[Harp-L] Comb Profile, Channel Volume

["Brendan Power" <bren@xxxxxxxxxxxxxxxxx>]

The relation between reed pitch and channel volume is an interesting
offshoot of the comb debate. As has been pointed out, it is all very
well swapping combs with identical reedplates and covers to test
perceived tonal differences, but if the different combs vary in
thickness and chamber size other variables comes into play. These should
also be eliminated in any credible test.
 
Rick Epping told me of an interesting experiment he once conducted. I
hope he can correct me if I've got this wrong, but here's what I recall:
He took a diatonic comb (I think a Marine Band), and covered the two
reedplate-bearing surfaces with a thin strip of material (wood or
plastic, I can't remember - it doesn't matter). The harmonica comb then
resembled a tiny Pan Pipe.
 
Rick pursed his lips (as one does when blowing into a bottle or a Pan
Pipe) and blew to get the resonant tone of each chamber. He noted this
down, and then experimented with putting different pitched reedplates on
an identical comb. As I recall, he said there was not a significant
effect on the lower reeds, but the relationship between comb volume and
reed pitch WAS significant in affecting response on the high reeds.
 
I realise this is quite well known anyway, but the salient fact (as I
recall) was that the resonant frequency of the comb channel should be
ABOVE the pitch of the reed for optimum response. If it is below the
reed pitch, it interferes with it and response is poor. Can anyone
confirm that?
 
On chromatics certainly, you do get much better high-end response by
dramatically reducing the channel volume of the top few holes. Most
commercial models come with this in some form: simple and basic on the
cheaper chroms and more refined on higher-end chroms like the Hohner
Meisterklasse and Suzuki Fabulous. Doug Tate took this approach to the
logical extreme on all channels with his innovative 'Tate Ramps', and
custom makers like Bill Romel also incorporate some radical comb-volume
reductions throughout the range in their chromatics.
 
As far as diatonics go, I have been trying a few subjective tests, with
interesting results. I tried switching the same reedplates and covers
between a Suzuki Overdrive comb and a Suzuki Harpmaster/Bluesmaster one.

 
The Overdrive has the smallest-volume chambers of any commercial
diatonic, partly because the comb itself is not as thick as others (it's
only 5mm thick, compared to at least 6mm for other combs). The chamber
lengths are very short also. At the other extreme, the
Harpmaster/Bluesmaster comb is 6mm thick and has the longest channels of
any commercial harp, to my knowledge. 
 
I like the Harpmaster/Bluesmaster comb because it means it is really
easy to half-valve the reedplates, as there is no danger of the valve
base getting sandwiched between reedplate and comb. However, I was
curious to check the effect of channel volume against the much reduced
size of the Overdrive comb channels. 
 
I found the reedplates on the Overdrive comb had a tighter, more
focussed sound than on the Harpmaster, which was a bit more 'airy'. I
like both sounds really, and think there is value in selecting them both
as appropriate: I think I'm going to use the Overdrive comb for my high
harps (D to F#) and the Harpmaster for the low ones.
 
Of course, the two combs are made of different materials, so my test is
not scientific. But I'd be interested to hear other opinions on this
interesting topic.

Brendan
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