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A new acoustic insulating sheet material comprises in laminar assembly a) a primary sound absorbing sheet, and b) a dense porous membrane that i) has an air-flow resistance of about 5000 rayls or less and ii) has an Acoustic Value Ratio as defined herein of at least 3000. Preferably, the membrane is

A new acoustic insulating sheet material comprises in laminar assembly a) a primary sound absorbing sheet, and b) a dense porous membrane that i) has an air-flow resistance of about 5000 rayls or less and ii) has an Acoustic Value Ratio as defined herein of at least 3000. Preferably, the membrane is about 200 micrometers or less in thickness, and more preferably is about 150 micrometers or less in thickness. Also, the Acoustic Value Ratio is preferably at least 7,000. The described membrane can also be used alone to acoustically insulate a space, e.g., by mounting it in planar array over an air gap and in position to attenuate noise from a noise source.

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What is claimed is: 1. An acoustic insulating sheet material comprising in laminar assembly a) a primary sound absorbing sheet, and b) a dense, porous self-sustaining membrane about 200 micrometers or less in thickness that i) has an airflow resistance of about 10,000 rayls or less and ii) has an A

What is claimed is: 1. An acoustic insulating sheet material comprising in laminar assembly a) a primary sound absorbing sheet, and b) a dense, porous self-sustaining membrane about 200 micrometers or less in thickness that i) has an airflow resistance of about 10,000 rayls or less and ii) has an Acoustic Value Ratio of at least 3000. 2. A sheet material of claim 1 in which the membrane has a basis weight of 100 g/m2 or less. 3. A sheet material of claim 1 in which the membrane has an airflow resistance of 5000 rayls or less. 4. A sheet material of claim 1 in which the membrane comprises at least two distinctive layers of fibers. 5. A sheet material of claim 1 in which the membrane comprises a densified SMS sheet. 6. A sheet material of claim 1 in which the membrane has a thickness of 150 micrometers or less. 7. A sheet material of claim 1 in which the membrane has an Acoustic Value Ratio of at least 7,000. 8. A sheet material of claim 1 in which the membrane has a thickness of 150 micrometers or less and an Acoustic Value Ratio of at least 7,000. 9. A sheet material of claim 1 in which the membrane has a thickness of about 50 micrometers or less. 10. A sheet material of claim 1 in which the membrane has a basis weight of 50 g/m2 or less. 11. A sheet material of claim 9 in which the membrane has a basis weight of 50 g/m2 or less. 12. An acoustic insulating sheet material comprising in laminar assembly a) a primary sound absorbing sheet, and b) a dense porous fibrous self-sustaining membrane that i) is about 200 micrometers or less in thickness, ii) has a basis weight of 100 g/m2 or less, iii) has an airflow resistance of about 5000 rayls or less and iv) has an Acoustic Value Ratio of at least 7,000. 13. A sheet material of claim 12 in which the membrane is 150 micrometers or less in thickness. 14. A sheet material of claim 12 in which the membrane has an Acoustic Value Ratio of at least 10,000. 15. A sheet material of claim 12 in which the membrane has an airflow resistance of about 2000 rayls or less. 16. A sheet material of claim 12 in which the membrane comprises a densified SMS sheet. 17. A method for acoustically insulating a space comprising mounting a membrane in a planar array over an air gap and in position to attenuate noise from a noise source, the membrane being a dense porous self-supporting membrane having a thickness of 200 micrometers or less, an airflow resistance of about 5000 rayls or less and an Acoustic Value Ratio of at least 3000. 18. A method of claim 17 in which the membrane has a thickness of about 150 micrometers or less. 19. A method of claim 17 in which the membrane has an Acoustic Value Ratio of at least 7,000. 20. A method of claim 17 comprising mounting a more lofty nonwoven fibrous sheet material in laminar assembly with the membrane.