in the previous update, i fretted about the 146Hz node in my listening room, and wondered if i could fix it. you can't just fix it by turning up the gain at 146Hz - it's like pouring sand into a black hole. the problem has to be fixed acoustically, so it's time to try bass-trapping.

bass-trapping is the process of shoving huge amounts of foam and/or fiberglass into the edges and corners of a room to dampen a room's low-frequency modes. it's a popular thing to do in listening rooms and/or home studios, and some people think it's the very first thing you should do. i'm not convinced. that much damping material is really going to cost you, and mounting it in a habitable space is quite non-trivial. but i like science, which means i have to try. i have to try to do it right.

so here's the experiment - get one block of acoustic foam at a decent price, mount it in the most theoretically sound place (ha), and see if the change is worthwhile enough to justify further expense. here we go!

question 1: where to put it?

i want to damp the 146Hz mode specifically. i have one big foam block, and it has to count. where should i mount it?

here's where some science comes in. sound travels through orthogonal fluctuations in pressure and velocity. my foam block should have the maximum effect if i place it in a high-velocity region of the 146Hz mode. somewhere in the room, the 146Hz mode has a velocity antinode. how can i find this antinode?

well, my speakers are pressure sources. my microphones are pressure sensors. and velocity is orthogonal to pressure.

so if i play a 146Hz tone through my speakers, and i can find a spot somewhere in the room where the sound pressure is abnormally low, that should be a high-velocity spot - the best spot to place my block of foam.

here's a needlessly complicated way to probe the room for sound pressure:

that's a Nexus 5 running AudioTool, connected to a calibrated Dayton iMM-6 measurement microphone. i probe the room with the phone mounted at the end of a long stick. it streams video (screen mirroring) through a Chromecast so that i can wirelessly watch the spectrum. and all of this works, but you know what.. forget it.

this is what i actually used:

[a $20 SPL meter and a long stick.]

the speakers are now blasting 146Hz. some rummaging reveals a spot in the room where sound pressure is >15dB lower than almost everywhere else. it's a winner!



[up there!]

up there is our low-pressure spot, which should also be our high-velocity spot. it's about 2 feet away from each wall, which is not very surprising. i mean, the wavelength of sound at 146Hz is 7.6 feet. it's like this:



[diagram of a 146Hz standing wave at a wall. a quarter-wavelength from the wall, velocity (red) is maximum and pressure (blue) is minimum.]

if 146Hz is reflecting off each wall, physics dictates that the lowest-pressure region is 1.9 feet from the wall. so far, everything makes sense. how can i put a foam block up there?

question 2: up there?

yes. all things are possible through my prodigious tripod collection.

[hammer a 1/4-20 T-nut through some scrap plywood, and up it goes.]

[it almost looks cool!]

question 3: does it do anything?

my first step is to place the SPL meter at each of my four listening positions, and record the loudness of the 146Hz tone with and without the foam block.

here are the results (before and after):

position 1: 51.6dBA to 50.5dBA

51.6dBA to 50.5dBA position 2: 53.3dBA to 53.3dBA

53.3dBA to 53.3dBA position 3: 50.7dBA to 50.1dBA

50.7dBA to 50.1dBA position 4: 54.2dBA to 53.6dBA

that's.. not good? the difference is so small that it might as well be noise. so let's break out the real microphones, and compared averaged room measurements.

[left speaker frequency response, averaged over 4 listening positions, with 1/6-octave smoothing. without (blue) and with (red) foam block.]



[right speaker frequency response, averaged over 4 listening positions, with 1/6-octave smoothing. without (blue) and with (red) foam block.]

aside from slightly less treble (i left the 8.7kHz filter on) they're essentially the same curve. the...