Mechanism of sound generation by electrical signals

What is sound?

To comprehend the type system needed for generating sound using an IC, it is important to first understand how electrical signals produce sound.
Sound propagates through vibrations, which can be represented as sine waves.
A sine wave has a frequency, with vibrations between 20Hz and 20kHz, known as the audible frequency range, can be heard by the human ear as sound.

Mechanism of sound generatio using a speech synthesis IC

Let's look at the mechanism of sound generation in a dynamic speaker. When an electric current flows through the coil inside a speaker, a Lorentz force is generated between the coil and permanent magnet, as described by Fleming’s left-hand rule. As the current fluctuates as an electric signal, the coil vibrates accordingly. If these vibrations fall within the audible frequency range, they are perceived as sound. Specifically, the vibration of the coil is transferred to the connected cone, which then causes the surrounding air to vibrate, generating sound waves.
In other words, when an electric signal corresponding to a sound is applied to the speaker, the speaker produces a sound that matches the signal. This is the fundamental mechanism by which electrical signals generate sound.

As we have seen, sound can be represented as a sine wave, which means that it can be reproduced using electrical signals. There are two common methods for generating these signals: 1) Analog signals that vary the voltage over time, controlling speakers through amplitude and frequency modulation, and 2) PWM (Pulse Width Modulation) signals that adjust the pulse width to replicate analog signals and control speaker output accordingly.

Analog Signal

A key characteristic of an analog signal is its continuity. Simply put, it has an infinite number of intermediate values, forming a smooth, uninterrupted line when graphed. In fact, analog signals are present all around us, with sound being a prime example. By transmitting an analog electrical signal that mirrors the sound waveform to the speaker, sound can be generated.

PWM Signal

PWM signals generate sound by controlling the energy applied to the speaker through variations in pulse width.
A pulse is a general term for a signal that undergoes rapid changes over a short period, including digital signals. Pulse widths consist of an H (High) pulse width and L (Low) pulse width, with their sum defining the pulse period.
The most important parameter of PWM is the duty cycle, calculated as the ratio of the H pulse width to the pulse period.
By finely adjusting this cycle, devices like speakers can be accurately controlled.

How a PWM signal produces sound

Sound is generated when a PWM signal is input to a speaker because the duty cycle of the PWM signal pulse replicates an analog signal. While a digital signal is not analog, the duty cycle introduces analog-like characteristics. Creating a PWM signal involves sampling the original analog signal and generating pulses with a duty cycle proportional to the amplitude of the sampled waveform. As a result, passing a PWM signal through a low-pass filter converts it into an analog signal.