Like many people today who are interested in sound and recording I’ve got a room which I use as my “Home Studio”.  As this is it’s only use, I’m fortunate in that I can treat it pretty much exactly how I want, with less need to make compromises which otherwise inevitably happen for a multi-purpose room.   I’m working on predictions/approximations from calculations, using the Sabine formula and from the subjective ‘feel’ when in the room and when comparing it to other rooms in my home.

To be clear at the outset – this isn’t about making the room soundproof.  That’s a completely different aspect of room, and indeed building, design which goes way beyond what can reasonably be achieved in a domestic home.  I’m merely making the room less reverberant, more like a recording studio control room.  I’m treating the room with absorbent materials to reduce the reverberation in it.  It will also be furnished in a manner which assists the acoustics in the room.

The starter for doing this is to calculate approximately what the RT60 reverberation time for the room, with no treatment is.  As the room measures 3.3m x 3.0m x 2.4m it’s not the ideal shape for a control room.  There’s a chimney area and a built-out cupboard/wardrobe along one of the side walls too.  For the purposes of approximate calculation they’re being ignored.   As with most rooms in a home, the walls are plaster over brick, wallpapered but then painted over.  There’s a regular carpet with underlay on the floor.  The ceiling is plaster, like the walls, papered and painted.   

This lets me look up and use the approximate absorption coefficients for the surfaces in the room. I can then using the Sabine formula for predicting and calculating the reverberation in the room.

I made a simple spreadsheet to calculate the RT60 times, so using the above information it looks like this :

Room Dimensions  (in meters)       
length (L) 3.3    
width  (W)  3    
height (H)  2.4    
       
Room has these surface  areas (in square m) Area Absorption Coefficent Absorption per surface
 wall 1 (side) 7.9 0.02 0.16
wall 2  (side)  7.9 0.02 0.16
wall 3 (front)  7.2 0.02 0.14
wall 4 (back)  7.2 0.02 0.14
floor 9.9 0.4 3.96
ceiling  9.9 0.03 0.30
 Calc’d total  Surface Area of the Room             (square meters)   50.0   4.86
       
and it has a total volume (in cubic meters)        
Total Volume of the Room 23.8    
       
Using these calculated numbers we can calculate the room’s total absorption in Sabines      
Area (per surface) x Absorption Coefficient 4.9 Sabines  
       
 the RT60 for the room can be calculated as        
(0.161 x  Total volume of room) /  Total Absorption  0.79 seconds  

Good control rooms generally have an RT60 of around 0.3 – 0.5 seconds. So my room is outside this – as a raw space.  For comparison purposes, a living room or  bedroom has a general RT60 of around 0.4 – 0.9 seconds.  My room’s already in the middle of that spread. However, as it’s easy to make some simple changes, I’ve chosen to make them.


Treating the rear wall.  

To do this, I’ve done two things.

1: Installed a full width sofa/bed.   (Ikea’s best sound absorber…)

2: Built a wall mounted absorption panel which is large enough to cover most of the remaining exposed area of the rear wall.

The absorption panel was built from a fabric print bought some time ago simply as decoration for my room.  The print measures 1m x 1m and is on a 4cm x 4cm wooden frame. To convert it to being a more absorbent panel I filled the back of the frame with 4cm thick fibreglass wool.  This process is shown in the photos.  To further improve this, instead of mounting it directly on the wall, it is spaced out from the wall by approx. 6cm.  This lets the absorbent fibreglass effectively work twice. Once on the direct sound waves and then a second time on any soundwaves which pass through and are reflected back from the wall.

The combination of the two treatments of the rear wall substantially reduce reflected sound coming back to the listening ‘sweet spot’ in the room.

Impact of changes

While you can certainly ‘feel’ that the room is now less reverberant,  changing the absorbency coefficients for the rear wall, and one of the side walls where thick curtains have been hung make the calculations look like this:

Room has these surface  areas (in square m) Area Absorption Coefficent Absorption per surface
 wall 1 (side) 7.9 0.3 2.38
wall 2  (side)  7.9 0.1 0.79
wall 3 (front)  7.2 0.02 0.14
wall 4 (back)  7.2 0.4 2.88
floor 9.9 0.4 3.96
ceiling  9.9 0.03 0.30
 Calc’d total  Surface Area of the Room             (square meters)   50.0   10.45
       
and it has a total volume (in cubic meters)        
Total Volume of the Room 23.8    
       
Using these calculated numbers we can calculate the room’s total absorption in Sabines      
Area (per surface) x Absorption Coefficient 10.4 Sabines  
       
 the RT60 for the room can be calculated as        
(0.161 x  Total volume of room) /  Total Absorption  0.37 seconds  

An improvement from 0.79 seconds to 0.37 seconds is remarkable. and I’m not sure I fully believe it. Whether this is actually what’s been achieved I can’t exactly tell. However, subjectively as you walk into the room you’re certainly aware of entering a far less reverberant space than any other room in the house.  

 

Further planned treatments

I plan to purchase cork tiles, or some other form of cork to treat the front wall and reduce direct reflections from the front wall.   At some time, I will also make a 1m x 1m skyline diffusion panel and swap it for the absorption panel – simply to hear what the difference is.  If I can borrow or buy a calibrated mic I’ll make actual measurements to back up the predicated/calculated  and subjective changes I’ve made.