Harmonic Damping Compared

Discussion of harmonic damping was initiated earlier within the comparison of the Wigand, Back and Young instruments. Let us now recapitulate, and understand the issue in full. Problems with harmonic damping happen because damping occurs at points that are proportional to the speaking string length. If, for example, we find that a damping point occurs at 1/3 the speaking length, then damping might well result in a pitch -

(an octave and a fifth) sounding if the string is strummed or caught in inaccurate pinch or pluck when the string is damped.

So despite the application of damper to string, sound still emerges in the form of high harmonics if there is any simple proportional relationship between the point at which the damper is applied and the length of the damped string. This presents in the form of constant noticeable high harmonic content, or “plink” during even the most accurate playing of the instrument.

Harmonic damping is a problem for autoharp and reverse action harps alike. In both it is compounded by the fact that each damper bar damps several strings from the same point and therefore solutions such as the one patented by Walton Page are not appropriate mitigation strategies — the damper bar cannot simply be moved to a suitable calculated point in relation to a single string. Moreover, the manifestations are so complex and unpredictable as to seem to bear little relation to the simple calculation above.

The problem is heightened on reverse damped instruments, firstly because of the reliance on a spring to provide the damping force, this is significantly less than that provided by pressing the damper bar against the strings, and secondly because each string is damped only once, from one position, as opposed to an autoharp where the chord bar structure will mean that each string is damped from many different positions on different chord bars. In the case of an autoharp system, ineffective harmonic damping for one string on one chord bar will only present as audible “plink” to the musician only when that particular chord bar is depressed. A reverse damped instrument however, will display these problems equally, throughout the instrument; resulting in a continuous “plink” that ruins the musical experience.

This was the key problem that emerged within my own design experience on completion of prototype 2 - the harmonic damping was very poor. The problem was so bad that it demanded urgent attention.

Two approaches may be taken for resolution:

1. The damper bar is moved to a different position, thus changing the proportional relationship to the strings. In practice autoharp players are often attached to chord bar combinations and do not like to do this, and would rather put up with a persistent harmonic or use the second approach.

2. The length of the damper may be extended slightly (a “fat” damper is added at the problem string - commonly termed an "outrigger").

Within the ReAPH formulation, an unexpected benefit of the independence provided by the pulley and string arrangement was that poor harmonic damping could be resolved using the first strategy without affecting the interface presented to the player. The fact that pulley and strings can easily be re-coupled effectively enables random access between keyboard and damping systems. The damper bars can be presented in any arrangement without affecting mechanical considerations, or affecting the playing interface in any way — it does not matter to the keyboard arrangement where the damper bar appears if the coupling is flexible. This allows 12! possible combinations of damper bar arrangements and this design property was one of the factors in securing the patent, and resolved all of the harmonic damping problems for prototype 3.

Of the historical inventors, only Millington/Young (limited to one additional C) and Page allow cross coupling of keys in their designs, and only Page appears to consider the issue of harmonic damping of sufficient significance to warrant discussion within the patents. This disbars one of the primary strategies to combat harmonics from all of these formulations. But to what extent did these historical prototypes exhibit poor harmonic damping characteristics?

Steve Brown’s Newton harp certainly does not exhibit excessive harmonics at all by the sound of his recordings, indeed it appears to be very good, and this is clearly achieved without recourse to cross-coupling. In email exchanges, Steve pointed out to me that the different geometry of the feet-through-the-strings design of the Newton (also Henner and Back) allow for easy extension of damping down (or up) the strings as far as necessary from node points without interfering with the movement of adjacent damper bars — a point, the importance of which I had previously missed. The potential to implement the second strategy of producing “outriggers” is therefore considerably enhanced on these instruments, which clearly compensates for the lack of cross-coupling potential. Indeed on further reflection, I realized that the potential to implement outriggers within reverse action formulation generally is enhanced over autoharp formulations because no string has to be damped from multiple positions and therefore there is no practical obstruction to producing a long outrigger even on an overdamped system. This proved crucial to resolving the damping problems on prototype 5 within my own design practice, and I am endebted to Steve Brown for his wise counsel.

Overall, I suspect that harmonic damping was an issue on the historical instruments and was perhaps a surprise for some of the early inventors, but clearly it was possible to combat this problem. The extent to which this was done is unknown.