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O Citizen Interests 0 briefly above. Rather like the wine glass that was shattered by the opera singer holding a sustained note, a structure has a "natural" frequency at which it will vibrate when "excited". When an incoming ground motion is at that same natural frequency, then the structure is "excited", and the resulting response can well exceed the intensity of the incoming o ound motion. 'Ibis again is the reason cahv the regulations permit higher vibrations at high frequencies than they do at low frequencies. Because of this also, since this phenomenon has already been taken into account, the regulations control the incoming ground motion only. From the point of view of perception, therefore, a person will be less disturbed by a blast when he is outside his house than when he is inside it. From the viewpoint of actual effects, however, the citizen may well have noticed the potted plants on his window sill vibrate, and he may consider it reasonable to req~iest that vibration measurements be taken on the window sill. The mine operator would be justified in declining that request, as it would only measure the response of the structure at that point, arui not be a valid measurement of ground motion under the OSMRE regulations. When frequencies are very low, about 4 to 12 Hz, and structures respond, then the maximum danger of actual damage occurring exists. A small possibility exists of damage at velocities even below 1.0 inch per second, which accounts for the low velocities allowed an the Blasting Level Chart, Figure 12 on page 24 of this manual. As frequencies increase, the likelihood of structural response decreases, and the allowable limits increase. The actual effect of a grourd vibration close to, but not exceeding 2.0 inches per second, at a frequency at or above 30 Hz would be a highly rice tib~le vibration, and probably a very annoying one, but one that would not cause damage. Incidentally, ground motion has to be far above these levels to threaten wells or aquifers, although these problems are frequently feared and complained of. AI~IIVOYANCE The above discussion on perceived and actual vibration effects has had the central theme of annoyance running through it. Unfortunately, annoyance is very difficult to quantify, and it is further complicated by the fact that not all human beings are annoyed by the same things. Some of us simply do not worry about things that drive others Crary. IL can also help to recognize that as far as a local surface mine is concerned, the citizen who originally tended to benefit from the operation, and who did not oppose the application, will be far less annoyed by the operation than the citizen who did oppose that operation, and who had to watch it become an established operation in the face of his objections. 117