Notes from the Test Bench
By Bruce Hofer, Chairman & Co-Founder, Audio Precision
We’re full steam ahead to release APx v2.4 on August 1st. This is a big release. We’ve already introduced the digital protocol analysis tools, and you can read below to learn about the new acoustic response measurements. A fully functional beta is available at http://ap.com/beta.
Before that though, I thought I’d share some of the responses from last month’s Future of Audio Test survey. Many of your answers confirmed what AP has been assuming for several years (this is a good thing!): the number of 2 channel applications is continuing to drop (85% of you expect to require 4+ channels in the next few years), and the demand for different digital interfaces is increasing (although after AES3 and USB, there are no clear winners).
One respondent included a really important point that we should all keep in mind:
Understanding how your vendor (or colleague, or customer etc) got to their results stops the potential for hours of wasted time. Of course, knowing that they have an AP, and being able to run their saved tests on your own bench helps a lot.
There was one other comment that had a lot of wisdom too:
No one knows the future, but it’s a safe bet that it’s going to be different than today. And chances are, you’re going to want a lot of bandwidth and a low noise floor.
Output: Tech Tips and New Applications from AP
Acoustic Response Measurement
Acoustic response is part of the upcoming APx v2.4 software, which will be released and ship with all new instruments beginning August 1st. Current customers will also be able to download a free v2.4 update on the 1st as well. A fully functional beta is now available at http://ap.com/beta.
Also available now for download is AP Applied: Acoustic Response, a look at some of the important features needed in an acoustic audio analyzer, the problems you may encounter in a non-ideal testing environment, the needs of good production line testing, and the range of measurements and results that should be produced.
The acoustic response measurements in APx audio analyzers are for audio engineers who need high performance acoustic as well as electronic audio tests. On the production line, APx’s ease-of-use and high speed are ideal for both speaker OEMs and electronics manufacturers conducting quality assurance on drivers from third party vendors.
Devices suitable for testing include loudspeakers and PA monitors, as well as drivers found in consumer products such as home theatre systems, portable DVD players, iPod docks, clock radios, TVs, and laptops.
Easy Test Setup
See the article below for more details on setting up an acoustic test.
Complete Acoustic Characterization In Six Seconds
Sound Advice: AP Knowledge Base
Setting Up and Making Acoustic Response Measurements with APx
This month, Audio.TST editor Adam Liberman writes a short tutorial on how to set up acoustic response measurements using an APx analyzer.
Acoustic response measurement is one of the major new features of the upcoming APx v2.4 software release. So, we thought it would be a good time to go over the basic steps of setting up an acoustic test. Although we’re focusing on the APx Series, the basic hardware setup and positioning applies to the 2700 Series as well.
Selecting a Room
With a distance of one meter between the mic and the speaker, and given that sound travels 1116.4 feet (340.29 meters) per second at sea level, the first reflection will arrive about 5 ms after the direct sound. This means that results over approximately 200 Hz will be reliable, and results below about 200 Hz will be uncertain (they may be OK, or may be tainted by reflections).
The APx software will recalculate the results as you adjust the time window (see below). If you can find a larger room, or can measure outside with a tall stand, you’ll be able to extend this range of reliable results lower.
Positioning the Speaker
Positioning the Microphone
The measurement microphone is set into a mic clip on a microphone stand or tripod. Raise the mic stand until the center of the mic is four feet off the floor. Four feet doesn’t have to be the exact height—what’s important is that the mic and speaker are at the same level and right on axis with each other. Now, adjust the spacing between the mic and the speaker until they are exactly one meter part. One meter is a standard distance used in many speaker measurements, but other distances like two meters are used as well. If the distance is too far, the signal to noise ratio will be poor, although this problem can be greatly minimized by using the synchronous averaging function in the APx software. Too close, and the three drivers in the loudspeaker won’t blend together.
The microphone needs a power supply (also part of the MMK2 kit) to operate its electronics. Connect a BNC cable from the microphone to the power supply, and another from the power supply to the analyzer channel one unbalanced input.
Calibrating the Microphone
Use a 1 kHz sine wave to set the APx generator level as desired. Many measurements specify a level corresponding to 1 Watt at the amplifier output. For frequency response measurements, the level isn’t too critical as long as the amplifier isn’t clipping and the speaker isn’t overloaded.
Taking the Measurements
Excluding the Reflections
Automatically Recalculating the Measurements
Note that although eliminating reflections will give the most accurate and repeatable results, there are times when their inclusion may be desirable to in order to better represent the real world conditions where the speaker will actually be used.
As you can see, it's very easy to set up and make comprehensive acoustic response measurements with APx analyzers. We've given you general guidelines to get you started. You may need to alter some of the parameters depending on your specific application.
Other Recent Knowledge Base Articles
Test Results: AP News & Events
AP in the News:
Pro Sound News Europe
IBC Amsterdam, Sept 11-15
AES New York, Oct 9-12
©2009 Audio Precision Inc.