Engineering acoustics relies on core fundamental metrics and equations to quantify sound pressure, energy, impedance, and room characteristics.

spl, sound pressure level

reference acoustic pressure by medium

Medium	Reference Pressure, $p_{ref}$ in ozf/in2 or osi
Air	46.4 billionths
Freshwater	2.32 billionths
Seawater	2.32 billionths

Sound Source	SPL (dB)	RMS acoustic pressure (osi)
	140
	130
	120
	110
	100
	90
	80
	70
	60
	50
	40
	30
	20
	10
	0

Measures the logarithmic ratio of a sound pressure wave relative to the standard reference threshold of hearing in air (20 μPa)

SPL = 20$\log_{10} (p/p_{ref})$ dB

acoustic impedence

$Z=p/v=\rho c$

• $Z$ is acoustic impedance (Pa*s/m) or Rayls
• $p$ is sound pressure
• $v$ is particle velocity
• $\rho$ is medium density in kg/m3
• $c$ is speed of sound

acoustic intensity

$I_{total}=\frac{p_{rms}^2}{\rho_oc}$

• $I_{total}$
• $p_{rms}$
• $\rho_0$
• $c$

sound intensity level

a logarithmic measure of sound power per unit area, expressed in decibels (dB), relative to the threshold of human hearing (10⁻¹² W/m²)

$IL=$10$\log_{10} (I/I_{ref})$

pressure spectrum level

the effective SPL of an acoustic signal contained within a frequency band of 1 Hz or 1 cycle/sec wide

PSL = SPL – 10$\log_{10} (\Delta f/1 \space\text{Hz})$

band level

BL = PSL + 10$\log_{10} (\Delta f/1 \space\text{Hz})$