Susan, this is what we refer to as an induction loop in the UK. It shouldn't be under the floor though. Typically at head height or in the ceiling.

The problem with house induction systems is that the amplifier may not be accessible or the sound engineer may not think to connect to it. It is certainly something I shall bear in mind in the future. If there is someone with a hearing aid around I shall recruit their help and experiment next time I find an induction loop.

I looked at the Wifi/smartphone approach which could easily be installed temporarily. Made by Sennheiser and selling at £10K! Maybe the price will drop once they've recouped the development costs.