Passive room treatments work well, and are the preferred method of room correction. However, there are circumstances where passive treatment simply is ineffective or impractical. The solution is then to use active controls.
The most effective form of active control is to utilize a parametric equalization. Parametric EQ is preferred over 1/3rd octave because of it's greater flexibility and control. Parametric equalizers range in price from $135-$5,000+ depending on their sophistication, accuracy, and sonic signature.
In order to use active equalization it's mandatory to have a basis for adjusting the controls. This is usually created by measuring the frequency response of the room. Near-field (3" or less), 1 meter, or GP (ground plane), are measurement distances. The least expensive and most simple method of measuring loudspeaker performance in a room; is by using a device like the Radio Shack SPL meter, while sending sine waves through the loudspeakers. The resulting meter readings can be plotted in Excel, and the FR (frequency response) of the loudspeaker's performance in the room can be easily seen.
If one wants to use their PC as a tone generator, the NCH Tone Generator by Swift sound is a great program ($20). The newest trial version (it has an expiration date) has a provision for creating and saving multiple tones to your hard drive as .wav files. Those files can then be burned to create a test CD. I have a copy of the older freeware version of the program. It's only a tone generator and as a result lacks the ability to automatically create multiple tones or save tones as .wav files, but it never expires. (email for info)
For those that don't have a PC available, There are many commercial CDs available with test tones burned on them.
Note that sine waves should not but run at high SPL's or for long durations.
One of the better investments for those needing help to EQ lower frequencies, is the Behringer DSP1124P. Available for as little as $120, this device offers 12 bands of stereo parametric EQ. It is a very flexible and powerful tool. But the supplied instructions leave something to be desired. Also, it should be noted that the Behringer unit is fine for subwoofer frequencies. Since it digitizes the signal it's not recommended for use with full-range audio signals
Sonnie Parker's archived SnapBug Website goes into great detail on the set up and use of the Behringer DSP1100P/1124P. Here's a LINK to a 820k .pdf copy of the Snapbug website. Here's a link to the Excel Worksheet talked about on Sonnie's website. Note that it has corrections for the Radio Shack SPL built in.
Another somewhat less intimidating website with info about using the Behringer DSP1124P is the Brinkster website. The original website has been taken down, but an archived version is available at THIS link from the Wayback Machine.
For those using the Behringer DSP1124P, Anthony Gomez has created a neat little program to program the equalizer. The program can be downloaded HERE
There are many good software programs for predicting/evaluating/measuring loudspeaker performance. CARA, ETF 5.0, and RPG Acoustics Room Optimizer look at loudspeaker placement and the interactions in with room.
TrueAudio's TrueRTA is an excellent program. The 1 octave freeware version will give the user a good idea of the program's abilities. The 1/24 octave version is an amazing tool when married to a proper microphone. The latest version has a .cal file for use with the Behringer ECM 8000 mic (see discussion below)
Speaking of microphones. People are stunned by the cost of true calibrated measurement microphones, that being several thousand dollars each. The cheapest usable microphone system is the RS SPL meter. It has an RCA output that can feed the signal to a computer soundcard for use by various software programs. There are limitations and draw backs to it's performance. Some of these can be decreased by integrating the correction curves into the FR plots. An interesting side note is that ETF 5.0 has a calibration file for the RS SPL meter
Below are the correction values for the RS analog SPL meter. Add/subtract the values to your measurements obtain a more accurate plot.
10Hz add 20
12.5Hz add 16.5
16Hz add 11.5
20Hz add 7.5
25Hz add 5
31.5Hz add 3
40Hz add 2.5
50Hz add 1.5
63Hz add 1.5
80Hz add 1.5
100Hz add 2
125Hz add .5
160Hz add -.5
200Hz add -.5
250Hz add +.5
315Hz add -.5
400Hz add 0
500Hz add -.5
630Hz add 0
800Hz add 0
1kHz add 0
1.25kHz add 0
1.6kHz add -.5
2kHz add -1.5
2.5kHz add -1.5
3.15kHz add -1.5
4kHz add -2
5kHz add -2
6.3kHz add -2
8kHz add -2
10kHz add -1
12.5kHz add +.5
16kHz add 0
20kHz add +1
For those interested more information about the RS meter there are multiple websites with that information. Instead of my posting the links please do a Google search for the sites using 'Radio Shack SPL meter' (no quotes in the search text) as the search topic.
There are other options for relatively inexpensive microphones useful for loudspeaker testing.
One such mic is the Behringer ECM8000 is a condenser electret, widely available for under $40. It requires 48volt phantom power and a mic preamp for it's operation. The Behringer Eurorack MX602A approx $70 retail, is a modestly priced unit that functions as a mic preamp with 48+ volt phantom power.
We have done a comparison tracking of the ECM 8000 against our laboratory standard B&K 4133. The little Behringer does a surprisingly good job. The top end rolls off above 12Khz but other than that it's pretty accurate.
The lowest price true calibrated microphone available is the $249 IBF-EMM8 Electret Measurement microphone sold by ETF Acoustics. The price includes their IBF-MP21 microphone preamplifier and a cal file referenced to a B&K 4133.
For those adventurous souls interested in building a DIY mic and mic preamp, there are plans on the net. Most use the little Panasonic electret capsules sold by supply houses like Digi-Key. Again a Google search will turn up multiple links to these projects.
This concludes a basic introduction to modestly priced equipment and software used for room analysis, loudspeaker measurement, and room correction.