Sound proof box 2
Still working on the isolation box. Today I made measurements with chirps inside moving the microphone around to see where sound is leaking out. I found that sound intensity inside the room has three general peaks:
- A low frequency peak corresponds to room resonance (near 150Hz?)
- Many mid frequency peaks (200-1000Hz) which seems to correspond to a rattle frequency of the snare
- A high frequency which seems to be a resonance of the speaker horn
Measuring the volume in the garage at various places it seems that the back of the box has a strong coupling of low frequencies to the back wall. The front of the box radiates more probably because of the placement of the speaker. There are subtle differences between the frequencies that get through the front door beam, panels and windows.
Looking at the problem frequencies up close with the SPL meter I find that
- The low frequency peak gets the whole room vibrating - walls and windows. Using the SPL meter I get
- 66dBA in the room
- 60dBA at the window (6dB attenuation)
- 59dBA at the front door panel
- 64dBA at the crack at the bottom of the door
- 56-58dbA on the side walls
- 63dbA at the crack at the bottom of the side walls
- 60-63dbA at the back wall
- 51-55dbA at the garage door
Sound levels seem to rise near the floor by 3-6dB perhaps from the focusing effect. There may be some ground vibration, however.
At these low frequencies, the isolation booth is largely transparent. We really have only 10dB isolation between the inside and the garage door, when really we want more like 20dB. Fixing cracks might buy us 5dB at most. We really need to push the resonance of the ply away from the resonance of the room. Probably looking at a second layer of ply inside the room with some kind of dampening fill. This should buy us another 10dB or so.
At the higher siren frequency (2kHz?) I measure
- 66-69dbA in the room
- 55dbA at the bench
- 52-55dbA at the garage door (15dB atten)
- 50-54dbA at the side wall
- 60dBA at the front door/window
- 65dBA at the crack
- 50dBA at the back of the box (16dB atten)
At this frequency it seems like the cracks matter but the window only attenuates the sound by 6-7dB.
Response to the high cheeep (6kHz) peak is similar, with about 10dB of attenuation at the front window and door but almost zero attenuation at the cracks.
Todo:
- Try to deaden the inside of the box and remove resonances that way
- Buy carpet strips for bottom of box, gasket bottom of door
- Install door latch and gaskets
- Buy plywood 6 sheets
- Buy 1-inch ply mounting screws