Symmetric glazing for improved sound attenuation

Abstract

A process for making a symmetrical glazing that has the same nominal weight as an asymmetrical glazing that has been determined to afford enhanced glazing strength, glazing rigidity, or stone impact resistance wherein the symmetric glazing has improved acoustic attenuation over coincidence frequencies of the asymmetric glazing design.

Claims

1. A process for making a lightweight symmetric glazing that has a lower cost and greater sound attenuation at a coincidence dip in comparison to asymmetric glazings having one transparency layer of aluminosilicate glass and another transparency layer of soda-lime silicate glass, said process comprising the steps of: (a) selecting an asymmetric glazing have a first outer transparency layer of soda-lime silicate glass, a first inner transparency layer of aluminosilicate glass, and a first interlayer for bonding said first outer transparency layer and said first inner transparency layer, wherein the thickness of said first outer transparency layer is greater than the thickness of the first inner transparency layer; (b) determining the total transparency thickness of the asymmetric glazing by adding the thickness of the first outer transparency layer of soda-lime silicate glass of the asymmetric glazing to the thickness of the first inner transparency layer of aluminosilicate glass of the asymmetric glazing; and (c) making the lightweight symmetric glazing by (1) arranging a sheet of polymer material between a first transparency sheet of soda-lime silicate glass that has a thickness that is one-half of said total transparency thickness of the asymmetric glazing and a second transparency sheet of soda-lime silicate glass that also has a thickness that is one-half of said total transparency thickness of the asymmetric glazing, said first transparency sheet of soda-lime silicate glass, said sheet of polymer material, and said second transparency sheet of soda-lime silicate glass forming a symmetric laminate stack; and (2) heating the symmetric laminate stack to cause said sheet of polymer material to bond to said first transparency sheet of soda-lime silicate glass and to cause said sheet of polymer material to bond to said second transparency sheet of soda-lime silicate glass to form said lightweight symmetric glazing wherein said lightweight symmetric glazing has a coincidence dip that has a greater sound attenuation that the sound attenuation in the coincidence dip of the asymmetric glazing, and wherein the cost of the first transparency sheet of soda-lime silicate glass in combination with the cost of second transparency sheet of soda-lime silicate glass in the lightweight symmetric glazing in less that the cost of the first outer transparency layer of soda-lime silicate glass in combination with the cost of the first inner transparency layer of aluminosilicate glass of the asymmetric glazing.

2. The process of claim 1 wherein said step of heating the symmetric laminate stack is performed in an autoclave and wherein the coincidence dip of the lightweight symmetric glazing occurs over a lower frequency range than the frequency range of the coincidence dip of the asymmetric glazing.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A presently preferred embodiment of the disclosed invention is shown and described in connection with the accompanying drawings in which:

(2) FIG. 1 is a top perspective view of a section of a symmetric glazing with portions thereof broken away to better disclose the structure thereof;

(3) FIG. 2 is a top perspective view of a section of an embodiment of the asymmetric glazing with portions thereof broken away to better disclose the structure thereof;

(4) FIG. 3 is a bottom perspective view of the section of the asymmetric glazing shown in FIG. 2 with portions thereof broken away to better disclose the structure thereof; and

(5) FIG. 4 is a graph showing sound attenuation of asymmetric and asymmetric glazings over a frequency range of 50 Hz to 10,000 Hz.

DESCRIPTION OF A PRESENTLY PREFERRED EMBODIMENT

(6) Significant aspects of sound attenuation in symmetric and asymmetric automotive glazings are discussed in “Practical Design Considerations for Lightweight Windshield Application” published Feb. 28, 2017 and filed by Applicant as U.S. Provisional Application 62/448,657 which document is hereby specifically incorporated herein by reference in its entirety.

(7) The presently disclosed invention concerns sound attenuation in connection with symmetrical glazings, especially in non-forward facing automotive glazings. The emphasis on weight reduction of automotive vehicles has tended to support the use of asymmetric glazings, especially in windshields and other forward-facing glazings. Weight reduction in asymmetric glazings sometimes results in an extreme degree of asymmetry—that is the thickness of inner transparency is much less that the thickness of the outer transparency in comparison to inner and outer transparencies of prior art asymmetric glazings.

(8) It has been stated that sound attenuation in asymmetric glazings is not significantly different than sound attenuation in symmetric glazings of comparable weight. Such assumptions tend to support extending the use of asymmetric glazings to automotive sidelights and backlights. However, cost and other factors may counterbalance such tendencies. For example, thinner inner transparency sheets for asymmetric glazings have been considered down to thicknesses of 0.7 mm. However, compared to a more conventional glass thickness of 1.4 mm, 0.7 mm glass requires a more expensive raw material and involves more process steps to manufacture. Specifically, 1.4 mm glass can be strengthened by thermal tempering whereas 0.7 mm glass generally uses more costly aluminosilicate glass (as opposed to soda-lime silicate glass) and is strengthened through an ion-exchange process rather than thermal tempering. Such differences in source material and processing steps may result in significantly higher manufacturing costs.

(9) FIG. 1 shows a symmetric glazing 10 of the type used in some automotive glazings. Symmetric glazing 10 includes an outer transparency sheet 12 that defines a first surface 14 and a second surface 16 that is oppositely disposed on sheet 12 from first surface 14. First surface 14 and second surface 16 are separated from each other by a thickness dimension 18 that is oriented orthogonally to each of first surface 14 and second surface 16.

(10) Symmetric glazing 10 further includes and an interlayer 20 that defines a layer of polymer material having a first surface 22 and a second surface 24 that is oppositely disposed on said polymer layer from first surface 22. The first surface 22 of interlayer 20 is opposed to the second surface of 16 of outer transparency sheet 12.

(11) Symmetric glazing 10 further includes an inner transparency sheet 26 that defines a first surface 28 and a second surface 30 that is oppositely disposed on sheet 26 from first surface 28. First surface 28 and second surface 30 are separated from each other by a thickness dimension 32 that is oriented orthogonally to each of first surface 28 and second surface 30. Symmetrical glazing 10 is “symmetrical” in that thickness 18 of outer transparency 12 is nominally the same as thickness 32 of inner transparency sheet 26.

(12) FIGS. 2 and 3 show top and bottom perspective views of an asymmetric glazing laminate 34. Much of the structure of asymmetric glazing laminate 34 is similar to the structure of symmetric glazing laminate 10, but there are also important differences.

(13) As shown in FIGS. 2 and 3, asymmetric glazing 34 includes an outer transparency sheet 36 that defines a first surface 38 and a second surface 40 that is oppositely disposed on sheet 36 from first surface 38. First surface 38 and second surface 40 are separated from each other by a thickness dimension 42 that is oriented orthogonally to each of first surface 38 and second surface 40.

(14) Asymmetrical glazing 34 further includes an interlayer 44 that defines a layer of polymer material having a first surface 46 and a second surface 48 that is oppositely disposed on said polymer layer from first surface 46. First surface 46 of interlayer 44 is opposed to the second surface of 40 of outer transparency sheet 36.

(15) Asymmetric glazing 34 further includes an inner transparency sheet 50 that defines a first surface 52 and a second surface 54 that is oppositely disposed on sheet 50 from first surface 52. First surface 52 and second surface 54 are separated from each other by a thickness dimension 56 that is oriented orthogonally to each of first surface 52 and second surface 54. Asymmetric glazing 34 is “asymmetrical” in that thickness 42 of outer transparency 36 is greater than the thickness 56 of inner transparency sheet 50.

(16) Interlayer 20 of symmetric glazing 10 and interlayer 44 of asymmetric glazing 34 may be a polymer material such as ethylene vinyl acetate, polyvinyl butyral, polyethane, polycarbonate, polyethylene terephthalates, and combinations thereof. Interlayers 20 and 44 bond the outer transparency sheet with the inner transparency sheet in the respective symmetric and asymmetric glazings 10 and 34 in accordance with autoclave processes that are known in the art. Following the autoclave process, the thickness of interlayer 20 or 44 may be in the range of 0.71 mm to 0.81 mm.

(17) Human auditory recognition normally occurs for sounds in the range of about 20 Hz to about 20,000 Hz, but humans are generally most sensitive to sound in the range of about 2,000 Hz to about 5,000 Hz. In connection with the presently disclosed invention, sound attenuation performance of symmetric and asymmetric glazings in the frequency range of 50 to 8,000 Hz are closely approximate except in the frequency range of about 2,500 to 8,000 Hz. In the 2,500 to 8,000 Hz range, asymmetric glazings exhibit a resonance condition (also herein “coincidence dip”) that is more pronounced (i.e. greater magnitude) than the coincidence dip in symmetric glazings of equivalent weight. For example, a symmetric glazing with a 1.4 mm outer transparency (i.e. the transparency that is exposed to external conditions) and a 1.4 mm inner transparency (i.e. the transparency exposed to the passenger compartment) has a weight that is equivalent to an asymmetric glazing with a 2.1 mm outer transparency and a 0.7 mm inner transparency. However, the coincidence dip of the symmetric glazing is more muted than the coincidence dip of the asymmetric glazing.

(18) It is believed that the “coincidence dip” is due to vibration frequency of the material matching the vibration frequency of the incident sound pressure waves. Frequencies that produce coincidence conditions in the glazing may be generally referred to as “coincidence frequencies.” At certain coincidence frequencies, sound waves that impact the outer transparency cause a glazing to resonate and enhance sound transmission from the glazing to the passenger compartment. Accordingly, sound attenuation is enhanced by damping coincidence frequencies, particularly frequencies in the 2,000 Hz to 5,000 Hz range where humans have higher sensitivity.

(19) In accordance with the disclosed method, different glazings may have different coincidence frequencies depending on the thickness ratio of the inner transparency vs. the outer transparency. The presently disclosed method exploits the relationship between coincidence frequencies and the transparency thickness ratios in combination with lightweighting of automotive glazings to produce lightweight glazings with a reduced coincidence dip.

(20) FIG. 4 is a graph that shows the “coincidence dip” of an asymmetric glazing having a 2.1 mm outer transparency layer and a 0.7 mm inner transparency layer. FIG. 4 also shows the “coincidence dip” of a symmetric glazing having a 1.4 mm outer transparency layer and a 1.4 mm inner transparency layer. The symmetric glazing has the same weight per unit area of the glazing as the asymmetric glazing. The asymmetric glazing has a coincidence dip in the range of about 2,500 Hz to about 8,000 Hz. The symmetric glazing also has a coincidence dip, but it is less pronounced than for the asymmetric glazing and, importantly, the symmetric glazing coincidence dip occurs at frequencies above 6,300 Hz.—out of the range of highest sensitivity for most humans.

(21) Comparison of the respective line graphs in FIG. 4 shows that a 1.4 mm/1.4 mm symmetric glazing produces the same weight reduction as a 2.1 mm/0.7 mm asymmetric glazing, but that the symmetric glazing has important advantages in sound attenuation. In the frequency range of highest human sensitivity, the symmetric glazing has approximately 5 dB greater sound attenuation in comparison to the asymmetric glazing.

(22) In addition, from the standpoint of cost, the symmetric glazing has important advantages over the asymmetric glazing. The symmetric glazing uses 1.4 mm glass in both the inner and outer transparencies. Compared to 1.4 mm glass, the 0.7 mm glass that is used in the asymmetric glazing requires a more expensive raw material and involves more process steps than 1.4 mm glass. Specifically, 1.4 mm glass can be strengthened by thermal tempering whereas 0.7 mm glass generally uses aluminosilicate glass (as opposed to less costly soda-lime silicate glass) that is strengthened through an ion-exchange process (as opposed to a less costly thermal tempering process). Thus, in comparison to the asymmetric glazing, the symmetric glazing reduces costs by avoiding the need for relatively costly glass and for more complicated strengthening processes.

(23) The disclosed invention includes a process for realizing improved sound attenuation advantages as well as other advantages of symmetrical glazings relative to highly asymmetrical glazings while maintaining the benefits of weight reduction. In a first step, a preferred asymmetric glazing is determined. Selection of the preferred thicknesses of the inner and outer transparencies of the asymmetric glazing is made according to factors that are known in the art. Such factors include requirements for overall glazing strength, glazing rigidity, stone impact resistance, and combinations thereof.

(24) After the asymmetric glazing design is selected, the total transparency thickness of the asymmetric glazing is determined. That may be done by simply adding the thickness of the outer transparency to the thickness of the inner transparency.

(25) Next, an interlayer made of a polymer material is placed between an outer transparency sheet and an inner transparency sheet to form a laminate stack. Each of the outer transparency sheets and the inner transparency sheets in the laminate stack has the same thickness which is determined as one-half the total transparency thickness of the asymmetric glazing.

(26) Thereafter, the laminate stack is heated in an autoclave or other heat process to cause the interlayer to bond the outer transparency and the inner transparency into a symmetric glazing.

(27) Through this process, a symmetric glazing is produced that has the reduced weight of the asymmetric glazing design and also the benefit of higher sound attenuation in the range of high human sensitivity.