When building a vocal booth, foam can be used to tame high-frequency reflections, while reintroducing some reflective surfaces (in this case old CDs) can counteract an excessively dry sound.
I'm having a small vocal booth built within an open-plan office to record voiceovers for games and DVDs. The main reasons for building a booth are to keep the sound of the air conditioning and the PC out of the mic, and to reduce the background noise of office staff charging past on their way to buy sandwiches! I think the higher fequencies will be OK, but lower frequencies, where male speech is pitched, travel well through this building.
I'm told that the studio in our last office had a floor floated on sand. It had a two-door atrium to cut out noise but it was small, with a tiny rectangular booth and a larger listening room about 8 x 10 feet, rectangular but with an undulating ceiling. The walls were plasterboard on a wood frame, hung with fabric stuffed with Rockwool. The same builders are available, but if anything we have less space here in the new office — the total available area is 18 x 10 feet. But we do have a floor that lifts so we can make a floating floor flush with the rest of the office.
So, what shape should a tiny booth be? Should the mic face into a corner, and is Rockwool behind fabric enough acoustic treatment or would some corrugated shaped foam on the walls help? We're making it free-standing to please the landlord. Should we just float it on neoprene to isolate it from the surrounding floor? As for the listening room, will foam provide better diffusion than Rockwool? Does the shape matter in such a small room or will the lower frequencies go right through the walls unheard? Or should we not have a booth at all, and instead create one larger room and isolate the sound of the PC using a cabinet? The benefits of booths seem to be debatable...
Jon Chan
Editor In Chief Paul White replies: I think that if your main business is voice recording, then a vocal booth would be desirable, and as voices have relatively little low-frequency content, you shouldn't get too many mode-related problems in your main control room as long as you put in some basic acoustic treatment. Without a booth you may have problems isolating the vocal from unwanted sounds, but you're also right to be suspicious of booths as badly made ones can sound dreadful.
Vocal booths need to be very well treated to avoid them sounding boxy, so you need to allow for at least six inches of acoustic treatment on the inner walls and ceiling. A plasterboard and studding construction with a double-glazed door is fine in most applications of this type and you can float the walls of the box on neoprene to add some isolation from the floor. The inside floor can be created by using high-density Rockwool slab and then resting chipboard directly on it with a felt or rubber buffer between the edges of the floor and the booth walls. Two layers of chipboard, glued and screwed, work better than one. Filling the space between the inner and outer plasterboard skins with Rockwool insulation will help improve sound isolation, as will using two or more layers of plasterboard on the outside of the booth. I'd be reluctant to make the finished internal size of the floor of the booth less than around 4 x 5 feet though it needn't be rectangular. There's no easy or cheap way to supply a constant, quiet air flow to such a booth while maintaining isolation, so opening the door regularly between takes is probably the most pragmatic solution.
A practical way to treat the inside is to use more high-density Rockwool (a minimum of 30mm thick) spaced away from the plasterboard walls by at least a couple of inches but ideally four inches. You can do this using wooden battens for support as high-density Rockwool is quite rigid. This air gap will reduce the boxiness that you may have experienced with your original design. Put fabric or visually attractive acoustic foam on top of this to contain any loose fibres, then use a perforated MDF sheet (the type used for covering radiators) to cover some of the side walls at head height as this will allow some high end to be bounced back into the booth. Without some high-frequency reflections, the sound may be too dry, but you can always add more HF reflectivity later by putting up more perforated sheet if necessary. During our Studio SOS visits, we've previously used old CDs taped to the walls at head height to improvise some high-frequency reflection. If you have room to hang a heavy curtain or rug behind the speaking position this will also help kill reflections. Normally the artist will use a cardioid mic and be facing the glass in the door so no significant reflections will reach the mic. It doesn't really matter whether there's a flat surface or a corner behind the vocalist as long as the rear and side surfaces are mainly absorbent.
For the main room, you'll need to place absorbers at the 'mirror points' at the very least. Again Rockwool slab spaced off the wall and suitably covered works well as does sticking acoustic foam to Rockwool or using thick foam spaced off the wall on blocks. In a room that size you'll probably find that a square metre of absorber above your listening position and on the walls at either side will make a big difference. You may also need absorbers on the wall behind you, especially if it is fairly close, otherwise shelves and furniture may scatter the sound adequately. If the walls are solid, you may also need some bass trapping, which can be accomplished by building more Rockwool slab traps across the corners or between the walls and ceiling. The triangular space behind these can then be stuffed with Rockwool insulation to improve performance but even without this you should get good results. If the budget allows, Real Traps Mini Traps are good in this application and save a lot of DIY work.
If you search our web site, you'll find a lot of practical articles on acoustics and soundproofing that may be helpful to you.