Out-of-band beamforming methods, frequency-difference and frequency-sum beamforming, are array signal processing methods that allow a user to process signals at either a lower, below-the-signal-band difference frequency, or higher, above-the-signal-band sum frequency. Frequency-difference beamforming has previously been investigated for shallow-ocean propagation with sparse vertical arrays, demonstrating its capability to overcome the negative effects of spatial aliasing [Douglass, Song, and Dowling (2017). J. Acoust. Soc. Am. 142, 1663–1673]. In the work presented here, data from the 2012 COAST (Cascadia Open-Access Seismic Transects) experiment is utilized to explore out-of-band methods with horizontal arrays, where variations in environmental characteristics (bathymetry, sound speeds, etc.) may be more pronounced across the array. The abundance of array elements (636 in total) and total array length (7.9 km) in this experiment provide a means for controlling the sparsity and signal coherence within a chosen subarray to determine the capabilities and limitations of out-of-band beamforming. In addition, the bandwidth of the signal of interest is sufficient to compare out-of-band methods to conventional methods where the difference or sum frequencies match the in-band frequency used with conventional beamforming. [Sponsored by ONR.]