The reliable field measurement of airborne sound insulation between rooms in the very low frequency range (20 Hz – 100 Hz) presents a substantial challenge for several reasons. Sound source and microphone placement can have a strong effect on the transmission, and diffuse field conditions are usually not possible to establish in medium-sized rooms. In this study we compare three methods that have been proposed previously for transducer placement with each other, and with mass law theory. Our results show that substantially different values may be obtained from each method of measurement. Furthermore, we examine the influence of the test signal on the measurement, and find that non-linearities in the building fabric can also substantially affect the apparent sound reduction index in the very low frequency range. We discuss how measurement techniques might be refined to increase their reliability.