This is an English version of the article from L’Audiophile no.2. december 1977.
Translated using source scans from:
Réalisation de l’enceinte grave ONKEN article scan in french at Onken.info
Réalisation de l’enceinte grave ONKEN article scan in french from http://6bm8.lab.free.fr
Planet10 Hifi english version of the article
Original text © By Jean Hiraga
L’Audiophile
Many audiophiles are in search of a true low-frequency reproduction. In order to achieve this, all possible means are often employed: very large loudspeaker enclosures, multiple loudspeakers, heavy amplification, active filters, correction systems (graphic equalizers), MFB systems (Motional Feedback, that is to say servo-controlled systems). Others prefer to modify and improve the acoustic characteristics of the listening room itself, which is certainly very important: placement of the enclosures, absorbing materials, selective reflectors by frequency, resonators to absorb several peaks and resonances. But a true low-frequency reproduction does not mean merely objective quality and measurement results, which are generally excellent for many well-calculated enclosures.
The present article will describe an acoustic enclosure of the ONKEN brand, designed in Japan. A very demanding enclosure, studied in 1965, then manufactured only in very small quantities in 1973. Of imposing dimensions (approximately 360 litres), it achieved considerable success and was used (and is still used) as a reference in a well-known Japanese magazine. In the magazine Radio Experimenter of October 1973, Mr. Koizumi described it in every detail.
Since then, a very large number of amateurs have built this enclosure as well as several manufacturers of acoustic enclosures. Despite the simplicity of this enclosure, its construction requires good woodworking knowledge. It is estimated that approximately 4,500 to 5,000 ONKEN enclosures have been built by Japanese amateurs. Given the publicity value of the ONKEN brand, it was truly necessary that the results be exemplary, so that amateurs would take it up seriously. Because choices of enclosures are not the same in Japan, comparisons are made in a very strict manner.
If midrange and tweeter units are manufactured on a small scale, audiophiles in France can fully appreciate the remarkable qualities of this enclosure, thanks to its design and its relatively low cost price.
Preliminary recommendations
This enclosure is recommended for demanding audiophiles who wish to build it, and who are prepared to respect all the details of its construction and the dimensions of the enclosure. Even though many factors come into play with regard to bass quality, the important points to be respected are as follows:
A listening room of sufficient size, that is to say larger than 4 × 6 metres.
A very rigid and well-damped floor (double carpeting).
Under these conditions, other points influence bass quality: the amplifier, the tonearm, the cartridge, the turntable platter cover — in short, the most sensitive elements.
ONKEN enclosure: wood clamped under ten tonnes
The original ONKEN enclosure is very often superior to imitations, even when these are perfectly executed. This perceptible difference upon listening seems to originate from the wood.
This wood, 25 mm thick plywood, is manufactured specially for Onken near Nagoya, where the majority of furniture and piano manufacturers are located. This wood, called either “shinaban” plywood or Canadian plywood1, has the particularity of being not only very heavy, but also of being composed of thin laminations strongly bent while hot, by an ultrasonic process, and then pressed by a ten-tonne press.
It should be noted here that before pressing, each lamination is strongly convex or concave, and that after hot pressing there remains, within the thickness, an internal stress which gives this plywood great rigidity.
By tapping such plywood with one’s fingertip, one can notice a pure and clear sound. In general, one more often obtains either a dull or a “cracked” sound, caused either by a press that is too light, by non-homogeneous bonding, or by insufficient glue.
The quality of the wood used is important for the 400–600 Hz range, a band that is difficult to reproduce and located near the crossover with the midrange loudspeaker.
It is therefore up to the reader to find a wood that is very heavy and hard at this thickness. Let us add that using a wood of lower quality but of double thickness improves almost nothing. Wood of quality close to that used for ONKEN enclosures can be found either in Switzerland or from small boat manufacturers (so-called “marine-grade” wood).
Felt
The felt used in the ONKEN enclosure is made of 100% pure wool, with a thickness of 15 mm, and is moderately compressed felt. Under no circumstances should glass wool be used, nor should a different quantity of felt be employed, this quantity having been chosen after long trials and both objective and subjective measurements.
Construction
Once the wood has been selected and found, it is always recommended to have it cut and glued by a well-equipped cabinetmaker. The final gluing of the six panels must be carried out using precise and heavy presses.
All battens and anti-vibration reinforcements are screwed and glued, another point that must be respected. Note that the front panel is removable and fixed with 16 screws (cross-head), approximately 42 mm in length. In the majority of cases, tightening these screws cannot be done by hand (use of T-handles, brace drills).
The parts to be assembled first are the two side panels with the filters. Once the side panels are assembled, these are mounted and glued in a single operation together with the top, bottom, and back. Before this assembly, note that battens are screwed and glued, which allows the screwing and gluing of the back panel. The glue used is hot glue, as used by cabinetmakers. Glue based on synthetic rubber is not recommended here.
Note the presence of numerous reinforcements on most of the panels, all of which are screwed and glued. The six panels are also screwed and glued, taking the precaution of masking the marks left by the screws on the external panels. For this purpose, one may use either a special glue (very thick), or a mixture of plaster and glue, which will be levelled after complete drying.
The finishing of ONKEN enclosures is done with a special paint applied in two coats. The first, after drying, renders the surface “cracked”, with a thickness of approximately 1 mm; the second is a paint applied by spray gun, of very dark grey colour, semi-matte. The front panel does not include any protection of the loudspeaker by grille or fabric, and it is preferable to leave it as such.
Another important point is to mount the loudspeaker from the rear of the baffle and not fixed from the front. Despite the many theoretical disadvantages, rear mounting is the one that gives the best subjective results. Note that the perforation of the baffle, with a diameter of 34 cm, has been calculated for the best phase response, and that thus a troublesome part of the outer suspension of the loudspeaker is concealed.
Fixing the loudspeaker
The basket of the 416-8A loudspeaker, although imposing, must be fixed with great care. This loudspeaker is of high efficiency and has a very narrow air gap. The slightest deformation is sufficient to cause the entire moving assembly to become off-centre.
It is therefore necessary to tighten each screw very slowly and little by little. One can also tighten the screws with the baffle separated from the floor by two battens while at the same time injecting a sinusoidal signal of very low frequency, 5 to 10 Hz, at the limit of bottoming. If the signal generator allows it, descend lower in frequency (1 to 2 Hz) so that at these inaudible frequencies the slightest misalignment will be felt.
This adjustment must be carried out in complete silence.
The rear panel includes a sealed opening, dimensions 6 × 10 cm, on which will be fixed, on the inside, a bakelite plate, thickness 1 cm, carrying the loudspeaker connection terminals. Choose terminals of good quality and generous dimensions. Those used on the ONKEN enclosure have an external diameter of 15 mm and a length of 3 cm.
Connect the terminals and the loudspeaker using quality wire, of the “LUCAS” type, which gives the best results here.
Conditioning of the loudspeaker
If the loudspeaker has remained unused or exhibits stiffness due to its storage conditions, conditioning is indispensable in order to obtain correct operation from the outset. This conditioning concerns mainly the outer suspension and the centring spider, which must under no circumstances be softened by hand.
Proceed as follows:
Inject, at 0.6 watt, the following signals:
- 25 Hz for 5 hours
- 110 Hz for 2 hours
- 1000 Hz for 2 hours
- 3000 Hz for 1 hour
in the order stated above, the entire sequence to be repeated four times.
This conditioning is important and avoids the numerous adjustments and readjustments during the months that follow.
Placement of the loudspeakers
Separated from the rear wall by at least 1 metre.
Separated from the side walls by at least 1 metre.
Placed directly on the floor or on a stone base.
Very demanding audiophiles may have granite slabs made, polished on five faces (edges + top), with dimensions 65 × 90 × 8 cm. In this case, insert hard felt, thickness 5 mm, between the slab and the enclosure.
The minimum distance between the internal walls of the enclosures is 1.80 m.
Crossover frequency
The most recommended crossover frequency is 600 Hz, with a 12 dB per octave slope. A high-quality active filter is advantageous; however, it is also possible to use a passive filter composed of a 3 mH inductor, made with large-diameter wire (3 to 3.5 mm, as used on ONKEN filters), and a high-quality, paper-impregnated, non-polarised capacitor of 24 µF (or alternatively two 12 µF capacitors connected in parallel).
Do not use inductors wound on laminated steel cores or on ferrite formers. Use large-diameter wire, approximately 3 mm, to connect the filter elements.
Advantages of the ONKEN enclosure
It is always possible, for example by treating the diaphragm, to make a loudspeaker mounted in an enclosure very linear in frequency response. Unfortunately, this is always to the detriment of efficiency and definition.
The average efficiency of the ONKEN enclosure is 96 dB per watt, that is to say roughly 10 dB more than conventional enclosures. In the majority of cases, an amplifier with a power between 5 and 50 watts is more than sufficient. This enclosure can accept, despite its exceptional efficiency, powers of 60 watts, which translate acoustically into an intense sound level that is difficult to tolerate, yet still not distorted.
By comparison, a low-efficiency enclosure would require more than 3000 watts to obtain the same level, which is technically impossible and would lead to the destruction of the voice coil. More commonly, this results in saturation and limitation of musical peaks.
This enclosure, fitted with the Altec 416-8A loudspeaker, gives results very close to those of the original ONKEN enclosure. For a level of 100 dB at 1 metre, distortion remains below 0.8% at 35 Hz, and 0.2% between 60 and 600 Hz. This performance, let us repeat, is difficult to obtain with such a high efficiency.
Musical qualities
This enclosure was described in La Revue du Son (No. 275). While bass quality also depends on the quality of the midrange and treble, the bass of ONKEN enclosures is characterised by great lightness, firmness, and exceptional dynamics.
Audiophiles who do not like large-diameter bass loudspeakers must first consider that the most important factor in a loudspeaker is not the mass of the diaphragm (which is very light in the case of the 416-8A), but rather the ratio of transmitted energy to diaphragm mass, which in this case is very high and plays a primary role in efficiency.
The magnet used on the 416-8A is a central magnet, not a ring magnet, which makes it possible to reduce magnetic leakage to a minimum (a ferrous metal object does not “stick” to the magnetic circuit, despite the high total magnetic intensity and the very generous flux in the air gap: 15,000 gauss (1.5 tesla)).
In the majority of cases, not only is the diaphragm heavier, the magnet less powerful and the air gap wider, but the ring magnet introduces an inevitable magnetic leakage, easily demonstrated; this leakage can reach 30%, which further worsens the energy-to-mass ratio.
Even without producing excessive coloration, such loudspeakers show that a large mass driven by weak energy makes perfect control of the diaphragm impossible. By touching the stylus of the phono pickup at a moderate level, one can hear the bass loudspeaker “pound” and “lag”. If acceleration is easy, the opposite is less so, because it is impossible to want to stop instantaneously a diaphragm of 100 to 130 g with such a weak force. This is comparable to the relationship between engine power and the mass of a car.
On this point, the 416-8A loudspeaker, which is by no means a recent design, combines the advantages of an ultra-powerful motor with a light and rigid cone.
In the ONKEN enclosure, it performs superbly: very sharp attacks, perfect clarity at low as well as at high levels. Thus this enclosure can reproduce simultaneously several very different low-frequency sounds at the same time: a very soft and diffuse bass foundation (hall, reverberation, distance effect), upon which other bass sounds can simultaneously stand out: double bass with “plucked” strings, cello with all the richness of the resonances of its body, the dry and percussive bass of percussion instruments, the broken and intense exhaust bursts of a tractor…
In general, even when no obvious coloration defects are noted on a large majority of loudspeakers, one must nevertheless observe that there exists a more or less marked “similarity” between several reproduced bass sounds, a sign of another form of coloration that one might call “musical expression coloration.”
When correctly adjusted, this enclosure is, as was described in the article on ONKEN enclosures, capable of remaining completely absent on certain programmes containing no bass, which may seem obvious but is rarely respected.
For adjustment, it is therefore recommended to begin with sound sources containing practically no bass and to adjust the bass level to the threshold of perceptibility. Without altering the levels, one then proceeds to sounds containing progressively more bass: cello, double bass, bass drum, organ. This adjustment is delicate and must be redone several times, at a medium listening level, with all tone controls left in the linear position.
Let us say that the system proposed here, for the bass, is not “a good loudspeaker,” but that it is certainly a faithful device, capable of reproducing a wide variety of low-frequency sounds with a remarkable freedom of expression.
Recently, the 416-8A loudspeaker has been replaced by the 416-3B, differing in its chassis and its magnet. It is nevertheless recommended to obtain the earlier version, the differences affecting mainly the extreme low frequencies (30–50 Hz band).
Thus proposed, its cost price is attractive in view of its performance, being reduced simply to:
— Two Altec 416-8A loudspeakers
— Six panels of 180 × 90 cm × 25 mm
— Pure wool felt
— Two 600 Hz low-pass filters, 12 dB
— And good ears for the final adjustment!
Let us hope that this enclosure will meet with the success it deserves in France.
- In the original French text, Jean Hiraga uses the term contreplaqué canadien (“Canadian plywood”). In the context of 1960s–1970s European audio and woodworking literature, this term generally refers to high-quality multi-ply birch plywood (often of Baltic origin), rather than plywood of strictly Canadian manufacture. The original wording has been retained for fidelity.
↩︎
[…] Construction of the ONKEN bass enclosure – Jean Hiraga, L’Audiophile 1977 Le Petite Onken – Jean Hiraga, L’Audiophile 1982 […]