Originally Posted by kwarny
I'm still trying to figure out what the rumbling is that I hear with other subs. The rythmik may still have it but other subs exaggerate whatever it is pretty clearly.
My explanation is that the clear and coherent reproduction of the original source and its echos is the reason. The microphones at the recording site will pick up the direct sound from the instrument (let us call that source) and its echos from near-by objects. If that object where the echo is reflected from is 4 ft away from the instrument plus if the echo signal is an inverse (meaning it has negative magnitude), the lowest frequency being affected has a half wavelength (180 degrees shift) of 4 ft. So the frequency is 1000/(2*4)=120hz. The effect is best demonstrated as an impulse response. In this case, we will see the source signal at time 0 (which looks like a normal impulse waveform), and then at 4/1000 sec (or 4ms) later, there is another blip with negative shape (which is similar in shape to the source, except it is going to the negative amplitude direction) and with smaller amplitude. When you do FFT (fast Fourier transformation), we get the frequency response of the composite signals. So you can see for frequency response, both source and echos contributes to it. But our brain does not treat them that way. In addition, if the echo is further delayed (because it is from farther objects), the affected frequency goes lower.
In short what the sub plays is the low frequency source plus its multiple echos. If the sub is "coherent" over time, our brain can still recognize the music rhythm of the source out of complex composite sound. At the same time, this complex composite sound makes it sound like the instruments is boxed-in in a vaguely large box (the echos define the box). In a less coherent sub, the bad distortion (arisen from memory effect) makes the source and echos hard to be individually recognized and therefore just seems to be some sort of rumble in the background. But in spectral analysis, the two have very similar. Crucial information is lost in analysis.
As we switch the extension filter setting on our plate amps, on good recording, the sound effect is the sound stage seems to become larger, deeper, and taller when we switch from 28hz to 14hz. In addition, as we do that, the sound lost its one note characteristic where some describes it as punchy. If some prefer to hear that type of sound, he should try 28hz/low damping and then boost signal around 80hz to 150hz.