arXiv subjects

David Politzer

Publications and source records attributed to David Politzer.

14 recordsLinked to original sources

arXiv2021

Arithmetic of Spring Forces on the Banjo Bridge

Spring-like forces on the bridge are key to a banjo's characteristic voice. These are due to the tension in strings and head. Conceptually distinct from the forces of waves in the strings and head that encode the underlying music, the spring-like forces impact the timbre of how those waves are converted to sound. This note presents a simplified model that allows the head contribution to be calculated (mostly) with paper and pencil. The key simplification is placing a circular bridge at the center of the head. The resulting formulae show how design elements and player's adjustments can effect the sound. The results also provide estimates of the magnitudes of the effects when evaluated with numerical values for Young's moduli and typical banjo set-up specs. For steel strings and tight mylar head, the head contribution is about three times as large as that of the strings.

physics.pop-ph↗

arXiv2021

Pickers' Guide to Acoustics of the Banjo, parts I and II

The results of an investigation of fundamental banjo acoustics and physics are now readily available. The papers describe measurements and calculations which demonstrate the extent to which the sound of a banjo can be represented by linear modeling of the basic parts common to the banjo family: strings, drum head, light floating bridge, and break angle. This note summarizes the conclusions of that work and outlines some of the physics involved. While the scrutinized banjo behaviors are well-known to discerning builders and pickers, the focus here is how these arise from the physics. A practical application (discussed in an addendum) concerns head tap-tuning: what's going on, how it works, and why some people and some tuners can't get it right.

physics.pop-ph↗

arXiv2020

Banjo Break Angle Tension Modulation as Parametric Oscillation

The motion of the floating bridge of the banjo, in conjunction with the break angle of the strings over that bridge, produces string tension modulation that is first order in the amplitude of the string motion. This note refines a previous suggestion regarding the impact on the frequencies of the strings' and bridge's motion. For a given mode frequency pair of string and bridge, the resulting tension modulation produces a new, additional motion characterized by the sum and difference of the original ones. Strictly speaking, this corresponds to canonical "frequency modulation" only in the limit of modulation slow compared to the string frequency. The more general result is precisely an example of what is known as "parametric oscillation," first analyzed by Rayleigh. The qualitative impact of tension modulation on banjo timbre remains as suggested previously. It is only the precise math and physics that warrants this correction.

physics.pop-ph↗

arXiv2019

Banjo Ring from Stretching String: A Zero Break Angle Demo

A novel bridge and tailpiece design allows direct comparison of the sound of zero break angle with same banjo (and all its parts) configured to have an angle of 13 degrees. This lends additional support to the 2014 proposal that a key element in banjo sound is the frequency modulation produced by string stretching due to a floating bridge, break angle, and head with substantial motion. When playing a banjo tune in the 0 degree configuration, there are enough audio clues that it still sounds like a banjo. The comparison allows you to judge for yourself to what extent it's lost its ring or sparkle.

physics.pop-ph↗

arXiv2018

Banjo Drum Physics -- sound experiments and simple acoustics demos

11" D mylar heads over a normal range of tensions (DrumDial 85 to 91) and "open-back" backed pots of depths 2", 2 3/4", and 5 5/8" are studied over the range 100 to 2000 Hz. Normal modes and resonant frequencies of the heads and of the pot air separately are easily identified and agree with simple expectations. The present focus is the head - pot air interaction. There is no "gold-plated" example of a pair of head-air interacting modes that are distant in frequency from all others. (Had there been such a pair, their interaction could have been isolated and studied in detail.) Nevertheless, there are a few cases where there are hints of the kind of interactions expected from a simple theory. The investigations also offer several examples of banjo physics, including aspects of bridge position and rim flexibility, and some dramatic examples of the perils of sound recording, including floor bounce and room sound.

physics.pop-ph↗

arXiv2018

Whither Tone Ring Ring?

An extremely simple model captures the essence of the interaction of a banjo tone ring with the wood rim. The large scale, low frequency resonances of the assembled system are related to the weights and resonant frequencies of the tone ring and rim separately. Very crude measurements satisfy the derived relations within about 5% for the lowest frequency modes and give qualitative agreement for the next ones on a particular, heavy-tone-ring resonator banjo. The two combined sub-systems become increasingly independent for higher frequency, shorter-lived modes. Nevertheless, the ringing sounds of the struck individual parts, which dominate the perception of their pitch and sustain, are related by the simple model to the sound when the parts are struck when combined into one.

physics.pop-ph↗

arXiv2017

Banjo Drum Physics - theoretical preliminaries

The interaction of a drum's head with its enclosed air is presented in the simplest possible form appropriate to the questions and issues that arise in understanding the timbre of the banjo. The inherent air-head impedance mismatch allows treating the head as driver of the air and the air's effect, in turn, as back reaction. Any particular question can then be addressed with a calculation in simple wave mechanics. The analysis confirms and quantifies the notion that internal air resonances enhance the response of the head at its and their frequencies. However, the details of just how are fairly complicated.

physics.pop-ph↗

arXiv2016

Air modes of the Bacon internal resonator banjo

Sound measurements on a sequence of related, similar constructions with slightly different dimensions confirm a simple picture of the air modes of the internal resonator banjo's body. For the purpose of this study, the air modes are decoupled from the soundboard (i.e., [drum] head) modes by replacing the head with 3/4" plywood. The resulting characteristic features survive the strong coupling of the air modes to the head and are in accord with the qualitative distinctions recognized by banjo players.

physics.pop-ph↗

arXiv2016

Banjo Rim Height and Sound in the Pot

Rim and back geometry determine much of the behavior of sound inside the pot, whose effect on total, produced sound is subtle but discernible. The theory of sound inside a cylinder is reviewed and demonstrated. And previous work on the Helmholtz resonance and the interplay between the Helmholtz resonance and the lowest head mode is revisited using some improved techniques.

physics.pop-ph↗

arXiv2016

Physics of the Bacon Internal Resonator Banjo

The internal resonator banjo, patented and first sold by Fred Bacon around 1906, remains something of a cult favorite and is still produced by some independent luthiers. According to enthusiasts, the characteristic design elements produce a sound that is mellower, richer, and of greater complexity and presence than without them. Aspects of that sound are studied here, comparing instruments that are otherwise identical and identifying physics mechanisms that are likely responsible.

physics.pop-ph↗

arXiv2015

The Resonator Banjo Resonator, part 2: What makes them really crack?

A simple experiment quantifies the difference between the sound production of a banjo with and without a resonator back. Driven by a small tweeter mounted inside the pot, for frequencies above about 4500 Hz, the produced external sound is 6 to 10 dB louder with the resonator than without. With the banjo played in any normal fashion, this gives a negligible contribution to the overall volume. However, that difference is clearly a reflection of the universally recognized resonator sound, in close analogy to plosive consonants in human speech. No direct correlation is observed between the head-resonator separation and the spectrum of the enhanced response. This suggests that direct reflection off the back is not a primary contributor to the resonator/openback difference, leaving differences in overall absorption as the major suspect.

physics.pop-ph↗

arXiv2015

Sound Hole Sound

The volume of air that goes in and out of a musical instrument's sound hole is related to the sound hole's contribution to the volume of the sound. Helmholtz's result for the simplest case of steady flow through an elliptical hole is reviewed. Measurements on multiple holes in sound box geometries and scales relevant to real musical instruments demonstrate the importance of a variety of effects. Electric capacitance of single flat plates is a mathematically identical problem, offering an alternate way to understand the most important of those effects. The measurements also confirm and illuminate aspects of Helmholtz's "bottle" resonator model as applied to musical instrument sound boxes and sound holes.

physics.pop-ph↗

arXiv2014

The plucked string: an example of non-normal dynamics

Motion of a single Fourier mode of the plucked string is an example of transient, free decay of coupled, damped oscillators. It shares the rarely discussed features of the generic case, e.g., possessing a complete set of non-orthogonal eigenvectors and no normal modes, but it can be analyzed and solved analytically by hand in an approximation that is appropriate to musical instruments' plucked strings.

physics.pop-ph↗

arXiv2014

Banjo timbre from string stretching and frequency modulation

The geometry of a floating bridge on a drumhead soundboard produces string stretching that is first order in the amplitude of the bridge motion. This stretching modulates the string tension and consequently modulates string frequencies at acoustic frequencies. Early work in electronic sound synthesis identified such modulation as a source of bell-like and metallic timbre. And increasing string stretching by adjusting banjo string-tailpiece-head geometry is known to enhance characteristic banjo tone. Hence, this mechanism is likely a significant source of the ring, ping, clang, and plunk common to the family of instruments that share floating- bridge/drumhead construction. Incorporating this mechanism into a full, realistic model calculation remains an open challenge.

physics.pop-ph↗