The internal spectra of several air jet excited Helmholtz resonators with natural frequencyf0of approximately 250 Hz, as a function of jet speedvbetween 0 and 5000 cm/s, were surveyed. Above the edge tone region the frequency of oscillation ranged between 140–305 Hz. Domains of jet speed in which a single frequency Helmholtz mode occurs are separated by narrow transition zones where Helmholtz modes of very low amplitude having two different frequencies either produce a heterodyne or alternate. The Helmholtz mode is excited throughout the range ofvconsidered. Harmonics of the Helmholtz mode occur in the highest and next to highest domains and frequency mixing between it and the lowest standing wave mode occurs in the highest domain. In the highest domain a standing wave mode at approximately 6f0cooperatively excites the Helmholtz mode. An approximate linear model incorporating an empirical dispersion relation is used to explain the major features of frequency versus jet speed behavior. The effects of varying jet width and neck length are examined.