Noise isolation is an acoustical issue we regularly confront. The two primary methods for noise to be transmitted is airborne and structure-borne. Airborne sound sources pass sound by the air which causes the partition (wall or floor ceiling) to vibrate.  The vibration is transmitted and produces noise on the other side. Airborne sound sources include: speech, TV (assuming the unit is not vibrating the partition), stereos, animals, etc.

Structure-borne sound is transmitted from a source vibrating the partition (wall or floor). The vibration of the partition produces sound on the other side. Examples of structure-borne sound sources include: sound from walking on a tile or wood floor, mechanical units (such as a roof top HVAC), plumbing in walls, etc.

The type of sound mitigation that will be effective depends on how the sound is transmitted. The sound path is not always easy to determine. Airborne and structure-borne sound will cause a partition to vibrate. For roof top mechanical sound, it may be necessary to predict the amount of airborne sound coming through the roof and ceiling, and evaluate the sound breaking out of the ducts and from the diffusers.  The remaining sound can be attributed to structure borne (vibration) sound.

The Sound Transmission Class (STC) is used to evaluate airborne sound and Impact Insulation Class (IIC) is used to evaluate impact (or structure borne) sound.

The STC is a single-number rating of the sound transmission of a partition tested over a frequency range. The higher the STC, the better the partition is at reducing sound transmission between spaces.   The following is a list of STC ratings that correspond to a subjective evaluation of a listener. The STC descriptions are based on the audibility and intelligibility of speech between two spaces.  It is assumed that there is a relatively low background sound.  The background noise level in the receiving room will directly impact the perception of noise. The subjective descriptions below are based on human speech. Low frequency sound, such as from the bass of music, will be more audible than speech.

STC 30        Speech can be heard and understood
STC 35        Loud speech can be heard and understood
STC 40        Loud speech can be heard and moderately understood
STC 45        Loud speech is audible, but will sound “muffled.”
STC 50        Loud speech is difficult to detect. An occasional word may be understood.
STC 55        Loud speech is not audible.

The following are a few guidelines for improving sound transmission:

  • Add acoustical absorption in the stud cavity – increase the STC by 5
  • Double the layers of drywall on one side of a wall – increase the STC by 3
  • Double the layers of drywall on both sides of a wall – increase the STC by 5
  • Double the size of the air cavity of the wall – increase the STC by 5
  • Change from single studs to staggered studs – increase the STC by 10
  • Add resilient channels to one side of wood studs – increase the STC by 5
  • Add resilient channels to both sides of wood studs – increase the STC by 10

To test the STC, broad band sound is created in one room at a very high level and the sound level in the source room is measured.  The sound level in the adjacent (receiving room) is also measured.  The difference in noise level is calculated then the value is normalize the number (to adjust for the size and absorption in the room).  The data is plotted (1/3 octave bands) and the STC is determined.

IIC is a single number rating for structure borne (or impact noise). The IIC is measured by having a tapping machine operating on the floor above and measuring the sound level in the room below. A tapping machine is a mechanism consisting of five 0.5 kg hammers which fall regularly and freely onto floor surface from 40 mm height at a rate of 10 impacts/second. 16 third-octave-band between 100 Hz and 3150 Hz are measured below the tapping machine, they are normalized and plotted on a standard graph to determine the IIC.

To improve the IIC:

  • Change from single joist to staggered joists (ceiling independent of floor) – increase the IIC by 7
  • Add resiliently suspended ceiling – increase the IIC by 8
  • Add floating floor (depends on thickness and type of isolation) – increase the IIC by 10 to 20
  • Add 3/32″ linoleum – increase the IIC by 5
  • Add carpet and pad – increase the IIC by 20 to 25
  • Add acoustical absorption in the cavity – increase the IIC by 3

Installing a floating floors is putting in a resilient surface under the hard floor. The hard floor may be wood, ceramic tile, concrete. The resilient (springy) surface underneath the hard floor could be compressed fiberglass, cork, rubber, even springs. The floating floor’s effectiveness depends on the type and thickness of the resilient underlayment.

It may be necessary to determine the background sound level for a space to be able to determine how much improvement should be made. ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers), has a standard that we often use that is for the level of the mechanical system background noise. They use a Room Criteria and Noise Criteria but it can roughly be converted into dBA.  Knowing the existing background noise level and knowing the noise level from the adjacent space allows you to determine how much improvement will be needed.

The other factor to consider is how much will the perceived reduction in noise.  For example, reducing the noise level by 6 dBA is a perceived significant noise reduction.  Reducing the noise level by 10 dBA is a perceived halving of the noise level.

Thank you for visiting Noise Expert Acoustical Consulting – Please let us know if you have any questions or if you would like our assistance with acoustical evaluations. 480-332-9325,,

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