ANALYSIS OF A LOW FREQUENCY MUFFLER BASED ON THE ACOUSTIC BLACK HOLE EFFECT

ANALYSIS OF A LOW FREQUENCY MUFFLER BASED ON
THE ACOUSTIC BLACK HOLE EFFECT
N. Sharma, Acoustics Research Centre, University of Salford
O.Umnova, Acoustics Research Centre, University of Salford
A.T. Moorhouse Acoustics Research Centre. University of Salford
1. Introduction
In order to keep up with the regulatory compliance norms, there have been continued efforts to install
and maintain quieter machinery. In several such instances, the industrial mufflers and silencers have
been the target areas of amelioration Essentially. a muffler (or silencer, often used interchangeably) is a
low pass acoustic filter☁, used to attenuate noise levels from machinery and other noisy sources Besides
automotive and aviation sectors, the other applications where components of low frequency noise are
considered disturbing, include, industrial plants, wind turbines, blowers and compressors, ducts and
vents, HVAC systems, high-speed trains and even exposure to loud music. To achieve low overall noise
exposures from these sources, machine designers, fabricators and operators have developed hybrid
mufers, unlike the conventional classication; make use of both absorptive and reactive design
elements. Managing the noise by absorption at source and/or receiver ends, in combination with
manipulating its flow along the path, the technological advancement for modern-day mufers seems
challenging and intriguing.
Amid the considerable amount of efforts for developing effective absorption and noise control
techniques, the concept of Acoustic Black Hole (ABH), as an effect, has gained popularity. ABH based
structures rely on the principle of impedance matching and, in principle, can achieve atotal absorption of
incident sound. Till date, ABH effect has been applied to several configurations such as, for waves
travelling over surfaces such as plates, wedges, beams, across cylinders and more recently, through
laminating tubular structures, ABH based studies2 carried out so far encompass☁ single and multi
dimensional structures, analyzed by not only analytical and numerical but alsoexperimental techniques to
achieve at least modest reductions in noise, pressure, size and cost, as applicable to above mentioned
sources of noise.
It has been, probably for the rst time, theoretically demonstrated by Mironov and Pislyakov3 that
ABH effect can be applied to a noisy medium through a terminating structure, rather than the surface. The
paper has presented that a total absorption of sound can be achieved in the tube with quasi-periodic
ribbed internal structures, whose radii decreased to zero with the structure's termination, i.e. devoid the use of absorbing materials. To complement this study, the only experimental investigation of the ABH based sound absorption through a terminating structure, considering both the exclusion and inclusion of
absorbing materials have been conducted by A. Azbaid, et al☁.
in the present paper, Mironov's ABH model has been extended and modied to deal with an open
chamber design and accounting for the presence of losses. The muffling section, embodying the open