Big hole & distortion at 100 Hz

[Retread]

My system is a Ventriloquist with VTF-3R driven by the Pioneer VSX-D912.

I have TrueRTA software loaded on my laptop. The laptop headphone output is connected into the front audio ports on the Pioneer. A Behringer ECM 8000 calibrated microphone/Shark DSP110 combination is connected to the microphone input of the laptop.

TrueRTA will generate sine waves, pink noise, white noise, and a quick sweep. It has oscilloscope and spectrum analyzer displays.

I've tried combinations of sweep and pink noise, front speakers both small and large, subwoofer LP on and off. There's a big hole in the response at 100 Hz. When I say "hole," I mean a drop of 18 dB moving from 113 Hz to 100 Hz. By 78 Hz, it's back up again.

The hole in the response is bad enough, but when I use a 100 Hz sine wave and look at it with the oscilloscope, it's almost a square wave. Actually, it looks like the exercises we did in school where we built a square wave graphically by successively adding odd harmonics to the fundamental. This looks like severe 3rd harmonic distortion.

Any ideas?

[Lwang]

Have you measured it with sub or Ventriloquist disconnected? Along with setting the mains to large so that there is no bass rerouting to the sub? In no way would a combined sound between a center channel speaker and a sub cause only odd ordered harmonic at 1/n'th intensity. Something must be clipping somewhere. I don't know if you observe a clipped signal or a square wave that is usually produced by an oversampling digital filter with pre and post echo.

[Retread]

I just ran some measurements using the Radio Shack SPL meter instead of the Behringer.

First I did a loop-back test feeding line out to mic in, to see if the TrueRTA was generating good sine waves. It is. Over the range at which I did the tests, the sine generator produced 0 dB sine waves +/- 2 dB with 3rd harmonic distortion at least 60 dB down.

With the sub powered off and the Ventriloquists set at small, and the SPL meter producing -5 dB in the range above 100 Hz, I got -9 dB at 100 Hz, with 3rd harmonic distortion at -34 dB. At 95 Hz, I got -10 dB with 3rd harmonic at -28dB, which is really pretty bad.

When I powered the sub on, and without changing anything else, at 100 Hz I got 2nd and 3rd harmonics of roughly equal size down 15 dB from the fundamental. Also fairly large higher order harmonics.

And the sound reflects the numbers.

[Lwang]

Are these near field or with MLSSA so as to eliminate boundary effects? If you measure harmonic distortion and you hav a suckout, the primary signal will be attenuated while the upper harmonics will not.

Anyway, to get anything close to a square wave that you are talking about, your 3rd harmonic would have to be -10dB, 5th @ -14dB, 7th @ -17dB, 9th @ -19dB, etc.

[Retread]

I ran the measurements in the open room, in the general area of listening. First I ran them with the SPL meter on a pillow. Then I picked it up and moved around. There was some change in the magnitudes of the fundamental and harmonics, but they tended to maintain genrally the same relative values.

There's a lot about sound in rooms I don't know yet. It's an ongoing learning process.