----- Original Message -----
From: "K9sqg" <[email protected]>
To: <[email protected]>
Sent: Saturday, November 13, 2010 8:00 AM
Subject: [Drakelist] MS-7 speaker improvement
Fellow Drake enthusiasts,
A number of people have contacted me about the Drake MS-7
speaker so I thought I would just spread the word via the
Reflector. I certainly don't want to encourage any debate
on the topic but your thoughts are always appreciated.
Although Drake gear is the mainstay of my station, the MS-7
speaker is basically not there. The reason is that the
frequency response and overall sound is not as good, in my
opinion, as the internal speaker in the TR-7 nor the MS-4
speaker. But here is something that I have done by
experiment to improve the MS-7. One thing is just to block
off the back of the speaker which seems to eliminate some of
the hollowness of the sound. However, what I find works
best is to find some acoustical foam, used to line speaker
cabinets, walls, etc. and cut a piece to fit inside the
speaker case, 6 inches by 4 inches seems to work fine.
Install it with the cones of the foam facing the speaker but
don't put it in all the way otherwise it might actually
touch the speaker cone itself. To me, this brings the MS-7
up to the level of "barely adequate". What I use as my
primary speakers is the MS-4 speaker which has better bass
response and intrinsically seems to filter out some of the
hiss and high frequency static that is more pronounced with
a smaller speaker. Too, adding acoustic foam to the MS-4
will improve it somewhat too. Listening to the MS-4 seems
to be less fatiguing. Your mileage might vary. Hence, I've
sold off the six or so MS-7 speakers I had and only have one
left that I use for experimentation and such.
The MS-7 is nice for rounding out the Drake 7 line but, for
enjoyable listening, I prefer the MS-4. I'm sure there are
many that find the MS-7 speaker to be preferable and that's
great. With ham stations, you can mix and match to your
satisfaction. Enjoy those Drakes!
73,
Evan, K9SQG
Probably the best material from an acoustic standpoint
is fiberglass batting. Fill the case with it. It serves to
reduce the standing waves and thus resonances in the case
and also increases the compliance of the enclosed volume a
little thus lowering the speaker resonance. The speaker
resonance will be affected by the compliance of the cabinet.
Essentially the speaker is a parallel resonant circuit with
the mass of the cone and compliance of the edge support and
spider providing the reactances. The frequency response of
the speaker will fall off at 6db/octave below the primary
cone resonance but there is also the effect of a mis-match
in the radiation impedance of the speaker causing an
additional roll off. The speaker is subject to much the same
conditions as an antenna and for low frequencies is like a
short antenna, that is, quite inefficient. The lower limit
is about a wavelength equal to about the circumference of
the speaker. Below this the match between the acoustical
radiation impedance of the speaker and the characteristic
impedance of the air becomes increasingly poor so the low
end rolls off. The low frequency resonance of the speaker is
used as a partial equalizer to overcome this over a small
range of frequencies.
Its possible to get some HF roll off using foam or
solid weave cloth over the speaker but its simpler to put a
capacitor across it. Most small speakers make pretty good
tweeters unless intentionally designed otherwise.
I have a Hallicrafters R-47, a small speaker designed
for SSB use which is probably much like the MS-7. It works
well with a Drake R-4B but I usually prefer to use an
ancient 8" speaker of unknown make in an old Bud cabinet
because its just easier to listen to.
--
Richard Knoppow
Los Angeles
WB6KBL
[email protected]
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