Behavior of Clamped-Edge Circular Membrane

  • Context: Undergrad 
  • Thread starter Thread starter Chrono G. Xay
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    Tension
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Discussion Overview

The discussion revolves around the behavior of a clamped-edge circular membrane, particularly in the context of selecting drumhead thickness for desired tuning and feel. Participants explore the relationship between tension, diameter, and material properties, while also addressing the mathematical modeling of membrane behavior under tension.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant discusses the equation for tension in a circular membrane and expresses confusion about the multiplier "0.766" used in the equation.
  • Another participant suggests that the factor "0.766" may relate to the inclusion of a factor of 2*Pi when frequency is expressed in hertz.
  • There is a mention of the term "basis weight" for the area mass unit of the membrane.
  • Participants discuss the applicability of the equation for predicting changes in diameter and express uncertainty about the correct formulation for a circular membrane.
  • One participant proposes a formulation for the area of the membrane's perimeter to calculate changes in diameter but questions its validity.
  • There is a discussion about the modification of generalized Hooke’s Law in the context of cylindrical coordinates and axisymmetric stress.
  • Another participant describes their understanding of tension as a unit that can be multiplied by the perimeter to find total force on the membrane.
  • Participants explore the relationship between the diameter of the membrane before and after stretching and the implications for tension and force distribution across lugs.
  • One participant outlines a three-part derivation process related to stress, strain, and restoring forces in the membrane.

Areas of Agreement / Disagreement

Participants express various viewpoints and uncertainties regarding the equations and concepts discussed. There is no clear consensus on the derivation of the tension multiplier or the correct approach to modeling the membrane's behavior under tension.

Contextual Notes

Limitations include potential misunderstandings regarding the definitions of variables, the applicability of certain equations to circular membranes, and the assumptions made in the derivations presented.

  • #31
Chrono G. Xay said:
I’m interested in them because they’re a continuous portion of the membrane; as the rim pulls down on the hoop, which is clamped to the edge of the membrane’s collar, naturally the length of the collar affects the amount of elongation experienced until the desired musical pitch (and therefore tension) has been achieved.

(Up until now I thought there was no technical term for the portion of the membrane between the drumhead’s speaking diameter and the hoop, hence my use of “skirt”, but it’s actually called the ‘collar’.)

For this analysis I do not consider the added magnitude of elongation afforded by the collar to be insignificant.
It is not insignificant, but you also need to account for normal forces that result in frictional forces which affect the skin stresess and strains in membrane skin in this region. This is what allows the skin to be pulled down.
 
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  • #32
Chestermiller said:
It is not insignificant, but you also need to account for normal forces that result in frictional forces which affect the skin stresess and strains in membrane skin in this region. This is what allows the skin to be pulled down.
It’s for that very reason that when lowering the pitch of a drum you intentionally take the pitch slightly lower then what you want it, then tune it *up* to where you actually want it. 😊 The same is done for guitar strings.
The rest of it will basically work itself out on its own once someone starts striking it with a stick.

I also just realized that I had mistyped a few of the earlier functions. I’m sorry. $$ω \left( f_0, D_s \right) = \frac {πf_0 D_s} {J_0 \left( \frac 2 {D_s} {α_1} \right)}$$ $$ρ \left( ρ_M, h_m \right) = ρ_M h_m$$ $$σ \left( ω, ρ \right) = \frac {ω^2} ρ$$ $$D_0 \left( D_s, L_{c,0} \right) = {D_s} + 2L_{c,0}$$ $$D \left( D_0, ν, E \right) = D_0 \left[ 1 + \left( 1 + ν \right) \frac σ E \right]$$ $$L_c \left( D, D_s \right) = \frac {D - {D_s}} 2$$ $$δ \left( L_{c}, r_h, h_c, h_r \right) = \sqrt {L_{c}^2 - r_h^2} - \left( h_c + h_r \right)$$ $$n \left( δ, t \right) = \frac δ {t^{-1}}$$ $$Δδ = δ - δ_0$$ In order of appearance…

##f_0## = Desired fundamental frequency
##D_s## = Speaking diameter of membrane
##α_1## = 1st zero of First Order Bessel Function (2.4048)
##ρ## = Basis weight
##ρ_M## = 3D density of Mylar
##h_m## = Height (thickness) of membrane
##σ## = In-plane axisymmetric stress on membrane
##D_0## = Unstretched membrane overall diameter
##L_{c,0}## = Unstretched membrane collar length
##D## = Stretched membrane overall diameter
##ν## = Poisson’s ratio
##E## = Modulus of elasticity
##L_c## = Stretched membrane collar length
##δ## = Required displacement of membrane rim’s tension rods after tension rods have been taken as low as they will go without increasing the in-plane stress ##σ## above theoretical zero.
##r_h## = Extra radius of membrane hoop (from the center of the shell’s bearing edge to the inside diameter of the membrane’s hoop)
##h_c## = Height (depth) of membrane collar
##h_r## = Height (thickness) of membrane’s rim
##n## = Decimal number of revolutions tension rods must make to achieve needed in-plane stress
##t## = Number of threads per inch/mm on tension rods
##δ_0## = Initial displacement of tension rods (assuming the membrane is already tuned to some initial fundamental frequency ##f_{0,0}##; i.e. ##σ_0 > 0##)
 
Last edited:
  • #33
Chrono G. Xay said:
It’s for that very reason that when lowering the pitch of a drum you intentionally take the pitch slightly lower then what you want it, then tune it *up* to where you actually want it. 😊 The same is done for guitar strings.
The rest of it will basically work itself out on its own once someone starts striking it with a stick.

I also just realized that I had mistyped a few of the earlier functions. I’m sorry. $$ω \left( f_0, D_s \right) = \frac {πf_0 D_s} {J_0 \left( \frac 2 {D_s} {α_1} \right)}$$ $$ρ \left( ρ_M, h_m \right) = ρ_M h_m$$ $$σ \left( ω, ρ \right) = \frac {ω^2} ρ$$ $$D_0 \left( D_s, L_{c,0} \right) = {D_s} + 2L_{c,0}$$ $$D \left( D_0, ν, E \right) = D_0 \left[ 1 + \left( 1 + ν \right) \frac σ E \right]$$ $$L_c \left( D, D_s \right) = \frac {D - {D_s}} 2$$ $$δ \left( L_{c}, r_h, h_c, h_r \right) = \sqrt {L_{c}^2 - r_h^2} - \left( h_c + h_r \right)$$ $$n \left( δ, t \right) = \frac δ {t^{-1}}$$ $$Δδ = δ - δ_0$$ In order of appearance…

##f_0## = Desired fundamental frequency
##D_s## = Speaking diameter of membrane
##α_1## = 1st zero of First Order Bessel Function (2.4048)
##ρ## = Basis weight
##ρ_M## = 3D density of Mylar
##h_m## = Height (thickness) of membrane
##σ## = In-plane axisymmetric stress on membrane
##D_0## = Unstretched membrane overall diameter
##L_{c,0}## = Unstretched membrane collar length
##D## = Stretched membrane overall diameter
##ν## = Poisson’s ratio
##E## = Modulus of elasticity
##L_c## = Stretched membrane collar length
##δ## = Required displacement of membrane rim’s tension rods after tension rods have been taken as low as they will go without increasing the in-plane stress ##σ## above theoretical zero.
##r_h## = Extra radius of membrane hoop (from the center of the shell’s bearing edge to the inside diameter of the membrane’s hoop)
##h_c## = Height (depth) of membrane collar
##h_r## = Height (thickness) of membrane’s rim
##n## = Decimal number of revolutions tension rods must make to achieve needed in-plane stress
##t## = Number of threads per inch/mm on tension rods
##δ_0## = Initial displacement of tension rods (assuming the membrane is already tuned to some initial fundamental frequency ##f_{0,0}##; i.e. ##σ_0 > 0##)
I really don't have anything to add to what I said in my previous posts.
 

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