* "Her voice had that tense, strained tone I know so well." This uses commas to separate the adjectives "tense" and "strained," emphasizing both qualities.
* "Her voice had that tense strained tone I know so well." This uses no punctuation between the adjectives, creating a tighter connection between them and emphasizing the combined effect of the tone.
* "Her voice had that tense, strained tone I know so well." This uses a comma to separate the tone from the clause about knowing it well, implying a slight pause between the two thoughts.
* "Her voice had that tense strained tone—I know so well." This uses an em dash to create a stronger pause, emphasizing the recognition of the tone.
The best choice depends on the specific effect you want to achieve.