No_4_December_1963 — Page 140

Measurements

have

fluence the noise level from traffic markedly. shown that noise at first and second storey levels is slightly less than at ground level, particularly where a podium screens the windows from direct view of the traffic. but at higher levels the noise does not fall off with distance above ground as might be expected. This is not only a 'canyon' effect, with both sides of a busy street flanked by tall build- ings; it also occurs with compara- tively isolated tall blocks where the screening effect to the sides and rear of the upper storeys is not so mark- ed as near ground level.

traditional

Since windows in structures are generally single-glazed and openable, all parts of such buildings exposed to similar noise climates will be affected in the same way. An open window provides little, if any, attenuation to external noise; elaborate double windows can virtually give complete protection. The degree of protection necessary in any particular case depends on the external noise climate and the standard of internal quietness desir ed.

Noise reduction of windows

In order to calculate the effective noise levels inside buildings for a given noise climate outside it is

necessary

to know the reduction provided by different kinds of window. For any type this is least at low frequencies and most at high frequencies. Normal measurements of the insulation of partitions, in- cluding windows, are made over the frequency range 100-3200 c/s. It would be very convenient if the single figure average of the insula- tion over this frequency range could be subtracted from the value for the external noise climate in dB(A) to predict the likely internal noise level, also in dB(A). Unfortunately this is not possible and the only accurate way of predicting inside levels is by subtracting the measured values of insulation at different frequencies from the spectrum of the outside noise and recalculating a single figure in dB(A) for the internal level obtained. Figure 2 shows a typical external noise spectrum, the insula- tion of a closed single window and the resultant intrusive noise level. The characteristic low-frequency rumble of traffic heard through clos-

132

REDUCTION DUE TO

SOUND PRESSURE LEVEL

dB ABOVE 0·0002 DYN/5Q.CM

9P MOONIM

TYPICAL SPECTRUM OF OUTSIDE TRAFFIC NOISE

(77AŁA

50

RESULTANT INSIDE SPECTRUM

(55 dBA)

30

20

10

100

200

TYPICAL REDUCTION OF WELL SEALED SINGLE WINDOW (32 oz GLASS IN METAL FRAME, BRICK WALL)

(AVERAGE. 100-3200 c/s, 24 dB)

1600

3200

6400

400

800 FREQUENCY </s FIG. 2. Noise reduction of window

ed windows is seen to be due to the relative emphasis of the low frequen- cies in the inside spectrum.

Typical average values of insula- tion for different types of window are given in Table 2. These are the effective single-figure insulation values for rooms of

average size having typical windows, not exceed- ing half the area of the external building structures. At positions close to or remote from the windows in large or highly absorbent in- teriors the intrusive noise level may differ appreciably from the average for the room. Values for curtain- walled buildings with areas of lightweight cladding which may itself have air gaps are not sufficient- ly well known to quote.

Table 2

INSULATION OF WINDOWS

AGAINST NOISE

(a) Open window, up to 10 dB de-

pending on area open

(b) Closed single

single windows

with some air-gaps at the edges, 15-20 dB

(c) Closed well-fitting single win-

dows, 20-25 dB

(d) Heavy (4-in. plate) single win- dows with edges well sealed, 25-30 dB

(e) Fully treated double glazed win- dows, sealed, with large (8-in.) airspace and absorbent reveals, 40 dB.

Buildings having 'thermal double glazing with two narrowly separated panes of glass, possibly sealed in the same frame, have no greater pro- tection against external noise than single windows having the same total weight of glass. A wide absorbent air-space is needed to develop the maximum value of 40 dB quoted in Table 2. Where such windows are provided with good edge sealing to cut down thermal losses this is also an advantage acoustically.

At the higher levels of insulation such as

are obtained with treated double windows, the insulation of the remaining structure, even in traditional buildings, may affect the overall reduction obtained in prac tice. For instance, the insulation through a pitched tiled or slated roof with thin plasterboard ceiling is of the order of 35 dB (average) and this limits the reduction obtainable in the upper rooms of such build- ings. (There is obviously an even greater limitation for rooms actually built in to such roof spaces).

Desirable limits for intrusive noise in buildings

Any figures for desirable limits are essentially approximations where they depend on subjective reactions to noise. Where the intrusive noise interferes with aural communication the acceptable levels in various cir- cumstances can be stated a little more accurately. Table 3 gives the levels,

THE HONG KONG & FAR EAST BUILDER-VOLUME 18, NUMBER 4

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