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I have a
box of Monacor waveguides cut to various depths for a quick test on
tweeters to pick those suitable for applying a waveguide. Just
throwing in a waveguide on a tweeter mostly doesn't work. Some
tweeters may require a shallow waveguide, some tweeters a deep
one.
The initial part of the waveguide, the throat, is critical in the way
the tweeter couples to the air and loads the "horn". There's
no clear distinction between a horn and a waveguide but if a waveguide
is deeper than 30-40 mm, I would start thinking of it as a horn. With
the TW034
tweeter a very shallow waveguide was possible and counts for its lack
of any "horny" sound. The shape of the dome loading a long
tube may cause phase irregularities and count for aggressive treble
performance, thus compression drivers are often fitted with phase
plugs meant to load the horn properly.
A horn provides an acoustic impedance match between the driver and
free air and improves the efficiency with which an e.g. dome loads the
surrounding air. As seen from the measurements below we have some 6 dB
increase in response around target point of crossover, thus when
equalised properly, we have a significant reduction is distortion.
The ScanSpeak tweeter is a high-sensitivity tweeter displaying an
impressive 95+ dB/2.8V sensitivity due to the low-ohmic voice coil and
strong neo magnet, thus a potential candidate for high-efficiency
speakers.
Initially
I picked a shallow waveguide taking advantage of the 710003
faceplate's initial rounding towards the dome. This turned out really bad and
produced a serious dip around 10-12 kHz. Thus, a deeper waveguide was
tried and produced the results seen below. Really nice!
The D3004/660000 tweeter has a different faceplate and does not take
the here shown waveguide, just be ahead of that question. You could
use the 7100 faceplate on the 6600 tweeter, but I don't think you can
buy one from ScanSpeak. In that case you will have to modify the 6600
faceplate to fit the Monacor waveguide. I have no measures for this
and no plans for doing so.
Before
you rush to buy a pair of 7100 tweeters and waveguides, think about
implementation with your other chosen drivers. Making a crossover for
a waveguided tweeter is much more difficult than a properly placed
tweeter on a flat baffle or properly faceted baffle. The latter may
produce a nice flat response easing crossover design. For the 7100
tweeter here the response at 3 kHz is some 6 dB higher compared to 1
and 10 kHz, thus good measurement data and simulation software is
necessary to get it right. Quite often it means we can use a much
smaller series capacitor, not unimportant when we apply for super
caps. Another possible feature is that the acoustic depth of the
tweeter is increased the same as the height of the waveguide, e.g.
20-35 mm, which may be the same acoustic depth of the middriver, thus
allowing experimentation on phase and time-coherent crossover
implementation.
Modification
of Monacor WG300 waveguide
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TO INDEX
Monacor waveguide, item #WG300, designed
for the Monacor DT300 tweeter.
We need to make two spacers to
accomodate the ScanSpeak D2904/710003 tweeters.
Here I used 5 mm MDF. Outer and inner diameters are 116 and 50 mm. 5
mm plywood works as well.
Either saw off the threads or make four
9 mm holes to allow the flange to rest on then ribs.
Fill cavity with filler, e.g. cheap acrylic filler or Superfix as used
here.
Smooth filler and press flange towards
the ribs. Apply filler round the throat and smooth with finger as seen on photo.
Make a gasket from
acrylic felt or similar fabric and mount the driver. Use washers not
to damage the rubber gasket of the tweeter.
I used 4 mm wood screws. Drill 2.5 mm holes before fastening the
tweeter.
Voila!
On my prototypes I used 5 mm plywood and
shaved the treads to mount flange with four countersunk screws.
Actually works a little better. Screws are better with plywood
compared to thin MDF.
Measurements
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All measuments
performed on IEC baffle at 0.5 meter distance, response normalised for
1 meter, 2.8 volts input.
Let's slowly zoom out over the next four
graphs and start looking at the 1000-10000 Hz range to the upper left,
the range where things really matter because most treble energy is in
this area. Well, not much to comment on because this all look smooth
and easy as should be.
These two graphs are an average of 0, 10, 20 and 30 deg. responses.
To the upper right I have included the 10-20 kHz range and averaging
the 10-18 kHz range we see a response of ~94dB/2.8 volts.
Overall, quite a potent system for high-efficiency systems.
To the upper left we have the 2-40 kHz
range at 0, 10, 20 and 30 deg angles. Right: 40 and 50 deg. angles
included.
For those interested, the impulse and
step response.
CSD @ 25 and 50 dB scaling. Note the
unusual clean 25 dB presentation.
50 dB scaling is hardly relevant as we're getting into the noise
floor, but it may give hints of trouble zones,
like the tiny wrinkle at 6 kHz, but it's so low it doesn't matter.
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