7 Key Automotive Audio Measurements You Can Make with SoundCheck
Car audio is complex, but measuring it doesn’t have to be! SoundCheck measures a range of automotive infotainment parameters from simple frequency response to complex communications measurements. These include:
Frequency response. Frequency response is the clearest indicator of overall audio system performance. We recommend evaluating it according to TC-AA recommendations using 80dBA monophonic, correlated pink noise on both left and right input channels on the head unit. SoundCheck also supports the use of uncorrelated pink noise, which may yield additional useful information.
Seat-to-Seat Spectral Uniformity. Ideally, each passenger in the car experiences the same high-quality sound performance from the infotainment system. However, it is extremely difficult to independently equalize the loudspeaker playback system for each seat in the car. Trade-offs are made in the infotainment system tuning, typically favoring the driver’s seat. This results in the frequency response in the other seats being less smooth or ideal. SoundCheck measures the frequency response in each seat, then compares it to the driver’s seat to identify the seats with the greatest differences.
Total Harmonic Distortion (THD). THD is a valuable measurement which indicates the overall non-linear performance of the device; a measurement outside the limits may indicate a poorly centered voice coil, or mechanical defects. SoundCheck measures THD using both conventional and frequency-normalized methods. Frequency-normalized distortion offers greater insight to what is causing distortion for in-car measurements where there are many reflections.
Intermodulation Distortion. Intermodulation distortion is useful for evaluating multi-way speaker systems with a crossover. Intermodulation distortion can be observed when a small driver is trying to play back low and high frequencies simultaneously. Measurements are made using a two-tone stimulus with a fixed tone set to 50Hz and a sweeping tone from 20kHz to 150Hz, and responses are power-averaged prior to display.
Maximum SPL. Maximum SPL makes it easy to compare the maximum volume of an infotainment system in a car. It is the maximum sound pressure level that a car’s infotainment system can reproduce inside the cabin with its windows, sunroof, and convertible top closed, and it combines all the components of the system – speakers, amplifiers, power supply, etc. SoundCheck measures both the overall Maximum SPL and also the Max SPL spectrum, displaying them simultaneously.
Max SPL versus Frequency for a given percentage distortion. While Maximum SPL is the commonly specified measurement, it’s important that the audio system sounds good when it’s played loudly – this makes Maximum SPL at the onset of audible distortion an important metric. While time-consuming and manual with some test systems, SoundCheck’s conditional looping makes it easy to configure and automate. For each measured frequency, the tone-burst stimulus is played multiple times, increasing the level in 3dB steps until the THD exceeds, for example, 10%. The level is then reduced by 3dB and increased in smaller increments of 0.5 dB to more precisely find the 10% THD threshold. The Max SPL is recorded at that frequency and the test sequence continues to the next frequency. Although time consuming, the entire process is completely automated via the test sequence to minimize human interaction.
Impulsive Distortion. Buzz, Squeak, and Rattle (BSR) caused by vibrating door panels, loose wire harnesses, fasteners, etc. degrades the listener experience. Impulsive distortion can be used to measure this, and also serves as a useful measurement to make before performing system characterization measurements to ensure the infotainment system is correctly installed in the vehicle under test. SoundCheck offers three different impulsive distortion metrics: Crest Factor, enhanced Perceptual Rub & Buzz (ePRB), and enhanced Loose Particle detection (eLP). While Crest Factor is suggested by the AES Technical Committee on Automotive Audio (TC-AA), ePRB results correlate better to human audibility and provide increased reliability in noisy environments. SoundCheck’s eLP algorithm records individual transient distortion events versus time which reveals any loose particles or rattling wires in the car with greater precision.
