🚀 go-pugleaf

The technique of recreating room responses in the manner 
described is called "Auralisation".

The basic idea is to measure or calculate the impulse 
response(s) of the room and filter "dry" (anechoically 
recorded) material with these impulse responses.

There are several software modelling programs available for 
calculating accurate room impulse responses (given room 
geometry, reflection of walls etc).  The one I have experience 
with is CATT Acoustic from Computer Aided Theatre Technique, 
Gothenburg, Sweden.  Such programs are used by the 
architectural acoustics people to try out hall designs before 
they are built.  Special hardware takes care of the real-time 
filtering.

If you want to do anything more than the first order 
reflections (from walls and other surfaces) you are going to 
need some very long FIR filters, as room impulse responses can 
last several seconds.  To get the proper effect you really do 
need to implement the reverberant late part of the response.

To do this in real time means you will need an FFT based 
filter implementation, which will give you more programming 
work to do.

There is a technique that reduces the computation required at 
the expense of quality.  You just pick out the major peaks in 
your long impulse response and only implement those.  This is 
essentially what you get when you press the "hall" or 
"stadium" button on your CD player or digital amplifier.  It 
doesn't sound really good.

Good luck,

Regards,

Adam McKeag
DSP Design Engineer
Lake DSP Pty. Ltd.
Sydney Australia

PS. You can find out (a bit) more about this by looking at 
http://www.lakedsp.com.  Sorry it's not more comprehensive.