IE870603L - Sound absorbing member - Google Patents

Description

o f n 7 - ^ U I The invention relates to a device and method for controlling sound reverberation in a room. It is particularly, though not exclusively, concerned with sound absorbing members which are located in the angle between the walls and ceiling, or in the 5 corners of a room.

Swedish Patent Application No. 8103345 discloses the use of planar diagonal absorbers arranged to adjust the sound pattern (including the time of reverberation) in a room. The frequency response is, however, not completely satisfactory as 10 it is not sufficiently uniform.

According to a first aspect of the invention there is provided adjustable sound-reverberation control apparatus comprising an acoustic absorber, means for mounting the absorber to a wall or ceiling of a room, and actuating means arranged to move the 15 absorber with respect to the mounting means into an area of locally high sound intensity, thereby selectively controlling the reverberation characteristics of the room.

According to a second aspect of the invention there is provided a method of controlling the sound reverberation 20 characteristics of a room by means of an acoustic absorber mounted to a wall or ceiling of the room, comprising determining an area adjacent the absorber where the sound intensity is locally high, and moving the absorber into the said area.

Where the absorber comprises a sound-absorbing panel, the panel may be generally triangular, the apparatus being constructed and arranged for positioning the panel diagonally between the ceiling of a room and two adjoining walls.

The invention may be carried into practice in a number of ways and some specific examples will now be described, with 5 reference to the accompanying drawings, in which Figure 1 illustrates a known diagonal acoustic absorber situated in a corner of a room, Figure 2 illustrates how the kinetic energy of the air molecules near the corner of a room varies with position 10 when sound waves are set up within the room, Figure 3 shows isoenergy lines based upon horizontal cuts through Figure 2; it also shows the possible positions of a number of diagonal acoustic absorbers, Figure 4 illustrates the way in which the absorption 15 coefficient x (in arbitary units) varies with frequency, for two different lengths i of the absorber, Figure 5 illustrates a number of possible absorber types and positions and shows how the absorption coefficient a varies with frequency for the various arrangements, and Figure 6 is a graph showing how, for acoustic absorbers of various types, the specific flow resistance through the absorber material varies with the absorber density.

In all cases, means for mounting the absorber to a support (e.g. a wall, a ceiling or both) are not shown, but are 25 implied.

At frequencies exceeding about 100-300 Hz, sound absorption in a thin absorber such as is shown in Figure 1 occurs as the air molecules move within the body of the porous material.

The frequency fj at which the absorption is a maximum can be determined from a theoretical analysis of the sound field in a corner, viz: li = 140 = 280 Hz d i sin 20 where d is the depth of the absorber in metres, % is the length of the absorber in meters, and 0 is as indicated in Figure 1.

At lower frequencies (50-200 Hz) and with diagonal absorbers where ^ is less than about 2m, most of the sound absorption occurs by the sheet material being induced to oscillate at its resonant frequency. Energy is therefore lost as a consequence of losses in the material and losses along the edges where the material is secured to the wall or ceiling.

For such resonant oscillations to be produced, the relative positions of the absorber and the regions in space at which the sound intensity is greatest (that is, where the pressure oscillation and mean molecule kinetic energy is greatest) are of importance.

For connection with a planar diagonal absorber of low bending resistance compared with the resistance of the confined air, the resonant frequency is: fQ = 1_20 Hz J m* £ • s 1 n t 2 0) where m is the mass of the sheet material per unit area in kg/m2 and I is the length of the sheet in metres.

The first process, at high frequencies, involves specific requirements as to size, shape, and flow resistance. The second process, at low frequencies, involves specific requirements as to the size and mass per unit area.

The highest possible sound absorption is required, for practical purposes, in the frequency range 100-4000 Hz, and the absorption should preferably be as uniform as possible in that range.

Experiments with planar diagonal absorbers have given promising results when * = 0.90 m and 0 = 30°, and still better results can be expected in connection with particular, non-planar embodiments. Conversely, poor uniformity over the frequency range is to be expected with particularly disadvantageous embodiments. Problems arise at low frequencies if the absorber dimensions are too small. Furthermore, the obtainable absorption area depends on the . surface area of the absorber.

As the resonant frequency fj, should, in practice, be about 100 Hz, if £ is assumed to be in the region of 0.90 to 1.80 m, the following requirements to the mass.per unit area appear: m = 1-2 kg/m2 The mass per unit area must be highest for small values of I. 10 Experiments have shown that the flow resistance of the material, which, as mentioned above, will affect its absorption characteristic, should be somewhat higher than that of traditional suspended ceiling plates, preferably about: r = 2000 - 2500 Ns/m3. These values depend on the thickness h of 15 the plate as well as on the material parameters according to the formulae: m = ph and r =XH h where p is the density andis the specific flow resistance.

»■ Thus, one obtains 'H/ ~ 1.250 s"1 P This can be recorded in a -p-diagram as a straight line, see Figure 6. Also indicated on this graph are the corresponding lines for various fibrous materials. The position of the line defined above indicates that a good practical absorber should have a fibre diameter somewhat smaller than the fibre diameter of the usual glass wool. As an alternative a coating (either of the individual fibres or of the surface of the material) suitable increasing the flow resistance can be used.

In order to avoid high-frequency signals from being reflected from a too strongly compressed surface the specific flow resistance should not be too high. The parameters should preferably be selected within the following ranges: h P mm 100 kg/m3 40 mm 50 mg/m3 Figures 5a-5e show how the absorption characteristics (in arbitary units) varies with frequency (in units of absorber size per wavelength) for differing embodiments of the absorber. Figure 5a shows an oval absorber providing a very non-uniform frequency response. As will be seen from the graph the frequency response shows a notch at d/\ — 0.7 125.103 Ns/m* 63.103 Ns/m4 corresponding to the absorber being situated, for sound of that wavelength, in positions with weak molecular oscillations. The diagonal absorber of Figure 5b also provides a very poor response since only a small part of the 5 absorber is situated at the spot of high sound intensity. This arrangement is the one disclosed in Swedish Patent Application 8103745, mentioned earlier. A minor improvement is obtained by situating the diagonal absorber asymmetrically with an angle differing from 45°. Figure 5c. illustrates the 10 effect of such an absorber at different frequencies. It appears that with a planar absorber the response must necessarily show a notch (here at about 500 Hz). Figure 5d illustrates an ideal embodiment of the absorbed member, the absorber being situated in areas where the sound is 15 particularly intense. The distance between the crests is preferably about one half wavelength. Figure 5e illustrates an absorber of an alternative asymmetric embodiment, (an asymmetric L-shaped embodiment).

The acoustic absorbers are of variable configuration, for 20 example so that the configuration, and thus the absorption characteristic with frequency, can be varied according to the resonance properties of the room at the time. These resonance or other acoustic properties may depend, for example, upon the number of persons in the room, and/or the position of articles 25 within the room. It is further desirable that the configuration may be altered in such a way that the frequency response remains substantially constant, as the natural acoustic properties of the room vary. In this way, the room can artificially be provided with a relatively constant reverberation characteristic, independent of the number of 5 persons or articles within the room. As an example, acoustic absorbers could be used in a concert hall, and adjusted in response to the particular requirements of an orchestra, with regard to the time of reverberation and so on, while a concert is actually in progress. Furthermore, allowances can be made 10 for variable numbers of people in the audience.

In one specific example, the adjustable configuration could be the angle between the absorber and the wall or ceiling of a room. This could be achieved by providing suitable means for varying the angle of the absorber, for example an electrical 15 stepping motor, positioned immediately behind the absorber.

As an alternative, the adjustable configuration could be provided by the use of a deformable absorber, and some means (for example a series of actuator pistons pressing against the absorber on its rear side) to produce the deformation.

Both of the above arrangements could be provided for the same absorber.

In one particular convenient arrangement, the configuration of the absorbers in a room is under microprocessor control. This could operate in several ways. If, for example, the natural resonance frequency of the room at any particular time is already known, and it is desired to change the acoustic properties of the room to give a different previously defined 5 frequency response, then the microprocessor could simply be instructed to change the configuration of the absorbers to produce the required response. Alternatively, the microprocessor could be instructed to change the configuration so that the room has a particular desired frequency response, 10 independent of the people or objects who are in it at the time. This could be achieved by providing means for determining the actual frequency characteristic of the room, comparing this with the desired frequency characteristic, and using the difference to make an appropriate alteration in the 15 configuration. Naturally, it is not essential for the entire frequency characteristic of the room to be measured; it is sufficient for the sound pattern (in particular the sound intensities) near the corners of the room in which the absorbers are situated to be measured.

As a further alternative, the frequency response, reverberation characteristic, or sound pattern could be calculated rather than measured.

In one particular arrangement, the absorber is arranged to be moved in response to the actions of a player of a musical instrument within the room. The player can thus control not only the musical tones being produced by the instrument, but also the resonance characteristics of the room within which he is playing, so being able to control not only the notes played but also the timbre of the notes.

The absorbers could be situated either in one or more of the corners of a room, or between the edges of two adjacent walls, or between one or more walls and the ceiling. In the latter case, the absorber can act as a cornice. With any of the embodiments, particularly that shown in Figure 5e, the 10 absorber may be optionally used in connection with a perforated sound permeable under-ceiling which is flush with the absorber. This provides a good architectural effect.

Claims (17)

1. Adjustable sound-reverberation control apparatus comprising an acoustic absorber, means for mounting the absorber to a wall or ceiling of a room, and actuating means arranged to move the absorber with respect to the mounting means into an area of locally high sound intensity, thereby selectively controlling the reverberation characteristics of the room.

2. Apparatus as claimed in Claim 1 in which the absorber comprises a sound absorbing panel.

3. Apparatus as claimed in Claim 2 in which the panel includes a coating to increase the flow of resistance thereof.

4. Apparatus as claimed in Claim 2 or Claim 3 in which the panel is asymmetrically L-shaped in cross-section.

5. Apparatus as claimed in Claim 2 or Claim 3 in which the panel is wavy in cross-section.

6. Apparatus as claimed in any one of Claims 2 to 5 in which the panel is generally triangular, the apparatus being constructed and arranged for positioning the panel - 13 - diagonally between the ceiling of the room and two adjoining walls.

Apparatus as claimed in any one of the preceding claims in which the actuating means are arranged to deform the absorber.

Apparatus as claimed in any one of the preceding claims including measuring means, arranged to determine the sound pattern in the vicinity of the absorber, the actuating means being arranged to operate in dependence upon an output of the measuring means.

Apparatus as claimed in any one of the preceding claims in which the actuating means is arranged to operate under microprocessor control to move the absorber into one of a predefined plurality of positions.

Apparatus as claimed in any one of the preceding claims in which the actuating means is arranged to operate under microprocessor control to move the absorber into such positions as to cause the room in which the apparatus is situated to take on one of a predetermined plurality of selected resonance characteristics. - 14 -

11. Apparatus as claimed in Claim 10 arranged to operate so as to maintain the resonance characteristics of the room substantially constant.

12. Apparatus as claimed in any one of the preceding claims 5 in combination with a musical instrument, the musical tones produced and their resonance characteristics being under the control of a player of the instrument.

13. A method of controlling the sound reverberation characteristics of a room by means of an acoustic 10 absorber mounted to a wall or ceiling of the room, comprising determining an area adjacent, the absorber where the sound intensity is locally high, and moving the absorber into the said area.

14. A method as claimed in Claim 13 comprising changing the 15 angle of the absorber with respect to the walls or ceiling of the room.

15. A method as claimed in Claim 13 comprising deforming the absorber.

16. Sound reverberation control apparatus as specifically 20 herein described with reference to any one of Figures 5a, 5c, 5d or 5e; and with or without reference to Figure 3 and to Figure 6. - 15 -

17. A method of controlling the sound-reverberation characteristics of a room substantially as specifically herein described. Dated this 10th day of March, 1987 CRUICKSHANK & CO. Agents for the Applicant 1 Holies Street Dublin 2 MM\A:RTB\9525.DEC