Measurements Of Input Admittance Research Articles

Investigating the effect of body geometry on the acoustics of electric guitars

Three electric guitars of different body geometries are investigated in an attempt to characterize their tonal differences. One guitar has a solid body, one is fully hollow, and one is semi-hollow which represents a midway point between the first two. The transfer functions of the electromagnetic pickups are determined by inducing a known signal through the pickup and measuring the output. Input admittance measurements are taken at the bridge and nut of each guitar to show the vibrational modes of the bodies. Wire break measurements are taken for different notes on each guitar, which are analyzed in conjunction with the admittance measurements. The results demonstrate significant admittance peaks in the hollow and semi-hollow guitars that are not present in solid body electric guitars. As well, “dead” notes with shorter decay times are found to be correlated with these admittance peaks, thus indicating that the bodies of hollow and semi-hollow electric guitars play a role in their tonal characteristics.

Langasite surface acoustic wave gas sensors: modeling and verification

We report finite element simulations of the effect of conductive sensing layers on the surface wave velocity of langasite substrates. The simulations include both the mechanical and electrical influences of the conducting sensing layer. We show that three-dimensional simulations are necessary because of the out-of-plane displacements of the commonly used (0, 138.5, 26.7) Euler angle. Measurements of the transducer input admittance in reflective delay-line devices yield a value for the electromechanical coupling coefficient that is in good agreement with the three-dimensional simulations on bare langasite substrate. The input admittance measurements also show evidence of excitation of an additional wave mode and excess loss resulting from the finger resistance. The results of these simulations and measurements will be useful in the design of surface acoustic wave gas sensors.

Investigating the effect of different strutting arrangements on the modes of a guitar soundboard

A spruce soundboard was fitted with a bridge and attached to an experimental rig which provided a fixed edge and uniform backing cavity volume. The rig allowed the same soundboard to be fitted with various standard strutting arrangements. The mode shapes and frequencies were investigated using holographic interferometry. Of particular interest were the small changes in mode shapes induced by the different strutting arrangements. Input admittance measurements were made to determine the effective mass of each mode as ‘‘seen’’ by the strings. Previous psychoacoustical work [H. A. K. Wright, Proc. Inst. Acoust. 19(5), 149–154 (1997)] has suggested that effective mass is an important parameter in determining a mode’s contribution to the overall sound output, and can be significantly affected by small changes in the mode shape. A new method of determining the effective mass is also presented. [Work supported by EPSRC.]

Effects of pars flaccida on sound conduction in ears of Mongolian gerbil: acoustic and anatomical measurements

This paper presents evidence on how the pars flaccida of the tympanic membrane affects the acoustic input to the middle and inner ear. Measurements of middle-ear acoustic input admittance and sound-pressure levels in both the ear canal and the middle-ear cavity were made in ears of eight gerbils before and after manipulations of the middle ear and tympanic membrane. The results are interpreted in terms of a model proposed by Kohllöffel [Hear. Res. 13 (1984) 83–88]. The input-admittance measurements show that the pars flaccida of gerbil acts as a resonator with a resonance frequency of approx. 500 Hz. The admittance of this resonator appears in parallel with the input admittance of the pars tensa and its ossicular and cochlear load. At frequencies below the resonance, the pars flaccida admittance is compliance-like and its magnitude is comparable to that of the pars tensa and its load; consequently, the presence of pars flaccida increases the overall middle-ear input admittance and decreases the pressure difference across the tympanic membrane. At higher frequencies, the admittance of pars flaccida is mass-like and small in magnitude, and it has negligible influence on the overall middle-ear input admittance and the pressure difference across the tympanic membrane. These results suggest that the presence of pars flaccida reduces low-frequency acoustic input to the middle- and inner-ear and consequently decreases hearing sensitivity in this frequency range. Our measurements suggest that with a constant sound pressure stimulus, stiffening the gerbil pars flaccida would increase the motion of the pars tensa by 3–10 dB at frequencies below the normal flaccida resonance. © 1997 Elsevier Science B.V. All rights reserved.

From twenty years of violin acoustics research

A very fruitful year as research associate to Arthur Benade in 1967 started my professional work as a scientist. This year formed much of my way of thinking and future work on strings, some of the “highlights” of which are reviewed. Normal modes of plates of a violin under construction were mapped by means of hologram interferometry (with Molin and Sundin). Experiments with the air cavity, employing Fransson's Ionophone, disclosed a large number of air volume modes. Long‐time‐average spectra of quality judged violins implied that high level below 1 kHz, low at 1.3 kHz, and high around 2.5 kHz are favorable (with Sundberg and Gabrielson). Normal modes of the complete violin were mapped with “speckle interferometry” and input admittance measurements, and implied that “equally strong” low‐frequency modes and a marked “bridge hill” at 2.5 kHz are positive for quality (with Alonso Moral). The influence of material properties and design of free plates were investigated in physical experiments and finite element...

Guitars: Steady state and transient response

Measurements of input admittance confirm that the guitar can be considered as a bass‐reflex enclosure with flexible sides. The effect of this is to increase the sound output without distortion below the first resonance of the top plate. The proposed equivalent circuit will be reviewed. New measurements on plucked notes using a digital analyzer show that the onset of sound contains two distinct parts. The main steady state resonances are shock excited at the beginning of the pluck, grow, and decay within the first few milliseconds of the onset. Simultaneously the harmonics of the string grow and remain as the sustaining vibration of the tone after the steady state resonances have died away. It appears that every played note of the guitar contains at its beginning sound representative of the steady state response which contains information about the type of instrument being sounded and its quality, and this is followed by the sustaining harmonics of the string. Experiments with the violin, cello, and harp s...

Scattering Matrix Analysis of Surface Acoustic Wave Reflectors and Transducers

Absmcr-The phased periodic array scattering parameters [l] have been modified and corrected for use as a single electrode mixed units scattering matrix consisting of one electrical and two acoustic ports. These scattering parameter analytical expressions are functions of material constants, electrode geometry, and frequency. The threeport single-electrode scattering matrices are acoustically cascaded to produce a three-port device scattering mat&x which takes into account all finger interactions (acoustic and electric) for the combined effects of piezoelectric and mechanical scattering. The analysis agrees well with experimental measurements of input admittance, electroacoustic transfer function, and acoustic transmission and reflection coefficients as functions of frequency. Analysis results for the complete modeling of solid and split electrode transducers and transducers with floating electrodes are also presented.

A transmission line and radiating system measurement

One of the major problems in mobile system maintenance is adequate evaluation of the transmission line and antenna performance. This paper describes a practical method of technique for making line loss and antenna match measurements to assure proper installation and continued satisfactory performance. Certain innovations have been incorporated in the process which are calculated to provide the maximum information with a minimum effort. The procedure consists essentially of making two transmission line input admittance measurements at widely differing frequencies. The results are plotted on a simplified version of the Smith Chart in a manner that yields transmission line loss and antenna match, These two properties adequately evaluate the transmission system for maintenance purposes. Both measurements are made at the input end of the transmission line without altering or disturbing the antenna or load end of the line. This is important where tall towers are involved or the antenna is otherwise not readily accessible. Accuracies are more than adequate for service purposes and the results are consistent under a wide variety of conditions. A simple technique for determining the distance in feet to a fault such as a break or short circuit is also described.

A transmission line and radiating system measurement

One of the major problems in mobile system maintenance is adequate evaluation of the transmission line and antenna performance. This paper describes a practical method of technique for making line loss and antenna match measurements to assure proper installation and continued satisfactory performance. Certain innovations have been incorporated in the process which are calculated to provide the maximum information with a minimum effort. The procedure consists essentially of making two transmission line input admittance measurements at widely differing frequencies. The results are plotted on a simplified version of the Smith Chart in a manner that yields transmission line loss and antenna match, These two properties adequately evaluate the transmission system for maintenance purposes. Both measurements are made at the input end of the transmission line without altering or disturbing the antenna or load end of the line. This is important where tall towers are involved or the antenna is otherwise not readily accessible. Accuracies are more than adequate for service purposes and the results are consistent under a wide variety of conditions. A simple technique for determining the distance in feet to a fault such as a break or short circuit is also described.

The Nature of the Uncorrelated Component of Induced Grid Noise

An investigation of induced grid noise in vacuum tubes has been made. It was found that the uncorrelated component of grid noise can be explained in terms of electrons elastically reflected from the plate of the tube. Experimental and theoretical justifications of this explanation are presented. The accuracy of methods for predicting grid noise from measurements of input admittance is affected because of a component of input admittance arising from reflected electrons. A table is included showing typical (measured) values of the induced grid noise of eleven modem receiving tubes.

The input impedances of thermionic valves at low frequencies

It is shown that accurate measurements of input admittance (or of input impedance) under various conditions can be made by means of the Schering Capacity Bridge. The input circuit is regarded as being equivalent to a condenser with a definite phase , or loss angle, δ=90° - , and the results are expressed by stating the effective capacity and value of tan δ for each set of experimental conditions. A series of measurements made on an R valve is recorded, and it is shown that the results are in good agreement with the theoretical investigations of Miller and Nichols. The theoretical investigation has been extended to allow for the effect of dielectric losses in the valve, since these were found by experiment to be rather large, and to have an appreciable effect on the capacity and phase angle of the input circuit. It is shown that the effective capacity may vary from about 10μμF. to 100μμF. for an R valve, and the phase angle may vary from about 80° leading to 126° leading, depending mainly on the load in the anode circuit. Values of phase angle greater than 90° correspond to a negative resistance, or negative power factor, and occur when the load in the anode circuit is inductive. The variations of input capacity and phase angle with filament voltage, anode voltage, input voltage, and frequency are also investigated.