Comments on my post
Winslow Yerxa:
You can directly observe the behavior of a reed in both opening and closing reed bends.
• Take an unattached reedplate, and watch yourself and the reedplate in a mirror.
• Hold the reedplate vertically, so that the reeds are horizontal.
• Make sure that the reeds are facing away from you so that you can see them in the mirror.
• Choose a reed. Longer reeds are easier to observe visually.
• Play the reed first as a draw reed, and note the gap at the free end of the reed.
• Now bend the pitch down. Notice that the gap decreases as the pitch goes down. The bend will choke out and the reed will stall at the point at which it no longer emerges from the slot.
• Next, exhale to put the reed into opening (overblow) mode, sounding nearly a semitone higher.
• Then bend the pitch UP, sliding it upwards from the default opening pitch. Notice that the gap at the tip now increases as the reed flexes up and progressively away from the slot. The reed will cease to sound at the point where the reed no longer enters the slot.
Does this help with your idea of the reed acting as a fipple?
Mark Harper:
### The Physics of Harmonica Overblows: Mechanical Gating vs. Fipple Theory
The argument that an overblow functions as a "fipple" mechanism is a simplified heuristic. While the analogy captures the effect—higher pitch through forced air interaction—it misidentifies the primary vibrating element.
#### The Mechanical Mechanism
* **Fundamental Frequency:** A standard reed’s frequency is defined by its effective mass ($m$) and stiffness ($k$):
$$f = \frac{1}{2\pi} \sqrt{\frac{k}{m}}$$
* **The Overblow State:** In an overblow, the reed is forced against its slot, effectively "choking" its fundamental oscillation.
* **Forced Vibration:** The resulting high pitch is generated by the opposing reed, which is driven into a higher-order mode by the pressure distribution across the slot.
* **Frequency Shift Approximation:** The pitch increase due to applied pressure ($P$) can be modeled as a stiffness correction:
$$f_{overblow} \approx \frac{1}{2\pi} \sqrt{\frac{k + \alpha P}{m}}$$
* $\alpha$ represents the coupling coefficient of the fluid-structure interaction between the air jet and the reed.
#### Technical Conclusion
The "fipple" analogy is technically inaccurate because, in a whistle, the air jet vibrates while the fipple remains static. In a harmonica overblow, the reed is not acting as a fipple; it is acting as a **mechanical gate** that forces the air stream to excite a higher-frequency mode in the opposing reed.
---
#HarmonicaPhysics #Acoustics #ReedMechanics #Overblow #MusicTech #FluidDynamics #HarmonicaStudies
Nikolaus Grill
I'd say that during overblows, (and in fact during bends, in which the same principle is at hand) the harmonica turns into a true free reed aerophone. During normal playing, the harmonica is not an aerophone at all, but an air-driven idiophone. The resonator of the aerophone during overblows is the oral cavity. In overblows and bending, the reeds function much like in a Sheng (but not a Hulusi, which due to its triangular reed functions as a single reed instrument rather than a free reed aerophone), even though in bending the opposite reed acts as an additional resonator, making continuous glissandi possible (or easier). So actually in overblows, our mouth becomes the resonator, and the player has to adjust rhe oral cavity to fit the frequency of the reed, much like the bamboo resonators do in Chinese free reed instruments.
Mechanical Gating vs. Fipple TheoryThere is a historical debate on the exact physical mechanism taking place during an overblow (or its draw equivalent, the overdraw).Mechanical Gating Theory (The Accepted Consensus): Understood as the standard acoustical explanation for the overblow, the reed acts as a physical gate. When overblowing, the player blows air while shaping the vocal tract to the resonant frequency of a higher pitch (for example, the note one half-step above the blow note on that hole). This specific air column limits the oscillation of the natural blow reed, forcing it to remain flat against the reed plate (choking). This sudden stop frees the higher-frequency draw reed to vibrate in the same direction of airflow.Fipple (Flute) Theory (The Oversimplification): Some early theories attempted to explain overblows by comparing them to edge-tone instruments like recorders or the flute's embouchure. In fipple theory, an air jet hits a sharp edge (the fipple) which sets a standing wave inside a tube. While altering the vocal tract shapes the internal volume, applying a fipple model to a free reed is an inaccurate analogy. Overblows are instead caused by aerodynamic reed-coupling and forced-mode vibrations.
No comments:
Post a Comment