"Cult of the Infinitely Baffled"
IB Subwoofer (aka IB subs)- FAQ Pages
Generic 'how-to' guide for designing, building,
installing, and fine-tuning your IB subwoofer
Page 3
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Testing 1, 2, 3.....
There are 3 ways to measure the output of your sub.
1) Manual: requires a SPL (sound pressure level) meter, a signal source (test tones recorded on a CD, or PC with tone generator) and graph paper or a spreadsheet.
2) Semi-automatic: requires a test mic, mic preamp with phantom power, PC/notebook with a duplex sound card and a software measurement program
3) Automatic: buy a self-contained RTA (real-time analyzer) like the Behringer DEQ2496
We're going to start off with an example of the manual method. ____________________________________________________________________________________________________________________________________________________
Initial Measurements, don't work in a vacuum, start with a near-field plot...
Start by making a measurement of the near-field response. Near-field means putting your microphone or SPL meter 2" or so in front of the woofer's dust cap. Using pre-recorded test tones or a tone generator take a series of measurements. If you look at the left-hand column in the graph below you'll see 1/6th octave spacing.
The near-field response tells you the performance of the sub without the effects of the room. This information is then compared to the listening position response. The difference between these two plots allows you to see the effect the room is having on the performance of your subwoofer.
NOTE: Only play each tone for 5 sec or so, just long enough to get a meter reading. Using pink-noise test tones for more than a few seconds is hard on amps and drivers.
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Initial Measurements, taken from the listening position.....
Once you've completed your near-field measurements, it's time to measure the performance at the listening position. Same procedure as before, but with the meter or mic located roughly at ear level.
Understand that moving the mic or meter as little as 12"-18" one way or another, can result in significant differences in the output levels. This is of course a factor of those room modes we talked about on page one of this guide.
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Now, let's compare those two plots.....
Compare the near-field plot with your listening position plot. The differences between these two plots is the effect the room placement is having on the performance of the sub. The easiest way to compare your plots is draw them using the printable plot paper, or use the Excel spreadsheet. If you're using the Radio Shack SPL meter download this CHART, it contains the corrections for the meter. If your not using the Radio Shack meter download this CHART it has no corrections. Both spreadsheets have the ability to make 3 separate plots, nearfield, listening position without EQ, and listening position with EQ.
Below are nearfield, (blue) and farfield (13') (magenta), plots from my small IB subwoofer. This makes it very easy to see what effect the room is having on the frequency response. The increase in output is a function of what's called 'room-gain'. Certain frequencies are boosted by the interaction of the sound waves and the geometry of the space/room.
Below we see the listening position (green), compared to the farfield 13" (magenta) compared to the nearfield (blue). The listening position is approx 3' further away from the sub and mic is approx 1' higher than the farfield plot. Note that the measurements for the green plot were taken at a slightly lower output level, so the plots wouldn't overlap.
This shows why trying to obtain a 'perfect' curve for a single position isn't a particularly good idea. Generally speaking before adding EQ, it's best to sample several locations in the room. Then average those locations and dial in EQ that works best for that 'average' location. _____________________________________________________________________________________________________________________________
Here are some links to additional spreadsheets. This first LINK is to a sheet that will plot 5 different graphs. It was made for IB users, so it has lower frequencies than those normally included on standard spreadsheets. Also it does not automatically include the RS corrections, but it lists them on the side. Finally HERE's another spreadsheet that will automatically average 3 different plots. And again it does not automatically include the RS corrections.
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