Handbook for Sound Engineers

72 Chapter 4

The low-frequency resonance effect is due to the
mechanical resonance of the barrier. For heavier
barriers, the resonant-frequency is usually below the
audible limit. As the panel vibrates at resonance, there
is virtually no transmission loss. At frequencies above
resonance, the mass law is in effect, and the function
stays fairly linear until the coincidence effect. The coin-
cidence effect occurs when the wavelength of the inci-
dent sound coincides with the wavelength of the
bending waves in the panel. For a certain frequency and
a certain angle of incidence, the bending oscillations of
the panel will be amplified, and the sound energy will
be transmitted through the panel with reduced attenua-
tion. The incident sound covers a wide range of
frequencies and arrives at all angles, but the overall
result is that the coincidence effect creates an “acous-
tical hole” over a narrow range of frequencies giving
rise to what is called the coincidence dip in the trans-
mission loss curve. This dip occurs above a critical
frequency, which is a complex function of the properties
of the material. Table 4-4 lists the critical frequency for
some common building materials.

4.2.4 Sound Transmission Class (STC)

The noise criterion approach is convenient and valuable
because it defines a permissible noise level and spec-
trum by a single NC number. It is just as convenient and

valuable to be able to classify the transmission loss of a

barrier versus frequency curve by a single number. The

STC or sound transmission class, is a single number

method of rating partitions.^9 A typical standard contour

is defined by the values in Table 4-5. A plot of the data

in Table 4-5 is shown in Fig. 4-9. Only the STC-40

contour is shown in Fig. 4-9, but all other contours have

exactly the same shape. It is important to note that the

STC is not a field measurement. The field STC, or

FSTC, is provided for in ASTM E336-97 annex a1. The

FSTC is often 5 dB or so worse than the laboratory STC

rating. Therefore a door rated at STC-50 can be

expected to perform around STC-45 when installed.

Nonetheless, the STC provides a standardized way to

compare products made by competing manufacturers.

Assume that a TL versus frequency plot of a given

partition is at hand and that we want to rate that parti-

tion with an STC number. The first step is to prepare a

transparent overlay on a piece of tracing paper of the

standard STC contour (the STC-40 contour of Table 4-5

and Fig. 4-9) to the same frequency and TL scales as the

TL graph. This overlay is then shifted vertically until

some of the measured TL values are below the contour

and the following conditions are fulfilled:^10

1. The sum of the deficiencies (i.e., the deviations
   below the contour) shall not be greater than 32 dB.


2. The maximum deficiency at any single test point
   shall not exceed 8 dB.

When the contour is adjusted to the highest value

that meets these two requirements, the sound transmis-

sion class of that partition is the TL value corresponding

to the intersection of the contour and the 500 Hz ordi-

nate. An example of the use of STC is given in

Fig. 4-10. To determine the STC rating for the

Figure 4-8. The performance of a barrier is divided into
four regions controlled by stiffness, resonance, mass, and
coincidence.

Table 4-4. Critical Frequencies

Material Thickness (Inches) Critical Frequency (Hz)

Brick wall 10 67

Brick wall 5 130

Concrete wall 8 100

Glass plate ¼ 1600

Plywood ¾ 700

*Calculated from Rettinger^8

Critical frequency

Heavy damping

Light damping

Transmission loss

Frequency

StiffnessResonance Mass controlled Co-

incidence

Figure 4-9. The standard shape used in determining the

sound transmission class (STC) of a partition (ASTM

E413-87).

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