sound transmission/insulation

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Bankole Folorunso

October 24, 2021 16:52

Hello All,

The below is a question from the BCPM & Systems (Mehta, 3rd Edition), Chapter 8.

And I thought the answer was (a.) True, however the review answer indicates (b.) False as the correct answer. How is that?, I thought dense materials are better sound barriers, can someone help me with these?

8. Compared to a lightweight building element, a heavyweight element
is generally a poor transmitter of airborne sound.
a. True b. False

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  mermann October 24, 2021 22:14

  You are correct….and the question is correct too! A poor transmitter is also a good barrier. It’s a funnily worded question for sure.

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  neadagolban November 26, 2025 04:07

  I have the same question... 

  In my notes I have: 

  CMU cells with air are better insulators than grout-filled, but obviously weaker than grout-filed reinforced block. Loose granular mineral poured into the CMU cavities for increased thermal resistance, increased sound transmission loss, and improved fire rating.

  And in the NCARD practice exam we have:

  

  I'm confused. :/

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  mermann November 26, 2025 05:08

  Lightweight materials = better for thermal insulation

  Massive materials = better for keeping airborne sound in one room from creeping into another.

  (This is different than what it takes to absorb sound when both the source and receiver are in the same room.)

  Michael Ermann

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  neadagolban November 26, 2025 05:29

  Thanks mermann. When answering this question, I was considering how sound-absorbing materials perform— materials with more air pockets tend to absorb sound more effectively

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  mermann November 26, 2025 11:46

  When thinking about the weather, there are (mostly) independent variables:
  Warm or cold
  Windy or still
  Sunny or cloudy

  You can take a walk on a warm, windy cloudy day…or a cold, windy, cloudy day…or a cold, sunny, still day..

  Likewise with architectural acoustics:

  Reverberant or dry (can I make out your words in the university lecture room?…sound absorption, fluffy lightweight surfaces, absorption coefficient and NRC)

  Has (almost) nothing to do with sound isolation (can I hear the music from your stereo? …Massive, airtight, structurally redundant assemblies. TL, STC)

  Has (almost) nothing to do with impact noise (can I hear you walking on the floor above my apartment?…squishy assemblies….IIC)

  Has (almost) nothing to do with noise control (How loud is the HVAC system? ….silencers, duct length, quiet equipment, dBA, NC)

  Has (almost)t nothing to do with environmental noise

  Has (almost) nothing to do with A/V design

  Has (almost) nothing to do with speech privacy.

  And so on.

  So an assembly may create a room that is
  Good for lecture halls, robust for stereos, and bad for footfall
  Or
  Good for lecture halls, poor for stereos, bad for footfall
  Or any combination, just like weather.

  Fluffy materials don’t make (much of) a difference in airborne sound isolation (the stereo next door). Hang a fluffy, sound absorbent curtain between you, on one side, and my stereo, on the other…and you’ll still be annoyed by my music.

  —Michael Ermsnn
  Amber Book Creator
  Author of Architectural Acoustics Illustrated

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