The Goal:

I want to be able to play guitar, or do some only moderately loud woodworking tasks in the garage without waking up the wife in the bedroom, which is 1 floor up and 1 room over from the garage, but with a bunch of HVAC ducts in the garage ceiling.

First, a little nomenclature:

• STC: Sound Transmission Class
  

  “The STC number is derived from sound attenuation values tested at sixteen standard frequencies from 125 Hz to 4000 Hz. … The measurement is accurate for speech sounds, but much less so for amplified music, mechanical equipment noise, transportation noise, or any sound with substantial low-frequency energy below 125 Hz.”

• dB: decibel
  • dBA:
    

    “dB(A), dB(B), and dB(C) These symbols are often used to denote the use of different weighting filters, used to approximate the human ear’s response to sound, although the measurement is still in dB. These measurements usually refer to noise and noisome effects on humans and animals, and are in widespread use in the industry with regard to noise control issues, regulations and environmental standards. Other variations that may be seen are dBA or dBA. According to ANSI standards, the preferred usage is to write LA = x dB. Nevertheless, the units dBA and dB(A) are still commonly used as a shorthand for A-weighted measurements. Compare dBc, used in telecommunications.”

• dB = 10 * log10( Power ratio ): Loudness Reference: http://www.sengpielaudio.com/calculator-levelchange.htm
• Note: for x power ration = n dB, 1/x ratio = – n dB. So, for example -50dB of attenuation means 0.00001 th of the original power, or 1/32nd as “loud”.
• You’ll often hear people (I’m certainly guilty) say “10 dB of attenuation”, which would be equivalent to “-10 dB of gain (amplification)”.
• We get a sound level drop of 6 dB per doubling of distance ref
  

  dB	Power Ratio	How much louder it sounds	Effective distance change (Inverse Square of Power)
  50	10^5	32x	~312x
  30	10^3	8x	~32x
  10	10^1	2x	~3x
  6	~4	~1.5x	~2x
  3	~2	~1.23x	~1.4x
  0	1	~1x	1x

• OITC
• NIC: Noise Insulation Class. Similar to STC, but without modifications based on the reverberation time, the size of the room and the size of the test partition

Some Overview Stuff:

• Some overall techniques
• Nice PDF with drawings
• Triple leaf effect
• Slight tangent: a cool room mode calculator

Some Product options:

• resilient clips or resilient channel drywall
• green glue: a sound dampening glue. From the website:
  

  “The rigidity and low damping of conventional adhesives works to worsen sound isolation by lowering the coincidence dip and raising low frequency resonances. Green Glue has a positive effect on sound isolation, making the performance difference between Green Glue and conven- tional adhesives considerable.”

• batting/insulation
• butyl rubber
• Mass loaded Vinyl:
  • via isostore
    

    “Mass loaded vinyl tests show little to no gain when included in assemblies using resilient clips or resilient channel. The gains achieved with the MLV in basic walls, floors, and ceilings are created by resolving resonance issues and not from the actual mass of the MLV. The standalone rating is only relevant when the material is exposed, i.e. around pipes, duct, hung from fences, in curtains over windows or doors, etc.”

  • via isostore
    

    “The goal of wrapping pipes and ducts is to minimize surface vibration
    resonance. Installation is simple by cutting the MLV to fit, overlapping
    slightly, and taping the seam with a basic duct or foil tape. Both tape
    products can be purchased from any local hardware store.”

  • 1lb MLV ~= 4dB Attenuation.

Standard construction = 2×4 studs, insulation (pink stuff?) 1 sheet of 5/8″ drywall on each side

References: * http://www.greengluecompany.com/download/file/fid/817 * http://www.greengluecompany.com/test-data

Some Sound levels:

• Quiet bedroom at night, = 30 dB (SPL)
• Conversational speech, 1 m = 60 dB (SPL)
• Hammer stroke on brass tubing or steel plate at 1 m distance = 160 dBA
• Hammer stroke in a smithy at 5 m distance (greatest level) = 150 dBA
• Angle grinder outside at 1 m distance = 90 dBA
• Hammer on Nail 104 dB
• Hand Saw 80-90 dB
• vacuum cleaner 70 dB
• washing machine dishwaser 65 dB
• http://physics.info/intensity/

Ok, so now some math:

 90dB Hand Saw (assume 1m?)
-20dB ~30ft = ~10m from workbench to bedroom = 1/10 distance
------ -------------------------------------------------------
 70dB ~30ft = ~10m from workbench to bedroom = 1/10 distance
-30dB "quiet bedroom"
------ -------------------------------------------------------
 40dB Attenuation needed in ceiling and walls

So, according to that math, I should already be have a “quiet bedroom”. Which means that either:

• The sound is coupling much better into the HVAC ductwork than calculated (I haven’t been able to find any data on coupling to ductwork).
• The garage is creating echos, or in other words, the 1/r^2 loss doesn’t apply because of the extra echos.
• Most likely, because one of the walls had about a 3″ gap in the dry wall along the bottom edge, I wasn’t even getting the ~33dB attenuation from “Standard wall construction”.

Cost Estimates