The ‘‘stack’’ used in conventional standing-wave thermoacoustics depends on intrinsic thermodynamic irreversibilities for its operation. One approach to increasing efficiency is to eliminate the stack, retaining a small gap between heat exchangers in a standing wave. While a ‘‘nostack’’ device has a temperature span that is limited by the achievable pressure amplitude, it is expected to have greatly increased thermodynamic efficiency compared to a stack-based device with the same power and operating temperatures. Losses associated with flow through the heat exchangers, however, are expected to be higher in a nostack device. The standard theory of low-amplitude thermoacoustics does not apply to nostack devices, where the displacement amplitude is larger than the heat exchangers and the gap. As an alternative, an idealized thermodynamic model of nostack is combined with separate calculations of conduction loss and flow losses, including so-called ‘‘minor losses’’ arising from sudden changes in cross-sectional flow area. The results suggest that by positioning the heat exchangers very close to a pressure antinode to reduce flow velocity, efficiencies similar to Stirling-like thermoacoustic devices might be achieved with the nostack idea. [Work supported by the Office of Naval Research and the Pennsylvania Space Grant Consortium.]