Emission Enhancement of Sound Emitters using an Acoustic Metamaterial Cavity

Abstract

The emission enhancement of sound without electronic components has wide applications in a variety of remote systems, especially when highly miniaturized (smaller than wavelength) structures can be used. The recent advent of acoustic metamaterials has made it possible to realize this. In this study, we propose, design, and demonstrate a new class of acoustic cavity using a double-walled metamaterial structure operating at an extremely low frequency. Periodic zigzag elements which exhibit Fabry-Perot resonant behavior below the phononic band-gap are used to yield strong sound localization within the subwavelength gap, thus providing highly effective emission enhancement. We show, both theoretically and experimentally, 10 dB sound emission enhancement near 1060 Hz that corresponds to a wavelength approximately 30 times that of the periodicity. We also provide a general guideline for the independent tuning of the quality factor and effective volume of acoustic metamaterials. This approach shows the flexibility of our design in the efficient control of the enhancement rate. Sound emission enhancement dates back to the late 19 th and early 20 th centuries before any electronic components were invented. During this pre-electronic era, emission enhancement of sound emitters was accomplished purely via mechanical means by utilizing a diaphragm, an air-gate, and compressed air. This form of sound emission enhancement quickly disappeared when a highly effective electronic form was introduced. Nevertheless, emission enhancement without the help of electronics has promising applications in areas such as remote acoustic sensors, energy harvesting, and acoustic filters in which no internal source of power is available. Limitations arise when the wavelengths become large (i.e., low frequency applications) because to confine and control the acoustic waves, structures with dimensions on the order of the wavelength are required, which may be tens of cm to tens of meters. Fortunately, this obstacle is becoming a thing of the past owing to the introduction of metamaterials [1][2][3][4] . The success of metamaterials in the Electromagnetic (EM) regime is carrying over to the acoustic regime where applications are being found that range from subwavelength focusing to acoustic cloaking 5,6 . This is possible because basic acoustic parameters such as the bulk modulus (B) and mass density (r) can be tailored by subwavelength elements analogous to permittivity and permeability in the EM regime [7][8][9] . Fabry-Perot-type (FP) resonance modes operating at low frequency are particularly interesting because they allow for full acoustic transmission similar to the extraordinary optical transmission through a subwavelength periodic aperture [10][11][12][13][14] . Recently, high-resolution sound imaging up to 1/50 th of the wavelength was demonstrated by using near-field propagation inside perforated metallic plates using FP resonance modes 12 . In addition to utilizing the resonance modes, acoustic metamaterials can also be used to realize h...

Acoustic Purcell Effect for Enhanced Emission

“…Emission enhancement of a sound source by environment changes has been achieved traditionally by employing coupling horns to loudspeaker drivers [14], employing various enclosure designs [15], or, more recently, by acoustic resonant metamaterial structures [16][17][18]. The emission enhancement is particularly important in lowfrequency emission since efficient emissions are highly demanded in audio acoustics.…”

“…The emission enhancement is particularly important in lowfrequency emission since efficient emissions are highly demanded in audio acoustics. The high refractive index in recently designed metamaterial structures by coiled up (zigzag) space configurations [19][20][21][22] enables the enhancement at low frequencies by resonances, as recently developed for enhanced emission of a monopole source by Fabry-Perot resonances [17] and for a multipole source of arbitrary orders by degenerate Mie resonances [18]. The low-frequency resonances for enhanced emission were proved by these designs which effectively change the source's emission environment and may be hence regarded as the Purcell effect in acoustics.…”

“…In traditional acoustics, enhanced emission (or acoustic Purcell effect, per se) has been generally interpreted by acoustic resonances [16][17][18] and radiation impedance [14,15]. The fundamental questions remain: How the resonant enhancement of sound emission can be understood by the DOS of sound waves?…”

“…2(a)]. The coilingspace (zigzag) configuration of the channels (width × height × length ¼ 3 × 10 × 170 mm 3 starting from an inner radius of 5 mm) elongates sound paths, resulting in a low effective radial sound speed (i.e., high refractive index [17][18][19][20][21][22]), consequently shifting the resonant frequencies down to attain low-frequency emission enhancement [18]. We employed a balanced armature speaker (Knowles, Model ID CI-26697-000, 7.2 × 9.5 × 4.1 mm 3 ) as the monopole source [ Fig.…”

Artificial Material Integrated Ultra-wideband Tapered Slot Antenna for Gain Enhancement with Band Notch Characteristics

“…In the past several years, artificial materials are studied by scientists and engineers because of some special electromagnetic (EM) properties which are very difficult to find in nature. According to the material properties and arrangements of meta-atoms (unit cells), an EM wave is able to have enhanced emissions [5], clear imaging [6], invisible cloaking [7], EM tunneling [8] and absorbers [9]. Among all artificial materials, the low or zero index metamaterial (LIM or ZIM) is widely applicable for its beam focusing properties.…”