Low-Mid Absorbtion - I'm missing something

[kEVINk]

I have some questions about low-mid absorption that I’m hoping someone can help me with.

Specifically, on the John Sayers site, he refers to hanging panels (Dacron over MDF), usually shown in a ceiling cavity (for example, of varying lengths in the back of a control room). Please note that I am not referring to what some people refer to a bass traps, which, as I understand it, are triangular shaped wedges that are applied to the corners on the finish side of a room. If I am using the wrong terms, please correct me.

So, while I think I understand the hanging panels in concept, in execution I’m still a bit in the dark. My questions are:

1. In one drawing on the John Sayers site, these low-mid panels are shown under a recessed studio monitor with a “port” at toe level, an opening for offending frequencies to go in and die. By this I assume the wall around the panels is something solid; say two layers of GWB of different thicknesses.

2. In another drawing, cut through the control room, shows the back of a control room with a ceiling sloping down toward the back of the room, with the ceiling space above the sloping ceiling filled with low-mid panels of different lengths, alternating in directions. At the back studio wall, these panels extend down to the floor in the cavity. The site gives a recommended width of panel (I think it was around 18"). This section does not show ports.

3. This is where I get confused. Are these panels in a “sealed” cavity between rooms? Or do they need ports for the sound to get in? What if the wall separating them from the studio was covered with some sort of “acoustically transparent” material? (For example, we have access to 100 yards of Marshall cabinet grill fabric.....don’t ask).

4. Is there a minimum size for these to be effective? For example, we are working in a basement with exposed ceiling joists. The depth of the joist is 8". My thought was to fill this space with very shallow panels in alternating directions, then to apply a fabric cover to the ends of the joists, covering the panels. Closer to the walls, the panels get longer. Is a panel of 8" worth it?

5. Is there a way to compute the size or area of panels needed; or, as I suspect, do you just put in as much as you possibly can?


I know I’ve asked a lot; thanks to anyone who is able to help

k

 

[John Sayers]

OK - the size of the hangers determines the frequency, i.e. small panels working around the low mids, larger/longer panels working on the low frequencies. They are normally used on open faced fronts getting bigger as they move toward the corner so the largest are in the corner. If you look at a Tom Hidley plan where he starts with a 35' x 35' space he will have cavities 3 - 4ft deep with many hangers of differing sizes, but you never see them as they are behind either timber panelled walls or cloth fronts. He will sometimes take up 1/3 of the floor space. The ones behind the timber walls are for the extreme low frequencies (below 100hz) and he claims they go down to 20Hz.

In the HR studio we usually can't afford the enormous space they take up but the ceiling is often an area that can be used...like in the pictures you mentioned in that small booth on the construction site. In that circumstance the sizes used would absorb around 80 - 250hz and the additional slot resonator would work from 250 - 800hz where finally the Insulco fibreglass would absorb up to 20Khz. The aim here is to have even broadband absorption across the frequency range.

The ceiling hangers you refered to are the low-mid range whereas the longer/bigger ones in the rear wall are the for the lower frequencies. The hangers also absorb the high end due to the mass of dacron. The timber slots work better for the low mids for me because they retain the highs and diffuse them so I have used them throughout the studio. When you walk into these rooms they still sound bright and clear yet they have a low reverb time.

I've actually just come upstairs after recording the first band in the studio. It was acc bass, drums and piano....jazz. It worked brilliantly. We were all impressed, it was relaxed and comfortable and the drums, totally flat, sounded great. There was no room sound, it was almost transparent. The bass in the smaller room was tight and true and just the single door was adequate separation. The piano had the other room and sounded great also. We had a great day.

Anyway I hope this sets the ball rolling on your questions.

cheers
John

 

[kEVINk]

Thank you very much for taking the time to reply. I'm not the first to state how much help your web site has given me in learning about studio design.

So let me see if I understand this correctly. The panels can be contained behind a 'rigid' cavity, or behind a fabric covered wall, which would allow them to do some high end absorbtion as well.

If the panels are contained behind a 'rigid' cavity, the bass will be abated based on the rigidity of the wall system in question. Based on the Construction section of the web site, substituting steel studs for timber studs, minimizing stud thickness, and increasing spacing, will INCREASE transmission loss, transmitting the sound energy to the cavity containing the low absorption panels.

I guess I'm still unclear on the need for a port, or why it's used in some locations and not others. Pardon my ignorance, but could you explain what you mean by "open faced fronts"?

 

[Razor]

Howdy,

I was hoping it might be possible to get more detail on the "hangers" mentioned above. I can't seem to find reference to them on your site, John.
It sounds like they might be perfect for my little set-up here. There are some nodes in the 150-300 Hz range that I would like to tame.


Thanks to all for all the great posts,
Charles