An apparatus is disclosed for reducing acoustic transmission from mechanical or acoustic sources by means of a double wall partition, within which an acoustic pressure field is generated by at least one secondary acoustic source. The secondary acoustic source is advantageously placed within the partition, around its edges, or it may be an integral part of a wall of the partition

Grosveld, Ferdinand M. W. A.

Shepherd, Kevin P.

English

NASA Technical Reports Server

United States Patent [191 
.Shepherd et id. 
[54] SOUND ATTENUATION APPARATUS 
[75] Inventors: Kevin P. Shepherd, Yorktown; 
Ferdinand M. W. A. Grosveld, 
Hampton, both of Va. 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[73] Assignee: The United States of America as 
[21] Appl. No.: 392,165 
1221 Filed: Aug. 10, 1989 
[51] Int. C l . 5  ........................... E04B 1/82; FOlN 1/06 
[52] U.S. Cl. .................................... 181/206; 181/286; 
181/290; 181/295; 381/71; 381/94; 52/14 
[58] Field of Search ............... 181/206, 286, 295, 296, 
181/30, 290; 381/71, 94; 52/144, 145 
[561 References Cited 
U.S. PATENT DOCUMENTS 
3,229,429 1/1966 Conrad ............................. 5 2 / 1 4  X 
3,879,578 4/1975 Wildi ................................. 381/71 X 
17 
d 
7 
;3 
[ i l l  Patent Number: 5,024,288 
[GI Date of Patent: Jun. 18, 1991 
4,177,874 12/1979 Angelini et al. ........._......... 181/206 
4,589,137 5/1986 Miller ................................ 381/71 X 
4,600,078 7/1986 Wirt .................................... 181/286 
4,715,559 12/1987 Fuller ................................ 381/71 X 
4,665,549 5/1987 Eriksson et al. ...................... 381/71 
4,783,817 11/1988 Hamada ................................ 381/71 
2550903 2/1985 France .................................. 381/71 
Primary Examiner-Benjamin R. Fuller 
Attorney, Agent, or Firm-George F. Heifrich; Harold 
W. Adams; John R. Manning 
1571 ABSTRACT 
Apparatus for reducing acoustic transmission from me- 
chanical or acoustic sources by means of a double wall 
partition, within which an acoustic pressure field is 
generated by at least one secondary acoustic source. 
The secondary acoustic source is advantageously 
placed within the partition, around its edges, or it may 
be an integral part of a wall of the partition. 
FOREIGN PATENT DOCUMENTS 
8 Claims, 2 Drawing Sheets 
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U.S. Patent June 18,1991 Sheet 1 of 2 5,024,288 
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US, Patent June 18,1991 Sheet 2 of 2 5,024,288 
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5,024,28 8 
1 
SOUND ATI'ENUATION APPARATUS 
ORIGIN OF THE INVENTION 
5 The inventiop described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 of the National 
Aeronautics and Space Act of 1958, as amended, Public 
BACKGROUND O F  THE INVENTION 
Law 85-568 (72 Stat. 435; 42 USC 2457). 10 
I. Field of the Invention 
The invention relates to the field of sound barriers 
and in particular to a sound barrier capable of attenuat- 15 
ing sound over a range of frequencies. 
2. Description of Related Art 
It is well known in the field of sound barriers that a 
single wall will reduce the sound transmitted by a 
source. In order to achieve the desired degree of sound 20 
attenuation, it may be neceSSary to use a wall so heavy 
or stiff as to be impractical, as for example, a wall on- 
board or as part of an aircraft or space vehicle, where it 
is desired to minimize weight. Further, the wall thick- 
ness required increases as the sound frequency de- 25 
creases. Thus, especially at low frequencies, particu- 
larly thick and heavy walls are required. 
One improvement to the use of a single wall is a 
double wall or other compound wall structure, which 
may have the capability of equivalent sound attenuation 30 
at lower total weight. The double or compound wall 
may also be stiffened, or fdled with a material which 
absorbs or damps sound, such as a fiberglass mat or 
blanket; also, viscoelastic damping materials may be 
applied to the walls. 
However, even compound walls with acoustic treat- 
ment may not be suitable. U.S. Pat. No. 4,600,078 
teaches the use of acoustical resonators with a double 
wall partition as a means of attenuating sound. Each 
resonator may be designed for a specific frequency. 
The use of resonators inherently imposes l i t a t i o n s  
which it is an object of the present invention to over- 
come. 
Accordingly, it is a primary object of the present 45 
invention to provide an improved sound barrier. 
It is another object of the present invention to pro- 
vide a sound barrier with increased sound attenuation 
effectiveness without increasing the total weight 
thereof. It is a further object of the present invention to 5o 
provide a sound barrier which conveniently attenuates 
sound of varying frequency, or sound of simultaneous 
multiple frequencies. 
It is sti l l  another object of the present invention to 
provide a sound barrier which incorporates acoustical 55 
elements. 
It is a further object of the present invention to pro- 
vide a sound barrier suitable for the construction of 
fuselage walls for surface vehicles, marinehbmarine 
SUMMARY O F  INVENTION 
According to the present invention, a double wall 
partition is placed between a primary acoustic source 
which it is desired to attenuate and a receiver zone. 65 
Within the partition is a sound-transmitting medium 
such as air. At least one secondary acoustic source, 
which may be a loudspeaker, is used to generate an 
35 
40 
craft, aircraft, and space vehicles. 60 
acoustic pressure field within the partition, which has 
the effect of reducing sound in the receiver zone. 
It is not an essential element of this invention that the 
secondary acoustic source(s) be within the double wall 
partition. Indeed, the secondary acoustic source(s) may 
be external to the double wall partition. However, the 
secondary acoustic source(s) or its (their) mountings 
may be an integral part of at least one wall of the parti- 
tion. Where the secondary acoustic source is a loud- 
speaker, it must be enclosed, and the enclosure may be 
an integral part of a wall or of the partition perimeter. 
The frequency of the primary and secondary acoustic 
source(s) must be substantially identical. It may be nec- 
essary, depending on the characteristics of the primary 
source, to adjust the amplitude and phase of the sound 
generated by the secondary source(s). Thus, the inven- 
tion has its broadest application where the frequency, 
amplitude and phase of the secondary source(s) are 
adjustable. 
Depending on the characteristics of the primary 
source, a greater degree of sound attenuation may be 
achieved for a given secondary acoustic source depend- 
ing on its location with respect to, or within, the parti- 
tion. It is especially advantageous if the secondary 
source is readily movable, especially by a means exter- 
nal to the partition. 
It is especially beneficial to have one or more sensing 
devices in the receiver zone or within the partition to 
measure sound level and to communicate with an auto- 
matic control device to vary the characteristics of the 
sound generated by the secondary acoustic source(s), 
viz., the frequency, amplitude and phase thereof, or to 
change the location of the secondary acoustic source@), 
so as to readily and continuously minimize the sound 
perceived in the receiver zone. 
Especially good results are obtained when the spac- 
ing between the walls of the partition is less than the 
wavelength of the sound to be attenuated. 
BRIEF DESCRIPTION OF THE DRAWINGS 
For a more complete understanding of the present 
invention, including its objects and benefits, reference 
should be made to the Description of the Preferred 
Embodiments, which is set forth below. This descrip- 
tion should be read together with the accompanying 
drawings, wherein: 
FIG. 1A is a schematic showing a side view of one 
embodiment of a sound barrier according to the present 
invention; 
FIG. 1B is a schematic showing nine measurement 
positions in a vertical plane parallel to the sound barrier 
of FIG. 1A and spaced 24 inches therefrom; 
FIG. 2A is a schematic showing a side view of an- 
other embodiment of a second barrier according to the 
present invention; and 
FIG. 2B is a schematic showing the positions of four 
loudspeakers within the sound barrier of FIG. 2A. 
DESCRIPTION OF THE PREFERRED 
EMBODIMENTS 
The invention may be understood by referen& to the 
following examples and their reference figures. 
EXAMPLE 1 
Referring now to FIG. lA, two rooms, a source 
room and a receiving room, were separated by a com- 
mon concrete wall 3 which had a 4 8 " x W '  opening 
therein. A double wall partition 4 was installed in the 
5,024,288 
3 
' opening, made of the following materials: wall 5 was 
0.063 inches thick aluminum and wall 6 was 0.25 inches 
thick plexiglass. 
A loudspeaker was placed in the source room, to 
serve as primary source 1, located 18-inches from parti- 
tion 4. Source 1 generated a pure tone signal at a fre- 
quency of 125 Hz. In the receiving room, a microphone 
7 was placed 24-inches from wall 6 and 18-inches below 
the horizontal centerline of partition 4. 
The signal generated by source 1 was measured at 84 
dB by microphone 7. 
A closepacked cluster of four juxtaposed loudspeak- 
ers 2A, 2B, 2C, and 2D was located at the center of the 
double wall partition to comprise secondary source 2. A 
125 Hz signal was fed to the cluster, and the amplitude 
and phase were adjusted until the sound detected by 
microphone 7 was at a minimum. The minimum sound 
level measured by microphone 7 was 41 dB when the 
phase difference between sources 1 and 2 was 164'. 
EXAMPLE 2 
Using the same room and conditions as in Example 1, 
noise reduction was measured at nine different locations 
(8, 9, 10, 11, 12, 13, 14, 15, and la) in a vertical plane 
containing microphone 7 as shown in FIG. 1B. With 
source 1 and source 2 at the identical frequencies, ampli- 
tudes and respective phases as in Example 1, an average 
sound pressure level for the nine locations was mea- 
sured first when source l alone was operating, and then 
when sources 1 and 2 were operating together at the 
identical frequency, amplitudes and phases as in Exam- 
ple 1. When sources 1 and 2 were operated, the average 
sound pressure level was decreased by 8.2 dB, com- 
pared to only source 1 operating. 
EXAMPLE 3 
Example 1 was repeated, but with microphone 7 
moved to a location 17, which is 60-inches from the 
partition rather than 24inches. At location 17, the mea- 
sured sound pressure level went from 88 dB without 
source 2 to 49 dB with source 2 operating; the phase 
difference between signals to source 1 and 2 was 161'. 
This demonstrates that a noise reduction is obtained at 
varying distances from the partition. 
EXAMPLE 4 
Example 3 was repeated, using the same location 17 
for the microphone, but at different frequencies. Results 
follow: 
5 
10 
15 
20 
25 
30 
35 
40 
45 
50 
4ooHs 55 Frequency 200 Hr 
Source 1 only, m a s u r d  dB 86 17 
Phue difference, degrees 220' 344' 
Source 1 and source 2 49 42 
operating, dB 
60 
4 
This Example shows that noise is reduced at different 
frequencies. 
EXAMPLE 5 
Example 1 was repeated, but the four loudspeakers of 
source 2 were placed in the four comers within the 
double wall partition 4. See FIGS. 2A and 2B. 
Results follow: 
~~ ~ 
Frequency 125 Hz 
source 1 84 dB 
source 1 + source 2 58 dB 
Phase Difference, degrees 291' 
This Example shows that the source 2 location affects 
results quantitatively, but in all cases studied, there is 
some noise reduction. 
What is new and desired to be secured by Letters 
Patent of the United States is: 
1. A sound barrier comprising: 
a multiple wall partition positioned between a pri- 
mary acoustic source producing a first sound of a 
frequency and a zone wherein the sound of said 
primary acoustic source is to be attenuated; 
a sound-transmitting medium within said partition; 
and 
at least one secondary acoustic source positioned to 
generate a second sound within the partition, the 
second sound having a frequency, magnitude, and 
phase selected so as to achieve attenuation in said 
zone. 
2. The sound barrier of claim 1, wherein the multiple 
wall partition is a double wall partition. 
3. The sound bamer of claim 2' wherein the at least 
one secondary acoustic source is placed within the dou- 
ble wall partition. 
4. The sound barrier of claim 2 wherein the at least 
one secondary acoustic source is external to the double 
wall partition. 
5. The sound barrier of claim 2 wherein the at least 
one secondary acoustic source is an integral part of at 
least one of the walls of the partition. 
6. The second sound barrier of claim 2 wherein the 
second sound generated by the at least one secondary 
acoustic source may be adjusted with respect to a mem- 
ber of the group consisting of: frequency, amplitude, 
and phase. 
7. The sound barrier of claim 6 wherein at least one 
sound-sensing device is placed in the receiver zone or 
within the partition and said at least one sound-sensing 
device is used to automatically vary a member of the 
group consisting of frequency, amplitude, and phase of 
the second sound generated by the at least one second- 
ary acoustic source so as to maximize the desired sound 
attenuation. 
8. The sound barrier of claim 3 wherein said at least 
one secondary acoustic source is readily movable by a 
means external to the partition. * * * * *  
65 


Sound attenuation apparatus

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