Happily, such a large chamber does exist, though it is apparently expensive to use and difficult to schedule for ongoing test work on audio projects. However, Paul took advantage of the larger chamber’s existence in order to create a low-frequency compensation curve for the smaller NRC chamber. His methodology was simple and elegant. First, he tested a speaker in the big chamber and then, on the same day and under identical atmospheric conditions, tested the same speaker again in the NRC chamber, and compared results. Predictably, results from the two chambers above 80Hz were essentially identical, but Paul was able to plot differences below 80Hz to create a compensation curve that could be applied for all future full-range (that is, 20 Hz – 20kHz) tests conducted in the NRC chamber. Clever, wouldn’t you say?
A Loudspeaker Test Session in the NRC Anechoic Chamber: Once Paul had given all of his guests a turn in the chamber, he offered to show us how an actual test session is conducted, using his own excellent and recently released PSB Imagine Mini small monitor speaker as the test subject. Part of the significance of this choice is that all of the journalists assembled in our small group had heard the Mini, while several (Playback included) had also reviewed the speaker. The test procedure went roughly as follows.
Step 1: Paul carefully placed the Imagine Mini on the test fixture in the chamber, connected speaker cables to the Mini, and then carefully shut both the inner and outer chamber doors.
Step 2: From within the control room, Paul turned on the laser-type aiming device in the chamber, and then, while watching the feed from the chamber’s video camera, carefully used remote controls to adjust the position of the test platform until the speaker was perfectly aligned with the microphone array.
Step 3: Paul set amplifier outputs to a level equivalent to the industry-standard of 2.83V at 8 Ohms, and then played pink noise through the speaker and conducted a check on each of the microphones in the five-mic array. During our session, this test turned up a faulty connection (thankfully on the control-room end of the signal chain), which Paul swiftly corrected.
Step 4: Paul fired up an oldie-but-goodie PC whose purpose is to control frequency response tests and to gather and analyze data. Making sure that amplifiers were connected to signal generators under control of the test PC, Paul initiated the test sequence. The process involved 20Hz – 20kHz test sweeps, where sweeps started at the upper frequency limit and worked downward to the lower frequency limit, with each sweep taking about 8 seconds to complete. The sweeps were, of course, repeated for each of the five measurement mics.
Step 5: After frequency response data has been captured, and taking care to make sure that the aforementioned low-frequency compensation curve is applied, Paul used the computer to analyze data and to generate a composite frequency response graph for the speaker. Paul demonstrated for us that it is entirely possible to view individual response curves from any of the test mics, or to overlay all five curves on top of one another, if desired. Any or all curves generated can be routed to an old-school X-Y axis plotter to produce hardcopy graphs for the designers’ logbooks.
And the Envelope, Please: The response curves for the Imagine Mini were impressively consistent from one mic position to the next, and they were stunningly flat (in essence, ± 1dB from a bit below 100 Hz all the way up to 20 kHz). It’s one thing to see results like these from monitor speakers costing thousands of dollars apiece, but it’s quite another to see them from small monitors costing around $760 per pair. PSB has long been known for building affordable loudspeakers that exhibit serious technical excellence, as these test results so eloquently demonstrate.