One of the biggest challenges in room acoustic measurements, according to the ISO 3382 standard (parts 1-3), is to use sources that maintain omnidirectional directivity (with small deviations) across the frequency range, from the 125 Hz to 4 kHz octave bands. Conventional omnidirectional sources can achieve adequately omnidirectional directivity with the use of more than one loudspeaker unit (e.g. twelve units in a dodecahedron). However, these sources tend to have poor performance at low frequencies, because of the restricted volume shared by the units, as well as non-uniform directivity at high frequencies. In addition, their size makes them far from ideal point sources and discourages acousticians from performing measurements more often, due to their reduced portability. In this paper, a novel sound source is presented, which solves the aforementioned problems. The source consists of a single loudspeaker unit, which keeps the size small and close to an ideal point source. The loudspeaker unit is backed by a cavity with sufficient volume for efficient low-frequency performance. In front of the loudspeaker unit is an acoustic lens with a doughnut-like shape, designed in such a way that the high-frequency radiation is close to omni-directional. This acoustic lens contains all electronics required to make the sound source fully portable and wireless. Free-field measurements of the sound source show that the frequency response is fairly flat over a wide frequency range and that the directivity pattern is well within the limits of ISO 3382, also for frequencies outside the default range (63 Hz and 8 kHz octave bands). An application example is presented, showing that the source is well-suited for room acoustic measurements in small to medium-sized rooms. In addition, the source can be used for measuring the room frequency response, e.g. from 20 Hz to 200 Hz, when positioned in a corner.