MAE PhD Defense- Chen Shen

Title: Design of Acoustic Metamaterials and Metasurfaces (Advisor: Yun Jing, PHD)

Date: Wednesday, August 10, 2016

Time: 2:00 PM

Location: EB3 3235

Summarized Abstract
SHEN, CHEN. Design of Acoustic Metamaterials and Metasurfaces. (Under the direction of Dr.
Yun Jing).
Acoustic metamaterials are artificially engineered materials or structures that exhibit exotic
properties which do not exist in nature. The recent emergence of acoustic metasurfaces, as a subgenre
of acoustic metamterials, opens up new possibilities in controlling acoustic waves. They
are subwavelength acoustic surfaces/screens that are commonly associated with unique
transmission/reflection behavior. Numerous novel or improved applications have been realized
by acoustic metamaterials and metasurfaces which are otherwise difficult or impossible to
achieve with conventional structures.
In this thesis, we start from two unique types of acoustic metamaterial, namely negative density
acoustic metamaterials and negative modulus acoustic metamaterials. The negative effective
density is introduced by periodically arranged plates in a waveguide and the negative effective
modulus stems from open-ended side holes. The underlying physics of negative effective density
and modulus are studied. Also, we present a design of utilizing multiple branch openings to
broaden the bandwidth of side-branch based negative modulus metamaterials. A lumped model
is developed for theoretical analysis and as much as 100% increase over the traditional single
branch opening structure in bandwidth is achieved.
By placing plates in only one direction, a broadband acoustic hyperbolic metamaterial can be
formed, where opposite signs of effective density in the x and y directions are achieved below a
certain cutoff frequency. The hyperbolic dispersion relation is obtained by calculating the
effective densities along orthogonal directions and numerical simulations. Partial focusing and
subwavelength imaging are demonstrated both numerically and experimentally at frequencies
between 1.0 and 2.5 kHz. The effect of material loss in the plates is also studied.
Furthermore, by combining the two types of acoustic metamaterials, an anisotropic
complementary metamaterial can be realized, which leads to dramatically reduced acoustic field
distortion and enhanced sound transmission through aberrating layers with proper designs. In the
example where a focused beam is studied, the acoustic intensity at the focus is increased from 28%
to 88% of the intensity in the control case which is in the absence of the aberrating layer and the
complementary metamaterial. The potentials of acoustic cloaking and canceling out multiple
aberrating layers are also studied numerically.
In the acoustic metasurfaces part, two kinds of acoustic metasurfaces with different
functionalities are designed and experimentally verified. A design of acoustic metasurfaces
yielding asymmetric transmission within a certain frequency band is presented, which consists of
a layer of gradient-index metasurface and a layer of near-zero index metasurface. Numerical
simulations and experiments are carried out to verify this phenomenon. In another particular
example of acoustic metasurfaces, they are used to construct an acoustic hologram. A series of
unit cells are designed and are able to modulate the transmitted phase across the cells. With an
iteration-based algorithm, certain images can be formed by manipulating the acoustic phase
across the hologram. We propose the design and experimental demonstration of an acoustic
hologram and a multifocus lens.

Personal bio
Chen Shen was born on Dec 18, 1989 in Wuhan, Hubei Province, China. He graduated from
No.1 Middle School affiliated to Central China Normal University in 2008 and in the same year,
he was admitted to Nanjing University, Jiangsu, China in department of acoustics. In 2012, He
was admitted to the direct doctoral program in Department of Mechanical and Aerospace
Engineering in North Carolina State University. Under the direction of Dr. Yun Jing, he
conducted research in acoustic metamaterials. In 2014, he received a MS degree from
Mechanical Engineering.