# Acoustics Corollary: Sound Pressure, Intensity, and Power
Just as in electricity, where Power=Pressure×Flow (P=V×I), sound operates on a similar principle. Understanding these three acoustic terms is the corollary to mastering Volts, Amps, and Watts.
## The Core Relationship: The Acoustic Equivalent
The acoustic relationship is defined by **Acoustic Intensity**, which is the sound energy equivalent of electrical power:
Acoustic Intensity=Sound Pressure×Particle Velocity
| Electrical Term | Acoustic Term | What it Measures | Analogy in Audio |
| ------------------------- | -------------------------- | ---------------------------------------------------------------------- | ------------------------------------------------- |
| **Voltage** (V, Pressure) | **Sound Pressure** (P) | Local change in air pressure due to sound waves. | The _strength_ of the wave measured at one point. |
| **Current** (I, Flow) | **Particle Velocity** (v) | The speed and direction of air particles moving due to the sound wave. | The _movement_ of air. |
| **Power** (P, Watts) | **Acoustic Intensity** (I) | The total energy flow per unit area (W/m2). | The _energy_ carried by the wave. |
## 1. Sound Pressure: The Voltage Equivalent
**Sound Pressure** is the instantaneous, local deviation from the ambient (still air) pressure caused by a sound wave.
- **Fixed vs. Variable:** Unlike the AC power **supply** voltage (120 V) which is constant, the **Sound Pressure** is the _signal itself_, making it inherently **variable** and constantly changing over time (just like the CV in a synth or the music signal in a wire).
- **Measurement:** Sound Pressure is measured in Pascals (Pa), but because the range of human hearing is so vast, it is universally reported using a logarithmic scale: **Decibels (**dB**)**, specifically **Sound Pressure Level (**SPL**)**.
## 2. Acoustic Intensity: The Power Equivalent
**Acoustic Intensity** (IA) is the measure of the average rate of energy flow per unit area perpendicular to the sound wave's direction.
- **Formula:** IA is the product of Sound Pressure and Particle Velocity.
- **Significance:** It tells you how much acoustic energy is being delivered to a specific point (like your ear, or a microphone capsule).
### Why We Use Power (Watts)
While Intensity (W/m2) is a precise measure, in large-scale acoustics, we often talk about **Acoustic Power** (W).
**Acoustic Power** is the total energy emitted by the source (the speaker). It is the acoustic equivalent of the electrical Wattage rating of a power amplifier.
- A speaker rated for high **Acoustic Power** can output a lot of energy, but the resulting **Sound Pressure (**SPL**)** you hear is still highly dependent on distance and room characteristics.
## 3. The Oscilloscope Analogy in Acoustics
You asked earlier about the oscilloscope measuring Voltage. There is an acoustic corollary to this in measurement:
|Electrical Measurement|Acoustic Measurement|What is Observed|
|---|---|---|
|**Oscilloscope**|**Microphone**(+ Analyzer)|The microphone measures the instantaneous **Sound Pressure** and converts it into an electrical signal (**Voltage**) for analysis.|
|**Voltmeter**|**SPL Meter**|The SPL meter takes the AC electrical signal (from the microphone) and calculates the **RMS (Root Mean Square)** average of the voltage, reporting it as a logarithmic dBvalue.|
In short, when you look at an acoustic waveform on a computer or analyzer, you are viewing the **Sound Pressure (Voltage)** changing over time.