Live Sound University Article Thu, December 04, 2008

LSI University | Acoustic Measurement |

Apples with Apples

By Pat Brown

Summary

  • It can be said that the most fundamental process regarding the reception and evaluation of sound is that of comparison. Any description of fidelity or the lack thereof must be in reference to something.

An audio system that amplifies the sound without producing any changes or distortions continues to elude designers. Most all categories of system components have their “golden” makes and models that the performance of all other components is compared too. But even these “standards” aren’t perfect, and relatively few people have them available for use in evaluating other products.

A very meaningful and more accessible method of comparison can be achieved with a number of PC-based analysis tools. Two or more audio or acoustic measurements can be compared to a great degree of accuracy with a few keystrokes. This can allow the differences between audio components to be determined and documented.

I used the Goldline TEF2OTM to perform some comparisons of acoustical measurement microphones. The testing session was initiated at the request of Brad Nelson of Sound Solutions Northwest. Brad had the misfortune of dropping his B&K 4007 measurement microphone. Since repairs on such a mic can easily exceed $1000, he asked me to perform a comparison between his mic and my own 4007 - a treasured possession that I acquired from Syn-Aud-Con founder, Don Davis.

I used a studio monitor as the acoustic source and made a measurement to use as a reference. Once stored in memory, this measurement becomes the benchmark that the next measurement (substituting the other mic) is compared too. Two identical mics would produce a fiat line in both magnitude and phase. While this is never achieved in the real world, one would expect microphones of B&K caliber to compare vary closely.

Two Earthworks mics compared.

While I was at it I decided to compare some of my other mics. Figure 1 shows the comparison of the Earthworks M30 with the newer M3OBX. Figure 2 compares two TEF 05 microphones, both of which have been fitted with new capsules this year. Figure 3 shows the comparison between Brad’s 4007 and mine. Assuming that mine is correct (it compares well with the Earthworks), Brad’s mic did indeed suffer some damage in the fall.

The difference measurement is indeed a valuable tool for answering some of the perplexing questions of audio. pb

The comparison of the two TEF 05 microphones shows that they are a very good match. The frequency and phase responses are nearly identical. Of even more importance, the levels are matched to less than one dB. This makes this pair especially suitable for stereo recording Prior to being damaged, these two mics had a 4 dB difference in sensitivity, requiring recalibration when switching between mics.

Bad news for Brad’s B&K;4007. The comparison between Brad’s and mine should have yielded a nearly straight horizontal line. The deviation from horizontal shows that the mics are not identical. Further comparisons between the two 4007 and the Earthworks M30 showed that Brad’s mic deviates from fiat response.