Stephen C. Thompson

Broadband multi-resonant longitudinal vibrator transducer

A broadband longitudinal vibrator transducer having a laminar head mass section including at least three layers coupled to electromechanical transducer elements. The head section includes a forward head mass, a compliant member abutting the forward head mass and a rear head mass abutting both the compliant member and the transducer elements. The compliant member allows the head mass section to mechanically resonate in at least two frequencies expanding the bandwidth of the transducer. The compliant member can be an active transducer element.

Noise in miniature microphones

The internal noise spectrum in miniature electret microphones of the type used in the manufacture of hearing aids is measured. An analogous circuit model of the microphone is empirically fit to the measured data and used to determine the important sources of noise within the microphone. The dominant noise source is found to depend on the frequency. Below 40 Hz and above 9 kHz, the dominant source is electrical noise from the amplifier circuit needed to buffer the electrical signal from the microphone diaphragm. Between approximately 40 Hz and 1 kHz, the dominant source is thermal noise originating in the acoustic flow resistance of the small hole pierced in the diaphragm to equalize barometric pressure. Between approximately 1 kHz and 9 kHz, the noise originates in the acoustic flow resistances of sound entering the microphone and propagating to the diaphragm. To further reduce the microphone internal noise in the audio band requires attacking these sources. A prototype microphone having reduced acoustical noise is measured and discussed.

The effect of the reed resonance on woodwind tone production

In normal woodwind tone production the nonlinear flow control properties of the reed transfer energy among the harmonics of the spectrum, and the favored playing frequency is one for which the air column input impedance is high at several harmonics. Above the middle of the second register, woodwinds have only one participating impedance peak; yet these notes can be played even without the use of a register hole, despite competing possibilities of low register intermode cooperation. Such notes are possible because enhancement of the reed’s transconductance A near its own resonance frequency can offset the small input impedance Z of the air column so that (ZA−1) ≳0, providing an additional means for energy production above cutoff. Spectral levels as a function of blowing pressure, air column impedance, and reed characteristics are derived. Experiments on the clarinet show that the player can adjust the reed resonance frequency from about 2 to 3 kHz. When the reed frequency is adjusted to match a harmonic co...

Analog model for thermoviscous propagation in a cylindrical tube

Modeling acoustic propagation in tubes including the effects of thermoviscous losses at the tube walls is important in applications such as thermoacoustics, hearing aids, and wind musical instruments. Frequency dependent impedances for a tube transmission line model in terms of the so-called thermal and viscous functions are well established, and form the basis for frequency domain analysis of systems that include tubes. However, frequency domain models cannot be used for systems in which significant nonlinearities are important, as is the case with the pressure-flow relationship through the reed in a woodwind instrument. This paper describes a cylindrical tube model based on a continued fraction expansion of the thermal and viscous functions. The model can be represented as an analog circuit model which allows its use in time domain system modeling. This model avoids problems with fractional derivatives in the time domain.

Performance of transducers with segmented piezoelectric stacks using materials with high electromechanical coupling coefficient

Tonpilz acoustic transducers for use underwater often include a stack of piezoelectric material pieces polarized along the length of the stack and having alternating polarity. The pieces are interspersed with electrodes, bonded together, and electrically connected in parallel. The stack is normally much shorter than a quarter wavelength at the fundamental resonance frequency so that the mechanical behavior of the transducer is not affected by the segmentation. When the transducer bandwidth is less than a half octave, as has conventionally been the case, for example, with lead zirconate titanate (PZT) material, stack segmentation has no significant effect on the mechanical behavior of the device in its normal operating band near the fundamental resonance. However, when a high coupling coefficient material such as lead magnesium niobate-lead titanate (PMN-PT) is used to achieve a wider bandwidth with the tonpilz, the performance difference between a segmented stack and a similar piezoelectric section with electrodes only at the two ends can be significant. This paper investigates the effects of stack segmentation on the performance of wideband underwater tonpilz acoustic transducers. Included is a discussion of a particular tonpilz transducer design using single crystal piezoelectric material with high coupling coefficient compared with a similar design using more traditional PZT ceramics.

Electrical compensation of the microphone sensitivities in a dual microphone directional hearing aid

Hearing aids with directional sensitivity have shown enhanced user satisfaction for their improved performance in noisy environments. One way to achieve the directional response patterns is to use two omnidirectional microphones with the necessary beamforming electronics to form a beam with the desired directionality. Proper operation of the beamforming circuitry requires that the frequency‐dependent sensitivities of the two microphones be accurately matched, or corrected, in both magnitude and phase. This paper presents a method of electrically compensating the response of the two microphones to provide an accurate match for beamforming without the need for special sorting of the pairs for matched response. The circuit is suitable for field recalibration if aging causes the sensitivities to vary, and can also be implemented as an adaptive circuit to update the compensation parameters with time for any slow drift with time.

Thermal boundary layer effects on the acoustical impedance of enclosures and consequences for acoustical sensing devices.

Expressions are derived for the acoustical impedance of a rectangular enclosure and of a finite annular cylindrical enclosure. The derivation is valid throughout the frequency range in which all dimensions of the enclosure are much less than the wavelength. The results are valid throughout the range from adiabatic to isothermal conditions in the enclosure. The effect is equivalent to placing an additional, frequency-dependent complex impedance in parallel with the adiabatic compliance. As the thermal boundary layer grows to fill the cavity, the reactive part of the impedance varies smoothly from the adiabatic value to the isothermal value. In some microphones, this change in cavity stiffness is sufficient to modify the sensitivity. The resistive part of the additional cavity impedance varies as the inverse square root of frequency at high frequencies where the boundary layer has not grown to fill the enclosure. The thermal modification gives rise to a thermal noise whose spectral density varies asymptotically as l/f(3/2) above the isothermal transition frequency.

Investigation of a balanced‐armature transducer for vibrational energy harvesting.

The ability to sustain or prolong the operation of devices which require electrical power by utilizing ambient energy is desirable in many applications. While a variety of transducers have been considered for harvesting vibrational energy, the balanced armature transducer has yet to be fully investigated for such purposes. Balanced armature transducers, such as those used in hearing aids, are promising as they offer a compact design and are commercially available. An analog circuit model for a commercial hearing aid transducer has been modified to include a vibration input and the results are compared with experiment. Using this model, the performance of the transducer can be assessed before and after possible design modifications are considered. [This work was sponsored by the Office of Naval Research.]

Variable compliance split‐cylinder transducer using Galfenol for frequency control.

The size of conventional split ring transducers is much smaller than a wavelength at operating frequency leading to a high Q resonance and limited operating bandwidth. This work investigated modifying the split ring structure to allow its resonant frequency to be adjusted under active control so that it is always operated at or near its resonance. Because the transducer is operated at resonance, the phase of its input impedance is approximately constant, simplifying the design of the transmit power amplifier and reducing the size and weight of the system. A high Q system is desirable for an actively tuned system resulting in very high efficiency and allowing the use of less expensive shell materials. The two variable compliance designs studied involve the use of Galfenol stiffener bars located at the nodal point/hinge point of the shell. By activating all or a portion of these bars, the compliance (stiffness) of the shell can be varied with a corresponding shift in resonance frequency. Two methods of compliance control are being investigated, one that switches the stiffness “on” and “off,” and one that allows continuous control using the “delta‐E” effect in Galfenol.

Acoustic noise in microphones from the thermal boundary layer resistance in enclosures

Many types of acoustic sensors include a small enclosure as an integral part of the device. For example, a condenser microphone has a volume enclosed behind its diaphragm so that the acoustic pressure drives only the front of the diaphragm. In other sensors the enclosure may exist mainly to protect the sensitive components or to provide electrostatic shielding to the high impedance parts of the electrical circuitry. Whatever their reason, these volumes can play a significant role in the performance of the sensor. The adiabatic compliance of the enclosure is commonly included in device models. However, these enclosures are often small enough that the thermal boundary layer thickness can consume a substantial fraction of the enclosed volume at low frequencies. Thermal effects modify both the real and imaginary parts of the cavity impedance in ways that may be important in determining the sensitivity and internal noise of the device. This paper examines these effects. One interesting consequence is a thermal...