The GeerS eVe II

By Edgar Beers
27 Feb 2001

The Final Design

After incorporating the new units, filter architecture and enclosure design, simulations showed that this would be an excellent speaker for smaller rooms, but that usage of a larger bass unit would greatly improve the easiness of bass reproduction, leading to a more natural sound. We chose the 21W4555-01 to replace the 18W8545K. However, this also means a larger cabinet. Based on Q=0.5, with 100 filling material, I would increase the cabinet volume from 27 to 46 litres. And so, the GeerS-eVe II was born, while the GeerS I (with the 18 cm woofer) never left the drawing board Fig. 2. GeerS - eVe II CAD drawing.

Phase Coherence
It is relatively easy to build a good sounding loudspeaker. It is quite hard to build a loudspeaker that is able to catch the emotion in music. For that you need detail and optimal imaging. In my personal opinion both are a result of phase coherence. So in the GeerS eVe we did everything to maintain consistent phase behaviour. First, the system is designed as a 2.5 way. Which means that the 18W8545 unit provides the most important area of the musical spectrum. The bass unit only works up to 200 Hz, where the mid unit takes over and runs up to 2500 Hz. There the tweeter takes over.

Second, The whole Baffle is slanted 6.85 degrees. This, in combination with critical placement of the units on the baffle, leads to near-perfect phase behaviour on a listening axis about 93 cm high, from 2 meter away from the speaker. Additional to this, the angle of the top enclosure can be adjusted through the spikes, as well as its position relative to the bass enclosure. However, the design is optimised for placement where the top enclosure's baffle aligns with the bass enclosure's baffle.

Third, The use of a series filter for mid-high, improves phase behaviour over a parallel filter. The parallel filter for the low end acts as a sub-woofer.

Finally, this speaker can be bi-amped or bi-wired. Although it is common marketing practice to proclaim a loudspeaker phase coherent and therefore fit it with one pair of binding posts (the other reason might be that series filters are more commonly used; a 100% series filter cannot be bi-wired), I say keep the option open! If two cables sound better, why not provide the option (hhmm. one reason might be the additional cost for two sets of WBT binding posts).

Again, many speakers are emphasising on phase coherence these days. However, phase coherence is the result of a cocktail of measures, and not "just" slanting the baffle, or put all the drivers magnets in vertical alignment. Cabling, filtering and the phase coherence of the used equipment might make all the difference.

The Tweeter
Scan Speaks D-9700 was chosen over the Revelator for a number of reasons. First, they're basically the same tweeters. Second the speakers I have heard using the Revelator (including Sonus Fabers' Amati Homage) had a tendency to be on the bright side. However, this might be personal preference rather then a quality statement. Third, the smaller front-plate of the D-9700 allowed me to design Avalon-like cut-off edges, which I like both from a design as well as a sonic point of view Fig. 3. Top enclosure.

The mid-woofer
Scan Speaks 18W8545 was the easiest choice. It's about the only remaining element from the Reference Plus as well as the Andromeda and its quality is proven by many top speaker designs who use this unit. Application of strips of lead bitumen on the unit's ribs clearly made the whole structure deader.

The Bass Unit
We basically choose a larger 18W8545, being the 21W4555-01. This unit has a nice long throw flexible design, strong magnet and is ideal for nice natural bass reproduction. Application of strips of lead bitumen on the unit's ribs clearly made the whole structure deader. I also wrapped the magnet in felt. I honestly didn't know what the effect would be, but it was also done on the Scan Speak units employed in the Audio Data Elance, and I would not let any chance on improvement go by Fig. 4. The bass unit and its enclosure.

The Bass Enclosure: Speakers Density
When I bought my Acoustic Energy's I was told that it was easy to build a large box, but hard to actually design a good sounding loudspeaker. Now I have to disagree. It is quite hard to build a large box with similar damping characteristics as a small box. I truly believe that a loudspeaker requires a minimum density. Simply devide its weight by its internal volume. Large speakers need to be heavy in order not to let the enormous amount of energy in the enclosure interfere in the audible spectrum.

Experience teaches me that good loudspeakers have a density of around one, which means that for every litre of internal volume, you need one Kilogram of mass to control vibrations. The Andromeda has a density of 1.05 (95 kilogram for 90 litres). The eVe II has a density of 1.00 (46 kilogram for 46 litres internal volume), even the AE1 has a density of around one (11 kilogram for 11 litres). I have for instance estimated Wilson Audio's CUB's at a volume of around 30 litres, with a weight of around 33 kilograms brings its density to around 1.1. However density cannot be the only indicator. The way mass was applied makes all the difference. Building thin plywood boxes and pouring concrete at the bottom to reach a certain density simply won't work. One can easily see that maintaining a proper density for larger loudspeaker designs is quite hard. Basic mathematics shows that a small volume has a relatively large surface (read: easy to apply mass) as an increasing volume has a relatively decreasing surface (read: harder to apply mass). So, if the minimal density statement is true, than it is much, much tougher to realise this density for large cabinets than is it for smaller ones.

The Andromeda needed a sandwich structure to do it, the eVe II doesn't. For the eVe II, I used 25 mm MDF for the sides, top and bottom. The baffle was made with double thickness: 50 mm. A similar 30 mm collar around Andromeda's top enclosure was used in eVe's bass enclosure, making wall thickness varying from minimum of 25 mm to 55 mm for most of the unit's side surface. Internal bracing was provided through 18 mm MDF matrix on all three axes. Here too, a cut out lined with felt provides additional support for the bass unit. Cutting out large holes out of the back of the bass units prior to installing the collar, added little, but useful volume while incorporating the back as internal bracing Fig. 5. The bass enclosures internals.

This enclosure was designed as a closed box with a Q=0.5. However, just to have the flexibility, a slot shaped bass port was placed at the top side of the enclosure, tuned to 29.5 Hz (this slot is now notoriously known as " the mailbox") Fig. 6. The mailbox.

The baffle is constructed out of two pieces of 25mm MDF. In order to give the bass unit the ability to move freely (remember, there is also a 18 mm matrix around the magnet), the hole in the inner piece of MDF was cut under a 15 degree angle, creating a trumpet like form in the baffle.
The inside, except for the baffle was lined with 4 mm thick lead bitumen and 40 mm thick Pritex foam. BAF wadding was used to fill the remaining volume.

The Top Enclosure
The original top section of the Reference Plus design was provided with a 50 mm thick baffle, similar in set-up with the bass enclosure. A simple 18 mm piece of MDF behind the cut outs for the tweeter created its own little chamber which is felt lined. A matrix on two axes provides necessary rigidity, in combination with lead bitumen lining and pure wool filling. To prevent back-lash, 40 mm pritex was used for the enclosure's back panel Fig. 7. The top enclosure prior to closing.

In order to reduce high frequency reflections, large cut-offs around the tweeter were used. The top enclosure rests on three adjustable spikes in order to minimise energy transfer from bass- to top enclosure.

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