When do we use I=P/a or I=P/4pi(r)^2?
for example question 1:
30 seconds of exposure to 115 dB sound can damage your hearing,but a much quieter 94 dB may begin to cause damage after 1 hour ofcontinuous exposure.You are going to an outdoor concert, and you'llbe standing near a speaker that emits 50W of acoustic power as aspherical wave. What minimum distance should you be from thespeaker to keep the sound intensity level below 94 dB?
- we used I=P/4pir^2
Question 2:
A woman wearing an in-ear hearing aid listens to a televisionset at a normal volume of approximately 60 dB. To hear it, sherequires an amplification of 30 dB, so the hearing aid suppliessound at 90 dB to the ear canal, which we assume to be circularwith a diameter of 7 mm. What is the output power of the hearingaid?
why did we use I=P/a not I=P/4pir^2?
