Handbook for Sound Engineers

Acoustical Noise Control 73

measured TL curve shown in Fig. 4-10, the STC

overlay is first aligned to 500 Hz and adjusted vertically

to read some estimated value, say, STC-44. The differ-

ence between the measured TL level and the STC curve

is recorded at each of the octave points. These data

are added together. The total, 47 dB, is more than the

32 dB allowed. The STC overlay is next lowered to an

estimated STC-42, and a total of 37 dB results.

Lowering the overlay to STC-41 yields a total of 29 dB,

which fixes the STC-41 contour as the rating for the TL

curve of Fig. 4-10.

The final illustration of STC methods is given in

Fig. 4-11. In this case, a pronounced coincidence dip

appears at 2500 Hz. This illustrates the second STC

requirement, “the maximum deficiency at any single test

point shall not exceed 8 dB.” This 8 dB requirement

fixes the overlay at STC-39, although it might have been

considerably higher if only the 32 dB sum requirement

applied.

The shape of the standard STC contour may be very

different from the measured TL curve. For precise

work, using measured, or even expertly estimated, TL

curves may be desirable rather than relying on STC

single number ratings. Convenience usually dictates use

of the STC shorthand system, but it is, at best, a rather

crude approximation to the real-world TL curves.

Assume that a goal of NC-20 has been chosen for a

sound room. The noise survey indicates a noise level

and spectrum as shown in Fig. 4-12. What wall

construction will bring the noise of Fig. 4-12 down to

the NC-20 goal we have set for the interior? Fig. 4-13

shows that a wall having a rating of STC-55 is required.

The next step is to explore the multitude of possible

wall configurations to meet the STC-55 requirement as

well as other needs.

If the NC curve in Fig. 4-12 is subtracted from the

measured noise curve, this will indicate the raw data

that indicates the amount of loss needed to achieve the

desired NC. This is plotted in Fig. 4-13. The standard

STC template is laid over the graph and the needed STC

is read opposite the 500 Hz mark.

Table 4-5. Standard STC Contour

Frequency

in Hz

Octave Sound

Transmission Loss

in dB

Frequency

(Hertz)

Octave Sound

Transmission Loss

in dB

125 24 800 42

160 27 1000 43

200 30 1250 44

250 33 1600 44

315 36 2000 44

400 39 2500 44

500 40 3150 44

630 41 4000 44

Figure 4-10. The method of determining the single-number

STC rating of a barrier from its measured TL graph.

(^1) » 3
(^1) » 3 1 » 3
50
40
30
(^20) 125 250 500 1k 2k 4k
160 315 630 1.25k 2.5k
200 400 800 1.6k 3.15k
(^1) / 3 octave band center frequency—Hz
3.03.85.25.2 5.0
3.7
2.0 0.8
STC-41
Sound transmission loss—dB
Measured TL
Figure 4-11. The second rule for STC determination that a
maximum deficiency of 8 dB is allowed.
Figure 4-12. Noise spectrum from noise survey.
STC-39
50
40
30
(^20) 125 250 500 1k 2k 4k
160 315 630 1.25k 2.5k
200 400 800 1.6k 3.15k
(^1) / 3 octave band center frequency—Hz
Sound transmission loss—dB
Measured TL
8 dB
32 63 100 200 400 8001.25k 2k 3.1 5k 8k
60
55
50
45
40
30
25
20
15
10
Frequency–Hz
SPL–dB