Sunday, 8 September 2013

A Tannoy 15'' Enclosure for the Enthusiast – Part II

My first listening evaluation with the test cabinets started in a sealed mode. For that reason I had build connecting boards into the inside to connect the baffle with the back board in the central area. Here in the center the resonances are most substantial, in particular with such sizes of enclosures. All reflecting walls inside needed to be damped with layers of woolen materials, the cover quilts for transport protection are a very good and cheap solution. These can be used folded to form several layers and fixed with cleats to get a good damping effect to the inside reflexions. Woolen materials damp a lot better than foam, it was introduced to loudspeakers in order keep the amount of labour low during production. Before commercial aspects got into account, several other techniques and materials like felt have been used with a lot better success. I will show this methods further on in this articles, but for now the woolen quilts will do well.

The first listening showed a quite good overall balanced soundstage, not that far from what I am used to. Despite the bigger volume of the crates, the general sound structure was very detailed and refined, in particular in the mids and upper tonal spectrum. These cabinets work with a quite slim lower end, but in a excellent and undistorted manner, which should be normal with huge sealed designs. It was clear before, that the missing of the typical bass boost (somewhere around 80 to 120 hz), which is always present with small reflex cabinets like the Berkleys. The small volume of the Berkleys boosts this upper bass even more than a bigger type ported design like the Arden would do.
I did listen for a while and did find it very refined with lots of air, details and depths, but for me it lacked some low frequent energy, even when they get a lot deeper , maybe down to 40 hz. I did find the sound bit thin, less authoritative and with reduced energy in the low frequency ends. I missed the expressive timbre and its dedicated "foot snapping" effect, which I am normally used to and which is very typical for a 15'' speaker. So I decided to open up the back boards of the cabinets completely to try them as open baffles and to use the backward radiated energy as well.

I did take this step into account, when I finished the test cabinets, so I did mount the electrical terminals at the outer frame. This option gave me the opportunity just to unscrew the back boards. I did add some more woolen damping material into the chambers, in order to act like a resistance for the backside radiated energy. Such open baffle constructions in history have been made with adjustable blinds by Western Electric, so the cabinets could precisely been adjusted for compensation. I am still thinking about such a feature for my final design, but I am not sure about. On the other hand this feature gives a good opportunity to adjust a speakers to a given room, but I don't move that much speakers around. So it might be a unrealized complication or a silly option?
In my room the speakers are positioned quite close to backing walls, normally they need a minimum distance of 50 cm between their back and wall to get the reflections reduced. It is always absolutely listenable, if the speakers are positioned close to the walls with a distance less than 50 cm or even 1 m as other extreme. Sitting in a listening position of 3,50 m away, these differences have a dramatic influence to the illusion of the stage of sound. Direct radiated waves will be mixed with indirectly ones, in addition reflectied waves from ceiling and walls will blurry the precision of the sound structure. Here the front loaded horns show their positive abilities. Their concentrated projection reduce wandering reflections in rooms a lot, so they give projected frequencies a tight imaging and sound stage.

Everybody with large middle frequency horns knows what I am talking about. In their best option, let's say with a typical middle frequency horn installation like the WE 15/16 or the late 1970ties japanese Sato-horns, the soundstage can be completely holographic and as well free of any room reflections. Wider dispersing speakers (like the most cone radiating speakers with a typical 120-180° degree directionality) will always bounce a lot of energy into the room, which will be reflected certain times and therefore mix up with the direct radiated waves. So the illusion of a precisely projected stage suffers a lot. This effect explains the inevitability for damping the walls in listening environments with conventional radiating speakers. The front loaded horn at my set up act that way similar like such WE-horns, the distributed frequencies get projected to the listening position and avoid bouncing of uncontrolled reflexions in the room. Finally it leads as well to a wider fanning of the frequencies from 400 hz to 2500 hz, the spectrum distributed from the cone acting like a full range speaker. In addition the HF-horn has a physical support for its lowest cut off frequencies as well.
The middle tone spectrum has the most important tonalities and covers the human voice and almost a important part of any musical instrument. A perfect undistorted sound stage here, is the basic key for a natural impression of a technical reproduced sound. The harmonic resonances of the northern wood as materials of the cabinets give a natural tonal support and are very similar acting like musical instruments. Using both waves of radiated tones lead to a increase of sensitivity, with the Tannoy in full range mode this means a increase of LF up to 4000 hz.

I will stay for a while with this set up, than I will do final decisions about the speakers to be build soon. In order to make sure that I don't take any sort of inhabitation of the Berkley cabinets into justice I will stay on listening to this test setup. In the moment I am not sure, if a smaller enclosure will work more universal and more independent from different room parameters. If I am going to integrate the funnel into a cabinet, it will take 35 l of its gross volume. If will be very rigidly made with lots of strives to strengthen the structure, with pillows at all surfaces for the damping of the interior reflexions (exactly like the original Tannoy cabinets were made in the 1950-60ties, a gross volume of 170 l will be a good starting point. This will make possible a room friendly design, where the center of the chassis is not to low located. With roughly 90cm x 55cm x 43cm (HxWxD) it will fit the most living rooms.

Read on soon, Volker