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General speaker questions for audio and home theater.

The beat goes on

The ear is indeed sensitive to the effects of sounds well above its normal hearing range, even if it can't hear those signals directly.

For example, trumpets produce very rich and loud harmonic overtone structures up to and above 50 kHz. Let's look at two hypothetical overtones, at 32 kHz and 34 kHz. These two overtones actually produce four distinct sounds - a 32 kHz signal, a 34 kHz signal, a 66 kHz "sum" signal, and a 2 kHz "difference" signal. The latter two are considerably down in volume as compared to the original signals, but they are indeed present. The difference signal is called a "beat".

Beat signals are present in all harmonic structures, and add richness to the timbre. So in our exmaple above, even though we cannot possibly hear the 32 kHz or 34 kHz harmonics, the 2 kHz beat they produce (albeit at lower volume) does contribute to the harmonic structure that is within our range of hearing.

One subjective effect of very extended high frequency response is greater harmonic richness and airiness because of the beat signals generated within the audible range.

Okay, all that being said, very few tweeters will take you up to 50 kHz. It's not just a matter of changing tweeters - it's more a matter of adding a supertweeter. Changing tweeters is not a simple matter, because the crossover would have to be re-designed, and a tweeter that goes up to 50 K is probably not going to go down as low as a tweeter that only has to go up to 20 K. Adding a dedicated supertweet would be much simpler.

I believe Tannoy makes a high quality add-on supertweeter, and Radio Shack used to market some inexpensive piezo units, but I never liked them much.

Getting back to your overall upgrade strategy, I'd say go with an SACD player before you start adding 50 kHz response to your speaker system. SACD is a clear audible improvement even on lowly 20 kHz speakers.

Best of luck to you!


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