Spheroidal Loudspeakers in White Marble.
Introduction to the SP62:The SP62 loudspeaker has the transparent and uncoloured sound of a full range electrostatic panel loudspeaker in a compact 20 cm / 8" sphere. The SP62 provides exceptional clarity of reproduction and imagery by the combination of a non-resonant, low diffraction enclosure with a single wide range point source driver.
The spheroidal enclosure shape results in much lower diffraction distortion than a conventional box enclosure, and the high mass of the marble acts as an acoustic sink for the driver. Carved from a single block of marble the non-resonant enclosure, unlike a wood based box, adds negligible colouration to the sound being reproduced, whilst maintaining the advantages of an enclosed loudspeaker.
The single 62 mm metal cone driver covers the entire audio spectrum from below 200 Hz to 20 KHz. There is no mid-band cross-over, with its power and phase abnormalities, to muddy the sound. The 1.88 gram moving mass cone has very low stored energy, and down to a tenth of the distortion produced by conventional drivers. The single driver connects directly to the power amplifier output (optionally with a first order high pass filter) and is an easy load for the power amplifier to drive.
The enclosure is not ported; low frequency roll-off is 12dB / octave and the speaker does not suffer from tuned port resonance effects, and cone excursion is well controlled. The SP62 positioning is very flexible and they may be used in free space, against a wall or in the corners of a room. Due to the exceptional phase accuracy the SP62s may be placed with wider than normal spacing for a larger sound stage.
The SP62s are designed from first principles to recreate as closely as possible the original sound field as recorded by a stereo microphone, without artefacts.
Cone / Chassis 62 mm (2.45") Controflex metal
foil. / Aluminium die cast.
Life expectancy: 1000 years plus for the enclosure/stand (the drive units may need refurbishing before this).
Design BriefTo produce a very high quality compact loudspeaker that will reproduce the original source material as presented by the power amplifier as accurately as possible, without adding or detracting from the original sound, with the aim of recreating the original sound field as recorded by a single point stereo microphone.
BackgroundModern professional and hi-end domestic audio systems can have very good specifications but are often badly let down by the loudspeakers.
The problems associated with loudspeaker sound reproduction can broadly be divided into two areas; the enclosure and the drive units.
The standard box enclosure is resonant and has sharp edges which cause diffraction effects. A typical veneered MDF and particle board loudspeaker box can resonate in conjunction with the drivers and radiate as much sound pressure as the driver itself at certain frequencies. Diffraction effects caused by a sudden discontinuity in the radiated sound field as it meets the edges of the enclosure results in secondary sound sources, contributing a three to five percent distortion figure. (It should be noted that amplifiers are typically specified in hundredths or thousandths of a percent distortion levels.)
Standing wave modes within a rectangular enclosure can cause pressure variations across the driver cone, causing it to flex and thus lose acoustic energy at lower frequencies.
The loudspeaker as a whole is light and vibrates on axis to the bass driver cone movements, so it must be solidly mounted to a floor or wall to damp these vibrations. Heavy metal stands with spikes are often used to try to control these vibrations.
The drive units used only cover a restricted part of the audio band so multiple drivers are used. One bass and treble driver for a two way, or with additional mid-range drivers for a three way design.
As each driver only covers a part of the audio band the sound is produced from different locations, depending on frequency. Many sounds contain rich harmonics, so for example a piano note fundamental may be heard from the bass driver, but its harmonics will come from the treble driver located 150 to 300 mm from the bass. The sound is not integrated as coming from one plane. In the crossover region the combined output from both drivers produces complex interference patterns, with areas of reinforcement and cancellation. These patterns are frequency and spacing dependant, so frequencies close to each other produce different patterns and intensities for the same listening position.
Also in the crossover region there is a discontinuity in the power response from the drivers caused by the beaming effect of a driver related to the diameter of its cone and the frequency of reproduction. As the frequency increases the sound is progressively focused into a tighter and tighter beam; so around the crossover region the bass driver is producing a relatively tight beam of sound as its cone diameter is large, whilst the treble with a much smaller diameter cone is radiating a very much larger area. This causes further anomalies to the sound.
Crossover networks are required to split the signal from the power amplifier into bands for each drive unit. These electrical filter circuits impart their own characteristics to the signal, thus altering the sound further.
The resultant multiple driver box loudspeaker cannot therefore produce an accurate reproduction of the original source material, there will always be colouration derived from the enclosure and various effects from the multiple drivers and crossover.
An alternative to the box loudspeaker is the electrostatic panel loudspeaker, where a very thin film conductive membrane is suspended between two charged plates and vibrated with the audio signal. These loudspeakers do not suffer from enclosure colouration or multiple driver problems, but they are large, require high voltage power supplies and other components, and cannot be placed close to a rear wall due to their open design, needing a large room.
Design ImplementationReturning to first principles in audio sound reproduction each problem area was considered and solutions to them devised.
Problem: Enclosure resonance and vibration, diffraction distortion
and standing modes.
Problem: Multiple driver and crossover effects.
The combination of a high mass, non-resonant, low diffraction enclosure with a single low mass, point source, metal cone driver solves or removes the problems previously described.
Marble was chosen for the enclosure as a homogeneous high density material that can be turned and hand carved from a single block to produce the required form. Marble is available in a wide variety of colours and textures from jet black to white, and can be polished to an attractive natural finish.
A spherical enclosure shape eliminates diffraction distortion as there are no sharp edges to act as secondary radiators.
Using marble, the high density spherical enclosure does not resonant to colour the sound and the high mass acts as an acoustic sink for the driver, so the enclosure does not vibrate.
The internal volume of the enclosure is spherical which maintains an even pressure around the rear of the driver cone so the acoustic energy at lower frequencies is preserved.
For the drive unit a special British designed and manufactured unit was chosen. This driver has a small 62 mm metal cone and it covers the entire audio spectrum from below 200 Hz to 20 KHz. Performing the same function as two conventional drive units, there is no mid-band crossover, with its power and phase abnormalities, to muddy the sound. The 1.88 gram moving mass cone has very low stored energy, and down to a tenth of the distortion produced by conventional drivers. A single driver is a much simpler load for a power amplifier to drive accurately, and it is a true point source reproducing the sound as it was originally recorded by the microphone.
ConclusionStarting from first principles, with the primary goal of the highest quality of sound reproduction, has resulted in a departure from the accepted norm for a loudspeaker. In doing so fundamental problems with the established format of box enclosures have been analysed and solved using a combination of modern and traditional technologies. The spheroidal loudspeaker reflects the source material and the amplification used, without imposing its own signature on the sound being reproduced. It is ideally suited as a near-field monitor in the recording and editing studio, as well as a domestic loudspeaker of great transparency.
The SP62 is a unique loudspeaker with the acoustical qualities of a full range electrostatic panel loudspeaker in a compact 20 cm / 8" spheroid form.
Design by: Susan Parker, MIEE.
This page last modified on: 15th November 2004