Subwoofer Equalization

  1. Why do I need to equalize my subwoofer?

    The subwoofer is probably the piece of equipment in your home theater that is most effected by room acoustics. You may wonder why a sub you heard in a store sounded great, but when you got it home it sounded boomy. The reason is probably your room and it's effect on your subwoofer. While your room can cause effects that can't be fixed even with an eq, an equalizer can go a long way towards improving the sound.

    Here is the in-room response of my subwoofer (Stryke Audio 15.2):

    Figure 1.1 - In Room Response of Un-corrected Subwoofer

    As you can see in Figure 1.1, my subwoofer measures very poorly in-room. Without the correction, it sounded boomy and unnatural, as if it played only one note. I actually prefered my shiva based sub in the same room without correction.

  2. What equipment do I need to equalize my subwoofer?

    In addition to your existing home theater equipment, you will also need a measurement microphone and some kind of parametric equalizer. This paper will focus on how to equalize your subwoofer using the following equipment:

    Behringer Feedback Destroyer

    Model: 1124P
    Available for purchase here: click here

    What is it?
    24-Bit DSP Based Parametric Equalizer

    Acoustisoft ETF

    Software Version: 5.8100
    Available for purchase here: click here

    What is it?
    Software based measurement tool for Windows based PC. This software has a special signal designed to obtain better low frequency measurements.

    Radio Shack SPL Meter

    Model 33-2050
    Available for purchase here: click here

    What is it?
    SPL Meter with built in preamp. This is used in conjunction with the ETF software to take measurements of the frequency response of your sub. It is somewhat inaccurate, however this can be overcome (not completely) by using a series of corrections at different frequencies (more on this later).

  3. OK, I want to equalize my subwoofer, but how do I do it?

    At this point, I will go through a step by step process on how I approached this task:

    1. Microphone/SPL Meter Setup:

      The first thing to do is to setup the Radio Shack SPL Meter in the listening position. Try to ensure that things are not directly behind the microphone that could reflect sound. The ideal solution would be to put it up on a mic stand. For my testing, I decided to point the meter in the direction my ear would face when sitting down (I chose the ear that faces the subwoofer).

      *Side Note: I took measurements in many different places throughout my room. Every place I measured, the response looked different, although the 16dB peak in my response was found in just about every measurement. After equalizing the subwoofer, it will probably only sound perfect in the position where you measured from. For this reason, it is best to figure out the spot in your room where the subwoofer creates the flatest frequency response before continuing.

    2. Equipment Setup/Connections:

      After you have positioned the SPL Meter, connect an RCA cable to the side of the SPL Meter, this is the preamp output. Run this over to your computer.

      This is a good time to get familiar with the ETF software. When you first run the software, there is a Getting Started guide that takes you through the setup process. Go through these steps until you get acceptable levels from the mic and soundcard output.

      After ETF has been properly setup, you need to hook up your soundcard to measure the subwoofer. To do this, you will need some RCA Cables and 2 Stereo RCA Y Adapters from Radio Shack. Start by plugging one Y adapter into the Line Input and the other into the Output of the soundcard. Then connect the left output of both Y adapters to each other using an RCA cable. Now take the RCA cable coming from the microphone and plug it into the Y adapter that is in the Line Input on the sound card. Then connect an RCA to the Y adapter in the Output of the soundcard and connect it to the input of the Feedback Destroyer using a Radio Shack RCA to Phono Plug Adapter. Using the same type of adapter go from the output of the Feedback Destroyer to the input of your amplifier. Doulbe check your wiring with Figure 1.2. You should now be set to take measurements.

      Figure 1.2 - System Connection Diagram

      Once the connections are done, we can finish configuring ETF to take subwoofer measurements. Make sure that the Measurement Type is set to Two Channel and that Test CD is unchecked. Select Low Frequency in the Bandwidth box and 0.68 sec in the Measurement Time box. Your measurement screen should look like Figure 1.3 when complete.

      Figure 1.3 - EFT Measurement Settings

    3. Microphone/SPL Meter Correction Files:

      Most good measurement microphones include a file that is designed to correct for response irregularities in the measuring device. While the Radio Shack SPL Meter is not exactly perfect, there are some reasonable correction curves available on the internet. I could not get this to work in the demo version, but here is a Radio Shack SPL Correction File that can be used with ETF for Low Frequency Measurements:

      Radio Shack Low Frequency Correction File for ETF 5.0

      10.000   20.000   0.000
      12.500   16.500   0.000
      16.000   11.500   0.000
      20.000   7.500    0.000
      25.000   5.000    0.000
      31.500   3.000    0.000
      40.000   2.500    0.000
      50.000   1.500    0.000
      63.000   1.500    0.000
      80.000   1.500    0.000
      100.000  2.000    0.000
      125.000  0.500    0.000
      160.000  -0.500   0.000
      200.000  -0.500   0.000
      250.000  0.500    0.000
      315.000  -0.500   0.000
      400.000  0.000    0.000

      You can download this file here (right click to save).

    4. Taking your first measurements with ETF:

      Now click the Level Check button and ensure that the Recording Level for both Channels is OK. It will show up in green and read Level OK. If it doesn't, change the gain controls for the soundcard (both input and output levels may need to be changed) until it says they are OK.

      It is now time to take the first measurement of your subwoofer. Make sure that the Feedback Destroyer is setup to bypass the filters. Usually this is done by hitting the on/off button on the top right. The green light should not be lit for this measurement.

      Once that is done, hit "Start Test" on the measurement panel in ETF. After the signal has stopped, hit the "OK" button. You should see the frequency response of your subwoofer. Repeat this measurement at least one more time and make sure that the two measurements agree.

    5. Building filters to correct the response:

      Behringer includes some software that can be used to model the effect of the filters you create. Unless I missed it in the directions, there is no way to import your frequency response into the program to see how it will actually affect your curve. In addition, there is no way to limit the plots to low frequencies. I opted not to use this software.

      Here is how I designed my filters:

      First, start by selecting the peaks and dips that you want to fix. Determine where the middle of the peak or dip occurs by clicking on the curve. The frequency will be displayed on the right along with the SPL reading at that frequency. Use the chart in Figure 1.4 to determine the frequency setting for your specific frequency.

      Figure 1.4 - Frequency Conversions for BFD

      Make note of how many dB (to the nearest dB) the signal is from your reference SPL Level (GAIN). Now determine the how wide this filter should be (Bandwidth). This is somewhat tricky since the measurement is done in octaves, so it may require some quess work. For the first filter, just select a number between 20 and 60.

    6. Entering the filter into the Feedback Destroyer:

      Start by designing one filter to fix a specific peak or dip in your frequency response. Once you have this design, you need to get the filter into the Feedback Destroyer. This is simple!

      Figure 1.5 - BFD Control Panel

      First hit the Filter Select Button. Move the rotary dial until "1" is displayed in the digital readout. This tells the Feedback Destroyer that you want to edit the first of twelve available filters. After you have selected the filter, hit the Filter Mode button. Move the rotary dial until the digital readout displays "PA". This tells the BFD that you want to use a parametric filter rather than one of the presets. Now look on the frequency chart in Figure 1.4. Find the desired frequency on the chart. Hit the frequency button the Feedback Destroyer and move the rotary dial until you get to a frequency closest to the desired frequency. Make sure that it reads Hz to the right of the digital readout. If you can't get the exact frequency that you desire, hit the Fine button below the Frequency Button. Now look on the cart in Figure 1.4 and move the rotary dial to the value corresponding to the frequency you desire. Now hit the gain button and rotate the dial to get the desired gain. Finally, hit the bandwidth button and select a number between 20 and 60 depending on how wide your peak or dip is (we will use measurements to fine tune this value). To store this filter, hit the store button which should be blinking red. This completes the process of entering a filter into the BFD. Now lets see how it worked:

      *Note - Ensure that the Green light is lit on the In/Out button. If it isn't, hit the button until it is lit.

    7. Testing your first filter:

      Once the filter has been stored, go back to the EFT software and hit the "Start Test" button again. Hit OK and look at the area on the Frequency Response plot that you tried to correct. How well did it work? Specifically check the gain and the bandwidth. Adjust both of these values until the filter has corrected the peak or dip adequately. Repeat this process for other peaks and dips in your frequency response.

  4. The Results of our Equalization:

    Figure 1.6 shows the frequency response of my subwoofer in-room after the equalization process. While it appears to roll off in the lowest frequencies, this curve does not take into account the Radio Shack SPL Meter correction values. With these values included, the response should be extremely flat. This response was the result of only 1 hours worth of work, almost no fine tuning has taken place yet. I will post updates as I continue to improve the frequency response of this sub.

    Figure 1.6 - In Room Response of Corrected Subwoofer