Decibels (dB) in Ultrasonic Measuring Technique
In this article:
- The Decibel (dB) System Quantifies Relative Signal Strength: In ultrasonic testing, decibels express the ratio between two signal amplitudes—typically pressure or voltage—using a logarithmic scale to simplify comparisons.
- dB Values Are Calculated Using Logarithmic Formulas: The standard formula is dB=20log10(A2/A1)dB=20log10(A2/A1), where A2A2 and A1A1 are the measured and reference amplitudes, respectively, enabling precise signal evaluation.
- Positive and Negative dB Indicate Gain or Loss: A positive dB value reflects signal amplification, while a negative dB value indicates attenuation—critical for interpreting echo strength in flaw detection.
- Using dB Simplifies Complex Calculations: The logarithmic nature of decibels allows multiplication and division of signal ratios to be handled through simple addition and subtraction, streamlining ultrasonic analysis.
- dB Scaling Supports Standardized Ultrasonic Testing: By referencing echo amplitudes to known standards, the decibel system ensures consistent, repeatable measurements across different probes, materials, and inspection setups.
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 = 100,15
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 • 100,3
A2/A1 = 0,1 • 2,0 = 0,2
