Decibels (dB) in Ultrasonic Measuring Technique

The ultrasonic measuring technique depends an relationships. The absolute values of many measuring parameters are not as interesting as are their relative relationshiPs to a reference quantity. In order to be able to give a relationship of quantities one uses a measure especially created for this purpose: the decibel (dB).

The decibel doesn't mean anything other than 20 times the decadic logarithm of a ratio.

In the ultrasonic technique lt is mostly the pressure amplitudes which are compared p2/pi or also electrical signal voltages U2/U1.

An example:

p₂ = 3p₁

p₂/p₁ = 3

20 Ig ( p₂/p₁) = 20 • (0.47712) = + 9,5 dB

or when the reverse applies:

p₂ = p₁/3

p₂/p₁ = 1/2

20 Ig (p₂/p₁) = 20 (- 0.47712) = - 9,5 dB.

lt the relationship of 3 dB applies between voltages U2/U1 how big is the ratio?

20 Ig (U₂/U₁) = 3 dB

Ig (U₂/U₁) = 0,15

U₂/U₁ = 10^0,15

U₂/U₁ = 1,4125

The ratios of quantities, and dB values are given in Table 3.

As the dB is a logarithmic value the advantage is offered, when dealing with rational expressions, of form

eqn. 

When using dB values the multiplication and division in eqn. (7) become simple addition and subtraction.

Which size ratio corresponds to -14 dB?

20Ig (A₂/A₁) = - 14 dB = - 20 dB + 6 dB

Ig ( A₂/A₁) = - 1 + 0,3

A₂/A₁ = 10^-1 • 10^0,3

A₂/A₁ = 0,1 • 2,0 = 0,2

What is the range of applications of ultrasonic testing?