An Introductory Guide to Subwoofers 
Measuring the frequency response of your subwoofer
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Deciding what measuring equipment to buy...

There are a couple decisions that when made earlier rather than later, make this measurement process less expensive and less complex. 

The standard procedure is to buy the Radio Shack or similar sound pressure meter, then use it to set up the system. This is a ~$40-$50 investment. It's the least accurate way to make measurements. 

Next for consideration is a system using a real test microphone. The Behringer ECM8000 or it's twin the Nady CM100 retail for $50, These are almost as accurate as some extremely high-end professional test mics. 

Using a test mic requires a choice, use computer based software measurement system, or a stand-alone hardware based measurement system. 

With the PC based system the user must buy a mic pre-amp with phantom power, have a PC with a duplex sound card (this can be internal or USB) and appropriate software. This is the technique for geeks and those with a propensity to tweak and fine tune their systems.

With a stand-alone measurement system, the mic is simply plugged in, and the unit is powered up and testing begins. All the equipment to run the mic is inside the device. And no software is needed. This is the approach for people who want to spend more time listening to their systems that adjusting with them.

The least expensive all-in-one device for measuring and EQing a subwoofer is the Behringer DEQ2496
This device combines a Real-Time-Analyzer, Parametric EQ, Graphic EQ with most sophisticated functions like AutoEQ, and DynamicEQ. More information about it's functions and operations are posted HERE

Regardless of which method testing method is used the basics presented below apply. Obviously if one is using something like the DEQ2496 one doesn't need to use Excel to create the plots. If one is using a more advanced software based measurement system the xcel plots aren't needed either.

The technique below based on using the RS meter and Excel, so adapt your technique according to the tools you decide to use. _________________________________________________________________________________________________

Initial Measurements, don't work in a vacuum, start with a near-field plot... 

After you've established the optimal location for your sub, you need to measure the near-field response. Near-field means putting your microphone or SPL meter 2" or so in front of the woofer's dust cap. Now make your same 1/6th octave measurements of the woofers output.

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.
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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.

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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