The transient modifier is a type of audio effect that changes the level of the transient parts in a musical signal while leaving the steady-state parts unchanged. This article presents a high-performance algorithm for transient detection and modification, one that is capable of modifying transients in polyphonic or multi-voiced signals, and capable of modifying both hard (percussive) and soft (non-percussive) transients. The detection and modification of transients are performed in the frequency-domain using a sub-band approach. Detection is based on both phase and energy information using an adaptive threshold, and modification is carried out independently at each sub-band. The performance of the proposed sub-band approach was compared with other transient-modification algorithms using subjective listening tests. We show that the sub-band approach with adaptive threshold mostly outperforms other approaches.