Essays - Blues Harp Science (en)

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This page offers downloads of essays in pdf format.

An  essay on the Physics of the Blues Harp has been published in the  conference paper of the FAMA (in Deutsche Gesellschaft für Akustik e. V.  DEGA) workshop in October 2015: Alfred Förtsch, Physics of playing bluesharp in: Musikalische Akustik zwischen Empirie und Theorie, Berlin 2016, p. 35-38.
A free download of the whole conference paper is offered here.
Erratum: On p. 35 (right column) read: "For air flowing through a series of appropriately chosen volumes this phase shift can be derived without referring to standing waves or inertia, presupposing ..."

During bending, self-excited oscillations of the air flow and the two reeds in the channel occur within a very short time with both reeds oscillating with equal frequency nearly from the very beginning. The essay Transient response of a half-tone bend on channel #3 of a blues harmonica can be downloaded here.

Fletcher 1979  and  Johnston 1987. Completely revised updates, status: March 2021

While searching for essays on the physics of the blues harp, the first paper I came across was by Robert B. Johnston:
Johnston, Robert B. "Pitch control in harmonica playing." Acoustics Australia 15.3 (1987): 69-75.
Johnston makes substantial use of formulas from a paper by Neville H. Fletcher from 1979:
Fletcher, Neville H. "Excitation mechanisms in woodwind  and brass instruments." Acta Acustica united with Acustica 43.1 (1979):  63-72.

Consequently, Johnston's work can only be "understood" if one has understood Fletcher's essay. Result of my occupation with the work of Fletcher is this essay Fletcher 1979, in which first the mathematical derivation of the formulas used by Johnston is calculated in detail (a nice application of elementary analysis). This presentation was inspired by a book by Robert D. Klauber: Student friendly quantum field theory, which actually fulfills its promise given in the title.
  • Linear acoustics and AC physics are closely related. Fletcher explains self-excited feedback in wind instruments by analogy with electrical feedback circuits using components with negative differential resistance (such as the Gunn diode).
  • From today's point of view, Fletcher derives the formulas used by Johnston as a linear approximation for harmonic perturbations. Neglecting an inertia term introduced by Fletcher (which he  omits in later works), his formulas for the admittance of the mouthpiece are identical to the formulas used today in linear stability analysis, as found in the textbooks by Hirschfeld or Chaigne/Kergomard. This is presented in a "student-friendly" way.
  • Work by Dieckman and Cottingham on Asian mouth organs successfully uses an approach going back to v. Helmholtz, who considered the admittance of the generator only to be the dynamic response of the oscillating reed. This approach is compared with Fletcher's approximation formulas.
  • Fletcher's formulas with and without mass term as well as the admittance as dynamic response are applied exemplarily to channel #4 of a blues harp in C in analogy to Johnston's work.

Robert B. Johnston was probably the first to deal with bends and overbends on blues harps from a physical point of view. In his 1987 paper "Pitch control in harmonica playing" he builds on an electro-acoustic model by Neville N. Fletcher for woodwind and brass instruments from 1979 ("Excitation mechanisms in woodwind and brass instruments" ). In Fletcher's work one finds formulas for the admittance of closing and opening reeds and criteria for possible playing frequencies. Johnston interprets the two reeds in the channel of a blues harp as a parallel circuit and uses Fletcher's formulas to calculate their total admittance. From the result he reads possible playing frequencies, which he associates with playing normal notes, bending and oververbending. A necessary condition for possible play frequencies is (as with Fletcher) a negative real part of the total admittance, which stands for energy supply to the resonator. Which play frequency actually occurs should be decidable by a comparison of the phase shifts of the admittances of generator and resonator.
  • My paper Johnston 1987 prepares the theoretical part of Johnston's paper in a "student-friendly" way.
  • Furthermore, Johnston's formulas are applied to channel #4 of a C-Harp. In particular, the curves for the real part of the admittance at first glance appear different from Johnston's example. Actually the curves are related by the damping factor as a parameter.
  • For a resonator with two modes and for a cylinder as resonator, we qualitatively investigate to what extent Johnston's phase condition yields playing frequencies that deviate noticeably from the resonance frequency of the resonator.
  • To actually verify Johnston's approach, one would need measured admittances or admittances calculated from MRI pictures of the vocal tract. To my knowledge, such data are not currently available.

Measurement of relaxation time and damping of a harmonica reed - download offered here.
Physical studies on playing the blues harmonica model the reeds as harmonic 1-point oscillators. The damping constant is needed for the equation of motion of a reed. In this paper two methods are presented to determine this damping constant without expensive laboratory equipment. An exemplary investigation is made of the draw reed in the fourth channel of a blues harmonica in C. In fact, after a transition phase, the decaying reed oscillation can be described as a monofrequent falling exponential curve. It can be seen that both methods, which are fundamentally different in their approach, provide the same results for frequency and relaxation time within this range, which indicates that they are indeed suitable for measuring the damping constant.

Forces on a blues harp reed - download offered here.
Blues harp reeds are free reeds. They can move freely due to the slots in the reed plate and oscillate almost sinusoidally in the first transverse mode of a clamped beam. The motion of a reed tip (and thus reed motion) can be modeled by an abstract 1-point oscillator equation, where effective values have to be used for some parameters. This linear equation can be used to compare the influence of different forces on reed motion.
The oscillation is generated and kept going by interaction with the fluctuating air pressure in the channel. Here, the amplitude of the oscillatory component of the pressure force on the reed in the oscillator equation is an order of magnitude smaller than the amplitude of the elastic restoring force. The influence of the pressure force is explained in frequency domain by a slight phase shift of the fundamental oscillations of reed and pressure, which provides for the necessary energy supply and for a playing frequency deviating from the resonant frequency of the reed. Apart from that, a blues harp reed oscillates "almost by itself" due to the interplay of elastic restoring force and mass inertia. This is ultimately due to the fact that the reed oscillates unhindered and weakly damped.
In a supplementary section, it is argued why it is better not to call the pressure force acting on the reed a "Bernoulli force", even if it is related to a Bernoulli equation.

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