Common dB References in Acoustics, Vibration, and Signal Processing
Decibels are always expressed relative to a reference value. Therefore, a dB value is incomplete unless the reference quantity is known. Different fields use different references depending on the physical quantity being measured.
In acoustics, the most common reference is the sound pressure reference in air p0 = 20 μPa
This is used for Sound Pressure Level, or SPL Lp = 20 log10 (p / p0)
This reference is approximately related to the threshold of human hearing at 1 kHz. For underwater acoustics, the standard reference pressure is different p0 = 1 μPa
Therefore, underwater sound levels are usually written as dB re 1 μPa
Sound power level uses a different reference W0 = 10 −12 W and sound intensity level uses I0 = 10 −12 W/m2
These are power-like quantities, so they use 10 log10 not 20 log10
In vibration, common references include acceleration, velocity, and displacement references. For example, acceleration level may use a0 = 10 −6 m/s2, velocity level may use v0 = 10 −9 m/s and displacement level may use x0 = 10 −12m
In signal processing and electronics, common references include
| Quantity | Reference | Notation |
|---|
| FFT Magnitude | 1 | dB |
| Power Spectrum | 1 (unit²) | dB |
| PSD (Power Spectral Density) | 1 (unit²/Hz) | dB/Hz |
| Sound Pressure Level | 20 μPa in air, 1 μPa in underwater | dB SPL, dB re 1 μPa |
| Sound Power Level | 10-12 W | dB SWL or dB re 1pW |
| Sound Intensity Level | 10-12 W/m2 | dB SIL or dB re 1pW/m2 |
| Acceleration | 1 m/s2 or 1 μm/s2 | dB re 1 m/s²... |
| Velocity | 1 m/s or 1 nm/s or 5x10-8 m/s | dB re 1 m/s ... |
| Displacement | 1 m or 1 μm or 1 pm | dB re 1 m ... |
| Voltage | 1V | dBV |
| Electrical power | 1mV | dBm |
MALMIJAL Example
Right-click the Y-axis label to open the dB Reference Settings dialog.
Refer to Samples/octave band.mmj
Summary
The key point is simple. A dB value is meaningful only when its reference is specified.
For example, 60 dB SPL, 60 dB re 1 μPa, 60 dBV, and 60 dBm all have completely different physical meanings. Therefore, when using decibels, the reference value must always be stated clearly.
Suggested Further Reading
You may also find these topics helpful:
Common dB References in Acoustics, Vibration, and Signal Processing
Decibels are always expressed relative to a reference value. Therefore, a dB value is incomplete unless the reference quantity is known. Different fields use different references depending on the physical quantity being measured.
In acoustics, the most common reference is the sound pressure reference in air p0 = 20 μPa
This is used for Sound Pressure Level, or SPL Lp = 20 log10 (p / p0)
This reference is approximately related to the threshold of human hearing at 1 kHz. For underwater acoustics, the standard reference pressure is different p0 = 1 μPa
Therefore, underwater sound levels are usually written as dB re 1 μPa
Sound power level uses a different reference W0 = 10 −12 W and sound intensity level uses I0 = 10 −12 W/m2
These are power-like quantities, so they use 10 log10 not 20 log10
In vibration, common references include acceleration, velocity, and displacement references. For example, acceleration level may use a0 = 10 −6 m/s2, velocity level may use v0 = 10 −9 m/s and displacement level may use x0 = 10 −12m
In signal processing and electronics, common references include
MALMIJAL Example
Right-click the Y-axis label to open the dB Reference Settings dialog.
Summary
The key point is simple. A dB value is meaningful only when its reference is specified.
For example, 60 dB SPL, 60 dB re 1 μPa, 60 dBV, and 60 dBm all have completely different physical meanings. Therefore, when using decibels, the reference value must always be stated clearly.
Suggested Further Reading
You may also find these topics helpful: