Removing echo in a 34m³ room with two slanted walls

I forgot to add a picture showing the recording setup. Here it is:

Hi Rock, thanks for your reply


I'm using an omnidirectional lavalier microphone. All the measurements were done using that microphone at its intended position and at the nominal recording levels (targeting -12 dBFS on the digital recorder). Attached  to this message there is a picture showing my setup and a sample sound.


Yes, I understand. My initial plan was to build a set of fiberglass panels and hanging them on the 2 parallel sidewalls. I went to a similar setup on the ceiling. Then I was told I couldn't do anything satisfying in such a small room without "activated carbon diafragmatic absorbers" and "Schroeder diffuser". I can understand those solutions might be better, but this is absolutely not the same budget as DIY fiberglass panels. I'm glad you bring back hope for something more affordable.


I notice you didn't mention the slanted walls. I thought they were a major problem. This may be naive, but I imagine the sound bouncing back and forth between the ground and those two opposite 45° walls. I even made some calculations: I found the echo I can see on the measures between 15 and 45m was consistent with the time needed for the sound to travel from the speaker to the front 45° slanted wall, down to the floor, and back to the mic. [ (4160mm+1100mm)×2/(343m/s) ≈ 30ms ]


Good idea to make the backdrop an absorbing panel! Concerning the ceiling: is there any advantage in suspending a cloud absorber rather than fixing a panel on the ceiling? I know when you mount a panel with some air gap it improves lower's frequency absorption. Is it for that purpose?



I record from a Talent-attached Rode Lavalier mic wired to a PCM recorder. That's what I have at hand, and I successfully used that setup in another location. I do not use the camera's mic. I made all my testings using those gears. I don't really need to be able to record anywhere else in the room. As long as I can record from the intended position, that's good. I have a picture showing the expected recording setup viewed from the camera's position if that helps:




Since the project was dormant, the room was "repurposed" as a storage room. It is no longer completely empty. I recorded a sample sound for you:
Voice: test.wav (1.66 MB) Hand clap: clap.wav (285.67 KB)

It seems better than when the room was 90% empty. But there is still audible reverb. Or am I wrong?


At some point I had something like that in mind:


The idea was to somehow (how?) absorb a max of energy when the sound reaches the floor after having bounced on the 45° front wall. Once again I was told it didn't make sense, and I gave up.


I hope I answered your question and made my project clearer!
Regards,
- Sylvain

Hi Hexspa,

Thanks for joining the conversation.


I'm not sure to understand.

• Do you mean putting absorbing panels also on the 45° walls? Maybe that's the only solution, but I do not see quite well how that could look in practice. Do you have pictures of such realizations? How to deal with the roof windows? Do you cover them as well?
  
• Or are you suggesting a "box-in-a-box" design, basically building a square recording booth inside the existing room?

Hi Sylvain,

I was checking back in and I re-read your questions to Hexspa: The "Box in a box" is usually used for sound isolation BUT as far as I can tell, that's not what you need. However, the moveable panels I suggested above IS an incomplete way create a "box" around your talent and camera using only acoustically transparent absorbers.

To take that idea to the next level, it is possible to actually build a "box" using only acoustical absorber material supported by 1x4 wood framing (which is thinner than the usual 2x4 wood faming). No plaster or drywall would be used as you will let the sound pass through to the outside room. Any sound passing through to the outside room will be reflected back to the "Box" but will pass through the absorber a second time and be further attenuated. The room won't be "sound proof" but will attenuate sound passing both ways; in and out.

For this idea, I would suggest using 4" thick rigid fiberglass or mineral wool. The ceiling would be easier to have absorbers attached directly to it. Using plaster or drywall will reflect any sound not absorbed back into the "Box" and cause a "boxy" sound from LF room modes, by eliminating rigid drywall, eliminates LF modes at the expense of not having as much isolation which you don't need anyway.

As you can tell, this "box in a box" variation is theoretical but I do in fact use movable panels surrounding the vocalist for recording vocals in my multipurpose room. If you do try it, you may save some money on absorber material. 

Here's some panels I use. The basic design is frameless. The ceiling clouds have a stiff wire screen called "Hardware Cloth" on the bottom (hidden by the fabric). Suspension wires hook on the "hardware cloth" and pierce the absorber, emerge on the top and fasten to the ceiling.

There's a loose one you can see leaning against the drum set. The ones in the stands are technically frameless too but you can see that the stand creates a frame on the bottom and halfway up the sides. Cutting slots or holes in the wood frame can allow some extra absorption.

One of the drawbacks of these is that the fabric is not stretched so the aesthetic is compromised. The fabric is cheap economy landscape fabric. The advantage of a frameless panel is a 25% increase in surface area (for a 4" thick panel).

As shown here, with a live band playing altogether, there is minimal attenuation or isolation provided between instruments but we mainly record rehearsals for playback and don't intend these recordings for commercial release, but since my interface has 16 analog inputs, we can track everything to separate tracks... it's just for fun but with some processing, we can get some decent mixes. But with overdubbing vocals alone, these movable panels do a good job of eliminating room reflections.
 

Pretty nice stands, rock!

How do you wrap the fabric around the fiberglass panels?

EDIT: Hexspa thanks for the link.

I admit that I don't really like the way mine look but I consider them "temporary" even though they have been in use for years .

I rolled out the landscape fabric on the dining table and trimmed it to length to overlap about 4" more than 2xL + 2xThickness and width 1xW + 2xT.

I placed the mineral wool in the center and wrapped the length around and fastened with E6000 adhesive.

Then wrapped the long edges and fastened with E6000 adhesive.

I actually used 2 layers of cheap LS fabric 'cause you can see the greenish color through one layer.