterns. These produce dips and peaks throughout the frequency spectrum known as the comb-filter effect, which result in variations of clarity over the audience area.
This is worsened by the treble-beaming experienced with all loudspeakers. Above 3KHz the response can drop to a half (-6dB) at 20° off-axis. For a conventional speaker facing downwards from a 3m high ceiling, this means that a circle with only 76cm (2.5ft) radius beneath it is within the 20° angle and so has reasonable treble. With LISCA, ALL the audience except the first row is within 20° and most within 10°. (Only a 46cm (1.5ft) radius circle is within 10° with a conventional speaker).
The effect of all of this is like the difference between a sharp photograph and one in which the camera moved. The blurring caused by multiple sources and beaming effect confuses the very short parts of speech - the short consonants, b, ch, d, g, j, k, p, t. This corresponds to the fine detail lost in a blurred photograph.
These short speech sounds are vital for intelligibility and clarity which are much degraded if they are impaired. Older persons are most affected, because hearing sensitivity for these sounds declines with age. Thus while older persons can hear speech, they have difficulty in understanding it.
The totally in-phase sound generated by LISCA causes all speech sounds to be reproduced clearly without loss of any part.
Natural Sound Location With most other systems the perceived sound source does not coincide with the visual location. With columns it is to one side and with multiple ceiling units it is overhead and even behind in some positions. The sound image with LISCA is normally centre-front, but due to the effect of LISCA's phase coherence on the audio/visual correlation of the brain, a pseudo stereo effect is often obtained whereby the sound appears to be coming from that part of the platform where the speaker is, -- the ultimate in natural sound!
Ease Of Installation All the units are installed in rows instead of all over the ceiling as with the conventional arrangement. This greatly facilitates mounting and wiring. Furthermore there are no holes to be cut in the tiles because each unit replaces one tile. Installation merely consists of dropping the unit with its base into the vacant tile position and then wiring.
Cost A LISCA system costs less than a conventional out-moded ceiling system. Fewer loudspeakers are required, although more are needed for a corner platform layout. 16 are needed in the average hall (100 people) against 20-24 for the conventional system. The cost of the LISCA housings are less than most conventional quality ceiling units and the cost of one tile is saved for each. Most conventional ceiling systems use high-impedance operation with a transformer for each speaker. LISCA operates at low impedance and so saves the cost of many transformers.
Fire Risk Many local safety authorities demand stringent specifications whereby fire is retarded from passing from the auditorium to the above ceiling area. The fibreglass LISCA units are fire retardant.
Units Each unit of the array occupies the space of a single 600mm suspended ceiling tile. These are mounted across the width of the hall, extending to one tile from each side wall. As the sound level from an unbroken line source is 3dB greater at the centre than at the sides, two tile positions are omitted to achieve an even level across the width of the hall. A space left either side of a central pair gives a good level response across the hall width. For a 17 tile width, 8 units would be used in the positions:
Sound Propagation Levels The significant dimension affecting all calculations, is h, the height of the array above the seated audience which is about 1m (3.5ft) above floor level. Thus h is the floor-to-ceiling height H, minus 1m (3.5ft). The first row of seats under the first array, is at an angle of 64° off-axis from the loudspeakers, which is the angle the line-source is tilted. The sound level there is therefore cos 64° = 0.44 times that of the same distance along the on-axis line. This corresponds to an SPL of -7dB.
The on-axis line converges with the audience level at a distance from the array of: h/sin(90°-64°) =h/0.44 or 2.3h. As the propagation loss from a line-source is 3dB for a doubling of distance, the loss along the on-axis line to the point where it reaches the audience is -3.5dB.
So with a level of -7dB under the array at the first row and -3.5dB at the on-axis point, there is a theoretical difference of 3.5dB. In practice, reflections from the inside top of the LISCA housing increase the level beneath it, so the difference is somewhat less, about 2dB.
The first rows thereby receive 2dB less sound than elsewhere. Thus the volume there is just enough to reinforce the natural voice and due to the Haas effect, the sound appears to come from the platform rather than the overhead array, so giving a perfectly natural effect.
The floor distance from under the array to the on-axis point is h/tan (90°-64°) = h/0.49, or approximately 2h. So 2h is the floor distance from the first row to the on-axis point, over which the sound level increases to a theoretical maximum of 3.5dB.
Beyond this it declines as the line of propagation goes off-axis again and the distance increases. However, distance has less effect than may be expected. The SPL decreases with distance because the wave expands; with a line-source it drops 3dB for a doubling of distance.
With LISCA, the wave expands from the array until it fills the space between floor and ceiling and up to there obeys the normal attenuation law, but beyond this, no further expansion is possible as it is constrained by the floor, walls and ceiling. So the effect is like sound travelling along a tube and in theory there should be little further loss beyond this point.
There are though losses due to absorption by the audience, carpet, curtains and padded seating. The range limit beyond 2h will thus depend on the furnishings. At 4h the propagation angle is narrowed to the point where the recessed loudspeaker cones begin to be masked by the ceiling. Low and mid frequencies are diffracted around the obstruction, but high frequencies are not, so intelligibility may start to deteriorate beyond this point. Also, the off-axis angle increasingly reduces the level. So 4h can be considered the maximum range to give adequate sound level with highest intelligibility. If the length of the hall from first row to last is greater than 4h, a second line-source array will be needed and in most cases this is so. There is no interference with the first one if correctly located and so there is no loss of clarity.
The Second Line-Source The second line-source is located, not where the output from the first is tailing off at 4h, but where it is strongest at 2h or a little beyond at 2.5h. Here, the output from the second array which is immediately overhead, is low compared to that from the first. So, with most of the sound coming from the first, the perceived location is still forward and there is virtually no awareness of the second array overhead.
Beyond this point, the output from the first line-source starts to diminish while that from the second increases, thus maintaining an even sound level throughout. Furthermore, as the second array is now forward relative to the listener, the frontal natural source location is maintained.
In very long halls or those with a short h dimension, the distance from the proposed second line-source location to the last row of seats may exceed 4h. In this case, the distance of the second array from the first may have to be increased to 2.5-3h.
In this case there may then be some awareness of an overhead source immediately beneath it. The situation where the length of a hall from the first row of seats to the last exceeds 6h is unusual.
The two arrays do not produce the mutual interference with subsequent comb-filter effect as produced by multiple ceiling speakers. For this, the volume levels need to be similar and the sound path difference between each source and the listener small. These conditions are experienced with the ordinary ceiling system, but nowhere with LISCA. Where the sound levels are similar, the path difference is large and vice-versa. So no interference problems are created by the two arrays.
Planning The System The first array is located just off the edge of the platform, determine the position of the second array from this as follows:
The significant factor is h, the ceiling height above a seated audience. The second array should be located at 2h distance from the first, or as near to it as possible. (See diagrams). For long halls or those with low ceilings in which the distance from the first array to the back of the hall exceeds 4h, the spacing between arrays may be 2.5-3h. Each array must have an even number of units for sound balance and impedance matching.
For halls with widths greater than 17t, the unit positions are:
Corner-Platform Layouts In Square Halls The first array is standard. The second array should be positioned just before the side corners of the walls, this will in most cases be somewhat less than 2h from the first array. The width of the hall at its widest point is greater tan the normal width of a rectangular hall, but the length from platform to the rear wall corner is less.
The second array must therefore be longer but does not need the normal range. Furthermore the required range is less at the ends of the array than in the middle. The units should therefore be mounted in alternate tile positions instead of in every one. The centre pair should be adjacent, then at alternate tile positions out to the walls.
As there is little sound propagation outside the ends of an array, two triangular sections at the sides where the walls diverge from the first array are left without coverage. These must be served by a single LISCA unit facing sideways toward the walls. Its location should be between the two arrays and in line with the end units of the first array. The reason for using a LISCA housing here, apart from visual uniformity, is that it will direct the sound sideways into the triangular areas and little if any will be propagated backward to cause interference with the main LISCA sound flow. A typical number of units needed for such a layout is 24, more than with a conventional system. Of course the actual number depends on the dimensions of the hall. (See Diagrams).
Installation Place the units in the required position in place of a ceiling tile and connect up. As only two rows are involved and no tile cutting is required, the installation is quicker than the old ceiling tile system.
There may be a reduction in sound level at seats adjacent to heavy side window curtains. Sound may be reinforced by putting an extra unit at the end of each array at the affected side, instead of the usual blank tile.
Connecting Up All speakers must be connected the same way round otherwise some will produce 'negative sound' that will cancel that of the others.
A series-parallel arrangement is usually best and depends upon the number of units used. All speakers in each bank are connected in series, positives (usually marked with a red spot) to negatives, just as the cells of a battery. The banks are then connected in parallel with the outermost positives and negatives wired together. (See Diagrams). The total impedance of the complete array is 8 ohms which perfectly matches a standard amplifier.
Frequency Equalising The bass response of a long line-source rises at 6dB per octave. The amplifier should have a bass cut facility to equalise this.
Hearing Aid Use The normal Inductive Loop enables hearing aid users to hear without the hollow, boxy sound due to reverberation caused by conventional speaker systems. With LISCA, reverberation is reduced and the hard-of-hearing can often get a better feeling of 'presence' by listening to the loudspeaker system through the deaf-aid microphone and not use the loop.