Mystery undercarpet sheet

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Hi. I have just bought an old house with a 1960s extension. I lifted the carpet and underlay in one of the rooms yesterday, to discover sheets of an odd material nailed to the floorboards. I have attached two photos. It has been severly dented by feet of bed etc, so is obviously a bit pliant, but quite firm to walk on. It has a plastic or foam look to it, but it clearly full of 'fibres'. Can anyone tell me what it might be made of please, and its purpose? Thanks in advance. Graham

[welsh brickie]

its called fibreboard and used for sound insulation.

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Hi Welsh brickie,

thank you very much for this. Given the age of the extension, I was worried that the sheets might contain asbestos. Being for sound-proofing this is unlikely then, presumably? I'm going to lay new underlay and carpet. Because the fibreboard is so dimpled, I plan to remove it first. But can the hollows be filled somehow instead? Thanks again.

[welsh brickie]

no the holes cant be filled in,either replace or remove it.

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Thank you very much Welsh brickie, much appreciated.