पानी के नीचे की वस्तुओं द्वारा उत्पन्न ध्वनियों को केवल प्राप्त करने के लिए उपयोग किए जाने वाले उपकरण को पैसिव (निष्क्रिय) ______ प्रणाली कहा जाता है।

A closed organ pipe and an open organ pipe are tuned to the same fundamental frequency. What is the ratio of their lengths?

Consider the following applications: I. Estimating the speed of an aircraft or ship by means of reflected waves II. Inferring the expansion of the universe through shift in spectral lines III. Determining the age of geological or fossil-bearing material through radiometric methods IV. Measuring blood flow in arteries using medical imaging techniques How many of the above involve the use of the Doppler effect?