Designing the Audio Subsystem for Smart Cockpits – Part 2

Acoustic Basics

Vehicle audio systems consist of microphones, speakers, power amplifiers, domain controllers and a system topology that interconnects these blocks.

Basic Sound Effects

Basic sound effects comprise multimedia audio processing such as equalizer (EQ) adjustments for bass, mids and treble, volume, balance and fader controls, and optional virtual surround simulation. They compensate for constrained speaker placement (e.g., door‑mounted speakers that are offset from the listener’s head) to achieve a virtual sound source directly ahead and to match the measured in‑car frequency response to a target curve.

Calibration workflow:

Measurement – place multiple artificial‑head microphones at representative cabin locations and record frequency‑response curves.

Offline virtual calibration – process the measured data with software to generate initial EQ settings.

In‑car final calibration – adjust speaker EQ, align high/mid/low speakers per channel, ensure phase and frequency alignment, fine‑tune gain and delay, and apply overall EQ presets.

Active Sound Generation

AVAS (Acoustic Vehicle Alerting System) – front‑mounted speakers emit a designed sound signal at low vehicle speeds to satisfy pedestrian‑alert regulations in Europe (UN/ECE R138), Japan, the United States (FMVSS 141) and China (GB/T 37153).

Internal‑combustion‑engine (ICE) active sound – sensors monitor engine speed, throttle position and vehicle speed; an audio processor synthesises engine‑like sounds for driver immersion, especially in high‑performance vehicles.

Electric‑vehicle (EV) active sound – similar sensor‑driven synthesis produces engine‑characteristic sounds while meeting environmental‑noise constraints.

Active Noise Control (ANC)

ANC operates in a closed‑loop:

Noise detection – cabin microphones continuously capture low‑frequency noise (e.g., engine rumble).

Signal processing – the captured signal is analysed for frequency, amplitude and phase.

Anti‑phase generation – an inverse‑phase signal with equal amplitude is generated.

Playback – the anti‑noise is emitted through speakers, causing destructive interference that reduces the original noise.

Road‑Noise Cancellation (RNC)

RNC follows the same principle as ANC but targets road‑noise frequencies of 30–500 Hz. It requires four vibration sensors mounted on the suspension and a dedicated processing chain. Typical reduction is about 10–20 dB in the targeted band.

Other Functions

3D Chime – uses multiple cabin speakers and spatial processing to create localized alert tones that appear to originate from specific directions, enhancing safety and user experience.

In‑Car Communication (ICC) – a microphone captures a passenger’s speech and nearby head‑rest speakers replay it, enabling conversation without raising overall cabin volume.

KTV – in‑car karaoke can be implemented either with existing cabin microphones or with a handheld microphone, each offering different trade‑offs in installation and acoustic isolation.

System Dependencies

Each acoustic function imposes computational (DSP) demand and latency requirements. Functions are mapped to approximate compute load and latency to guide algorithm placement on hardware blocks.

Microphone specifications required for these functions:

Sensitivity ≈ ‑26 ± 2 dBFS re 11 Pa at 1 kHz

Frequency‑response deviation < 1 % at 94 dB SPL (1 kHz)

Sample‑rate coverage 30 – 500 Hz

SNR > 60 dB (A‑weighted)

Three‑axis vibration sensor specifications for RNC:

Water‑proof, shock‑resistant packaging

Resolution ± 16 g (14‑bit)

Bandwidth 20 Hz – 4 kHz

Low latency, low noise

Operating temperature ‑40 °C – 125 °C

Audio System Design

Vehicle‑wide Audio Strategy

Audio channel definition – enumerate all audio functions and assign matching speakers.

Audio arbitration strategy – define mixing and interruption rules for every pair of functions, establishing how they share or pre‑empt each other.

A sample mixing matrix illustrates the pairwise arbitration decisions.

Audio Route Design

The Audio Route derives from the complete set of audio functions and determines the software architecture, which in turn informs hardware design reviews.

Typical Audio Route diagrams show the mapping from high‑level functions to processing blocks and hardware interfaces.

Design must also consider integration with other subsystems such as ASR and Bluetooth telephony.