As pointed out in the last section, it is not yet possible to build a perfect loudspeaker, each design must be according to a set of particular priorities, and people differ as to what they consider the most important parameters. The Kapellmeisters were designed with certain definite priorities in mind; if these happen to be in accord with your own, the Kapellmeisters can be strongly recommended. The design was first published in Electronics Today International.
The first and rather discouraging restriction, which was forced by necessity, was that of size. Space was severely limited, so the size had to be kept down. Measuring the available wall space having due regard to all the other necessary furnishings revealed that the only space available for the width of each speaker was an unpromising 8 inches.
However, depth was less restricted and so was height, so if maximum air volume was to be achieved, the result had to be in the shape of a narrow column. This is not ideal for an infinite baffle speaker owing to the pipe-like behavior of such a shape but perhaps a different type of enclosure could be engineered into that shape.
Natural uncolored reproduction in the bass and in fact all registers was a high priority, but it did not seem attainable in view of the size limitation. However, an extended frequency response was not high on the list. Up to 15 kHz in the treble and down to 45 Hz in the bass was considered acceptable for the reasons outlined in the last section. Certainly undistorted bass was rated more highly than having lots of it! Having established that, it did not take long to throw out the idea of a multi-driver speaker with a crossover network and all the phase and other distortions they generate. Instead, a single well-designed full-range driver was decided on. To reduce doppler and intermodulation distortion that are produced by large cone excursions at bass frequencies though, the driver needed to have a fairly large cone area so as to obtain the maximum acoustic output for the minimum cone excursion.
With a maximum width of 8 inches which must include the thickness of the sides, a large cone seemed impossible, but a reasonable area was in fact achieved by using an elliptical driver 5-inches by 8-inches mounted vertically.
There are two bonuses that arise from this: one is that a narrow cone and enclosure give a wide dispersion of high frequencies and an enhanced stereo effect, the other is that the single elliptical driver occupies less space than a multiple system. This is not only helps with the space problem, but simplifies construction and makes an alternative enclosure type to the infinite baffle feasible.
High power was given a low priority. Volume levels in excess of those heard in the concert hall with a full orchestra were considered unnecessary, in fact undesirable. Elimination of the crossover with its often-needed level balancing resistors meant fewer losses and higher sensitivity thereby permitting high acoustic output from low power. The drivers chosen are rated at 8 watts, and in the Kapellmeisters they produce well over 90 dB from a 4 watt per channel amplifier. A bonus here is that a truly hi-fi class A amplifier can be used as these can be easily designed to give low powers. It is high power operation at class A that produce the problems.
The problem remained as to which type of enclosure. The infinite baffle was not regarded with much favor because of the inherent non-linear air load distortion and resonance hang-over effects which both serve to distort the bass. These too are accentuated with smaller enclosures.
A transmission line was felt to be highly desirable but impractical because of the size and shape. But before abandoning the idea, a deeper investigation was decided on to see if any way could be found around the practical problems.
Conventional designs usually house all the drivers in a cubicle which extends to the back of the enclosure. From there the path, considerably reduced in area descends to follow its course through horizontal and vertical sections. A wave can be reflected from the rear of the cubicle back to the drivers just as in an infinite baffle enclosure, and so upset the smooth progression of the pressure wave along the transmission line.
Other disturbances can occur at the bends with some back reflections, and storage and release of energy by the panels facing the bend thus producing coloration. Ideally, the transmission line should have no bends for this reason, although this really is a practical impossibility.
Furthermore, the line should be of the same area throughout to prevent unequal pressures along its length. Actual designs usually taper the line in an attempt to broaden and flatten the resonance peak introduced by each section.
By using a single driver, the cubicle can be dispensed with and the transmission line can start immediately from the speaker. Thus the back reflection is eliminated and the sudden decrease in area from the cubicle to the start of the first section. It also saves a lot of space.
If the bends could be eliminated, or more practically, if they could be designed to afford an almost perfect reflection of sound around them, the line would behave as it should, as one long pipe rather than a collection of small pipes each with its own resonance. Then the single resonance and its harmonics could be dealt with, tapering would be unnecessary, and the line area could be maintained throughout its length at its ideal optimum, which is the same area as the driver cone.
The whole thing can be thus be seen to hinge around the near-perfect reflection of sound around the bends. Some form of deflector placed at the correct angle at each bend is called for, but wooden panels are not the answer. They vibrate, store energy and release it, and are not perfect reflectors.
One of the best materials for reflecting sound is concrete, which it does some ten times better than thick wood. The only improvement on this is when the concrete is faced with highly glazed ceramic tiles. (Notice how the sound bounces around at a swimming bath.) If two tile-faced concrete wedges are arranged at a 45° angle even a U-bend can be negotiated, something which would be inviting trouble in a transmission line of conventional design.
The design thus materialized as a transmission line in three sections, one behind the other, each 33 inches (83.8 cm) long and having ceramic and concrete reflectors at the two U-bends (Fig. 41). This arrangement satisfied the minimum width requirement of 8 inches, while taking advantage of the less
Fig. 41. Side plan of Kapellmeister speaker.
stringent restriction on depth. Actual depth of the completed Kapellmeister is 11 inches (27.94 cm), which although greater than the width is by no means large. The height puts speaker center at 2ft 6ins (76.2 cm) above the floor which is just about at head level when seated in a reclining chair.
The exit vent is at the rear facing downward, so the speaker is supported on three small legs some 2 inches above the floor to allow what remains of the rear wave to escape. In addition to the length of the line which is over 8 ft, the distance from the vent to the front of the cone is at least a further 3 ft, making a total distance of over 11 ft. Thus no cancellation can occur at any frequency higher than that having a wavelength of 22 ft which is 50Hz, however the rear wave is so attenuated having travelled through 8 ft of absorbent material, that little cancellation occurs anyway.
The tiles are set at an angle of 45° to reflect the sound accurately around the bends. It will be noticed in the illustration that there is also a concrete and title wedge at the top just above the driver. The purpose of this is to reflect sound generated by the top part of the cone, downward.
The wave-front radiated by the back of a loudspeaker cone travels outward along an axis that is perpendicular to the surface of the cone. If the cone is at an angle of 22° which it is for the specified driver, the tile should be set at an angle of 56°. If the speaker has a different angle from this, the difference for the angle of the title should be halved, so a 20° angle cone should have a tile angle of 55°. Most loudspeakers of this size have cone angles in this region, but the angle varies slightly with cone curvative anyway, so the above figure can be used in most cases and is not too critical.
As the sound pressure on either side of the baffles is almost the same they are not excited into vibration to produce coloration, and this is true also of the back. The top and bottom pieces are of stout timber lined on their insides with the concrete and tiles, so they too are prevented from vibrating.
Only the front panel has a sound pressure differential between its internal and external faces but this is much lower than with a totally enclosed cabinet. Also its narrow width makes for high rigidity which inhibits vibration. The structure of the enclosure thereby adds very little coloration to the sound.
Closed Pipe
The transmission line is as we have seen, a pipe that is closed at one end but open at the other. As such it exhibits a fundamental resonance plus odd harmonics; there are no even harmonics. The fundamental resonance occurs where the total length equals a quarter wavelength of the frequency. Here, the length is just over 8 feet, so the resonance is at 35Hz, which is below the lower frequency limit of the driver at 45 Hz.
However, being broadened by the dampening material in the pipe, it can influence the range of the speaker usefully extending it below 40 Hz.
The antinodes (points of maximum air motion) at the third harmonic occur at the third and two-thirds positions, that is at the bends. Extra wadding at these points serve to suppress this harmonic. Extending the extra wadding up the first channel to the fifth position, and also at the exit, dampens two out of the three antinodes of the fifth harmonic. Above the fifth, the harmonics are smaller and the normal wadding fitted throughout the length of the pipe virtually tames these.
The specified speaker unit is the Altai 8553 DU, an 8 x 5 inch full-range elliptical which allowing for the frame and surround, has a 7 x 4 inch cone. Thus the approximate area is 22 sq. ins, while the area of the channels is 7 x 3 inch rectangular, which at 21 sq. ins is almost the same. The area remains constant right to the exit vent. A high frequency horn is fitted at the apex of the cone to improve the high-frequency efficiency and the response is 45 Hz- 16 kHz, a little better than the response decided in the brief. Resonant frequency is 50 Hz, and the power rating is 8 watts rms.
Parameters of the driver are not too critical, so if the specified one is not available any similar unit will do. The size is important, and this includes the front to back measurement which should be about 24 inches, (63.5 cm). It should be a full-range speaker with a response from 45 Hz to 15 kHz or better. Generally you will find that an extension at one end is at the expense of the other. High power is not essential because of the good acoustic and electrical efficiency, but 8 watts rms minimum, the same as the specified unit, is recommended.
General good quality construction with a magnet of not less than 9,000 gauss, and a foam or similar surround should be looked for.