Characteristic impedance
The characteristic impedance of a medium (usually air, but also for instance exhaust gases in a silencer) is the ratio of sound pressure p to particle velocity v in open field (i.e. in a condition of no reflecting waves). This impedance is a material constant and equals the product of the density of air ρ (rho) and the speed of sound c:
ρ (rho)=density of air in kg/m3
c=speed of sound (acoustic wave velocity) in m/s
Note: Z0 is expressed in Pa·s/m. Before the Pascal was introduced by the SI, impedance was expressed in N·s/m3.
The characteristic acoustic impedance of air Z0 equals:
410 Pa·s/m at a temperature of 25°C (77°F)
413 Pa·s/m at 20°C (68°F).
°C=degree Celsius and °F=degree Fahrenheit.

Clarity C80, C50 (dB)
The measurement of Clarity is the ratio of the energy in the early sound compared to that in the reverberant sound, expressed in dB. Early sound is what is heard in the first 80 msec (C50 - 50 msec) after the arrival of the direct sound. It is a measure of the degree to which the individual sounds stand apart from

one another. If the clarity is too low, the fast parts of the music are not “readable” anymore. C80 is a function of both the architectural and the stage set design.
If there is no reverberation in a dead room, the music will be very clear and C80 will have a large positive value. If the reverberation is large, the music will be unclear and C80 will have a relatively high negative value. C80 becomes 0 dB, if the early and the reverberant sound is equal. Often the values for 500Hz, 1000Hz and 2000Hz are averaged.
This will be expressed by the symbol C80(3). For orchestral music a C80 of 0dB to -4dB is often preferred, but for rehearsals often conductors express satisfaction about a C80 of 1dB to 5dB, because every detail can be heard. For singers, all values of clarity between +1 and +5 seem acceptable. C80 should be generally in

the range of -4dB and +4dB. For speech, in comparison to music, the clarity will be measured as the ratio of the first 50 msec (C50) instead of 80 msec (C80) for music.

Critical frequency
In building acoustics (sound transmission) the frequency, where the sound of speed in air equals the propagation speed of bending waves in the partition.
The main mechanism of sound transmission through the partition changes at the critical frequency. At this frequency the sound reduction index drops down drastically.
The critical frequency depends on the material type (bending stiffness) and thickness of the partition.