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🪵 The Threshold

Adding more loudspeakers can worsen speech intelligibility by shrinking critical distance

Properties2
tagsja-picks, define, addendum, question, share, seed, contrarian_or_surprising, trade, corroborated, xref, candy, category/contrarian-or-surprising
aliases@howard_acoustics_2017, loc. 2454

5 min read

Adding more loudspeakers can worsen speech intelligibility by shrinking critical distance

Evidence

Notice that the %Alcons increases as the number of sources increases. This is counterintuitive because you would think that more loudspeak­ ers would mean they are closer to the listener and therefore should be clearer. … Unfortunately increasing the number of speakers decreases the intel­ ligibility, because only the loudspeaker that is closest to you provides the direct sound. All the other loudspeakers contribute to the reverberant field, and not to the direct sound! The net effect of this is to reduce the critical distance and make the problem worse. — David Howard, Jamie Angus, coustics and Psychoacoustics, 2454

There is a huge SPL variation from front to back: −22.7 dB! People in the front row are being deafened (one of the authors has seen people close to the speakers wince in a York church installation), while people in the rear row are straining to hear anything. … Unfortunately, this is not an unusual situation. Furthermore, the critical distance for this speaker is 2.63 m! So the %ALcons for this space will be 15.5%. Note that adding more speakers does not make the problem any better. Instead it makes it worse, because the extra loudspeakers further reduce the critical distance. In fact, in a typical multispeaker installation, it is possible to show that an unaided human voice can be more intelligible than the PA system, provided the speaker can project their voice sufficiently! — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2518

If one assumes that all the loudspeakers radiate the same power, the critical distance becomes: … So, in this case, more is not better! Ideally one should have the minimum number of speakers, preferably one, needed to cover the space. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2470

Notice that the %Alcons increases as the number of sources increases. This is counterintuitive because you would think that more loudspeak­ ers would mean they are closer to the listener and therefore should be clearer. — David Howard, Jamie Angus, coustics and Psychoacoustics, 2454

Unfortunately this is not an unusual situation. Furthermore, the critical distance for this speaker is 2.63 m! So the %Alcons for this space will be 15.5%. Note that adding more speakers does not make the problem any better. Instead it makes it worse, because the extra loudspeakers further reduce the critical distance. In fact in a typical multispeaker installation it is possible to show that an unaided human voice can be more intel­ ligible than the PA system, provided the speaker can project their voice sufficiently! — David Howard, Jamie Angus, coustics and Psychoacoustics, 2510

Notice that the %ALcons increases as the number of sources increases. This is counterintuitive, because you would think that more loudspeakers would mean they are closer to the listener and therefore should be clearer. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2467

Categorization

  • DEFINE: Explains the mechanism for why the counterintuitive %ALcons increase happens: only the closest speaker contributes direct sound, the rest feed the reverberant field.
  • ADDENDUM: Connects to the soundscape/silence themes from One Square Inch of Silence and the reader’s interest in auditory environments — a concrete mechanism for why ‘more sound equipment’ degrades rather than improves listening.
  • QUESTION: Worth chasing: what is the specific math linking speaker quantity/density to reduced critical distance? The 22.7 dB front-to-back spread seems like it roughly doubles with each added speaker row.
  • SHARE: Send to anyone who does church audio or PA system design — the counterintuitive claim that more speakers make intelligibility worse, and an unaided projected voice beats the system, is a punchy design insight.
  • SEED: Angles for audio technicians or PA system designers: the counterintuitive ‘more speakers actually hurts’ insight could anchor a practical essay on sound system design.
  • XREF: Connects to the general information-overload theme: more channels of the same signal can degrade rather than clarify the message. — connects to: Early room reflections benefit speech intelligibility as much as raised direct sound level — Toole, Floyd E, Sound reproduction- the acoustics and…, 557; Statistical acoustics and classical critical distance do not apply in small rooms — Toole, Floyd E, Sound reproduction- the acoustics and…, 1312; A meaningful speech-in-noise benefit requires at least 6 dB of SNR improvement (+4 more)
  • CANDY: In many multi-speaker installations a bare human voice out-communicates the sound system.
  • corroborated: 9 independent passages support this claim — David Howard, Jamie Angus, coustics and Psychoacoustics, 2454; David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2518; David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2470 (+2 more)

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