Re: [Harp-L] Reed Chamfering

[Rick Epping <rickepping@xxxxxxxxx>]

Hi Vern,

>From what I've read, those most versed in the mathematical fluid dynamics
of the Western free reed do not as yet have a complete model with which
they are in agreement.  Certainly, the mathematical discussion goes well
beyond my capabilities.

There is, however, a theory that states that as a reed is on its return
cycle, the tip having reached its lowest point through the slot and now
re-approaching the bottom of the slot, it encounters resistance from the
higher air pressure contained within the slot.  The practice of widening,
or flaring the underside of the slot that's found in accordion reeds, and
even more so in English-made concertina reeds, is said to be in order to
allow air to spill to the sides as the reed re-enters the bottom of the
slot, thereby relieving the pressure and allowing the reed to more easily
pass through the slot.  It's my guess that chamfering the upper edges of a
reed works in a similar manner as the reed approaches the lower side of the
slot on the return stage of its cycle.  However, the secondary effect of
the chamfering of the upper edges of the reed slot as a result of embossing
may work to relieve pressure from the upper side of the slot in a similar
manner during the reed's initial downswing into the slot.  Do you think
that the reduced thickness of the reed's edges achieved through chamfering
may also act as a means of pressure relief as the reed approaches the upper
edge of the reed slot?

I agree that it seems likely that reed chamfering is more important on the
return swing from the far side of the plate, particularly since the reed is
moving against the airstream at this point.

The tests Mr. Glunz and I conducted at the Hohner factory used only one
degree of air pressure.  I do notice, however, that chamfering a
harmonica's reeds seems to improve its speaking at lower pressure.

I chamfer the entire upper edges of the reed and have not tested whether
chamfering is only effective along that area of the reed that actually
enters the slot.  Chamfering is also effective on high reeds that do not
fully clear their slot.

One other reason I chamfer reeds is in the hope that, by relieving the
sharp upper edges of the reed I might be reducing the concentration of
stress there, thereby delaying fracture propagation and reed failure.  I
don't know whether this idea is really justified or not.

Sorry, I cannot give you any exact values regarding this test.  The data
and the minutes from the subsequent discussion with R&D relating to them
are most likely archived with Hohner.  Eberhard Glunz is no longer with
Hohner and the last time I inquired, the reed tester was not set up to make
this kind of test.

I recall that the vacuum was turned on at the motor to start the reed
playing rather than by means of a valve or solenoid.  Also, the air chest
to which both the vacuum and the reeds were attached was fairly large -
perhaps 0.5 cubic meters, so the actual time it took to make the reeds
speak, while seemingly immediate, may not be indicative of how much time
would be required to make the reeds speak when played by mouth.  The
pertinent information here was the comparative times between the
un-chamfered and the chamfered reeds.

All I can say regarding the effect of reed chamfering on response is that,
like slot embossing, it seems while playing the harmonica that chamfering
allows the reeds to speak more quickly and at lower breath pressure than
before they were chamfered.

Best regards,
Rick




On Tue, Jan 1, 2013 at 4:42 AM, Vern <jevern@xxxxxxx> wrote:

> Thanks for the pictures and descriptions for chamfering.   Knowing that it
> speeds up the reed response is very interesting.
>
> Does chamfering make the reed begin to speak at a lower pressure?  i.e.
> does it facilitate soft playing?
>
> Is chamfering effective near the rivet where there is little air flow?
>
> Do you agree that the major effect may have been the force on the reed
> when it was on the far side of the plate? Was chamfering equally effective
> on short reeds that don't exit the far side of the plate?
>
> I imagine that the number of milliseconds to reach steady-state amplitude
> must vary with pitch.  However, the number of cycles might be fairly
> consistent.  How many cycles did it take the chamfered and un-chamfered
> reeds to reach steady state?  If you only have the information for one or a
> few reeds, what were the pitches and times?
>
> Is this less time than that required for a player to raise or reverse the
> pressure in his mouth? i.e. does it place any limitation on playing speed.
>
> How did he suddenly turn on the vacuum?  The time required for a
> switched-on pump to reach full vacuum would be longer than reed response
> time.  Did he have a quick-acting valve to turn on the vacuum such as a
> windsaver-shaped pad jerked away from the plate by a solenoid?
>
> Vern
>
> On Dec 31, 2012, at 1:35 PM, Rick Epping wrote:
>
> > Hi folks,
> >
> > The page on chamfering I posted a link to the other day was set up for
> > viewing by invitation only.  Sorry about that!  It's now available for
> open
> > viewing.  Here's the link again:
> >
> https://docs.google.com/document/d/1hHeMSL1Pt3MV2U9HWGoPBCYicfukRP6W09CpfVssinU/edit
> >
> > I have expanded a bit on the article, including an account of tests that
> > were made of the process at the Hohner factory, as well as a mention of
> > David Payne's interesting test of chamfering a reed slot without
> narrowing
> > the reed-to-slot clearance.
> >
> > Best regards and a Happy New Year!
> > Rick
>
>
>