Room acoustics
We often hear that an audio system sounds different at home than we remember from listening to it in a high-end dealer’s listening room. The reason for this is that the sound emitted by the loudspeakers is reflected by the walls and objects in the room and reaches the listener with a time delay. The more sound the room reflects, the less the listener perceives the actual sound of the original recording. The room therefore has a significant influence not only on the sound impression (due to the frequency response at the listening position), but also on the spatial resolution of the musical events.
Room resonances
Not only the time delay of the reflected waves in the room (reverberation time), but also the standing sound waves that form, so-called room resonances, pose a major problem for the reproduction quality of the sound transducers (Fig. 1 shows typical room resonances).
Fig. 1: Different room resonances on two fixed walls (https://www.hifizine.com/2011/09/prototyping-dipole-bass-system/)
Room resonances are resonances that can occur between parallel walls of a room if they are half a wavelength or a multiple of this distance apart. However, a room resonancy or standing wave also occurs if there is only one wall or sound boundary surface. The reflections of such a wave add to the original signal, resulting in an increase or decrease in the sound level, depending on the frequency and position in the room. As a result, the auditory impression differs considerably from the original.
Tackeling room resonance
Room resonances in particular lead to distortion of the music signal at low frequencies. Frequencies can be canceled out, which leads to a weak bass, or amplified, which is perceived as booming. The room also has a slow transient response. This leads to a blurring of the impulse behavior of sound bodies, such as the attack of a drum, a guitar string or the reed attack in singing.
The room acoustics can of course be improved by damping measures. However, this does not always make the room more beautiful and does not solve all problems. In order to improve the situation, you have to ensure that the room is not stimulated to vibrate in the first place. The only way to prevent this is to ensure that the sound transducer system emits as little sound as possible into the room, so that as little indirect sound as possible reaches the listener. This means that you have to use transducers that do not emit the sound in a spherical shape, i.e. in all directions, but instead emit the sound directly to the listener. To achieve this, we use dipole radiators over the entire frequency range. Fig. 2 shows typical radiation characteristics.
Fig2a.: Spherical radiation, typical for low frequencies of a conventional subwoofer, which leads to excitation of the entire listening room.
Fig2b.: Typical radiation pattern for a dipole radiator.(https://commons.wikimedia.org)
We therefore develop dipole radiators for the entire frequency range. For the bass range up to a maximum of 150Hz, we use a dipole subwoofer based on the Linkwitz principle. The subwoofer only radiates to the front and rear. The great advantage of such a system is that the sound reflected by the walls is almost completely canceled out the next time they meet. As a result, room modes are only minimally excited and therefore only slightly interfere with the original signal. Impulses are not softened as a result, but are retained as in the original. In the bass range, the musical instruments can still be distinguished from each other, whereas with conventional systems there is a blurring.
In the mid-high range, room modes play an increasingly subordinate role. However, the wider the dispersion angle, the more you can hear the listening room, again due to reflections that reach the ear with a time delay. We reduce this considerably by using large-area electrostats (see below), which also work as dipole radiators. Here too, reflections caused by rear radiation are suppressed.
Housing
The overall performance of a speaker system also depends, among other things, on the acoustic effect of the enclosure. The diaphragm vibrations of the speaker lead to high air pressure differences between the front and rear walls of a speaker enclosure, particularly at low frequencies, which causes the wall surfaces of the speaker cabinet to vibrate. As the surface area of the cabinet is much larger than the usual cone surfaces of the speakers, the cabinet has a very large influence on the sound result. This is why we prefer to use high-performance composite concrete or specially glued wood construction with a high proportion of carbon fibers for bass cabinets (unless otherwise requested by the customer). The high damping properties of this material suppress the cabinet’s natural vibrations like no other. In addition, the design principle of the bass speakers of a dipole bass working against each other also helps to avoid natural vibrations of the cabinet. In this way, we have succeeded in achieving a high level of resolution and differentiation, particularly in the bass range, as well as a natural reproduction of musical instruments without coloration.
Electrostats
In the mid-high range, reflections from the ceiling and floor in particular lead to a diffuse and harsh sound. The dipole characteristic of the radiation of high-end electrostats prevents these reflections. This is a fundamental advantage for most living rooms and a prerequisite for a natural three-dimensional sound.
Design
While the world of traditional loudspeaker technologies consists of cones, domes, diaphragms and ribbons that are moved by magnetism, the world of electrostatic loudspeakers consists of charges. The principle is that like charges repel and opposite charges attract.
An electrostatic transducer is made up of three components: the stators, the diaphragm and the spacers.
Fig. 3: Design of an electrostat
The wafer-thin polyester membrane (brand name: Mylar), which is tensile in both the longitudinal and transverse directions, is provided with an electrically conductive layer. The coating developed by us is highly resistant to ageing and UV radiation.
The membrane is stretched between two rigid perforated plates, the stators. Together with the membrane, they are bonded at the edges with special acoustically neutral adhesive tapes to form a sandwich. The membrane can vibrate freely in the middle. The sound can escape unhindered through the perforations in the metal sheets. The perforated sheets are coated with a polyester resin for electrical insulation. To generate sound, a high voltage (polarization voltage) of 2500 – 5000 volts is applied to the membrane, depending on the size of the stators. However, a voltage is applied to the stators whose strength and polarity is modulated directly by the music signal from the amplifier. The signal is transformed to 70 – 100 times the voltage with the help of a special transformer. This causes the membrane to be rhythmically attracted or repelled by the stators (push-pull principle). In this way, air pressure waves are generated, which we perceive as sound (see Fig. 4.).
Fig. 4: Technical structure of an electrostat
Advantages of the electrostatic principle
The technique of push-pull operation is a key contributor to the sonic purity of the electrostatic concept due to its exceptional linearity and low distortion. Since the diaphragm of an electrostatic loudspeaker is driven uniformly over its entire surface, it can be extremely light and flexible. This makes it very responsive to transients and therefore tracks the music signal perfectly. As a result, great impulse fidelity, nuance and clarity are possible. The cones and hoods used in traditional electromagnetic drivers cannot be driven uniformly due to their design. Cones are only driven at the tip of the cone. Hoods are only driven at their circumference. This means that the rest of the cone vibrates without a drive. This leads to natural vibrations, which can only be avoided if the cone or hood were completely rigid, damped and massless, but this is not physically possible. In order for these cones or hoods to move, all electromagnetic drivers must have voice coils wound on bobbins, centering spiders and mounts for secure positioning of the cone or hood. These parts, in combination with the large mass of the cone or canopy materials used, result in an extremely complex device with numerous weak points and potential faults.