Handbook for Sound Engineers

Fundamentals and Units of Measurement 1655

48.3 Decibel (dB)

Decibels are a logarithmic ratio of two numbers. The
decibel is derived from two power levels and is also
used to show voltage ratios indirectly (by relating
voltage to power). The equations or decibels are

(48-39)

(48-40)

Fig. 48-4 shows the relationship between the power,
decibels, and voltage. In the illustration, “dBm” is the
decibels referenced to 1 mW.

Table 48-3 shows the relationship between decibel,

current, voltage, and power ratios.

Volume unit (VU) meters measure decibels that are

related to a 600: impedance, O VU is actually +4 dBm

(see Chapter 26). When measuring decibels referenced

to 1 mW at any other impedance than 600: , use

Figure 48-3. Power, voltage, current wheel.

Table 48-2. Frequency Classification

Frequency Band

No.

Classification Abbrevi-

ation

30–300 Hz 2 extremely low frequencies ELF

300–3000 Hz 3 voice frequencies VF

3–30 kHz 4 very low frequencies VLF

30–300 kHz 5 low frequencies LF

300–3000 kHz 6 medium frequencies MF

3–30 MHz 7 high frequencies HF

30–300 MHz 8 very high frequencies VHF

30–3000 MHz 9 ultrahigh frequencies UHF

3–30 GHz 10 super-high frequencies SHF

30–300 GHz 11 extremely high frequencies EHF

300–3 THz 12 – –

R

Z

E^2

(PF)

E^2

or

R

E or

Z

I^2 R or I^2 Z(PF) E

EI or EI(PF)

WR or

(PF)

WZ

I

W

I(PF)

or W

IR or IZ

or

E(PF)

W

E

W

or

Z(PF)

W

R

W

I

E

I^2

W

I^2 (PF)

or W

W

W

E^2

E(PF)

2

or

WI

E R or Z

Power dB 10

P 1

P 2

= log----- -

Voltage dBv 20

E 1

E 2

= log----- -

Figure 48-4. Relationship between power, dBm, and

voltage.

Milliwatts

Power VU dBm Volts

Watts