The Noise of Evil

Have you ever noticed that the only adult who can smile at Chuck E. Cheese is the older gentleman shown in the next photo?

Photo credit: Manas  Chaturvedi

He is probably about sixty-eight or seventy.  He is wearing hearing aids.  They are turned "OFF".

Noise is different than sound
Sound is acoustical energy.  It is often run through filters called dBa, or dBb or dBc to mimic how the human "ear" weights acoustical energy of various frequencies.  Beethoven's Ninth Symphony, Pink Floyd, (c)Rap music....all are sound.

"Noise" is sound that the listener finds objectionable.  Sometime noise is in the ear of the beholder.  I like Pink Floyd and Dire Straits but hate (c)Rap music.  My dad hates both.  One of Kubota's friends, Wild Willie, enjoys both.

There are some sounds that are always perceived as noise.

Joseph Bavonese once worked with the psychometrics of noise.  He determined that rapid rise times are fatiguing.  I suspect that is because it automatically triggers our fight-or-flight response.  A breaking twig...might have to fend off a sabre-tooth tiger.

But there is more to it than that.

We are hardwired to pay attention to children, so let's start by looking at the voice of a single child.

"Awww."  The sound of a disappointed child.  Freesound.org was the source of all graphics.

A close-up of a characteristic chunk.  I can count 13 clear harmonics before it gets cloudy.  I doubt this child indulges in cheap whiskey and cigars.

The lowest tone is quite high.  There is very little noise polluting below that.  The four lowest tones have near-equal power.  Harmonics 1, 2 and 4 show fade.  Harmonic three and five show some growth.

Now let's look at a happy sound, one that does not trigger out alarm responses.  Let's look at the spectrogram a laughing child.

Freesound laugh.  You can click on the links and activate the sound that this spectrogram represents.

Fuzzier harmonics.  But what is notable is that the harmonics form "smiles" or are "U" shaped.

Note that there are lots of fast rise-times when the sound goes from virtually nothing (black) to high power (red in the trace).  But most people are not alarmed by laughter.

Shrieking child

Freesound shriek

"All you have to do to turn that frown into a smile is to turn that frown upside-down."

The shriek is almost the opposite of the laugh.  It starts with vocalization and then the vocal cords first tighten, then relax.  The periods are longer.  More lung capacity is expelled with each vocalization.

The tones remain muddy, probably due because the sound is always in transition and transients from initiation never have a chance to dampen out.

A test case, the Evil Laugh
Evil Laughter makes us uneasy.  What do you suppose it looks like?

Freesound evil laugh

Sure enough.  Short vocalizations but the tones are upside-down smiles.

It is creepy because it is not coherent.  One part of the vocalization is a "laugh" but the tone shaping-through-time looks like a "shriek".  That causes dissonance in our brains.

Bottom line
You might be able to take the edge off the shrieking kiddies at Chuck E. Cheese by tuning out the treble but it will be a while before the logic in your hearing aid can recognize "shrieks" and block them out.

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.