[Harp-L] The results of the "cupping back" challenge by David Pearce

Vern jevern@xxxxx
Mon Jul 31 17:29:29 EDT 2017

There are two acoustic effects that prevent shape changes from affecting the perceptible volume of a harmonica.

Diffraction says that sound emanates from openings of less than a wavelength in a spherical wavefront pretty much equal in all directions.  There is no dimension of a harmonica that approaches the wavelength of sounds in the instrument’s range.  Therefore,  the shapes of covers have no directional influence.

The response of the human ear is logarithmic.  The sound power must increase by about 100% or decrease by about 50% before you notice a difference.  There is a story that may or may not be true about the young engineer who promised to redesign the exit nozzle of a jet engine to reduce the sound power by 50%.  He succeeded but his boss couldn’t notice the improvement and fired him.

Try closing a door with sound on the other side.  You will notice that there isn’t much difference between the door barely cracked open and standing completely open. There is a big difference in completely closed and barely cracked open. 

In harmonicas, the biggest sound change occurs between completely closed and a barely open hand cup.  The change between barely and completely open is much less. 


> On Jul 31, 2017, at 10:55 AM, David Pearce via Harp-L <harp-l at xxxxx> wrote:
> Will a cupping back on a chromatic harmonica that blocks holes 1 & 2 reduce the volume of those notes and change their volume or tone?
> Conclusion:  There is no discernible difference of volume or tone in the comparison of a harmonica with a cupping back on and one with no cupping back.  A cupping back can be used to make hand cupping a chromatic harmonica easier due to its smaller size without negatively impacting the sound or volume of the instrument.  I came up with this idea when I read about Vern Smith's experiment where he covered up 50 percent of the back of the harmonica and was not able to measure any difference in volume with an audio meter.  Thinking about the science behind a harmonica practice and testing it to determine its efficacy will help to advance the instrument make more productive use of precious time.
>  https://www.youtube.com/watch?v=5JBAp6xPYHY
> Keep on harpin!
> David Pearce

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