Sound isolation is a common acoustical concern. There are two main ways sound is transmitted from one space to another – airborne and structure-borne. Airborne noise sources transmit noise through the air which causes the wall or floor/ceiling to vibrate, the vibration is transmitted and causes sound on the other side. Airborne noise sources include: human speech, television (assuming the loud speakers are not vibrating the partition of concern), stereos, dogs barking, etc.

Structure-borne noise is transmitted from a source vibrating the wall or floor/ceiling. The vibration of the partition causes noise on the opposite side. Examples of structure-borne noise sources include: noise from hard shoes on a hard floor, mechanical systems (such as a roof top unit), plumbing noise, etc.

The type of noise mitigation used will depend on how the noise is being transmitted. The noise path is not always obvious. Airborne and structure-borne noise will cause the partition to vibrate. In the case of roof top mechanical noise, it is often necessary to predict the airborne noise coming through the roof and ceiling, evaluate the noise breaking out of the ducts and through the diffusers and the remaining noise can be attributed to structure borne (vibration) noise.

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

The STC is a single-number rating of the sound transmission performance for a partition tested over a standard frequency range. The higher the STC, the more efficient the partition is for reducing sound transmission between spaces.   The following is a list of STC descriptions which corresponds the single-number STC rating to a subjective evaluation of a typical listener. The STC descriptions are based on the audibility and intelligibility of speech between two spaces, and assume relatively low background noise. Keep in mind that the subjective descriptions below are based on typical human speech. Low frequency noise, such as from music, will be more easily audible than speech.

STC 30        Normal speech can be heard and easily understood
STC 35        Loud speech can be heard and easily 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.

Here are a few guidelines for improving sound transmission:

  • Adding acoustical absorption in the air cavity +5 STC
  • Double layers of dry wall on one side of a wall +3 STC
  • Double layers of dry wall on both sides of a wall +5 STC
  • Double air cavity of the wall +5 STC
  • Change from single studs to staggered studs +10 STC
  • Add resilient channels to one side of wood studs +5 STC
  • Add resilient channels to both sides of wood studs +10 STC

To test STC you play broad band noise in one room very loudly and measure the noise in the source room and in the receiving room. You subtract those, normalize the number (for the absorption in the room), plot the 1/3 octave data, and determine the STC.

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 noise 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) +7 IIC
  • Add resiliently suspended ceiling +8 IIC
  • Add floating floor (depends on thickness and type of isolation) +10 to +20 IIC
  • Add 3/32″ linoleum +5 IIC
  • Add carpet and pad +20 to +25 IIC
  • Add acoustical absorption in the cavity +3 IIC

Floating floors means adding a resilient surface under the hard floor. The hard floor may be tile, wood or concrete. The resilient surface underneath may be rubber, compressed fiberglass, cork, springs. The effectiveness of the treatment depends on the type and thickness of the underlayment.

Many times it is necessary to determine the appropriate background noise level for a space to be able to determine how much improvement should be made. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), has a standard based on the level of the mechanical system background noise in octave band frequencies. They use a Room Criteria and Noise Criteria but it can roughly be converted into dBA, as shown in Table 1. The other factor to consider is how much will the perceived reduction in noise.

Table 1
Recommended RC ratings for acceptable mechanical system background noise within various spaces

Intended Use of AreaEquivalent dBAIntended Use of AreaEquivalent dBA
Broadcast Studio26-30Cafeteria47-52
Recording Studio26-30Restaurant42-47
Concert or Recital Hall26-30Open Plan Office Area42-47
Production Studio30-34Courtroom42-47
Theatre30-34Hospital Corridor42-47
Hotel/Motel Unit34-38
Hospital Patient Room34-38Reception Area47-52
Executive Office34-38Gymnasium47-52
Large Conference Room34-38Indoor Swimming Pool47-52
Sanctuary34-38Computer Equipt Room47-52
Teleconferencing Room34-38Large Dining Room47-52
Music Room34-38Hospital Exam Room47-52
Meeting Room34-38Corridor47-56
Private Office38-42Restroom47-56
Cinema38-42Laundry Room52-56
Small Conference Room38-42Industrial Shop52-56
Small Dining Room42-47
Hospital Operating Room42-47


Thank you for your interest in the Noise Engineers podcast.

Noise Engineers provides information and resources to help people address acoustical issues. In these episodes my goal is to provide resources, inexpensive tools, rules of thumb when dealing with acoustical issues. I would like to explain basic acoustic principles and answer any questions. I will describe actual projects to make this as practical as possible.

You can find our other podcasts at Noise Engineers podcast and iTunes

I welcome suggestions, comments, and questions. You can contact me on Facebook, Twitter , LinkedIn, email me ( or call 520-979-2213.


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