Behind the Scenes at Secrets - Number 1 - June, 2008



Here is a photo of a recent test I performed on some speakers, using the AP SYS-2722.
















In order to set the test up, one has to configure all the screens such that the frequency range and type of signal being delivered are standardized. There are a lot of options, so it is easy to mess things up, but as long as you remember to make sure the output level is very low when you fire up the test, so that you don't put too much voltage through the test circuit, nothing will be damaged. But, in any case, there is a whole lot of trying this and trying that at first, before the final test procedure is set. Then, it is just a matter of opening a basic template file the next time you want to run that test on a new product in for review.

If one is testing speakers, obviously a microphone has to be used, and it needs not only to be a very high quality one, but it has to be calibrated because different microphones output different amounts of millivolts at any one SPL that is fed to it.

Calibration is accomplished by using an output of 1 kHz at known value and setting whatever millivolts that turns out to be incoming to the test instrument to that value. We do this by using a microphone calibrator that has a small opening where you stick the tip of the microphone, and setting the calibrator to play a 1 kHz tone at either 94 dB or 104 dB. Another way is to play 1 kHz through the speaker, and using a Radio Shack (RS) SPL meter – which is accurate at 1 kHz – one places the Radio Shack meter microphone tip at the same place as the microphone in need of calibration, adjusting the SPL to read 104 dB on the RS meter, and then setting the voltage incoming to the test instrument from the microphone needing calibration to be 104 dB as well.

So, after the microphone is calibrated, you need to make sure you use a standardized distance from the speaker, because SPL decreases exponentially as you move farther from the speaker. So, if you measure 0.5% THD+N (Total Harmonic Distortion plus Noise) at 100 dB and 1 meter, but the next speaker you put the microphone at 4 feet, you would have to turn up the volume in order to get that 100 dB. Even if you used the same speaker, the distortion reading would be higher, because every speaker produces higher distortion as the output is increased. So, in order to be fair to all the speaker manufacturers, not to mention making the results reproducible, you need to standardize the distance. In our case, we use 1 foot, 1 meter, and 2 meters for various tests.

In the next installment, I will discuss the other test systems that we use (primarily software-based) and go through an actual measurement, with details on all the parameters that have to be paid attention to during the tests.