Harmonic + Noise model (HNM) is a hybrid model of speech with a harmonic component and a noise component. While harmonic part describes efficiently the periodicities in speech signals (voiced parts), modeling of noise part introduces artifacts primarily because of the specific time-domain characteristics of noise in voiced speech. In this paper, we are concentrated on the modeling of noise in voiced frames. To model the temporal characteristics of noise, we study three time envelopes in the context of HNM; Triangular envelope, Hilbert envelope and Energy envelope. Listening tests showed a clear preference on the Energy envelope and Hilbert envelope for male voices and in a less extent the same conclusions can be drawn for female voices.