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Heavy Lifting: Early Testing Days

Let’s go back to the beginning. When the TubeTrap was invented I began to test it. We got the local university to loan us their concert hall reverb chamber. It wasn’t being used because they changed from acoustic to electronic reverb reinforcement. We lived in that concrete room for 5 years. I had one tech there almost constantly. We were testing the sound absorption of every model of TubeTrap at every frequency from 25 Hz through about 700 Hz. That’s almost 700 data points per test run. In a 10 second reverb chamber such as this, it takes about 10 seconds to charge the room with sound and 10 seconds for it to discharge.

At about 30 seconds per test point, that amounts to 350 minutes, or 6 hours per test run. At first it was thrilling but after a solid year, it was getting tedious.

Progress: Making Better Use of Our Time

So we experimented with speeding the test up. We took known traps and did the test faster, comparing the results with the known data for the product. We managed to speed the test up to a 1/8th second tone burst test cycle. That’s 8 tone bursts per second and each tone burst was a different frequency.

This wasn’t FFT testing; it was just a tone generator, sound meter and strip chart recorder. This direct testing system did not have any df x dT = 1 problems that we knew of and we always got great pure tone absorption data. If we ran the test any faster than that, things got blurry and we lost our ability to resolve details. By then our testing had become automatic, and each 700 point data run now only took about 1 ½ minutes. We had nailed high speed bass trap testing.

Behold: Modulation Transfer Function

So, I’m heading to Syn-Aud-Con with my dT x dF = 1 problem in mind and finally am sitting in the class with quite a few people who would become audio industry leaders in the future. To measure speech intelligibility, the test they used was called an MTF, short for Modulation Transfer Function. B&K made the RASTI testing system (RApid Speech Transmission Index). MTF is sort of like Morse Code, da da daaa da da. And the question is: how fast can you send the code before it loses the clarity and becomes a garbled blur of sound instead of a set of discrete tone bursts? And guess what the RASTI tone burst rate was? Right, 8 bursts per second.

By the way, photography engineers learn a lot about MTF. In photography it is about the silver crystal size. If we have lots of light, we can have slow film, lots of small silver crystal which produce sharp differences in brightness, and the ability to see very fine lines. If we don’t have much light we use fast film which has large crystals which have the effect of producing gradual differences in brightness. MTF measures how strongly the light changes from bright to dark, depending on the closeness of line spacing.

This is similar to MTF in sound, where we measure the difference in loudness of a gated sound, measuring the sound level change between when the sound is on and off and on again.

Eyes and Ears: Multi-sensory Stereo

Notice that we humans have 2 eyes and 2 ears. They are the same kind of system: stereoscopic is 3d visual imaging and stereophonic is 3d auditory imaging.

The eye event reaction time is 1/30th second, which lets fast slide shows become moving pictures. The ear event reaction time is the same, only in this case all the reflections inside of that time window blend into one, and outside of that time become separate echoes.

Even more intriguing to me is that photography is a recording of optical spectrums that vary depending on where in space you happen to be looking. On the other hand, audio is the recording of sonic spectrums that vary depending where in time you happen to be listening. Pretty cool…space-time interchanging. And finally, notice photography engineers and their gear manufacturers are all about controlling the level of visible detail, the photographic MTF for their customers, while audio engineers and their gear manufacturers see nothing, know nothing and say nothing about audio MTF for their customers. In fact, most have never even heard of it.

Read Art Noxon’s full article!