|Elliott Sound Products||Project 105 - Part II|
System Design Using ESP1
Tony Rendle & Rod Elliott (ESP)
The electrostatic panels described here are not available, and it is not expected that this will change. The set of articles has been retained for interest only, so please do not ask questions or ask about prices or availability - there is no point because the panels are not available.
The ESP1 panel is designed for use in hybrid ESL/dynamic systems. An array of two or four panels takes the place of both mid range & tweeter drivers in conventional 3-way systems. There are three basic issues which must be addressed to create successful system:
1. Extending the low frequency response.
Without any baffle, the response of an ESP1 panel falls off rapidly below about 700Hz. This is common to any sort of transducer which is small compared to a wavelength & not provided with a baffle. The solution is to provide a baffle, which can conveniently be the box housing an associated woofer.
2. Providing satisfactory high frequency dispersion.
This is a much harder problem to solve. To date it has largely remained in the too-hard basket in ESL system design. Typical efforts to alleviate the problem have been to use very narrow tweeter elements, or to use a curved shape for the panel. Another approach is to arrange a number of panels in a curved horizontal array, but this suffers from interference between the panels, giving a ragged polar diagram. None of the methods currently in use (excepting the QUAD ESL63) has been really effective.
A much better method is to stack a number of panels vertically, with each panel horizontally offset from the one above by a small angle. With suitable panel dimensions, and the correct offset angle, this type of array provides a good solution to the problem of high frequency dispersion. Using four ESP1 panels in a vertical array, a consistent wide dispersion can be obtained reducing to about 90° at 10kHz). In a vertical array, there is no phase cancellation caused by interference between the panels.
Controlling the fundamental resonance.
Controlling the fundamental resonance is a simple matter of providing some acoustic resistance near the panels which completely controls the resonance. In practice, the damping is provided by a strip of felt behind the ESP1 array. Felt, which is a thick non-woven material, is available at any shop specialising in fabrics for dressmaking etc.
The key factors in designing systems for the ESP1 are ...
Use a 24dB/Octave crossover, to avoid over-driving the panels with low frequency signal content.
The ESP1 may be built into many different types of system: the individual DIY constructor can let their imagination run free. For the purpose of demonstrating the principles, a sample system is shown in the sketch & description below. This is for a compact loudspeaker, conceived to demonstrate that a full range ESL/Dynamic system can be designed to suit a fairly small domestic situation.
It uses an array of four ESP1 panels, allied to an "isobarik" woofer (or alternative single driver enclosure - see below), using two good quality 170mm (6½") inch woofers, coupled together. Coupling two drivers in this way halves their equivalent volume (Vas), compared to a single driver and therefore halves the size of the enclosure without compromising performance.
Full range Electrostatic/Dynamic System
The system described here was an exercise in minimising the size of a full range ESL/dynamic system (approx, dimensions are 900 x 330 x 210 mm), without unduly compromising performance, although the power available at low frequencies is obviously limited.
The system is bi-amped with fourth order (24dB) Linkwitz-Riley crossover networks, the crossover frequency is 500 Hz. The details of this, together with transformer details and the bias voltage supply are given separately.
A stack of four ESP1 panels needs an area of about 850mm high & 120mm wide alongside the woofer box, which adds significantly to the overall size. To compensate for this, a so-called "isobarik" woofer system was chosen (as mentioned above). Vifa P17WJ 170mm (6½") drivers were used in a vented enclosure of about 21 litres. Box tuning details are given below. There is a choice of where to locate the power amplifiers, active crossovers, power supplies etc. The only parts which must be located close to the panels are the high voltage bias supply, & the step-up transformers for the panels. Everything else can go into a box near the CD player etc., or it can be built into the loudspeaker box.
In the prototype system, the crossover was included in a separate box containing a pre-amplifier. The power amplifiers, power supply (for two amplifiers), and bias voltage supply were all mounted on the removable opening on the rear of the woofer box. Low level signals are supplied to the speaker boxes from a pre-amplifier.
Dimensions for the loudspeaker boxes are shown below. Note that the right-hand edge (for the left loudspeaker) of the box is chamfered at 45° to allow the top two panels to radiate freely. Likewise, the right-hand edge of the right loudspeaker is treated the same way.
Mounting the Panels
The ESP1 panels are mounted on a series of platforms, as shown. Strips of 12mm material (wood or MDF) are glued on to the platforms, and then the panels are attached to these strips with double-sided tape. Similar strips (not shown) are also provided on the underside of the top box panel, & the top of the bottom panel.
Wiring the Panels
The three terminals on the panels are connected in parallel with similar connections on the other panels in the array, and wired back to the transformer assembly. In view of the high voltage drive to the panels, the terminals need to be well insulated after soldering the wires. The recommended insulation is self-fusing butyl rubber tape (NITTO No. 15, or equivalent). Ordinary insulation tape tends to unwind in time. The use of heat shrink tubing is not recommended, because the heat involved might distort the stretched polyester diaphragm.
|Wiring detail for ESP1||ESP1 Connections|
The voltages required for operating ESP1 panels are much less than those used in large panel full range ESL systems, where you can find 10kV plus. For the ESP1, the bias voltage is 1.5kV, but fed from a high resistance source, so is not life threatening. The drive voltage to the stators is a maximum of about 2.1kV RMS between stators. The strategy recommended is to earth the front stator, and cover the rear stator as much as possible with felt. However, it is also strongly recommended that the system be provided with a well constructed grille, to prevent children of all ages from touching the rear stator. The grille can be made in a lop-sided "U"shape, wrapping around the woofers & ESP1 panels, or divided into a removable "L", covering woofers & ESP1 units, plus a fixed frame (with grille material) attached to the back of the box.
Because the high frequency dispersion is symmetrical, two identical enclosures may be used for a stereo sytem, but it is aesthetically more pleasing if the left and right speakers are built as a mirror-image pair. The ESP1 stack for the left speaker should be on the right-hand side of the box, and the reverse for the right speaker.
Isobarik Woofer Mounting
The two 170mm (6½") Vifa woofer drivers (P17WJ) are mounted with a small sealed cavity between them, as indicated in the sketch. The distance between the two drivers should be not less than 60mm. The two drivers are connected in parallel, giving an impedance of 4 ohms.
Box tuning details
The net volume of the box is about 21 litres, so the tuning was chosen for this volume, but a good response can be obtained with smaller boxes, down to about 15 litres (with appropriate box tuning).
|Vifa P17WJ||Isobarik||Single Driver|
|fs||37 Hz||37 Hz|
|fb||38 Hz||44 Hz|
|f3||35.5 Hz||44.7 Hz|
|Vas||17.35 litres||34.7 litres|
|Vb||21 litres||22 litres|
|Port length (50 mm)||165 mm *||100 mm|
|Port length (2")||165 mm *||106 mm|
The port is made from 50 mm (2 inch) plastic water pipe. For the Isobarik design you need a right-angle bend plus a straight length. The straight length is pushed firmly into the right angle bend. Glue the right-angle bend so it is flush with the surface of the box. The length of the straight pipe is:
50 mm pipe: 85 mm
2 inch pipe: 90 mm
The net length of the port is about 165 mm in both cases. The port for the single driver system is made from a straight length of water pipe, with length as shown above.
Damping of the loudspeaker box
Especially if relatively thin material is being used for the construction of the loudspeaker box, it is strongly recommended that the enclosure is heavily braced, and that most of the inner surface of the woofer enclosure be covered with damping material. Suitable material is widely available from electronics & Hi-Fi retailers, and is effective in reducing vibration of the walls. It is usually made from a plastic loaded with mineral compounds, and is surprisingly heavy to handle. Although not shown in the drawings, bracing should be used to prevent panel vibrations.
Damping the ESP1 Panels
As mentioned earlier the electrostatic panels must be damped, to control the fundamental resonance of the diaphragm. A satisfactory material for this is a non-woven material called felt, which can be obtained from any fabric shop.
Rear view of panel array, showing felt damping
A one metre length of felt (100 mm wide) is stapled to the woodwork, close to the rear stators of the panels. The distance between the felt and the panels is not critical, but tucking it in close to the panels prevents prying fingers being exposed to high voltage drive signals, although the loudspeaker should not be operated without the grille being in place except for testing.
Because the high frequency dispersion is symmetrical, two identical enclosures may be used for a stereo sytem, but it is aesthetically more pleasing if the left and right speakers are built as a mirror-image pair. Placing the ESP1 stack for the left speaker on the right-hand side of the box and vice versa for the right speaker.
The following drawings show the major dimensions of the enclosures (all are in millimetres). It is almost certain that individual constructors will have their own ideas for the system, and the main points to consider are the enclosure volume and ESP1 panel positioning.
Front and Side Views Of the Box
Total internal box volume with the dimensions shown will be 28.8 litres - not allowing for the 45° chamfer. The volume occupied by this depends on the material thickness, but can be expected to be only around 0.5 litre or so at the worst (depending on how you actually make the chamfer). The total volume allows plenty of space for the drivers, port and bracing.
View Showing Placement of ESP1 Panels
There will be sketches of the three platforms (A, B & C) for mounting the ESP1 panels located in the secure site for those who purchase panels. There is nothing secret about these, they are not shown here simply to conserve bandwidth on my hosting server. The platforms are made from 12mm MDF and will be shown (approximately) full size to allow you to copy the shapes and the locations where the mounting strips are positioned.
Part 1 Part 3
|Copyright Notice.This article, including but not limited to all text and diagrams, is the intellectual property of Tony Rendle and Rod Elliott, and is Copyright © 2004. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro-mechanical, is strictly prohibited under International Copyright laws. The authors (Tony Rendle & Rod Elliott) grant the reader the right to use this information for personal use only, and further allow that one (1) copy may be made for reference while constructing the project. Commercial use is prohibited without express written authorisation from Tony Rendle and Rod Elliott.|