Acoustic absorption is a typical open cell foam application like nicely explained in this handbook for acoustics or in this wikibook. In some cases, fibrous materials like mineral wool are not allowed and an open cell foam is chosen. But if also combustibility is an issue, we have to work with a mineral foam. A typical example (and maybe the only one today on the market) is Reapor.

In the datasheet, I found the following absorption spectrum.

This means that 1m² of this material is equivalent with an open window of 1m² for the noise in a room, which is almost perfect. In another post, we found what was already possible in 1963. In this case, we reach 70% to compare with 100% for the Reapor product.  This means we the 1963 product, you have to install 30% more to obtain the same result. The composition on the Reapor datasheet discloses that the material is in fact foamed bottle glass.

Both products are two steps process. Reapor starts from foamed glass granules, which are sinterd to obtain a irregular structure of small and larger pores. The 1961 process starts from closed cell cellular glass. The cells are opened with hydro-static pressure and the holes are introduced on both sides to improve the absorption.

If a material could be foamed in large boards with 100% open cells and an irregular hole pattern could be introduced during the finishing, we have a one step process, which should be competitive. But today, such a process is still not public domain. Typical (one step) open cell foams have about 70% open cells.