Trouble Playing High Frequency 20khz - 100khz - I'm a little stuck

[Speedskater]

Not many DMM's (Digital Multi-Meter) are happy reading very high frequencies.
But many budget Oscilloscopes are happy. (not the ones that use a sound card)
The may not be super accurate, But they will show signal levels at the speaker terminals compared to say 1 kilohertz.

 

[danadam]

Also, high frequencies are attenuated/absorbed in the air more than lower frequencies. Over short distances it shouldn't be a problem but from what I've read, with the distances in a movie theater there is a few of dB attenuation at the highest audio frequencies. (Of course they don't care about ultrasonics in theaters but it would be worse.)

Post in thread 'Looking for music with meaningful ultrasonic content'

Feb 9, 2024

View attachment 348379

https://www.ti.com/lit/slaa907

For easier reading of the distance I added log grid:

• danadam

• 

 

[AceOverlord]

hee

Be a bit cautious with all this. You may not be able to hear it, but the energy is still there. I was doing frequency sweeps with REM with a pair of Neumann KH120s where I couldn't hear anything in the high frequencies, but one of our cats was in the room and started crying and ran out of the room.

Thanks for the warning. I definitely don't want to make the experience unpleasant for them.

I would be inclined to test with a cheap piezo tweeter instead of a so-called super tweeter. However, I do suspect that the preamp portion of the amp will truncate the frequency response to 20KHz to avoid interference; the power amp might work up to 200KHz but it is unlikely they allowed this throughout.

There is actually a very well written article by someone here on the forum that did a thorough review on preamp I'm using. It seems to suggest it won't truncate the response to 20khz. I actually have a piezo tweeter laying around somewhere, I think I'll try this. Thanks for the idea.

A couple more things...

I assume you are aware of the Nyquist Theory which says that the signal can't go over half the sample rate. And even though your interface might go to 192kHz, that doesn't guarantee that that the "audio" can go to 96kHz. Usually it can, but audio interfaces are intended for audio and they sometimes have additional filtering.

Thanks for the tips! I definitely don't understand the nuances of the nyquist theory, but I did get a preamp up to 200khz for this exact reason.

Obviously, it's "easy" to make analog electronics that goes way beyond the audio range and I believe your amplifier's specs. But frequently there's a DC blocking filter (and sometimes it's easier to build it that way) and often there is a low-pass filter to block-out RF and avoid any bad side-effects.

...This isn't a problem for you since you have a "good" audio interface, but any-old cheap soundcard can seemingly record and play at high sample rates even if the hardware isn't capable of it, because the drivers will up-sample or down-sample without telling you. (And sometimes the manufacturer isn't honest about it.)

I was definitely worried about some sort of low pass filter that is installed. I wonder if I could just remove the component from the amplifier. Finding it would be a challenge with my level of skill, might be better off taking it to an audio repair shop. There is a pretty good local one here.

You also have to be careful with power ratings. Speakers are rated for "normal program material" so a "100W" speaker is supposed to be safe with an amplifier hitting 100W on the musical peaks, with maybe 20W average. If you play 100W continuous test tones into it, you can burn it out! Even worse.... A "100" tweeter is rated only for the high-frequency part from a 100W amplifier which is much weaker than the midrange and bass. If you open a regular music file in Audacity and apply a high-pass filter, you'll notice the wave gets "smaller". the higher you go, the weaker the signal. It might seem misleading but a tweeter or super-tweeter manufacturer can legitimately specify a higher power rating by simply specifying a higher crossover frequency.

One thing that might help to protect the tweeter at extremely high frequencies is that the speaker/tweeter will probably become inductive (higher impedance at higher frequencies). That means less power with the same voltage.

Also, high frequencies are attenuated/absorbed in the air more than lower frequencies. Over short distances it shouldn't be a problem but from what I've read, with the distances in a movie theater there is a few of dB attenuation at the highest audio frequencies. (Of course they don't care about ultrasonics in theaters but it would be worse.)

I'm not exactly sure how to handle this yet. The super tweeter I was thinking about getting is rated for 20watts and 40watts max. What do you think about a 0.5 amp inline fast burn fuse and a 1 µF, 250V Polypropylene capacitor and perhaps two inline l-pad attenuators for tuning. If I'm interpreting your message right, by cutting off the frequencies I will use less power, therefor protecting the tweeter. This method would cut the low frequencies which I believe use more power, rather than the high, or do you think it would be better to cut off the high frequencies, say over 60khz?

 

[AceOverlord]

Not many DMM's (Digital Multi-Meter) are happy reading very high frequencies.
But many budget Oscilloscopes are happy. (not the ones that use a sound card)
The may not be super accurate, But they will show signal levels at the speaker terminals compared to say 1 kilohertz.

This is a fantastic idea as well, I seem to have lots of options at this point.

Post in thread 'Looking for music with meaningful ultrasonic content'

Feb 9, 2024

View attachment 348379

https://www.ti.com/lit/slaa907

For easier reading of the distance I added log grid:

• danadam

• 

I didn't realize the higher the frequency the steeper the falloff, thanks for this beautiful chart to illustrate this! I plan to use it within 3 meters, so luckily this won't be too much of a problem.