Limp Mass Research Articles

Sound transmission loss of meta-acoustic barriers with anomalous effective-mass

Meta-acoustic barriers (MABs) that are inspired by acoustic metamaterials (AM) offer opportunities to defy mass law-driven sound transmission loss (TL) performance that is characteristic of conventional acoustic barriers. In this analytical parametric study, the TL behavior of MABs that incorporate various local oscillator configurations are analyzed using an effective-mass modeling approach. Resonant, damped and inertant local oscillator configurations and combinations thereof that result in negative and complex effective-mass for the meta-acoustic barrier are considered. The influence of the local configuration’s orientation, oblique incidence and the presence of multiple oscillator types are examined using nondimensionalized metrics and compared to the performance of static mass and volume-equivalent limp mass and double wall barriers. It is shown that due to their anomalous effective-mass, such barriers display tunable TL bandwidths exceeding that of mass-equivalent conventional barriers indicating new device implications. The effective-mass modeling approach can easily be adapted to account for the presence of different dynamic features within various types of inclusions considered and is thus an efficient tool to evaluate meta-acoustic barrier designs. Inasmuch as these meta-acoustic barrier structures can be realized using recent advances in additive and hybrid manufacturing techniques, opportunities exist to create lightweight barriers for tonal and broadband acoustic applications.

Acoustical Isolation Floor Underlayment System

An acoustic isolation medium configured for placement between a subfloor and a finished floor with a poured underlayment, includes a first layer being a sound reduction mat disposed upon the subfloor, a second layer placed upon the first layer and being one of a sheet of fibrous material and a web of hi-density limp mass material with a high internal damping coefficient, and a third layer placed upon the second layer and being the other of a sheet of the fibrous material and a web of the hi-density limp mass material.

Low‐mass–high‐transmission loss portable speech isolation room for very important person protection.

This case study examines the design, construction, and testing of a lightweight, easily transportable speech isolation room capable of providing substantial anti‐eavesdropping countermeasures for use by high‐level government and intelligence agencies as a secure meeting and communications center that can be readily deployed worldwide. The performance criteria requiring a minimum field STC‐35 utilizing a modular, scalable mechanical design capable of repeated assembly without the use of tools, a maximum component weight of 2.65 lb/ft2 a 2 in. maximum wall thickness, and the incorporation of active and passive ventilation required the development of a robust, multi‐layered composite panel and joining system as well as ancillary components. Using a combination of non‐metallic limp mass and rigid metallic barriers separated by decouplers of varying properties in an aluminum shell brake‐formed to distort natural resonance, we were able to achieve significant transmission loss (up to 45 dB) in the speech range of frequencies equaling the performance of commercially available products of nearly twice the weight and bulk. Interior room acoustical characteristics were optimized by using modular demountable sound absorptive units.

Development of an analytical solution of modified Biot’s equations for the optimization of lightweight acoustic protection

During lift-off, space launchers are submitted to high-level of acoustic loads, which may damage sensitive equipments. A special acoustic absorber has been previously integrated inside the fairing of space launchers to protect the payload. A new research project has been launched to develop a low cost fairing acoustic protection system using optimized layers of porous materials covered by a thin layer of fabric. An analytical model is used for the analysis of acoustic wave propagation within the multilayer porous media. Results have been validated by impedance tube measurements. A parametric study has been conducted to determine optimal mechanical and acoustical properties of the acoustic protection under dimensional thickness constraints. The effect of the mounting conditions has been studied. Results reveal the importance of the lateral constraints on the absorption coefficient particularly in the low frequency range. A transmission study has been carried out, where the fairing structure has been simulated by a limp mass layer. The transmission loss and noise reduction factors have been computed using Biot's theory and the local acoustic impedance approximation to represent the porous layer effect. Comparisons between the two models show the frequency domains for which the local impedance model is valid.

Overview of low frequency control options in rooms

Low frequency control of modes and speaker boundary interference in rooms has been a persistent problem that affects the sound field in rooms. However, many innovative techniques have been developed to address these problems. This presentation will be an overview of approaches that address the optimization of room dimensions to provide the flattest room response and speaker/listener placement to minimize speaker-boundary interference; the mechanism and design of passive absorption devices based on Helmholtz resonators, diaphragmatic limp mass membranes, plate resonators and microperforated panels will be described and characterized; as well as electro-acoustic approaches, using equalization, active absorbers and the optimal placement of multiple in-phase subwoofers.

Sound isolation at low frequencies, only an extension of the traditional frequency range?

The expression ‘‘low frequencies’’ implies a frequency range where the rooms’ dimensions are comparable with the wavelength of sound in the rooms. In this range (below 150 Hz for normal room sizes) the sound field in the rooms consists of a few modes only, and changing of parameters that are not connected to the partition influences the sound insulation. This is not a problem of a correct measurement procedure. It is far more the description of the sound insulation itself in which problems arise. The measured or calculated sound insulation is only valid for the specific case under consideration. Numerical studies of the sound insulation of a flexible partition (characterized by a limp mass) between two rooms show that the dimensions of the sending and receiving rooms have the most important influence. This has to be taken into account when extending the frequency range in the standards concerning sound insulation which is necessary since neighbors low frequency stereo equipment, as well as the all present traffic noise from outdoors, demands a sufficient sound insulation even below 100 Hz.

Noise transmission through an acoustically treated and honeycomb-stiffened aircraft sidewall

The noise transmission characteristics of test panels and acoustic treatments representative of an aircraft sidewall are experimentally investigated in the NASA Langley Research Center transmission loss apparatus. The test panels were built to represent a segment sidewall in the propeller plane of a twin-engine, turboprop light aircraft. It is shown that an advanced treatment, which uses honeycomb for structural stiffening of skin panels, has better noise transmission loss characteristics than a conventional treatment. An alternative treatment, using the concept of limp mass and vibration isolation, provides more transmission loss than the advanced treatment for the same total surface mass. Effects on transmission loss of a variety of acoustic treatment materials (acoustic blankets, septa, damping tape, and trim panels) are presented. Damping tape does not provide additional benefit when the other treatment provides a high level of damping. Window units representative of aircraft installations are shown to have low transmission loss relative to a completely treated sidewall.

Practical designs for noise barriers based on lead.

Abstract Acoustical theory predicts that sheet lead and soft plastic materials loaded with finely divided lead should be excellent sound barriers. Seven case histories are given to show how this “limp mass” has been applied in practical problems.