All instruments (including the human voice) ... have characteristic "sound envelopes" ... when a pitch is produced.
An attack period ...a sustain period ... and a decay period.
Put them together ... and you have that instrument's "Timbre".
But every instrument's "sound envelope" profile ... will be different ... hence different timbres.
A violin and a cello will have profiles that are more closely related than ... say ...
A violin and a banjo.
As far as steel guitar and human voice ...
While the sustain and decay periods of a steel guitar (acoustic or electric) ... can be more "human like" than many instruments (basically a function of vibrato) ... hence it being so readily adopted by early Hawaiian musicians ...
The attack profile of a steel guitar ... specifically the
transient sound produced (which lasts bout 0.005 seconds) when the picks hit the string ...
Will be markedly dissimilar to ... a singers attack profile ... as air is forced over tissue.
I've done my best to show that here ... with my equipment, samples and ability.
I'll be glad to run and/or post a sample (waveform & full spectral analysis) of ...
A singer singing an
A 220 note ...
And that same A 220 note being played on a tricone, frypan, stringmaster ... whatever ...
I just can't sing ... at all !!!