Modeling and Characterization of an Air Motion Transformer
Open Access
- Author:
- Riccardi, Peter
- Graduate Program:
- Acoustics
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- April 13, 2022
- Committee Members:
- Andrew Barnard, Program Head/Chair
Dan Brown, Thesis Advisor/Co-Advisor
Richard Joseph Meyer, Jr., Committee Member
Dave Swanson, Committee Member
Tom Blanford, Special Signatory - Keywords:
- AMT
Air Motion Transformer
Air Motion Transformer Technology
Air Motion Tweeter
tweeter
ESS AMT
LTspice
equivalent circuit modeling
equivalent circuits
electroacoustic modeling
electroacoustics
transducer modeling
transduction
in air tweeter
acoustics - Abstract:
- Lumped element, single degree of freedom models of moving coil loudspeakers have been widely used by acoustical engineers to predict their electroacoustic response. These models characterize the electrical terminal characteristics, as well as the acoustical radiated output of the device. This thesis presents a linear, lumped element, single degree of freedom model of the air motion transformer. Air motion transformers are electromagnetic transducers which are considered to be an upgrade to conventional moving coil tweeters, yet no published literature is available which explores its unique construction and transduction. The model derives the lumped element components using the material properties and dimensions of the diaphragm thus shedding insights into the subtleties of the device and provides the opportunity for optimization of a desired response. A real device, the AMT2-4 from Dayton Audio, was used to perform a full characterization of a typical air motion transformer. Experimental data was captured in both the electrical and acoustical domains to establish a baseline understanding of the complex electrical impedance and the acoustic sensitivity. The Lorentz forces acting on the diaphragm and voice coil were studied to establish idealized mass-spring-damper free body diagrams which captured the basic motion of the diaphragm. The free body diagrams allowed equivalent circuit flow networks to be constructed utilizing lumped element components. Two checks were performed to validate the linear model: the fit between measured data and the model output of the complex electrical impedance and the acoustical far field pressure response. It will be shown that the model has good agreement in both the real and imaginary parts of the complex impedance in the vicinity of fundamental mechanical resonance as well as in the trend of the complex impedance at higher frequencies. It will also be shown that the model has good agreement with the acoustical frequency response and can predict the low frequency roll-off as well as the trend in the sensitivity in the passband. Finally, known nonlinearities in moving coil devices will be reviewed. These known nonlinearities will be cross checked with measured data of both an air motion transformer and a traditional moving coil tweeter. Both the harmonic distortion in the acoustical response and shifts in the complex impedance data at higher drive levels will be reviewed. It will be shown that the dominant nonlinear mechanism in the air motion transformer is an asymmetry in the magnetic circuit in addition to other forms of clipping such as nonlinear stiffness in the diaphragm.