We seem to have audio equipment all around us -– TVs, home entertainment systems, radios, computers, phones, tablets, and, yes, stereo systems all have speakers. Sound is ultimately a change in air pressure, but it travels in electrical form over analog cables, digital cables, and digital wireless connections.
Testing takes place in amplifiers, codecs, digital transmitters, and speaker drivers. To find out what's going on in audio testing, I spoke with Tom Kite, vice president of engineering at Audio Precision, by telephone.
An Audio Precision APx585 audio analyzer has analog
and digital inputs and outputs.
Martin Rowe: With all of our portable devices having speakers and microphones, have mobile devices become a significant part of audio testing?
Tom Kite: Everything seems to have an integrated speaker these days. When we plan new products, we no longer think exclusively about audio-visual receivers with many inputs and outputs. AV receivers keep getting loaded with more and more I/O, but the audio world is moving to mobile devices. Everything needs to connect easily to a mobile device. Audio receivers seem to be giving way to sound bars. Today you beam your audio to your sound bar from your phone. It's the mobile device manufacturers that seem to have the budgets for audio testers these days. We're also seeing new chips coming out that handle audio. It's still a young market.
Martin Rowe: I have a stereo, and I never listen to it. All my music is either contained or controlled by a mobile device. Has audio quality improved in mobile devices?
Tom Kite: Some people complain about audio quality on mobile devices, especially when streamed over Bluetooth, but it's all about being mobile now. People want the convenience of having mobile audio, but they're used to good audio quality. Bluetooth can provide good quality audio, because codecs have improved.
Martin Rowe: Despite all the digital connectivity in audio devices, sound is still sound. How is audio test performed on today's devices, given all the digital connectivity used today?
Tom Kite: We noticed that our customers started asking to use AP's audio test equipment for acoustical analysis in addition to decoding digitized audio on mobile devices. We already had the hardware and measurements for that, but just needed more measurements to round out the electroacoustic testing. More and more devices have both electronics and transducers today. TVs and cellphones have finely tuned electronics to compensate for deficiencies in their driver, producing a better sound.
Because there are so many different forms that audio can travel –- analog, SP/DIF, Bluetooth, HDMI, etc. -- we've developed modular test equipment. You might test an amplifier by direct connections, or you can analyze digitized audio to test codecs. For testing drivers, you might test them when they come in from the supplier, but you'll likely test again once assembled into the final product as a complete system.
Martin Rowe: How do you test drivers?
Tom Kite: Makers of Bluetooth audio products want to test their devices with a known quality analog audio signal, send it over Bluetooth, and measure audio quality coming out of the driver. They also want to measure the audio quality along the entire chain -- analog and digital. Acoustical measurements are made with microphones. We are a technology partner with GRAS Sound and Vibration, and we use their microphones. They plug into the analog inputs of our test equipment.
Bluetooth powered speakers let you stream audio directly from your phone, tablet, or computer.
Many of our customers like having one piece of test equipment for everything. That's why we developed modular audio testers that can test analog, digital, and acoustic signals. For testing speakers, we can measure the impedance of a speaker driver and its acoustical outputs. Frequency response is an important test, but we can also measure rubs and buzz that cause unwanted noises.
Martin Rowe: What is rub-and-buzz testing, and what does it tell you?
Tom Kite: Rub and buzz is an acoustical measurement. You have two ways to tell if a driver meets its spec before it rolls off the product line.
There are different ways to approach rub and buzz. You can look at crest factor, higher-order harmonics, or both. Comparing rub-and-buzz testing with frequency response measurements lets you see a driver's performance differently, but there are correlations between them. For example, undetectable rub and buzz -- the ear may not hear it, but the equipment does. What do you do? Some companies look at that and figure that it might cause an unwanted failure later in time, and that should be addressed. Other manufacturers say, if it sounds good now, ship it. Some people say they should use psycho-acoustical analysis and decide whether or not to ship a driver. We can provide the data, but it's up to the manufacturer to decide what to do with it.
—Martin Rowe, Senior Technical Editor