US2160638A

US2160638A - Sound-absorbing unit

Info

Publication number: US2160638A
Application number: US159840A
Authority: US
Inventors: Eual H Bedell; Leon J Sivian
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1937-08-19
Filing date: 1937-08-19
Publication date: 1939-05-30
Anticipated expiration: 1956-05-30

Description

May 30, 1939. BEDELL AL 2,160,638

SOUND ABSORBING UNIT Filed Aug. 19, 1937 FIG.

ATTORNEY l/V l E N 7095 i Patented May 30, 1939 UNITED STATES PATENT OFFICE SOUND-ABSORBING UNIT Application August 19, 1937, Serial No. 159,840

3 Claims.

This invention relates to the acoustic treatment of rooms and the object of the invention is a sound absorbing unit of high efllciency which is well adapted for the treatment of rooms with minimum expense and loss of time.

Many of the more emcient sound absorbing materials are either not self-supporting or they have other physical properties which make them unsuited for direct exposure to the action of in sound waves. Other materials such as the wellknown fibrous wall boards are less efllcient and incapable of being suitably decorated with further impairment of their acoustic properties.

A sound absorbing unit according to the invention comprises a large thin perforated metal casing containing highly efficient sound absorbing material and adapted to be mounted with both sides of the material exposed to the sound waves in the room. The perforations in the metal are 20 small enough to be inconspicuous and are of such spacing as to make the casing substantially quired. In general, however, it should be thick enough to receive a layer of damping material which will absorb a substantial part of the energy of the impinging sound waves.

In the drawing,

Fig. 1 is a side view and Fig. 2 is an end view of sound absorbing unit according to the invention suspended from the ceiling of a room; and

Fig. 3 shows a spring mounting for the unit and an alternate construction in which spacers are used between the sound absorbing material and the casing.

Referring to the drawing the casing ll may be formed by bending the perforated sheet material to a U shape as shown and closing the ends by means of wood or metal strips II. In one very satisfactory construction, the sheet material is No. 24 gauge steel with inch diameter holes spaced on inch centers in staggered rows inch apart. This gives a substantially acoustically transparent surface over the sound frequency range and one which may be readily painted without impairing its sound transmitting properties.

When the units are suspended from the ceiling, absorption is increased and ventilation of the room facilitated by leaving a clearance of about two feet between the top of the unit and the ceiling. It is usually desirable, however, to limit the vertical dimension of the unit itself to about three feet, or less if necessary, to avoid undue reduction in head room and interference with the lighting system of the room. The lengths of the units will vary with local conditions but in general it is convenient to make them in several lengths such, for example as 4, 6 and 8 feet. In most casings the absorbing material may be in direct contact with the casing but if desired internal spacers may be used to hold the material out of contact with the metal so as to permit the sound waves to have free access to the whole area of the material. When they are used the spacers i3 may be of wood 34 inch thick, for example, disposed a few inches apart and they are preferably assembled externally of the casing to form a unitary structure. The casing is made of sumcient thickness to receive at least a one inch blanket of rock wool l9 or other suitable materialsuch as spun glass or gypsum. After the blanket has been inserted the top piece it is then put in place and the sides of the casing fastened to it. Eye bolts or hooks ii are placed in the piece It and the unit is then ready for hanging as shown in the drawing.

when these units are used in buildings subject to considerable vibration, shedding and sifting of the damping material through the perforations will be largely prevented by using resilient suspensions such as springs i8 (Fig. 3) of such stiifness that the resonant frequency of the suspended unit is lower than that of the more important disturbing vibrations.

It is, of course, essential for efllcient sound absorption that these suspended units have sufllcient inertia to prevent appreciable swaying of the unit under pressure of the sound waves. While the construction described above has been found very satisfactory from this standpoint it will be understood that when necessary heavier construction or lumped additional masses may be used as required. In some cases it may be desirable to mount the units as a canopy over the machines or other sources of noise in the room and when this construction is used the units may be rigidly mounted on supports secured to the machine or they may be flexibly suspended at the proper height from the ceiling by springs of appropriate stiffness.

In either case it is found that by exposing both sides of the material to the action of the sound waves a given amount of absorbing material will provide from 60 to '70 per cent more sound absorption than when it is attached to the walls of the room in the conventional manner and the use of the perforated metal casing reduces the fire hazard to such an extent that even inflammable materials may be used as the filler with comparative safety. Moreover, since these damping units are prefabricated they may beused to treat a room with minimum interruption in its normal use and they are particularly advantageous for temporary installations in that they may be quickly installed and easily removed to a new location at very small expense.

What is claimed is:

1. A sound absorbing unit for the acoustic treatment oi rooms comprising a thin casing with sheet metal sides having small distributed periorations sumcient in number to render the casing substantially acoustically transparent, a blanket of sound absorbing material within the casing and a unitary spacing structure on each side of the blanket holding it out 0! contact with the sides of the casing.

2. A portable sound absorbing unit for the acoustic treatment of rooms comprising a layer of fibrous sound absorbing material, a thin flat casing surrounding and supporting the material and having sheet metal sides with small distributed perforations sufllcient in number to make the sides substantially acoustically transparent, and means for suspending the casing on edge from the ceiling of a room whereby both sides of the casing are exposed to sound waves within the room.

3. A sound absorbing unit according to the preceding claim which is of large mass to prevent swaying of the unit under the pressure of the sound waves and in which the suspending means is of such low stiffness that the suspended unit resonates at a frequency lower than that of the room structure vibrations to minimize the shedding of the fibrous material through the perforations.

EUAL H. BEDELL. LEON J. SIVIAN.