Scott P. Porter

The Singing Shoebox: A

Moving‐coil loudspeakers typify the interdisciplinary nature of acoustics. In order to reproduce sound, these devices employ principles of electricity, magnetism, mechanics, and acoustics. The widespread use of loudspeakers has made them familiar to students and thus a valuable opportunity to introduce students to acoustics. In this paper, the authors demonstrate a loudspeaker‐enclosure system that is easily built from scratch. The system is mostly constructed from common household supplies, making it low‐cost and accessible to a wide audience. In addition, this project is well‐suited for use by the K‐12 educator as the content can be scaled to fit a variety of different academic levels. To this end, the speaker’s design and construction will be presented and its relevance as an educational demonstration discussed. Finally, the loudspeaker will be auditioned and its performance demonstrated.

5 Loudspeaker Project

Moving-coil loudspeakers typify the interdisciplinary nature of acoustics: in order to reproduce sound, these devices employ principles of electricity, magnetism, mechanics, and acoustics. The widespread use of loudspeakers has made them a familiar and valuable opportunity to introduce students to acoustics. A low-cost loudspeaker project/demonstration is presented here that is built from scratch using common household items and craft supplies. A variety of educational topics may be illustrated with this device, making it appropriate for a wide range of academic levels.

A craft-project loudspeaker to serve as an educational demonstration

The headmass is a key element in tonpilz transducer design. As an acoustic radiator, a successful headmass must be built from a material that is both light and stiff. To assess the suitability of ceramics for this application, the authors used the mechanical properties of candidate materials to perform a theoretical comparison based on the flexural behavior of square plates. Although not a comprehensive metric for identifying the best headmass materials, the headmass flexure may be usefully employed as a first-level selection criteria. A software routine based on thin plate and thick plate theory was created to evaluate the flexural behavior in candidate materials.

Material selection for acoustic radiators that are light and stiff

Moving‐coil loudspeakers are an excellent resource for introducing students to acoustics: their widespread use makes them familiar to nearly everyone and the interdisciplinary nature of the device forms a technically rich problem. To exploit this, the authors have created a simple, low‐cost loudspeaker in a shoebox to function as both a science project and demonstration. Because of the many technical concepts illustrated by this device, it is an appropriate demonstration for a wide range of academic levels. Already, the shoebox speaker has been used as a teaching demonstration in a previous session [J. Acoust. Soc. Am. 127, 1910]. In this talk, the design and construction of the shoebox speaker will be shown, highlighting the incorporation of everyday materials. The focus of this talk will be on the educational opportunities afforded by this demonstration and its suitability for Project Listen Up. Lastly, the shoebox speaker will be demonstrated.

The singing shoebox. Part 2: Educational opportunities in a

Magnetostrictive drives require features that are not standard in piezoelectric motors: a magnetic return path, a biasing mechanism, and eddy current mitigation. In spite of this, there is not a standard design for a self‐contained magnetostrictive drive. This work investigates a configurable high‐power magnetostrictive drive design built from Galfenol that can be tailored to fit a variety of applications. Based on this design, a prototype drive has been constructed from laminated Galfenol structures and neodymium biasing magnets. In this paper, the author will discuss the fabrication and characterization of the device and compare the results to simulated performance.

5 loudspeaker.

An ultrasonic vibrator has been developed to serve as the drive mechanism for an electroacoustic transducer. This design explores the unique characteristics of Galfenol, a recently invented giant magnetostrictive material. In addition to possessing competitive strain capabilities, strong mechanical properties, and a high‐magnetic permeability, Galfenol does not require a prestress mechanism and can be laminated to effectively mitigate eddy current losses. Designing the vibrator required the authors to carefully engineer the magnetic circuit so that proper bias fields could be established using a permanent magnet. This step will be demonstrated with one‐ and two‐dimensional models. Drive coil considerations will also be discussed and the fabrication and assembly of the vibrator will be shown along with in‐air measurements.

Characterization of a configurable magnetostrictive drive.

Optimum performance of a tonpilz transducer head‐mass section is often achieved when the mass is minimized and the stiffness is maximized. High stiffness keeps head‐mass modes away from the band of interest with minimal material; low mass maximizes the motor section aspect ratio and reduces the stored energy in the head‐mass, raising the electromechanical coupling coefficient. Therefore, the material selected for the head‐mass construction plays an integral role in transducer performance. In the practical design of tonpilz radiating heads, a tool which can rapidly identify the materials best suited for a given design is desirable. The figure of merit, Eρ3, has traditionally been used. This figure of merit, however, is based on thin‐plate approximations. Most actual tonpilz head‐masses require thick‐plate theory for accurate analysis. In this paper, the authors use a numerical routine to rapidly assess the thick‐plate behavior of various materials and rank their suitability for a given head‐mass geometry. ...

An ultrasonic vibrator constructed from laminated Galfenol steel.

Galfenol is an attractive giant magnetostrictive material for transducer design because of its competitive strain capabilities, mechanical robustness, and high magnetic permeability. The latter two properties are particularly interesting as they allow exploration of a design space inaccessible to other active materials, both giant magnetostrictive and piezoelectric. Especially noteworthy is the ability to roll Galfenol into thin sheets from which laminated assemblies may be economically produced. Lamination is an effective mitigation scheme against eddy current losses and in this paper the authors explore transducers based on this construction. Much of this work builds on the designs of World War II era magnetostrictive transducers developed at the Harvard Underwater Sound Laboratory. Modeling these transducers as electro‐magneto‐mechano‐acoustical devices can be first achieved in a limited sense with a one‐dimensional model; this model is valuable for rapid simulation during the early design process. Man...

Rapid identification of candidate materials for tonpilz head‐mass design.

Previous studies have initiated an investigation into a new method of measuring the d33, μ33T, and s33H magnetostrictive constants of a given sample using a pair of Helmholtz coils, a laser vibrometer, and a sense coil. These parameters are calculated from measurements made as a function of dc magnetic bias and ac drive amplitude. This method contrasts with the traditional measurement technique which involves building the sample into a transducerlike device. The new method was demonstrated for laminated stress‐annealed Galfenol rods of high aspect ratio. Measurements obtained by this method appear credible and demonstrate good repeatability. For verification, the authors have returned to the established approach and designed a simple transducer using one of the measured Galfenol rods. The transducer was driven in air as a function of dc magnetic bias and ac drive amplitude. Data obtained from this transducer are compared to an analytical model that uses the material properties obtained experimentally with...

Multiphysics modeling of laminated Galfenol transduction devices.

Many of today’s college students experience music in compressed formats on a regular basis. While most are familiar with the term “MP3,” only a few know of the psychoacoustic principles and perceptual coding schemes which this and other compressed music formats rely upon. In fact, it is not uncommon that they do not realize the audible differences that exist between the CD‐quality audio files and those compressed at common commercial bit rates. To illustrate this, the author used freely available software (WINABX and AUDACITY) to demonstrate the differences with double‐blind A/B testing as part of an introductory course in acoustics offered at the Pennsylvania State University. The software will be briefly explained, the questionnaire used to acquire the students’ responses will be shown, and an interactive demonstration will be included.