Acoustical Interactions

Soundproofing fundamentally involves the means by which we can control the propagation of sound through acoustical interactions.  It is therefore important to understand how acoustical interactions with physical objects, such as walls, establish the groundwork for how we will control sound with soundproofing materials and products.

The various possible acoustical interactions of sound with a typical solid surface, such as a wall is shown in Figure 1.

Acoustical Interactions

Sound actually travels in waves, but for simplicity we are showing sound traveling as a vector.  When the incident sound wave encounters an acoustical absorption material, a portion of the acoustical energy is “lost” within the absorption material due to internal friction.  The remaining acoustical energy encounters the solid wall material, and a portion of the acoustical energy is reflected off of the wall, through the absorption material and back into the room.

Most of the acoustical energy is reflected at a complementary angle to the incident acoustic wave, however, based on the quality of the wall surface, some of the energy is reflected at non-complementary angles, known as acoustic diffusion.  The acoustical energy that is not absorbed or reflected is transmitted into the wall, where it may change direction or refract through the wall.  The acoustical energy that passes through the wall to the other side is the transmitted acoustic wave, which typically has a lower magnitude than the incident sound wave.

The difference in magnitude between the incident sound wave and the transmitted sound wave describes the Sound Transmission Class, or STC of the wall.  In general terms, the STC describes how well a barrier or wall reduces (or attenuates) airborne sound from one side of the wall to the other.  For example, a typical wall in a house or office has an STC of 33, meaning that it can attenuate 33 dB from one side of the wall to the other.  If a person is talking in the next room at a level of 65 dB, then you could hear 32 dB (which is the level of a whisper), although the speech would not be intelligible.  Loud music, however, could reach levels of over 85 dB, which would be attenuated to 52 dB through the wall and would clearly be heard.

The fractional ratio between the reflected sound wave and the incident sound wave describes the Noise Reduction Coefficient, or NRC of the acoustical absorption material.   In general terms, the NRC provides an indication of the amount of sound energy absorbed upon striking a particular surface.   An NRC of 0 indicates perfect reflection with no loss of acoustical energy (such as a smooth concrete surface) and an NRC of 1 indicates perfect absorption (such as our Sonic-Fiber product). All other types of materials and surfaces fall in between 0 and 1.  We have all experienced such rooms (or restaurants) with acoustically hard and reflective surfaces (NRC=0), where unwanted noise is amplified, making it difficult to hear speech or music.  These acoustical interactions make us speak louder or increase the volume of audio equipment, further increasing the overall sound levels within the room.

In order to arrive at an optimum soundproofing solution, it is normally necessary to achieve an optimum balance using a combination of sound absorption and sound barrier materials.  Our team of acoustic experts can help you characterize noise sources and assist you in selecting the appropriate materials and products to eliminate, block or absorb the noise through understanding acoustical interactions.