In-Vehicle Distortion Measurement
Authors: Zarina Bhimani, Steve F. Temme (Listen, Inc.), Patrick Dennis (Nissan Motor Co.). Reprinted from the 2017 Loudspeaker Industry Sourcebook.
Steve Temme and Patrick Dennis discuss their research exploring test methods that help determine audible distortion and enable manufacturers to test sound equipment after it is installed. Configurations for measuring in-car audio are shown. Objective measurements are made and correlated with subjective analysis, and conclusions drawn as to the level at which music sounds distorted.
Abstract
Although most automotive speaker manufacturers carry out thorough end-of-line (EOL) driver testing (in many cases, 100% of product), many automotive manufacturers do not test the speakers once they are installed. It is possible for a speaker to develop a fault through damage in transit, handling, or installation. Furthermore, the simple act of installing a loudspeaker into a car can result in vibration issues caused by mounting and other components in the car. Such issues can prove costly for automotive manufacturers. It is not uncommon for a car dealer to install a new set of speakers in a car if a customer complains about sound quality issues. It is, therefore, advisable for automotive manufacturers to invest in both incoming speaker QC and complete EOL testing of installed systems.
Automotive Audio Test Equipment
The test equipment for incoming QC and in-vehicle testing is similar to EOL production tests. In fact the test setup for incoming QC is practically identical to that used in driver manufacturing facilities worldwide. This simple setup consists of an amplifier to drive the speaker, a measurement microphone, and software to measure frequency response, distortion (particularly Rub & Buzz), and polarity. In-vehicle testing is implemented with similar equipment, but the setup differs in that the audio signal is transmitted from the measurement software via an audio interface to the auxiliary, Bluetooth, or USB input to the head unit.
The test signal is played through the speakers, and the signal is picked up by a centrally positioned microphone. Care must be taken in positioning the microphone to ensure that the path from speaker to microphone is not blocked by seats or other parts of the car’s interior. Usually the best position is on, or suspended above, the front seat arm rest.
A single measurement of frequency response and Rub & Buzz is usually sufficient to ensure that the audio profile measured in the car meets specifications. If there are discrepancies, each speaker can then be measured independently (including additional measurements such as polarity) to help identify the cause. Any microphones in the car (e.g., part of a voice control/ telematics system) can also be tested using the same equipment and the car’s own speaker to play the test signal).
A similar test setup can be used for R&D testing (e.g., for voicing the audio system to the car). This might include speaker positioning and equalization of the system for correct tonal and spatial balance including left/right (L/R) and front/back balancing. It may also be used for microphone positioning and directivity measurements and noise cancellation performance.
More about SoundCheck for Automotive Audio Measurement
More about automotive distortion (Buzz, Squeak and Rattle, Impulsive Distortion) measurement.
AES Technical Committee on Automotive Audio
