[Harp-L] What Cover-Plates Don't Do

[Jonathan Ross <jross38@xxxxxxxxxxx>]

The title of this thread is from a comment a friend made.  His comment was that he pretty much knows what cover-plates don't do, it's what they do he's still a bit unsure of.  

To that end, I thought I'd post my thoughts on the subject.  For the most part I wanted to focus on what covers don't do.  For this I'm mostly focused on traditional style covers.  Significantly different designs for cover-plates like the Hohner CBH and the Suzuki Overdrive are going to behave differently in some regards, and will mostly be used to highlight these differences from from the standard design.

First, covers don't significantly affect the timbre of the harmonica.  They don't do this in a variety of ways.  The cover-plates don't enclose an area in such a way as to create a tuned resonant cavity which would effect either pitch or timbre.  They don't vibrate in such a way as to effect the column of air moving through the instrument, not set of a resonant body of air which would then amplify their vibration with an audible effect.  They don't reroute the air to the reed in such a way as to affect the timbre of the reed, nor to affect the reeds performance (with specific exceptions as mentioned above).

So, let's take these in order and look at them in more depth.  The area under the covers is not specifically tuned to any pitch--the covers don't create a resonant chamber or body.  The openings in all traditional covers are way too large to create a Helmholtz style resonator.  Even if they were, the resonator would be set at a very specific pitch, and that would cause mismatches with the pitch of the reeds--reeds at a related pitch to the resonator would interact with it and have their timbre altered, reeds that were not connected harmonically would either be unaffected or fight with this resonator. But this doesn't happen.  Most cover-plates are the same size and shape for the entire range of the harmonica, from lowF to highG, and the timbre of the harps are the same throughout that range.  There is no fighting due to pitch, no interference and no amplification.  It doesn't happen because the area underneath the cover-plates doesn't function as a resonator.

Next, consider vibration.  Yes, covers vibrate.  But the important question is not why they vibrate, but if that vibration has a significant effect on the timbre of the harmonica.  There has been speculation on why covers vibrate, I would argue the most likely cause is that they are vibrating in sympathy with the vibrating column of air moving underneath them.  But that's not particularly important.  So the cover-plates vibrate, are they moving a large, enclosed body of air that would amplify that vibration?  No.  The air under the cover-plates is not enclosed and thus doesn't amplify the vibration.  Tap the cover-plates.  Unlike a guitar or other instrument where the sound is produced by a vibrating membrane over a resonant chamber of air, all you hear is a dull thud.  Moreover, let's assume the vibration of the cover-plates is causing a body of air to be set in motion (it doesn't, but let's assume it), any sound that might emanate from them is masked by the massive column of air moving being set into vibration by the reed.  The vibration of the plates would need to be amplified to a volume where it would significantly affect the column of air and alter the timbre.  But as pointed out, the vibrations aren't amplified and so can't have that affect.

As for the routing of the air--the air that goes through the reed comes from all around the reed.  It doesn't matter what route the air takes to get there--as long as no interferences occur and the air is not blocked off it doesn't even matter if the cover-plates are there or not timbre-wise.  Cover-plates can affect pitch and performance of the reeds, as can be seen by the CBH and Overdrive.  But consider these plates.  These covers are designed with specifically tuned and sized chambers which do act as resonant chambers (not majorly, but a bit--it's not like attaching the reed to a tube the length of the standing wave of the pitch of the reed).  So the reeds react differently to having the covers removed.  The pitch can change a bit with the covers and without, and the reeds can behave in odd ways (particularly the decoupling during bending which can occur with the Overdrive).  And the timbre is a bit altered due to the interaction of the reed's pitch and the chamber's pitch.  A bit being the important point here--this difference can be (and usually is) overwhelmed by attaching the instrument to a larger, flexible resonant chamber, ie the player.  


So, cover-plates don't significantly affect the timbre of the harmonica (with noted exceptions which prove the rule for traditional designs).

What do cover-plates do.  I'm not really sure either.  Certainly they can be comfortable or not, which is important--comfort being the most likely reason why they were introduced to the harmonica in the first place.  Possibly they can have a directional impact to the sound of the harmonica, but I wouldn't overstate that and most likely the listener would be less effected than the player in these cases.  But for the most part, it's easier to say what the covers don't do than what they actually do.



JR Ross