Trappage of the Bassage

So, some years ago I attended Sydney uni to do a Masters degree in Design Science (Audio). Basically they (the academics who ran the course) intended it to be a post grad coursework program for folks interested in acoustics. It was run by the dept of Architecture, so yes, it had(has) good acoustics cred. Problem was, acoustics is a tricky subject and after studying it part time for a year, on top of full time work, I decided it was better left to folks who were far more interested in it. So I did all the TV and film electives and still graduated. Possibly to the chargrin of my fellow acoustic students.

Point is, one of those wonderful fellow students went on to greater and greater feats in the field of acoustics, is currently doing his PhD at the old alumni and wanted some more data from the field for his Phd. So David Spargo, of Praxis Acoustics, has been my guiding hand throughout the build of the new mix room. We’ve bounced ideas back and forth, he’s taken quite a few measurements of the room for his Phd (before and afters as we progress) and Ive been mostly cutting back on all his dreams to meet my budgets and no doubt depressing him immensely.

So, from watching and learning as David used Fuzzmeasure to analyse the room, we noticed a few key room modes popping up.

David using the “dummy head” mic to measure the room

So, how best to tame them?

Panel traps and tuned bass traps was the answer. The two best tools for the maths involved were this spreadsheet and this link to a list of specifications on insulation. I could then punch in the numbers of the various Bradford products available locally, such as Ultratel and Fibretex 350 and see what results I would theoretically get from various sized panels.

First the killer mode in the room, 60Hz. It simply wasn’t decaying like it should. This was visible in the waterfall plots taken using Fuzzmeasure. So, for a specific frequency I turned to Helmholtz resonator traps.

Construction was fairly straight forward, if laborious, involving many many 2mm holes into a panel of MDF…

Grids for the perforated absorber

Drilling the holes for the perf panel

3 ozito drill battery recharges later...

I then needed to mount the absorbent material inside the trap, but with an air gap. I decided to leave the air gap at the front of the panel and the absorbent at the back, for ease of construction. Basically attaching it with a membrane of material loosely into the cavity, then batts, then seal it up with more upholstery material…

laying the batts between 2 membranes of material to hold them up

As above, then repeat…Now I had 2 perforated panel absorbers, tuned to 60Hz with a fairly tight Q. I then built two panel absorbers, with the stuffing in front of a larger cavity, that should be effective down to 100Hz thereabouts.

Once placing them in the room, I did some checks on where they were most effective, again use fuzzmeasure, before settling on the back wall array as seen here:

The new rear wall of bottom end absorption

The difference with them in place was, as you’d expect, night and day! I had to completely re-calibrate the subwoofer split coming from the satellites to the LFE channel in addition to fairly dramatic gain changes in the LFE levels. Once there, the bottom end was tight, controlled, articulated and everything I could have hoped for!

The top end was now less masked by room modes and on consultation with Andy Stewart, Brad Watts and Chris Holder from Audio Technology magazine, it was felt some more absorption aimed just below the sibilance range was in order.

I then took two wall panels apart and bolstered them with 50mm of Fibretex 350 rockwool, and lo and behold, that was taken care of as well!

More to come shortly!