Quote:
Originally Posted by St. Kelly  Care to comment on the different between straddling the corner with a 6" thick board, verses "filling" the corner with a triangle shaped "super chunk" trap? |
Leaving a gap, which is what the triangular space, is done in home studio building in order to save money on material. Sound is reflected to a certain degree whenever the sound goes through a change in the medium it is in. Air, dense insulation, thin insulation, are all different mediums to sound. Denser insulation has greater acoustic resistance per inch of thickness than lighter insulation. To absorb effectively at low frequencies, the absorbent material has to be where the sound wave velocity is high. All sound waves have zero velocity at hard surfaces, like a corner.
Putting the two together a denser absorber will be thinner while having some refections from the inside surface of the absorber, and be more reflected by the material. Filling the space with a lighter material will have no reflections inside and less will be initially reflected by the absorber.
Note that the internal reflection is quite minor.
The big factor that is so often overlooked, even by acoustics experts is cost. 70x is the great informal material used for most absorbers. By the point where material is 4" thick, 701 (1.0 lbft^3) is more effective than 705 (5.0 or 6.0lb/ft^3 depending on which data sheet you use) at 125 Hz. All other things being equal, which they are not, 701 is one fifth the price of 705, material being the major cost. So at that point in absorber design, the more expensive material is less efficient!
A big factor also is the suitability of home construction insulation for filled bass traps. The cost savings here are due the different marketing channels. Home insulation is almost sold on a cash (or credit) and carry basis. 70x and similar products are through industrial suppliers where price includes service etc. The cost difference is amazing. In my part of the world about 12 to 1. 703 is around $4.00/ft^2 in 4" thickness and Roxul Safe n Sound (3.5" thick) is 27 cents per square foot.
Straight substitution is a cost savings of a factor of 12. A filled corner with Safe n Sound as opposed to 703 straddling the corner would cost one quarter in insulation costs and give better performance.
I used density in the writing above. The actual physical property that is important is gas flow resistance. For (a rather long winded) discussion on this topic
this thread will help.
I am trying keep this relatively simple, but few things in acoustics are simple and intuitive. I hope this helps.
Andre