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Cement board has a higher density than 5/8 drywall. It also probably, and this is just a guess, probably has a lower resonance as well (being cement), so less sound should get through.

5/8" drywall is about 2.2lb per sqft.
1/2" cement board is 2.355 lbs per sqft.

The price is comparable between the two, with cement board being a little more expensive but not by much. 1/2" cement board comes in 3' x 5' sheets, at about $10 each (about 67 cents per sqft). 5/8" drywall comes in 4' x 8' sheets at about $15 each (about 47 cents per sqft).

Is 1/2" cement board a better soundproofing material than 5/8" drywall?

diy user
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    "Cement board has a higher density than 5/8 drywall. It also probably, and this is just a guess, probably has a lower resonance as well (being cement), so less sound should get through." Water has a lower density than air but it transmits sound better than air. Insulation is less dense than drywall and transmits less sound. It's not how dense it is but rather how well the material transmits vibration. – JimmyJames Jan 23 '19 at 18:10
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    Related: https://www.avsforum.com/forum/19-dedicated-theater-design-construction/1176902-cementboard-sound-isolation.html – UnhandledExcepSean Jan 23 '19 at 18:15
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    Correction - water has a higher density than air. – SteveSh Apr 12 '20 at 21:10
  • Mass absorbs energy and also resists vibration.. Mass is absolutely a factor in soundproofing, in fact, one of the greatest factors. Also, insulation DOES NOT transmit less sound than drywall. It also is not an apples-to-apples comparison. – diy user Dec 05 '21 at 17:02

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The less dense material would be better for soundproofing, the 5/8" drywall.

There's a lot that goes into soundproofing and there's even professions within the construction industry that specialize in it, acousticians. That being said I did a bit of research with this in college and it turns out that less dense materials provide better soundproofing. The more air bubbles in the material, the more the sound bounces around before reaching the other side of the material.

represton
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  • Wouldn't a more dense material have less air bubbles? Between (2.2/0.625 = 3.52) and (2.355/0.5 = 4.71), the cement board is more dense... – adamaero Oct 19 '21 at 16:39
  • Less dense material is incorrect. It's not just air bubbles preventing sound transmission. If more air meant more sound reduction, then 10 feet of air would be better than a 10 foot thick cement wall, whereas the former would block no sound, while the latter would likely block all of it. Your answer is fundamentally incorrect. – diy user Dec 05 '21 at 17:03
  • I'm referring to the material, not an empty room. If there's more air, that means there's more cavities and thus more attenuation... I'm not applying this to all applications of sound transmission. – represton Dec 08 '21 at 20:18