Sunday, August 2, 2015

Fixing the symptoms first

I heard Jesse J from Oregon in a live concert Saturday, bought the CD, got it signed by Jesse, played it at home, was wonderful.  Though I though a bit more wonderful when I lowered the TacT level (92.9=0dB) to 79.1 on cut three, that may have reduced some critical rattling in my living room.  This CD is no wimp when it comes to deep bass!   The next day, my friend thought it sounded a bit mechanical.  Right at that time, there were several rattles obvious to me, I've set about fixing them, which brings me to the title point.

As a general rule in such situations, it's better to fix the symptoms first.  By symptoms I mean the effects farthest from the original causation.  Why?  Because if you wait until after fixing the primary cause, you might not be able to find the symptoms again.  So I will fix the rattling before fixing the frequency response.  The frequency response doesn't require a non-linear failure like rattling to measure and can be fixed after the rattling is fixed.

But actually the frequency response is not bad.   I went through the test frequencies I have used to make adjustments before.  I have a series of tones recorded at something like -20dB, in Hz:  16,18,20,22,25,28,31.5,36,40,45,50 and up to 160.

Sitting at the Kitchen table listening casually, it seemed like 36Hz and 40Hz seemed a bit low.  But measured at the listing position, 22Hz-45Hz are within 1dB, which is pretty incredible and testimony to my hand-tuning of the EQ's, especially to fight the 10dB room mode at 45 Hz.

Playing the subwoofers alone makes it easy to find the rattles.  The bass is amazing tuneful considering the 80 Hz lowpass and everything.  I did think there was a kind of excessive bounce, excessively electronic sound in the bass, which I think is an unfortunate attribute of my system (I am not faulting the recording, which sounded just fine and natural on the bass on another audiophile's system which doesn't go quite as deep).  So, for that reason, and also because of booming and rattling all over, I dialed back a 20 Hz boost I had in one channel to zero.  Now there is no boosting below 31.5 Hz (at one time I had several boosts below 31.5Hz to enhance the lowest bass) and in fact there was a -3dB Q=1 cut at 20Hz, which I dialed back to -2dB after removing the 1dB Q=5 boost.  I can see the trick I was playing here (high Q boost combined with lower Q cut) but it's precisely that sort of tricky EQ that could add an artificial sound from extended reverberation.  Anyway, it's hardly any different from a 1dB change overall.

I only measured 20Hz after removing the 1dB boost, and it was actually 2-3dB below the 22-45Hz level.  But 20Hz is quite problematic, it causes a lot of wall flexing and rattles the kitchen range, among other things, which I can't fix.

The rattling wall clock was fixed by applying self-stick felt to the back, top and bottom.  Now this had an unintended effect of forcing the bottom of the clock away from the wall because of constraining the space for the wall hook on top.  At first it appeared as though the bottom was entirely away from the wall, but after some time, one side is almost touching the wall, the other side about 1/3 inch away.  I pound on the wall and it doesn't seem to rattle at all, so this is fixed for now, but may need to be monitored.

The rattling china cabinet was investigated, and finally the source of rattling was found: two vessels on the bottom shelf were touching.  That was it!  I was prepared to use felt, whatever, but that wasn't necessary.  Going one step further, I pounded on the shelves with my hand, and found one other potential rattle in the teapot and butter dish, with both have top and bottom parts.  I wedged pieces of paper in between top and bottom pieces and now they seem quiet.

The rattling in the range hood and back plate has eluded me for now.  Nothing I could hold with my hand changed it.  Anyway, this was not the rattle I heard on Jesse J's album, but something else that turned up playing spot frequencies up to 100 Hz.

The "cause" of lack of LF absorption will be a long time in fixing.

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