Stereo presentation masks audible differences between loudspeakers found in mono.

Evidence

In monophonic tests listeners reported large differences in both sound quality and spatial quality. However, in stereo listening most of the differences disappeared in the average ratings for all programs. The two highly rated loudspeakers kept their high sound quality ratings, but the loudspeaker with low ratings in mono became competitive in stereo. — Toole, Floyd E, Sound reproduction- the acoustics and…, 443

Categorization

• QUESTION: The author never fully resolves why stereo masks these defects — is it masking, distraction, or something the stereo mix adds? Worth revisiting how the mechanisms interact.
• XREF: Connects to the earlier discussion in this same chapter about phantom center imaging and the 2 kHz dip; also parallels visual masking/superiority effects in perception. — connects to: First-arriving direct sound dominates localization and loudspeaker images are built from mono and double-mono; Mono listening produces stronger and more consistent loudspeaker quality opinions than stereo — , Toole, Floyd E - Sound reproduction-…, 236; Mono reproduction exaggerates recording-room coloration that stereo cancels — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 1832 (+5 more)
• corroborated: 2 independent passages support this claim — Toole, Floyd E, Sound reproduction- the acoustics and…, 443

Broader Connections

• Mono listening produces stronger and more consistent loudspeaker quality opinions than stereo #corroborates (0.92) — , Toole, Floyd E - Sound reproduction-…, 236
• Loudspeaker evaluation must be done in mono because multichannel adds forgiveness #expands (0.90)
• Stereo and multichannel listening make listeners more forgiving of loudspeaker flaws #complicates (0.88) — Toole, Floyd E, Sound reproduction- the acoustics and…, 467
• Mono reproduction exaggerates recording-room coloration that stereo cancels #complicates (0.85) — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 1832
• Stereo reproduction injects room and speaker location cues unrelated to the recording #expands (0.82)
• Room differences dominate loudspeaker differences when listeners cannot adapt #qualifies (0.75) — Toole, Floyd E, Sound reproduction- the acoustics and…, 547