Music to your ears

FRAMINGHAM, MA—Whether you listen to music by Bach, Ellington, or Clapton as you drive, if your car has a Bose automotive sound system, you can thank the design-quality engineering department of Bose's Automotive Systems Division for your listening enjoyment. Headed by manager and jazz fan Dave Leis, this 33-member group shakes, rattles, and rolls speakers and amplifiers in ways your car can't. Their attention to detail results in music that sounds like the band is traveling with you.

Bose engineers aren't just designers of audio amplifiers and speakers—they're music fans and musicians, too. Each year, Bose musicians get together for a "Bose Jam" in a local hotel. Started 10 years ago in engineer and drummer Bud MacLellan's basement, the jam now spans an entire evening and includes professional audio and lighting systems.

Manager Dave Leis heads the design-quality engineering department, a 33-person team of engineers and technicians.                 In Bose's Automotive Systems Division's Electrical Lab, David Anderson, laboratory operations manager, uses an ATE system to characterize amplifiers before and after environmental and EMI tests.

Bose developed the automotive industry's first custom-designed, factory-installed sound system for the 1983 Cadillac Seville. Since then, engineers have refined test methods to improve the listening experience. Every new Bose automotive amplifier and speaker design undergoes a rigorous design-verification process prior to production that includes electrical, climatic, dynamic, reliability, and EMC tests, with each suite of tests performed in a separate lab.

As part of a component's design validation, electrical engineers develop tests for parameters such as frequency response and output power. Mechanical engineers develop shock, vibration, and climatic tests. Software engineers test code in amplifiers for proper digital-filter algorithms and for the serial communications buses that send control commands. Leis says that Bose engineers can perform 95% of design-validation tests in house; for the rest, they contract with outside environmental and EMC labs.

When design engineers complete an amplifier or speaker, they build a single prototype. Engineers in Bose's design-quality engineering (DQE) department, along with design engineers, test the first piece to determine if the product is worthy of a preproduction build. The suite of tests performed on prototypes varies widely, because DQE engineers look for risks associated with a design's new technologies before initiating a full design validation.

If the engineers conclude that a design's new technologies pose risks worth taking, they authorize building a preproduction lot of up to 100 pieces. Figure 1 shows how DQE engineers divide a 100-piece sample lot into groups for each of the test labs. Typically, 44 pieces go through design-validation testing, and 56 are units divided among Bose design engineers, Bose audio-system engineers, and the customer.

Figure 1. Amplifiers and speakers undergo electrical, reliabilty, climatic, dynamic, and EMC tests as part of a design validation.

After an amplifier or speaker undergoes a test in one of the labs, electrical engineers rerun the parametric measurements and compare the results to the baseline measurements. A speaker or amplifier may undergo more than one set of lab tests. Each time a component goes through a test lab, it undergoes parametric tests before proceeding to another lab.

Bose engineers use an ATE system from Mindready Solutions to perform 18 parametric measurements on preproduction amplifiers. Laidlaw says, "We chose Mindready because the company already had experience with automotive and telematics testing." The system tests six amplifiers at once and performs the tests with the DUTs in a temperature chamber. The system contains three chassis that house 18 instrument cards consisting of programmable loads, analog-to-digital converters (ADCs), digital-to-analog converters (DACs), and digital signal processors (DSPs), which the system uses to measure frequency response, total harmonic distortion, weighted noise, DC offset, power output versus frequency, and common-mode rejection.

To make the measurements, the tester applies random noise to each amplifier input, and it connects each amplifier output to a programmable load that simulates a speaker. The ADCs, DACs, and DSPs digitize an amplifier's output signals, produce the random-noise signals, and process the output signals, respectively. "The ins and outs of a Bose amp "  describes an amplifier's inputs and outputs.

Because every amplifier's design is unique, each design requires a custom test suite. The Mindready system contains a setup file for each amplifier. A graphical-user interface developed with TestStand from National Instruments provides drag-and-drop selection of amplifier tests. Test data goes to a database where engineers compare lab data against pretest results or against production data.

In the electrical lab, Bose engineers use the Mindready system, which includes an ATS-2 audio analyzer from Audio Precision as a reference standard and diagnostic tool. Design engineers also use many Audio Precision analyzers as development tools. (To learn more about audio amplifier testing, you can download "Measuring Switch-mode Power Amplifiers" from Audio Precision, Ref. 1.)

For speaker parametric measurements, Bose engineers use an acoustic anechoic chamber built in-house. The virtual acoustic speaker test (VAST) system contains a microphone mounted inside the chamber that connects to a National Instruments data-acquisition card in an adjacent test stand. The card digitizes amplified microphone signals and the computer calculates output power level, frequency response, resonant frequency, and voice-coil DC resistance. The speaker-under-test mounts to the chamber's outside wall through a mounting plate.

In the electrical lab, engineers must prove that amplifiers and speakers can function properly over a wide range of electrical conditions. For example, if you need to jump start your car, you expect its sound system to operate once the vehicle starts. You also expect the system to operate after it experiences voltage drops, polarity reversals, surges, shorts, and a combination of thermal and voltage conditions. Bose engineers test new component designs under these and other electrical conditions.

In the climatic lab, amplifiers and speakers go through temperature, humidity, and ultraviolet light tests. Design quality engineer and alternative rock fan Ron Holmes says that speakers go through hundreds of hours of temperature/humidity cycling in environmental chambers. This test subjects the speakers to conditions more severe than they will encounter in a vehicle. The high-humidity part of the cycle saturates the speaker's cone with water. The high-temperature/low-humidity part of the cycle "boils" the water away.

When installed in vehicles, speakers that mount in dashboards and rear panels often encounter sunlight for hours at a time. Bose engineers test for that, too. One environmental chamber in the climatic lab contains an ultraviolet light that illuminates speakers for several hundred hours because, as Leis notes, "There's no material that won't discolor in sunlight." The test tells Bose engineers that their speakers will survive many summers in places like Phoenix.

In the dynamics lab, shock and vibration testers from Unholtz-Dickie shake and shock preproduction speakers and amplifiers. "A speaker door-slam test simulates 100,000 slams at 25 g," explains Holmes. "Each simulated slam lasts 11 milliseconds and is generated by a half-sine pulse." The shock chamber is also a thermal chamber, so Bose engineers shock test the speakers at ambient and at extreme temperatures.

Following a shock test, technicians connect speakers to a test stand that contains a Crown power amplifier, a Tenma function generator, and an Agilent Technologies data-acquisition system. The technician checks the speaker's audio quality by listening. A warped cone, for example, produces poor audio. Holmes says that it takes a trained ear to listen for subtle problems.

Another subset of the sample speaker lot goes for a reliability test. In a room known as "The Bunker," speakers play elevated levels of a selected random-noise profile for many hours. Even though engineers wear ear protection when they enter this room, they can't stay inside for more than a few minutes because of the sound pressure. The sound is so intense that you can't hear yourself talk.

In the EMC lab, engineers test amplifiers for emissions and immunity. Amplifiers must not only withstand the harsh electrical environment of a vehicle, they must not cause other electronic vehicle subsystems to fail. Bose's EMC lab consists of a semianechoic chamber that engineers use to measure emissions and to subject amplifiers to controlled EMI. Antennas from ETS-Lindgren and EMI analyzers from Hewlett-Packard (now Agilent) measure and display test results. Applicable test standards include CISPR 25 for emissions and ISO 11452-2 and 11452-4. Amplifiers must also withstand ESD tests to ISO 10605 and continue to function properly (Ref. 2, 3, 4, 5).

Following the design-qualification tests, DQE engineers review the test results and consult with design engineers about any risks the tests uncover. If necessary, design engineers make changes to the products before production begins. If everyone accepts the design, the product is ready for production—almost. Next, a product's production line goes through process validation. The actual production line for the product is built in the company's Framingham, MA, headquarters. When engineers certify the line is ready for production, it moves to one of Bose's manufacturing plants.