IPA: /ˌsaɪ.nəˈsɔɪ.dəl.i/
KK: /sɪˈnuːsɔɪdəli/
In a way that follows a smooth, wave-like pattern, similar to a sine wave.
The sound waves traveled sinusoidally, creating a smooth and rhythmic tone.
Sinusoidally → It is formed from "sinus" (from Latin, meaning curve or bay) and "-oid" (meaning resembling) and "-ally" (meaning in a manner). The word describes a manner that resembles a curve, specifically the smooth, wave-like pattern of a sine wave.
Think of a 'curve' ('sinus') that 'resembles' ('-oid') a wave, and remember that 'sinusoidally' means moving in a wave-like manner.