Broadband seismic data recorded 2.3 km from the active vent of Arenal Volcano, Costa Rica, provide new constraints on tremor source processes. Arenal's tremor contains as many as seven harmonics, whose frequencies vary temporally by up to 75 percent, from initial values of 1.9 Hz for the first peak immediately following explosive eruptions to 3.2–3.5 Hz several minutes later. We infer that gas bubble concentration is variable within the conduit and also changes as a function of time, thereby changing the acoustic velocity. We infer that the source is a shallow, 200–660 m-long, vertically oriented 1-D resonator with matched boundary conditions, radiating seismic energy from a displacement antinode. Polarization analyses show that particle motion azimuths abruptly rotate, which may be explained by a decrease of the incidence angle. We suggest that energy is radiated predominantly from a displacement antinode that is changing position with time. Tremor consists mainly of transverse waves that travel at speeds of about 800 m/s. P waves in the magma conduit will couple very efficiently into S waves in the surrounding medium when there is virtually no impedance contrast between the two media for these two types of waves. The tremor at Arenal is similar to tremor at nine other volcanoes.