Why are some sounds(scratching nails on a blackboard) very irritating?

Question

Is there a technical parameter to measure the level of irritation of any sound?

Answer 1

Beauty is in the eye (or ear?) of the beholder. Sound can be propagated as organized harmonic sinusoids or disorganized random noise. As you mention, nails (or improperly held chalk) on a blackboard produces unpleasant noise. The chalk sticks and releases arbitrarily along its path making irregular noise. The eardrum likely enjoys sounds that cause it to resonate like the pure sound of a flute or a full orchrestra (except when tuning up?). Interestingly the bow used to sound a violin is much like the nails or chalk on a blackboard. The bow is made with horses hair (from the tail) and hair when forced out of a pore consists of overlapping plates kind of like pushing a stack of pancakes through a hole with a rope in the middle! You can feel the difference by scraping you nails along the horse hair in both directions (smooth in one direction rough in the other). This is true of your own hair which can easily get stuck on your rough tongue, right? Well, in the bow half the horse hairs must point in each directoin to work. Using rosen on the bow to increase friction, the bow can be drawn across the catgut string to cause vibration and the body of the instrument resonates and amplifies the sound. It is because the tension and finger-stopped length of the string resonates at a given pitch that the stick-release-stick-release action of the bow is converted into a beautiful tone, except of course for beginners who also sound like poorly held chalk on a blackboard. I'm glad you asked.