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RE: Ultrasonics?

Posted by Charles Hansen (M) on October 11, 2017 at 15:48:54

In Reply to: RE: Ultrasonics? posted by Kingshead on October 11, 2017 at 12:13:12:

Hello Kingshead,

Those tweeters were originally made by the OEM company Bohlender-Graebener, in Reno, Nevada. Soon Infinity had copies made in Taiwan, and later (when trade relations were "normalized") China. B-G has gone on to produce products aimed at cinema applications, and also marketed to the audiophile market under the trade name "Wisdom". All of these are planar-magnetic ribbons, meaning that the electrical conductor is printed onto a plastic (typically Mylar) substrate. In contrast a true ribbon has the conductive element (typically aluminum) pressed into a foil and used as the radiator itself.

Not many reviews publish measurements of the speakers tested, Stereophile being the main exception. Linked below is the "Measurements" page of the Stereophile review of the Modulus + subwoofer package. The frequency response on-axis is rising smoothly toward the 30kHz measurement limit. However JA uses a Bruel & Kjaer (the semi-pro model JA uses was from a division spun off as DPA) electret condenser microphone with a 16mm diaphragm. The response of a capsule this large gets squirrely above ~15kHz. At some point in the 1990s he realized this and entered a correction curve for that particular microphone into the MLSSA measurement system he uses and he reprinted the measurements of many previously-measured speakers. I believe the response at 20kHz was about 2dB off due to the microphone error.

Going beyond 20kHz with any accuracy, whether generating sound waves or attempting to measure them is rather difficult. As 20kHz is generally considered the upper limit of human hearing, almost all equipment is focused on that range. The most accurate measurement microphones are condenser microphones. The larger the diaphragm, the more sensitive it is and the better the S/N ratio. The largest measurement microphones use a 1" capsule, but the frequency response begins to vary above 5kHz or so. Virtually all loudspeaker measurements are made with a 1/2" capsule, which starts to vary above 10kHz or so. They can measure up to around 30kHz with reasonable accuracy but require an understanding of both the correction curve and the angle of incidence between the sound source and the plane of the microphone diaphragm.

You can see this more clearly with specialized very small diameter capsules. The original B&K is still making measurement-grade 1/4" and 1/8" capsules for specialized high-frequency measurements. But take a look at the datasheet for the 1/8" capsule here:

https://www.bksv.com/-/media/literature/Product-Data/bp2030.ashx

Figure 1 on page 2 shows the correction curves that must be applied, depending on the angle of incidence. Even with this very tiny measurement element, its physical external dimensions are large enough to disturb the sound field of very high frequency (short wavelength) sounds.

If measured directly on-axis (mic pointed at the sound source), the frequency response will be boosted by +11dB at 100kHz. For most measurements the microphone should be oriented at 90º to the sound source (so that the sound waves graze across the diaphragm). When used like this the response is within ±1dB all the way out to 100kHz.

The OP did not specify how high in frequency he wanted to go, nor how loud the sound source must be. Since he said that he was attempting to perform materials testing, I would imagine that he is looking for very high SPLs that would deform or stress the material. The EMIT planar ribbon tweeters are not very efficient, nor do they have very high power handling, so they likely would not generate the SPLs that he is trying to achieve. But just getting accurate measurements above even above 10kHz is much more difficult than expected.

Hope this helps.

• Stereophile Modulus review   (Open in New Window)

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