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Sound reduction index for ACRYLITE® acrylic sheet

As with most plastic materials, ACRYLITE® acrylic sheet provides sound as well as thermal insulation. A storm window made of ACRYLITE® sheet will significantly reduce the transmission of outdoor noises into a house or building. For general reference, Table I gives examples of the sound level produced by common noises. The sound levels in Table I are measured in Decibels, a logarithmic unit of loudness.

Table 1 — Sound Levels of Common Noises

Auditory Impression Sound Level
(in decibels)
Extremely Painful 130-140 Jet Engine
Painful 110-120 Shrill Horn
Extremely Noisy 100 Machine Room
Very Noisy 80-90 Road Traffic
Low Level Noise 50 Office
Quiet 20 Radio Studio

Sound Reduction Index (R, Rm, and Rw)

Figure 1 is a schematic for sound transfer between two rooms. In one room, sound is generated by music, voices, etc. The sound travels through air in the form of oscillating pressure waves. These waves will cause surfaces they strike to vibrate, which continue the sound transmission.

Figure 1.

The effectiveness of sound insulation is described by the Sound Reduction Index, R, which is defined below.

Equation 1.


  • R = Sound reduction index, in dB
  • P1 = Acoustic power from outside
  • P2 = Acoustic power emitted inwards

The sound reduction index can be considered to be the difference, in dB, of sound from one side of a structural panel to the other; i.e., outdoors to indoors. R is measured according to DIN standard 52221, part 1, over the frequency range of architectural acoustics (100 to 3200 Hertz). R, or sound insulation, rises with increasing frequencies and also with increasing material thickness, or weight. Sound levels are reduced when the surface receiving the sound waves does not vibrate at the same frequency as the sound waves. The higher the frequency of sound, the less able the receiving surface will be to vibrate at that higher frequency. Also, as the mass of the surface increases, it will be less able to vibrate at the frequency of the incoming sound waves. This is why insulation increases with either mass (panel thickness) or sound frequency.

The fact that R is a function of frequency is shown in Figure 2 for ACRYLITE® sheet of different thicknesses. To take into account how R varies with frequencies, and to account for the fact that the human ear is more sensitive to higher frequency sounds, a weighted sound parameter, Rw, was developed. Rw is a weighted average of R at various frequencies that accounts for the human ear's varying sensitivities to sound at different frequencies. Rw is a single value that describes the sound reduction of an insulating unit.

Figure 2.

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