Automotive Max SPL Measurements
Automotive Max SPL measurements provide a way to compare how loud an infotainment system can play in a car— manufacturers like to quote this in specifications, and consumers enjoy bragging rights about the sound level of their car stereo. However, a numerical value that is published by multiple manufacturers needs to be measured the same way by everyone in order to be meaningful.
Max SPL is defined as the maximum sound pressure level (SPL) that a car’s infotainment system can reproduce inside the cabin with the windows, sunroof, and convertible top closed. There are many ways this can be measured, but the the Audio Engineering Society (AES) technical committee working group on automotive audio recommends just overall Max SPL and Max SPL Spectrum regardless of distortion level. They do not recommend including distortion measurements as it is difficult to characterize distortion in a modern-day infotainment system—these devices frequently contain much signal processing, and this makes them unsuitable for playing back the sine wave stimuli that are typically used for harmonic distortion measurements.
SoundCheck, along with the AmpConnect 621 audio interface and 6 measurement microphones measures automotive Max SPL to the standards proposed by the AES working group. SoundCheck offers consistency, control and automation advantages over using a sound level meter – the entire test is automated, with SoundCheck interfacing with the head unit, controlling the stimulus playback and making Max SPL and Max SPL spectrum measurements simultaneously.
In-Car Max SPL Measurements Test Configuration
In this test, the precisely and specifically positioned six-microphone array replicates the position of an average person’s head in the driver’s seat. The precise seat location and angle, microphone positioning with respect to the seat, and the height and the angle of the microphones are defined in the proposed standard to ensure measurement consistency. Similarly, the sound system settings on the head unit, such as volume, tone control, fader are specified.
SoundCheck plays the stimulus through the head unti, either from a memory stick, CD, or Bluetooth from a smartphone or auxiliary line in and the average SPL in dB(C) is measured for 30 seconds. This is called a Leq measurement, and it takes the spatial average of the six-microphone array, power averaged, to get the overall Max SPL level. The Max SPL Spectrum is measured using a real-time analyzer. The Max SPL is recorded at each microphone simultaneously from 20Hz to 20kHz and the power average calculated.
Video Demonstrations of Automotive Max SPL Measurement
This short video explains how we can make automotive SPL measurements using SoundCheck.
We also offer a much more in-depth demonstration of all the measurements in the AES Technical Committee on Automotive Audio’s working group proposals. This includes frequency response and impulsive distortion as well as Maximum SPL.recorded seminar, along with a information on the various methods for connecting to an automotive head unit.
SoundCheck Test Sequences for Automotive Max SPL Measurement
A free pre-written SoundCheck test sequence is available to measure both the Max SPL Spectrum and a single, power averaged value for Max SPL in line with the working group’s proposed guidelines.
Learn more about the Automotive Max SPL Test Sequence for SoundCheck.
Additional Resources for Automotive Max SPL Measurement
Learn more about Automotive Max SPL Measurements using SoundCheck.
The best source of additional information on in-car Max SPL measurements is the Audio Engineering Society Technical Committee on Automotive Audio (TC-AA). Their in-vehicle measurements white paper (draft) aims to define repeatable and defined car audio system measurements and in addition to the measurement methods, contains information on standardized test configuration, for example microphone and seat positioning.