We investigate the observability of higher harmonics in gravitational wave signals emitted during the coalescence of binary black holes. We decompose each harmonic into an overall amplitude, dependent upon the masses and spins of the system, and an orientation-dependent term, dependent upon the inclination and polarization of the source. Using this decomposition, we investigate the significance of higher multipole moments over the parameter space and show that the $\ell =|m|=3$ harmonic is most significant across much of the sensitive band of ground-based interferometric detectors, with the $\ell =|m|=4$ harmonic having a significant contribution at high masses. We introduce the higher harmonic signal-to-noise ratio (SNR), and show that a simple threshold on this SNR can be used as a criterion for observation of higher harmonics. Finally, we investigate observability in a population of binaries and show that higher harmonics will be observable in a few percent of binaries, typically those with unequal masses and viewed close to edge-on. We find that he majority of binaries with mass ratio greater than $4:1$ will have an observable $\ell =|m|=3$ harmonic.

• Received 6 August 2020
• Accepted 8 December 2020

DOI:https://doi.org/10.1103/PhysRevD.103.024042