Anechoic chamber

Details Anechoic chamber Anechoic chamber

2000-05-23

2002-04-16

Donels, Jeffrey (Department: 2837)

Acoustics

Auditorium

C181S295000, C181S292000, C181S296000

Reexamination Certificate

active

06371240

ABSTRACT:

TECHNICAL FIELD
This present invention relates to the field of anechoic chambers, and in particular to a modular anechoic panel system and method.
BACKGROUND OF THE INVENTION
The character and quality of noise emitted from manufactured products has become increasingly important to the function and marketability of such manufactured products. Product manufacturers, governments, and standard setting organizations often require consumer and industrial products and equipment to comply with increasingly stringent sound emission specification. Accordingly, a large number of consumer products and industrial equipment must now undergo sound emission testing.
Anechoic chambers, constructed using acoustical anechoic wedges are frequently employed in such sound emissions tests. According to previous techniques, an anechoic chamber consists of a shell constructed of material to provide structural stability and predictable transmission loss characteristics from the exterior of the anechoic chamber to the interior of the anechoic chamber and an array of sound-absorbing anechoic wedge devices (“anechoic wedges”) lining the shell's interior surfaces to eliminate interior reflected sound. Materials used in the construction of shells for anechoic chambers have included various materials, such as masonry, wood, and metal. Shell designs have included permanent shell structures, as well as semi-permanent shells constructed of modular interlocking structural panels. Anechoic chambers with anechoic wedges or other linings on all interior surfaces are typically referred to as “full” anechoic chambers while chambers having linings on only the walls and ceiling are referred to as “hemi” anechoic chambers. Anechoic chambers, both hemi and full, are used in the testing and or measurement of sound characteristics emitted by a specimen being tested or calibrated.
An anechoic chamber is a room that is used for precise acoustical measurements. Therefore, the room must be designed so that acoustically free field conditions exist. For practical measurements, the room also must be free of extraneous noise interferences. Anechoic chambers are widely used in the development of quieter products, including automotive and aircraft products and other products for use in transportation, communications, computers, security, and medical research.
An acoustical free field exists in a homogeneous, isotropic medium that is free of reflecting boundaries. In an ideal free field environment, the inverse square law would function perfectly, so that the sound pressure level generated by a spherically radiating sound source decreases approximately six decibels (6 dB) for each doubling of the distance from the source. A room or enclosure designed and constructed to provide such an environment is called an anechoic chamber.
Also usually an anechoic chamber must provide an environment with controlled sound pressure (L
p
) free from excessive variations in temperature, pressure and humidity. Outdoors, local variations in these conditions, as well as wind and reflections from the ground, can significantly and unpredictably disturb the uniform radiation of sound waves. This means that a true acoustical free field is only likely to be encountered inside an anechoic chamber. For an ideal free field to exist with perfect inverse square law characteristics, the boundaries must have a sound absorption coefficient of unity at all angles of incidence.
Anechoic chambers are characterized by anechoic elements that are attached to the walls, ceiling and floor of the chamber. If the anechoic elements are attached to the walls and ceiling but not the floor of the chamber, the chamber is termed a hemi-anechoic chamber. Such chambers also are used for acoustical measurements. The anechoic elements may be attached so that they are essentially in contact with or spaced from the supporting walls, ceiling and floor, depending on what is considered to be the optimum design for the chamber based on its intended use.
An anechoic element is commonly defined as one that should have less than a 0.99 normal incidence sound absorption coefficient through the frequency range of interest. In such case, the lowest frequency in a continuously decreasing frequency sweep at which the sound absorption coefficient is 0.99 at normal incidence is defined as the cut-off frequency. Thus, in an anechoic chamber, 99% of the sound at or above the cut-off frequency is absorbed. For less than ideal conditions, different absorption coefficients may be established to define a cut-off frequency. Heretofore anechoic elements for anechoic chambers have commonly been designed in the shape of a wedge.
As already noted, a characteristic of a true free field is that the sound behaves in accordance with the inverse square law. In the manufacture of anechoic elements, those elements are tested in impedance tubes as a means for qualifying them for use in chambers simulating free field conditions. A fully anechoic chamber can also be defined as one whose deviations fall within a maximum of about 1-1.15 dB from the inverse square law characteristics, depending on frequency. According to currently accepted standards, semi-anechoic rooms or chambers, i.e., those with anechioic walls and ceilings but with acoustically reflective floors, e.g., floors made of concrete, asphalt, steel, or other metals or materials, can deviate from the inverse square law by a maximum of about 3 dB depending on frequency.
Because of the very high degree of sound absorption required in an anechoic chamber, conventional anechoic elements typically comprise sound absorptive material covered or contained by a cage or cover that is made of a wire cloth (mesh) or a perforated sheet metal. For many years anechoic elements typically embodied a wire mesh cage that typically was characterized by a 90-95% open area to allow maximum exposure of sound absorbing material to the sound waves.
A disadvantage with anechoic construction elements as explained above is that in highly industrial environments the wire mesh structure may not provide sufficient physical protection for the elements. The sound absorbing material can therefore become easily disfigured by unintentional impact that is quite foreseeable in a heavily industrial environment.
Another disadvantage of the conventional anechoic elements is potential medical hazards. The sound absorptive materials such as fiberglass, rockwool or foams can be highly erosive. Over a period of use such materials could erode into particulate matter floating in the air which could be inhaled into lungs.
A further disadvantage of the conventional anechoic elements and their wire mesh coverings is that in highly industrial applications, oil spills and dirt may rapidly accumulate on the sound absorbing materials. This may impede sound absorption performance of the material and additionally may impose a fire hazard. Cleaning the sound absorptive material is difficult and not efficient.
More recently, the wire mesh covering has been replaced by a perforated sheet metal, with the an open area provided by the perforations falling within a relatively wide range: usually the open area falls within the range of about 23% to about 52% of the entire area of the sheet metal covering.
The earliest practical design for sound absorbing units of the type used in making anechoic chambers was a wedge-shaped unit fabricated from or comprising fibrous glass. That geometry of anechoic wedges has been employed as the basis for anechoic chamber design and construction in the past. Examples of prior art anechoic elements and chambers made using such elements are provided by U.S. Pat. Nos. 2,980,198, 3,421,273, and 5,317,113, and the technical publications by L. L. Beranek et al, “The Designs And Construction of Anechoic Sound Chambers”, J. Acous. Soc. of America, Vol. 18, No. 1, pp.140-150, July 1946; and B. G. Watters, “Design of Wedges For Anechoic Chambers”, Noise Control, pp. 368-373, November 1958.
The cross-section of the conventional wedge shaped anechoic element consists of a square or rectangular ba

Affiliated with

Also associated with

Rating

Anechoic chamber does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Anechoic chamber, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Anechoic chamber will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2818480