Ambient noise vertical directionality measurements at low frequencies have shown that noise arrives from long distances at near horizontal angles, from moderate distances at larger angles, and from local sources at high angles. Therefore levels of noise from different sources can be measured by examining vertical linear array beam powers at the appropriate angles. However, when measurements are made, it is important to be able to distinguish between acoustic signals, ambient noise, and data contamination. Some experimental results are examined, using conventional and adaptive beamforming and eigenvalue–eigenvector decomposition, and used to establish criteria useful in deciding whether the measurements are ambient noise or otherwise.