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2. Dynamic ranue and resolution: If PPV is expected to exceed 10 in/s or frequency is <br />expected to exceed 250 Hz, special sensors approved by the Vibration Specialist should <br />be used to measure blast effects. In these cases, the Vibration Specialist should also <br />determine a digital sampling rate that should provide accurate recordings. <br />3. Recording duration: Set the record time for 2 seconds longer than the blast duration plus <br />1 second for each 1100 feet from the blast. <br />Part III Air-overpressure Monitoring <br />The following procedures should be used as possible when setting up instruments to measure <br />blast-induced noise. <br />A. Microphone placement <br />The microphone should be placed along the side of the structure nearest the blast. <br />1. The microphone should be covered with a windscreen and mounted near the velocity <br />transducers. <br />2. The preferred microphone height is 3 feet above the ground or within 1.2 inches of the <br />ground. Other heights may be acceptable for practical reasons. (ANSI S12.18-1994, <br />ANSI S12.9-1992/Part2) (USBM RI 8508) <br />3. If practical, the microphone should not be shielded from the blast by nearby buildings, <br />vehicles or other large barriers. If such shielding cannot be avoided, the horizontal <br />distance between the microphone and shielding object should be greater than the height <br />ofthe shielding object above the microphone. <br />4. If placed too close to a structure, the airblast may reflect from the house surface and <br />record higher amplitudes. Structure response noise may also be recorded. Placing the <br />microphone near a comer of the structure can minimize reflection of over-pressure <br />energy. (RI 8508) <br />B. Programming considerations <br />Site conditions dictate certain actions when programming the seismograph to record air- <br />overpressure. <br />1. Trigger level: When only an airblast measurement is desired, the trigger level should be <br />low enough to trigger the unit from the airblast and high enough to minimize the <br />occurrence of false events. The level should be slightly above the expected background <br />noise for the area. A good starting level is 120 dB. <br />2. Recording duration: When only recording airblast, set the recording time for at least 2 <br />seconds more than the blast duration. When ground vibrations and air-overpressure <br />measurements are desired on the same record, follow the guidelines for ground vibration <br />programming (Part II C.3). <br />ISEE Monitoring Standards Page 4 of 4 <br /> <br />