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RE: Ultrasonics?

Posted by Charles Hansen (M) on October 11, 2017 at 21:41:55

In Reply to: RE: Ultrasonics? posted by Kingshead on October 11, 2017 at 16:35:28:

Thanks for the image! I've not seen a Stereo Review for a long time... :-)

I don't remember what equipment Stereo Review used to use to measure loudspeakers, as I stopped reading it in the 1970s. Measuring speakers is really far, far more difficult than measuring electronics. There are two problems that are related:

1) The audio band varies by a ratio of 1000:1. (Compare this to the visual spectrum, which is less than 3:1 in wavelength.)

2) When you measure the loudspeaker in a room, you are actually measuring the combination of the loudspeaker *plus* the room. The only way around this is to use an anechoic chamber, where all 6 surfaces are designed to absorb sound. Essentially there is "no" room. Since it is impossible to make any wall be *completely* non-reflective, even in an anechoic chamber one wants to put the sound source as far away from any of the walls as possible. This means that the "floor" of the room is actually made from a grid of steel ropes under high tension, and the loudspeaker, technicians, microphones, et cetera are all placed on this (essentially) acoustically transparent "floor" that is midway between the actual floor and ceiling.

Even then very low frequencies are difficult to absorb, so an anechoic chamber has a rated "cutoff" frequency (lower limit) of usually between 50Hz and 100Hz. They are massive, expensive, and very rare - with only a handful around the world.

Since very few have access to one, it has become common to use one of two techniques to make "quasi-anechoic" measurements. The first to be developed was by Richard Heyser of CalTech, called TDS for "Time Delay Spectrometry". Several years later Doug Rife developed a digital tool that could perform many of the same functions and he called this MLSSA (pronounced "melissa") for Maximum Length Sequence System Analyzer. Both tools have the same limitation, in that they are only accurate up to the point that the first reflected wave hits the microphone.

Using really rough numbers, sound travels through air at about 1 foot per millisecond and one meter is roughly 3 feet. If the microphone is 3 feet from the sound source and the sound source is 3 feet off the floor, the initial arrival time will be 3 milliseconds after the signal starts and the first reflection will be 3.7 milliseconds later. This only allows measurements down to around 300 Hz with any degree of accuracy. There is an excellent article by Keith Howard on the Stereophile website that explains some of the limitations of this method. (See link below.)


And that is just the tip of the iceberg. Stereo Review simply *describes* the results of Julian's measurements, whereas Stereophile actually prints them. You can see these at:

https://www.stereophile.com/content/infinity-modulus-loudspeaker-modulus-subwoofer-measurements

While measuring relatively flat, JA's measurements were not close to JH's ±1.5dB. Furthermore in looking at both the frequency response of the tweeter and its waterfall plat in Stereophile's measurements, it is easy to see that the ribbon tweeter has a cluster of moderately severe resonances centered around 10kHz. It seems to clean up above that frequency, but again all sorts of things come into play. From 12kHz or so the tweeter's response rises fairly steeply, reaching +7dB or +8dB by 30kHz.

Without the use of a special high-frequency measurement microphone (1/4" or smaller), it is difficult to say what is happening here. It looks to be a resonance of some sort, perhaps the cavity resonance created by the metal front plate.

Again, without knowing more of the actual frequencies and SPLs that the OP is trying to reach, recommending a 27 year old, long discontinued planar-magnetic ("quasi-ribbon") tweeter of likely limited power handling might not be all that helpful for what he is attempting. But you are correct that it has a more extended treble response than typical 1" soft-dome tweeters.

• Measuring Loudspeakers   (Open in New Window)

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