As vehicles become more intelligent, electrified, connected and shared, the cabin is no longer just a physical space where the driver controls the car and passengers ride. It has become an important environment that delivers personalised service and comfort. In this context the role of the in-car audio system has evolved from a simple broadcast and playback device into a key part of the smart cockpit experience and a focus for consumers. Future cabin audio will need not only high quality sound hardware but also intelligent, personalised and immersive listening experiences based on the cabin environment, vehicle status and user needs, supported by vehicle connectivity, OTA and artificial intelligence.

Background to automotive speaker development

Background to automotive speakers

Once a vehicle is electrified, the absence of engine noise makes the cabin quieter, so drivers and passengers become more sensitive to the sound field and place higher demands on speaker quality. New energy vehicle makers are therefore more willing to add speakers to their models to improve cabin sound quality. A conventional combustion vehicle usually has 4 to 8 automotive speakers, about 6 on average, whereas flagship models from some new entrants reached more than 20 speakers in 2023. New energy models clearly carry more speakers than conventional vehicles, and premium new energy vehicles tend to fit more.

Speaker layout in a vehicle

The growth of the automotive industry brings both opportunities and challenges for speaker makers. The in-car acoustic system is a key element in improving in-cabin entertainment, and a good acoustic system is readily perceived by users. Consumer demand is moving towards high end, entertainment-focused and high fidelity audio. As the number of speakers fitted rises and their quality improves, the whole industry is thriving.

How automotive speakers differ from ordinary speakers

In basic acoustic performance such as impedance, power, frequency response and sensitivity, automotive speakers are broadly similar to ordinary speakers. The technical depth and difficulty, however, are clearly different and often much higher, including materials, structural design and manufacturing processes. Unlike most speakers, which operate in relatively simple and open environments, the vehicle interior is cramped and contains irregular objects, making the acoustic environment complex. Noise diversity and the dynamic nature of driving further increase the difficulty of designing and laying out automotive speakers. The complexity of the driving environment demands good resistance to vibration, shock and humidity. Automotive speakers face problems that ordinary speakers do not, and their operating environment differs in the following ways.

(1) Temperature. Automotive speakers mainly work outdoors, and temperatures vary widely with the weather, so they must resist thermal stress without ageing or deforming. In summer they are affected by high temperature, in winter by severe cold. Ageing prevention and thermal stress resistance must therefore be strengthened continuously.

(2) Humidity. A humid environment is unavoidable for automotive speakers and has a very significant effect. In the rainy season, for example, standing water on the road and higher air humidity strongly interfere with automotive speakers. Moisture protection technology must be improved to reduce the damage and loss caused by humidity.

(3) Dust. Because vehicles drive outdoors and windows and doors are often opened, dust enters the cabin, so automotive speakers must improve dust protection to reduce noise caused by dust ingress.

(4) Vibration. Vehicles vibrate while driving, and rough roads produce vibration and shock, so automotive speakers are constantly exposed to mechanical stress. Vibration cannot be ignored in vehicle development and production. Automotive speakers must use robust materials in design and processing and be mounted firmly, so that they do not break or detach under vibration.

Acoustic parameters of automotive speakers

To meet consumer expectations for comfort and reliability, the reliability of automotive speakers matters greatly, which requires strict combined testing of acoustic performance and reliability.

The main acoustic parameters are rated power, rated impedance, resonant frequency, frequency response range, Qts, directivity, sensitivity and distortion.

Rated power. Also called continuous power, this is the input electrical power a speaker can handle for long-term stable operation, in watts, reflecting thermal capability. Automotive speakers commonly use 30 W to 100 W. Peak power is the short-term maximum the speaker can withstand, usually two to four times the RMS value.

Rated impedance. The equivalent electrical impedance at a specified frequency, usually 1 kHz, in ohms. Automotive speakers are mostly 4 Ω to match factory head units, while 2 Ω can be chosen for upgraded high end systems to raise amplifier current efficiency. If impedance falls below the amplifier minimum load, protection circuits may trip or the amplifier may be damaged.

Resonant frequency. The frequency, in hertz, at which the diaphragm amplitude is greatest in free air.

Frequency response range. Frequency response describes how output sound pressure level varies with frequency. Plotted as a graph it is a curve with frequency on the horizontal axis and output sound pressure, or its decibel value, on the vertical axis. The curve is broadly flat in the mid band and rolls off at the low and high ends. The two points where output falls 3 dB below the mid band level are the low and high cut-off points, since the ear readily detects level changes beyond 3 dB, and the band between them is the frequency response range of the speaker.

Qts. The quality factor, or damping factor, is a key dimensionless parameter reflecting the damping of the speaker's moving system. It works like a vehicle shock absorber, helping control cone movement, and indicates the damping state, meaning how quickly vibration decays, and the sharpness of resonance. A low Qts means vibration stops quickly, a high Qts means it does not. If Qts is too low, output falls rapidly before reaching Fo, the speaker is over-damped and low frequency response is attenuated excessively. If Qts is too high, a peak appears at Fo, the speaker is under-damped and low frequencies are over-emphasised; the higher the Qts, the steeper the peak.

Directivity. The fall-off of sound pressure with radiation angle, usually shown as a polar plot. Radiation varies in sound pressure and frequency response with direction, and this is the directivity of the speaker. It depends on cone diameter - a large diameter gives narrow directivity, a small diameter gives wide directivity - and also on frequency.

Sensitivity. This reflects the loudness a speaker produces. The industry tests sensitivity by feeding a 1 kHz signal at 1 W and measuring the sound pressure level at 1 m. If sensitivity is too low, the same input produces less volume, so more drive is needed to reach a given loudness and distortion increases. Higher sensitivity generally means higher total harmonic distortion, especially at high output.

Distortion. Total harmonic distortion occurs because the output cannot exactly reproduce the input, producing waveform deformation or changes in signal content. Harmonic distortion exists in any signal and represents spurious content beyond the original signal. Because audio signals cover a very wide frequency range, this parameter is usually quoted as a nominal value, and the figure in a speaker data sheet is typically measured at a single 1 kHz tone and is only representative. Distortion varies with frequency but generally stays in a similar range. Speakers that sound tiring or irritating usually have higher distortion.

NTEK laboratory and capabilities

Testing automotive speakers requires first a full anechoic chamber with excellent specifications. NTEK Group operates a full anechoic chamber with an internal volume of 3 m by 4 m by 5 m, an average background noise level of 9.5 dBA, a cut-off frequency below 100 Hz and a free field radius greater than 1.2 m, which is more than sufficient for testing automotive speakers.

Besides the anechoic chamber, NTEK Group also operates a two-channel audio analyser with the highest performance and widest function set in the industry. It offers typical residual distortion of -120 dB, bandwidth up to 1 MHz, 1.2 million point FFT, full 24-bit resolution and a sweep range from 0.001 Hz to 80 kHz. These demanding specifications allow perfect testing at both the high and low frequency ends of a speaker.

NTEK Group also has high end accessories including a simulated torso, mouth simulator, artificial ear and free field microphone, giving it the capability to complete the full set of acoustic parameter tests for speaker products.

Acoustic performance test setup

Example sensitivity test data

Example distortion test data

In addition, NTEK Group has strong reliability testing capability, including environmental reliability such as high and low temperature, low air pressure and IP rating, and mechanical reliability such as vibration and shock. This allows NTEK to combine reliability testing with acoustic performance testing to evaluate the overall performance of automotive speakers, helping speaker makers identify weak points and improve their products in support of China's new energy vehicle industry.