Sound Insulation. Light Sound-damping Construction

Even if propagation of sound is avoided, complete elimination of a noise is impossible. If the sound source and the hearer are located in the same room, then some reduction takes place through sound absorptivity. If they are in separate rooms, then sound insulation is the main remedy.

A distinction is made between sound insulation of airborne sound and sound insulation of structure-borne sound: airborne sound sources initially disturb the surrounding air, e.g. radio, shouting or loud music; with structure-borne sound, the sound source is propagated directly through a structure, e.g. movement of people on foot, noise from plant and machinery. Sound from a piano is an example of both airborne sound and structure-borne sound.

Sound is propagated by mechanical vibration and pressure waves - very small increases and decreases in pressure relative to atmospheric pressure of the order of a few microbars (pb). (The pressure fluctuation generated by speaking in a loud voice is about one millionth of atmospheric pressure.) Sounds and vibrations audible to humans lie in the frequency range 20Hz-20000Hz (1Hz = 1 cycle per second). However, as far as construction is concerned, the significant range is 100-3200Hz, to which the human ear is particularly sensitive. In the human audible range, sound pressures extend from the hearing threshold to the pain threshold - (1). This hearing range is divided into 12 parts, called bels (after A. G. Bell, inventor of the telephone). Since 0.1 bei (or 1 decibel = 1 dB) is the smallest difference in sound pressure perceptible to the human ear at the normal frequency of 1000Hz, decibels are a physical measure of the intensity of sound, related to unit surface area - (1). Usually, noise levels of up to 60dB are expressed in dB(A); those of more than 60dB in dB(B), a unit which is approximately equivalent to the former unit, the phon.

For airborne sound, the sound level difference (between the original sound level and the insulated sound level) serves to indicate the degree of sound insulation. For body-propagated sound, a maximum level is given, which must remain from a standard noise level. Sound insulation, principally due to mass, is provided by the use of heavy, thick components in which the airborne sound energy is initially dissipated through transfer of the airborne sound into the component, then through excitation of the mass of the component itself and then, finally, by transfer back into the air. If the component is directly excited (body sound), then its insulation is naturally lower.

Light sound-damping construction - (6) makes use of multiple transfer (air to component to air to component to air) in providing sound insulation; better insulation, relative to that expected due to component mass, only occurs above the resonant frequency, however, which consequently should be below 100Hz. (This is comparable to the resonant frequency of the oscillation of a swinging door which is already swinging due to light impacts. It is simple to slow the motion of the door by braking; to make it move more quickly is more difficult and requires force.)

The intermediate space in double-shell construction is filled with sound-absorbing material, to avoid reflection of the sound backwards and forwards. The sound propagates in the air as a longitudinal wave (3), but as a transverse wave in solid materials. The speed of propagation of longitudinal waves is 340m/sec but, within materials, this depends on the type of material, layer thickness and frequency. The frequency at which the velocity of propagation of a transverse wave in a structural component is 340m/sec, is called the boundary frequency.

At this frequency, the transfer of sound from the air into the component and vice versa, is very good; therefore, the sound insulation of the component is particularly poor, poorer than would be expected from the weight of the wall. For heavy, quite inflexible building components, the boundary frequency is close to the frequency range of interest and therefore exhibits reduced sound insulation properties; for thin, flexible components, the boundary frequency is below this frequency range – (5).

With airborne sound, the aerial sound wave excites the component - (1); hence, the effect of the boundary frequency on the sound insulation increases – (5).

The standard curve shows how large the sound level difference must be at the individual frequencies, as a minimum, so as to achieve a level of sound insulation of ±0dB. Prescribed values - (2); required wall thicknesses – (7).

However, the effect of sound transmitted by 'secondary paths' (e.g. sound from foot steps) can be more disruptive than that from impact, so these must be taken into account in the sound insulation calculations. (For this reason, test results should always be drawn up for sound insulating walls with due consideration of the usual secondary paths.) Components which are stiff in bending, with weights per unit surface area of 10-160kg/m², are particularly likely to provide secondary paths. Therefore, living room dividing walls - which are contacted by such components in the form of lateral walls - should have a weight of at least 400kg/m². (Where the contacting walls have a surface weight of over 250kg/m², this value can be 350 kg/m².)

Doors and windows, with their low sound insulation properties - (6), have a particularly adverse effect on insulation against airborne sound; the small proportion of the surface occupied by the openings is usually subject to a sound insulation value which is less than the arithmetic mean of the sound damping of wall and opening. Therefore, the sound insulation of the door or window should always be improved where possible. Walls which have insufficient sound insulation can be improved through the addition of a nonrigid facing panel - (6) p. 117.

Double walls can be particularly well soundproofed if they contain soft, springy insulating material and are relatively flexible – (6) p. 117, or if the two wall panels are completely separately supported. Flexible panels are relatively insensitive to small sound bridges (by contrast to rigid panels). Type testing methods of construction should always be employed on sound insulating double walls. Covering layers of plaster on insulation materials of standard hardness (e.g. on standard styrofoam) considerably reduces the sound insulation.