Electrostatic loudspeaker
The electrostatic speaker (abbreviation: ESL) is a loudspeaker design can be used wherein elements that use electrostatic attractive force in order to generate acoustic waves.
- 2.1 Eintaktaufbau
- 2.2 Push-pull construction
- 3.1 Conventional
- 3.2 inverter principle
- 4.1 Power Control
Physical Basics
Electrostatic speakers do not use the Lorentz force ()
But the electrostatic attraction force
Voltage drive
For voltage control applies: with.
As you can see, the force is not linear to the current, but the square of the voltage. To achieve a useful playback so that a bias is required.
Constant charge
For push-pull ESL according to the constant tension principle, and these are almost all built ESL today, the driving force is linear.
For a constant charge ESL for the force applies to the membrane ( which carries the charge ):
It forms a homogeneous electric field between the stators with the strength
The force on the charge of the membrane is then →
In this equation, F is proportional to and thus linear because Q and 2d are constant.
This is true under the condition that the charge Q is small and has virtually no separate field effect. For large force effect, however, Q must be large. This leads to a constant displacement of the diaphragm from the zero position to a stator out to where there is a new zero point together with the mechanical pretensioning of the membrane. But even under these conditions it can be shown mathematically that the force / voltage characteristic curve remains linear.
Construction
Eintaktaufbau
Eintaktlösungen operate stably. Here to keep an attractive force by a constant electrical bias and mechanical membrane voltage at equilibrium. The modulated audio signal ensures appropriate force effect and thus deflection around this equilibrium point. This system is due to the quadratic power / voltage characteristic distortion only for very small deflections. Single-ended ESL be used as tweeters are in new products but virtually impossible to find.
Push-pull structure
1957 appeared the first commercial electrostatic full-range speaker area on the loudspeaker market: it was the QUAD ESL ® of the British hi-fi manufacturer's quad, who worked for the push-pull principle. As stators are two solid, mechanically stable grid electrodes between which the vibrating membrane film is. For electrical insulation, the stators are coated with polyester resin. The stators are perforated to allow the sound to escape. Force field and have it in operation in the same direction.
The membrane is thin, capable of vibrating and electrically conductive ( but with a high surface resistance). As a material for the membrane is, inter alia, Polyester and Polydisulphenol used which is provided with an electrically conductive layer. The membrane is about 2 to 20 microns thick extremely thin
Control
In contrast to dynamic speakers no high power (5 to 10 A), but a high voltage ( 1000 V to 4000 V ) is required for the activation.
This high voltage is produced directly by the high voltage amplifier ( tube amplifier or MOSFET) or stepped up by the transformer from the commercial power amplifier voltage (eg 20 V) (e.g., 1000 V).
Conventionally
In the conventional control, the tone frequency is out of phase is applied to the stators, when the bias voltage is applied to the membrane:
- Stator 1: úton
- Membrane: UPOL
- Stator 2 - úton
- Differential voltage 1: UPOL úton
- Differential voltage 2: UPOL - úton
By far the most to place on the market ESL operate on the constant tension principle. Here, the music signal is given with different polarity on the stators and the charged membrane foil by means of a voltage source to a constant charge. The membrane sheet is usually high impedance by an extremely weak conductive coating. Between the stators is a homogeneous field is formed, whose power / voltage relationship is completely linear. For this reason, the distortion behavior of this type ESL is the best of all the principles involved.
Inverter principle
When controlling the inverter according to the principle of the polarization voltage is applied in opposite phase to the stators, whereas the audio frequency is applied to the membrane:
- Stator 1: UPOL
- Membrane: úton
- Stator 2 - UPOL
- Differential voltage 1: úton UPOL
- Differential voltage 2: úton - UPOL
The membrane must still be mechanically biased, since the rest is unstable ( at magnetostats she is indifferent ).
The inverter design is patented.
Triggering two direct voltages of different polarity on the stators ( the grid electrodes ) is only one and rarely used option. It requires a highly-conductive membrane, on which the music signal is provided. Thus there is a constant voltage ESL, which is subject to the quadratic power / voltage relationship and thus does not have a linear drive. The system is distortion only for small deflections.
Force on the electrode in the rest position
Voltage drive
- Preload:
- NF- voltage:
- Distance between a GE and membrane:
- Tension between GE1 and membrane:
- Tension between GE2 and membrane:
- Capacitance between a GE and membrane:
- Force between GE1 and membrane:
- Force between membrane and GE2:
- Resultant force on the membrane:
The resulting forces are compared with electrodynamic speakers ( where values up to 50 N are at full usual), very small. Values of 2000 V, 1000 V, 4 mm, 1.5 m 0.4 m = 0.6 m lead to gerademal 2.6 N. However, since the driven mass is considerably lower, a direct comparison is not meaningful.
Benefits
Electrostatic speakers allow many design freedoms, because they can be made very thin.
Disadvantages
Despite push-pull actuation produce larger vibration amplitudes audible distortion (the two distances to the fixed electrodes are no longer identical, so that quadratic shares lift no longer in the bill up and out ). The design problem is that for large vibration amplitudes dramatically reduce the necessary larger distances between the electrodes the mean sound pressure. In the bass range is added as a further problem in that it comes through equalization of pressure between the front and back to acoustic short circuit, which further reduces the bass response and the oscillation amplitude is further increased.
The sound radiation occurs inherently directed relatively strong in electrostatics, that is, for a stereo installation creates a very narrow range of optimum listening (also called "sweet spot" ).
By appropriate structures is trying to counter this phenomenon:
- Curvature of the surface of the electrostatic
- Segmentation of the radiating
- Use boss acoustic lenses
A bass reproduction requires disproportionately large electrostatic surfaces, so this converter principle is not particularly suitable for the bass and the bass is often supported by additional electrodynamic transducers.