Page 211 - Master Handbook of Acoustics

Eyring-Norris Equation

Eyring and Norris, and others, have presented reverberation equations which overcame the

limitations of live rooms and can be used in more absorptive rooms. They are used for rooms in
which the average absorption coefficient is greater than 0.25. For rooms with an average absorption
coefficient of 0.25 or less, these equations are basically equivalent to the Sabine equation.
In particular, Eyring and Norris proposed an alternative equation for absorptive rooms:

where V =
room volume, ft 3
S =

total surface area of room, ft 2
ln =
natural logarithm (to base “e”)
α average =

average absorption coefficient (ΣS α ΣS )
i
i i
Young points out that the absorption coefficients published by materials manufacturers (such as
listed in the appendix) are Sabine coefficients and can be applied directly in the Sabine equation.
Young recommends that Eq. (11-1) or (11-2) be used for engineering computations rather than the
Eyring-Norris equation or its several derivatives. Two unassailable reasons for this are simplicity

and consistency. Many technical writings offer the Eyring-Norris or other equations for studio use.
There is authoritative backing for using Eyring-Norris for more absorbent spaces, but commonly
available coefficients apply only to Sabine. For this reason, we use the Sabine equation in this
volume. Other researchers have suggested alternative reverberation equations; these include Hopkins-
Striker, Millington, and Fitzroy.

Air Absorption

In large rooms, where sound travels over long path lengths, the passage through air can effectively
add absorption to the room, lowering reverberation times. Air absorption is only significant at higher
frequencies—above 2 kHz. Air absorption is not a significant factor in small rooms, and can be
neglected. When accounted for, a term 4mV is added to the denominator of reverberation time
equations, where m is the air attenuation coefficient in sabins per foot (or sabins per meter) and V is
room volume in cubic feet (or cubic meters). For example, the Sabine and Eyring-Norris equations,

respectively, are modified as: