US1726501A - Acoustical study and correction of rooms - Google Patents

Description

Aug. 27, 1929. R F NORRl's 1,726,501

ACOUSTICAL STUDY AND CORRECTION OF ROOMS Original Filed April 13 1927 Patented Aug. 27, 1929.

UNITED' STATES PATENT OFFICE.

RALPH F. NORRIS, OF AMADISON, WISCONSIN, ASSIGNOR TO C. F. BURGESS LABORA- TORIES, INC., OF MADISON, WISCONSIN, A CORPORATION F DELAWARE.

ACOUSTICAL STUDY AND CORRECTION 0F ROOMS.

Original application filed Apri1 13, 1927, Serial No. 183,343. Divided and this application led April 22,

1929. Serial No. 357,285.

My invention relates generally to the acoustical study and correction of rooms, such as theaters, churches, or other auditoriums, and has particular reference to the 5 control or elimination of echoes in such rooms.

One of the objects of my invention is to provide a method for showing and determining which surfaces of a room, either proposed or existing, cause echoes.

Another object of my invention is to provide a method for showing and determining the direction or directions in which sound is traveling at predetermined given points in rooms, such as theaters, churches, or

other auditoriums.

Another object is to provide a method for making a record, as by photographyz of the sound paths and sound concentrations in rooms.

This application is a division of my. copending application Serial Number 332,118, filed January 12, 1929, which in turn 1s a division of application No. 183,343, filed April 13, 1927. y

Sound waves consist of a traveling succession of relatively high and low pressure areas. These areas move forward in air at approximately 1088 feet per second at ordinary room temperatures. When these waves strike a comparatively smooth and nonporous surface, they are reflected from it. As with light waves, if the sound Wave strikes a reflecting surface in a direction normal to its planeit is` reflected back toward its source. If the wave strikes the surface at an angle, it is reflected fromathe surface at the same angle. After a sound wave has been generated in a room, it is v reflected from surface to surface, producing an enormously intricate sound pattern. in the room. The laws of sound reflection are like those of light, and every surface that the sound strikes maybe considered, in :some respects, as a secondary source of sound. As-

a mirror absorbs some of the light that falls upon it, so surfaces struck by sound waves absorb a portion of the energy in the waves, the percentage being dependent upon the nature ofthe surface. The result is that the reflected sound in a. room becomes conl tinually weaker as its energy is partially ab sorbed by the successive reflecting surfaces.

The length of time that it takes for a sound of standard intensity to be so reduced by absorption that it becomes inaudible, is called the period of reverberation of the room. y

An echo is closely related to reverberation. The human ear is so constituted that it cannot distinguish two sounds less than onefifteenth of a second apart as two sounds, but receives them as one prolon ed s ound. If the sounds are more than onefteenth of a second apart, they are heard by the ear as two distinct sounds.

Sound travels about 1088 feet per second under ordinary room temperature conditions, so that if a sound generated at a given point can reach the lauditors ear over two paths whose lengths differ by one-fifteenth of 1088 feet, or approximately 72 feet, the sound will reach the ear of the auditor as two distinct sounds, T15 of a second a art. When this occurs, the sound wave reac 'ng the ear latest is called an echo. When the difference in the length of the two paths is more than 72 feet, the echo will follow the direct sound by a time interval even greater than 15 of a second. When one or more echoes are produced in a room, a confusion of sounds results and the room is described as having poor acoustics. My invention is directed to the method .of determining' those surfaces which produce confusion echoes.

The usualgmethod now employed for finding echo-producing surfaces in rooms is graphical.' In using this method it is necessary to construct scale drawings of crosssections of the room, to lay out .rays of sound directly from the source of sound, and indirectly as reflected from smooth nonabsorbent surfaces, making the angle of reflection of the sound ray equal to the angle of incidence. In this way, a series of lines ma be built up which will show the sound pat s and areas of sound concentration. Echoes may be detected by determining the difference. in the len lths of the respective paths by which soun will travel from the source` to any iven point lin the room, an

echo being produced when the difference in the paths is, roughly, 7 0 feet lor more. This method is very laborious and less accurate than my improved method.

The present invention dispenses with the graphical effects and substitutes visual effects. The newmethod providing visual effects is capable of retaining all the advantages of the graphical method upon'photographing the visual effect. n

In carrying out my invention, I utilize the similarity as to reflection between sound and light waves. However, the high speed with which light travels 187 ,000 miles'per second) and the ease with which light rays may be photographed, as for example, by allowing them to act directly on sensitized paper, makes it decidedly advantageous to use light rays in determining the acoustics of a room. This will be explained more in detail hereinafter. It is possible to photograph the ei'ect of sound waves or pulses but such photographs only show the momentary curvature or contour of the wave front. By taking a succession of photographs, the changing contour of a sound wave may be shown as it strikes obstructions, such as surfaces corresponding to those of a room, or as it interferes with other sound waves, or where it is reflected by surfaces. During the period that the Fwave front is relatively simple in form, this method may give satlsfactory results, but when the sound wave is broken up by striking various obstructions it becomes diiicult toobtain satisfactory results (see Acoustics in Auditorium Design by I. E. Sabine in American Architect, J une 18, 1924). In this method a small model of a room is erected and successive photographs of a single sound wave emanating from the designated source of sound (such las the stage in a theater) are made. This method is very cumbersome and requires a highly developed technique. It has the further disv advantage that it shows the progress of a singlevwave only, whereas sound is coinposed of a rapid succession of waves which will often create' acoustic conditions diiering widely from the pictures of the single wave. .y

In my improved method, I replace the sound waves'emanating from the designated Sound source by light rays. I am often able to obtain complete information about the echo-producing surfaces of a room or auditorium in one picture or photograph, and nearly always in three or four. I not only locate the source of the loudest echoes, but I also locate the source of secondary echoes,

and I am also able to show visually the direction and intensity-of sound at any lpredetermined point in a room. My method 1nstantaneously shows the sound pattern produced by a multiplicity of sound waves rather than that produced by a single wave front, and therefore shows the normal condition in' a room. The simplicity of my method will become more apparent in the following description, taken in connection with the accompanying drawings, in which Figure 1 illustrates a photo-picture made by my process which shows the acoustics of a room, such as a theater auditorium having.

or vertical cross-sections, depending upon the particular problem under consideration. These miniature contours may preferably be from 12 to 24 inches long, but must be faithful reproductions as to the form of the original. When the contour has been reproduced on paper or cardboard, or in other convenient form, a strip of polished metal or other reflecting material, about one t0 two inches wide, is erected .on edge along the plane contour of the room, providing a fence-like contour that may be considered as a slice-section of the auditorium. The fence may be a single strip, or a number of sections. In Fig. 3, the characteristic portions of the fence are designated by the numerals 1, 2Q 3, 4 and 5. It is usually unnecessary to erect a strip for the iloor of the room, as I have done at 6, since the audience is on the floor. The audience, to gether with' vthe seats and carpets, are excellent absorbers of sound and therefore a light-reflecting surface does not represent the true condition. Accordingly, in Fig. 3, the strip 6 is not light-reflecting. It thereA fore becomes necessary to know the types of surfaces in the room in order to duplicate exactly the conditions in miniature. Surfaces vary in their ability to absorb sound, as is well known to those skilled in the art. Since continuous smooth surfaces are good `reflectors of sound and are potential sources of echoes, I simulate these surfaces in the minature contour by leaving the Ocorresponding areas of the reiiecting strip polished. The areas which are sound absorbing are treated so that they do' not refleet light, or these sections of the contour strip may be entirely omitted. Occasionally, it only becomes necessary to erect the contour of the ceiling in miniature, since the ceiling is sometimes the only portion having sound-reliecting areas. Side walls may also be sources of echoes. If the room.

,under consideration is the auditorium-of a sov small light 9, preferably an incandescent bulb, is placed at this point.

In the application of my invention, claimed herein, after the miniature contour fence is completed, it ma;1 be slipped over a sensitized surface, such as a sheet of photographic paper, the strips of polished metal or other reflecting or treated surface forming the contour being at right angles to thesensitized surface. This operation is of necessity carried out in a dark room., or in properly subdued light. The light 9, if an incandescent bulb, should have a concentrated filament and should be placed so as to illuminate both the contour strip and the entire sensitized surface which corresponds to the area traversed by the sound. This light,

being connected to a suitable source of elec` trical energy, is then fiashed. The intensity or length of Hash should besuch that any concentrations of light which are caused by the curvature or angle of the metal wall will stand out in contrast to the balance of the sensitized surface. These concentrations of light closely approximate the concentrations of sound in the auditorium. The exposed sensitized surface may then be developed or otherwise treated to fiX the image permanently, as in a photoprint.

When it is desired to know in what direction sound is traveling at definite points, as l0 to 29, throughout the room or auditorium, pins, needles, pegs, or similar articles, hereinafter called pins, may be inserted at these points of the surface on which the contour is'erected. It is especially desirable to know the `source of sound at the points where the audience is located. The pins cast shadows, as shown in Figs. 1 and 2 by dotted lines of which the spacing increases as the shadow fades out. The intense end of a shadow points in the direction of the light source, either direct or reflected. In this way, it is often found that at some points in a room, as in Fig. 1, the audience hears the sound from as man as five points at one time, one sound slightly after the other. This causes a series of disturbing echoes or reverberations, resulting in poor acoustics. Once the reflecting surface is found, it is a simple matter to apply a highly absorbent acoustical material to it, as is understood by 1 those skilled in the art.

Fig. 1 illustrates the appearance of a photo-picture made by my method. The picture is cut in the form of the contour. It is the sound pattern obtained for a theater having poor acoustics. In the miniature, or slice model, the light was placed at 9 on the stage 8. The print shows thatv the ceiling curvature at 1 causes echoes in the audi- -ence at 12, 13, 14, 15, 16 and 17. The pins inserted at these points show that the sound will be heard prinicpally from two points, the stage 8 and the curved surface 1. Since the distance from the stage to the curved surface and back to points 12 to 17 is more than 72 feet greater than the direct distance from the stage, the curved surface produces bad echoes for the audience, especially in the region from 12 to 17. The curved surface 2 also causes some echo at 16, 17, and 18making the acoustics Very poor at 1G and 17. At 18 sound is also heard reflected from section 3 of curved surface 3. Section 3 also reects the sound into the rear of the balcony, at 22, thereby re-inforcing the sound. This re-inforcement betters the acoustics at 22, since the length of the longer sound path is less than 72 feet more than the length of the direct path. Fig. 2 shows a photo-picture of the acoustics of the same theater after sound absorbing materials 80, 31 and 32 were installed on the echo producing sections of curved sufraces 1, 2 and 3. The echoes are completely eliminated and the audience on the lower floor hears the sound from only one source, the stage.

As an example in studying the acoustics of an auditorium, it is customary to take one picture of the reflection of the side walls on the floor plan; another picture of the reflection through the longitudinal section of the room; and a third picture on the transverse section of the room. In this way all of the concentrations which cause echoes can be detected, together with the surfaces from which ,they emanate. In the case of new buildings, the angles and curvatures of the surface may be changed to eliminate these echoes, but inthe case of buildings already constructed, sound-absorbing material may. be placed on these surfaces in such a way as to eliminate the echoing effect.

Various departures from and modifications of the method and apparatus herein described are, of course, contemplated. In the foregoing example, the reflection of light .rays proper has been employed to indicate the course of sound by relative intensities of the light. It will be observed that in Fig. 2, the pin 24 casts a shadow 24 which is reflected forwardly as a shadow 24h by the rear wall 5. Thus, a beam of light or a shadow may be employed and its reflection studied.

What I claim is:

1. The method of picturing the paths and intensity of sound waves from a iven source in a room, which comprises ma ring one or more plane miniature contours of sections of said room from a metalrefiecting strip, treating said reflecting strip surfaces to conform to the sound absorbing characteristics. of corresponding sections of said room, placing said contour strip on a sheet ofsensitized paper at right angles therewith, inserting pins at right angles Iinto said sensitized paper at predetermined points, placing a source of light at the source of sound,

llO

and flashing said light to appreciably affeet those portions of the sensitized paper which are subjected to the greatest intensity of light.

2. rlhe method of picturing the paths and intensity of sound waves from a given source in a room, which comprises making one or more plane miniature contours of sections of said room from a reecting strip, treating said reflecting strip surfaces to conform to the sound absorbing characteristics of corresponding sections of said room, placing said contour strip on a sensitized surface at right angles therewith, inserting pins at right angles into said surface at predetermined points, placing a source of 1i ht atv the source of sound, and dashing sai light to appreciably aect those' portlons of the sensitized surface which are subjected to the greatest intensity of light.

3. The method of picturing the paths and intensity of sound Waves from a given source in a room, `which comprises making a conweasel tour fence of a reduced section of said room, the fencing material having light-redecting and light-absorbing properties comparable to the sound-reflecting and absorbing roperties ofthe corresponding room sur aces, placing said contour fence on a sensitized surface at righ angles therewith, placing a source of light at the source of sound, and dashing said light to appreciablyatect those portions ot the sensii'nizedy surface which are subjected to the greatestintensity of light. 4. The method of determining sound-reecting surfaces in a room, which comprises putting a source of light rays at the sound source in a miniature llght-redecting fencedin contour of a cross-section of said room mounted on a sensitzed surface, and subjectin(r said encing surface'and said sensitized surface to said light rays; substantially as described.

ln testimony whereof, ll have subscribed my name.

nanrn r. Nonnrs.

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