Monday, September 16, 2013

Psychometrics of noise

It is difficult to map from sound (a form of energy that can be measured) to noise (a form of energy that annoys).

One psychometric study determined that components with rapid rise times are particularly irksome.  The speculation is that those kinds of sounds; rattles, squeaks, snaps, cracks, chugs (and rap) light up our alarm response.

Those ancestors whose muscles tensed up at the unexpected breaking of a twig were more likely to outrun the pouncing Smilodon than those whose muscles remained relaxed.  Guess who contributed more to our gene pool.  So sounds with rapid rise times tend to cause muscle tension....which morph into headaches, adrenaline surges and fatigue.  Sounds with rapid rise times are noise.

Old cars have many rattles.  The old Cavalier has more than its fair share.  Most originated from the rear package shelf.

There are two and a half ways to deal with rapid rise time sounds (which are usually caused by impacts).  One is to separate the parts so they never contact.  Two is to join the parts so they never separate.  Two-point-five is to pack cushioning material between the parts so any impacts are gentle.

Closed cell foam rubber used for door seal strips makes outstanding material to pack between impacting parts.

Sources of rattles can be found by gently thumping parts with one fist while "calming" suspects with the other.  Foam strip can be packed in promising nooks and crannies and the results verified by gently thumping with your fist and not calming the suspect with the other.

Some parts are not ammenable to packing foam between the parts that rattle.  Sometimes they can be calmed down by packing foam to increase pre-load or increase separation by packing foam near the impacting regions.
Butyl rubber and vinyl have good dampening characteristics.

Hooks that hold rear speaker to "package shelf" were rattling because they were loose.  Four inch foam rubber strip packed in region highlighted with green lines lifted speaker and caused hook to not lift off shelf when excited.  Seen from bottom.  Both left and right speakers were packed.

Center High Mounted Stop Lamp (CHMSL) bulb holder (ivory) was not securely locked in holder (translucent red).  Bulb holder secured.

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