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Velodyne SMS-1 Digital Drive Subwoofer Management System

Part III

December, 2005

Roger Welch

 

I had previously determined what I felt was the best position for the subwoofer using a good deal of trial and error, just listening by ear. This turned out to be between my front left speaker and the center channel. I had also calibrated the relative subwoofer level using Video Essentials and a Radioshack SPL meter.

Until the arrival of the SMS-1, this was the best that I could achieve. I could hear that the response was not flat, but there was little I could do about it. The SMS-1 demonstrated all too clearly the lumpiness of the response with peaks at around 22 Hz and 40 Hz, as well as dips at 50 Hz and 85 Hz, as shown below in the screen shot.

Since several of the peaks and dips in this response are significantly above the crossover frequency (63 Hz), the SMS-1 will be powerless to do anything about them because it can only influence what the subwoofer produces.

The first stage in improving the sub’s performance was to get it positioned in the optimum location. One traditional method of finding this is to play a bass-heavy CD with the sub located in the main listening position and then crawl round the room on hands and knees, listening for a place where the bass is tightest. With the SMS-1 you can make this process a little more scientific (and less embarrassing) by placing the sub in the listening position and just moving the microphone to potential sub positions while the SMS-1 generates test sweeps and measures the room response.

Some examples of my experimental positions were the front left corner, rear left corner, just inside the RH front speaker, and halfway down the right hand wall. Using the SMS-1 in this way demonstrated the huge difference that room placement makes to the response, sometimes by moving only a few inches, and eliminated the guesswork involved in finding an optimum position. A digital camera to capture the different screenshots for later comparison is also a good idea.

Boosting dips in the response using EQ is best avoided because it tends to eat up headroom from the subwoofer’s amplifier and/or lead to premature compression when the sub is pushed hard. So, instead, you should reduce the peaks.

The best placement is generally where there are fewest significant dips. If this is not a practical location, then you can choose the next best and so on. I chose to relocate the sub just inside my right front speaker. Returning the microphone to the back of my chair, I then re-verified that the response was the same with the sub and microphone positions reversed.

I next turned the main speakers back on and proceeded to smooth the response around the crossover frequency using the polarity, phase and low-pass crossover controls on the SMS-1. These can be found on the second OSD screen page which is entered by selecting NEXT.

This screen initially looks quite complicated, but if you restrict yourself to making changes in the Setup column only to start with, these get copied automatically across all the presets. Later on, when you get more adventurous, you can start changing settings for each individual preset.

I mentioned earlier that the SMS-1 can be connected into an existing setup in several different ways. For example, where the front left/right speakers are full range, the system may sound best if they receive a full range signal and the sub merely fills in the extreme bass where the main speakers naturally start to roll off. Achieving a seamless transition between the sub and main speakers in this scenario requires the sub’s low pass crossover frequency and slope to be perfectly mated to the natural in-room roll off from the main speakers. This is an area where the SMS-1 excels because it provides precise control over both parameters. Moreover, this is performed in the digital domain and will typically be more accurate than the equivalent analog crossover circuit in most subwoofers. The SMS-1 allows the low-pass crossover to be precisely set in 1 Hz steps in the range 15 Hz to 199 Hz. By default, it is set to 80 Hz. The filter slope can be set to values of 6, 12, 18, 24, 30, 36, 42 or 48 dB/octave, with a default setting of 24 dB/octave.

I chose to disable the Low-Pass Crossover in the SMS-1 by setting it to OFF, leaving my SSP to supply a suitably low-pass filtered subwoofer signal. However, this is by no means prescriptive. The tools are there to be used, and in some circumstances it may be desirable to use the LPC in the SMS-1 in conjunction with that in the SSP. The excellent article Bass Management Woes: Trouble on the Slopes gives some reasons why this may be necessary, and although it is probably not the right fix for the problem, the SMS-1 may still be able to help.

You can set a suitable subsonic filter frequency to help protect the subwoofer from trying to reproduce signals below its design limits. By default, this is set to 15 Hz with a 24dB/octave slope, but you can set the frequency in the range 15 Hz to 35 Hz, and the slope can be 6, 12, 18, 24, 30, 36, 42 or 48 dB/octave. I decided to set it to 20 Hz to match the tuning point of the sub and left the default slope alone.

I then tried both settings for polarity (+/-) followed by each phase setting from 0° to 180° in 15° steps, using the TEST key on the remote control to flick to the sweep screen and check the response each time. I was looking for settings that gave the flattest sweep response around the crossover frequency region (63 Hz). I finally settled on polarity + and phase 90° settings to obtain a basic system response.

So far, so good. There was already some improvement from my original response and I had yet to apply any equalization.

Click Here to Go to Part IV.

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