What are benefits of adding HF driver 7khz up?

[Pano]

FWIW, I heard no 2K tone, even tho the mic picked it up. I think I should have, just barely.

[weltersys]

Originally Posted by Pano

OK, I've done the test with 18KHz in one tweeter, 20Khz in the other. Playback and recording were 96Khz sampling rate to keep things clean.
Results:

• With one or the other tweeter playing a single tone, distortion is low. I just see H2.
• With both tweeters playing - 18K in one, 20K in the other, there is a definite 2K tone showing up at about 69dB below the fundamentals.
• moving the mic back to twice the distance of the first recording brings the 2K tone to about 72dB below the fundamentals. Or 3dB less.

Mic, preamp, air? I don't know. Any ideas?

Even wide dispersion tweeters are pretty directional at 18-20 kHz, unless they are about 1/4" diameter, in which case they are not capable of enough SPL for the beat tone to be audible.

"My Radio Shack meter said 92dB at 3.5" for the first measurement (both tones playing). "

Were the tweeters toed in to the microphone?
What was the level of each tone individually at the microphone location?

Two non-correlated tones add about 3 dB to the measurement level, 89 dB at only 3.5" is not much level, though it could be far higher because of the RS "A" scale rolloff.
The RS meter on a dBA setting will be rolled off that high, but your measurement mic could give a relative idea of SPL.

[Pano]

I did toe in the HiVi done tweeters. From what I can find, the RS meter is within a decibel or so at 18-20Khz. (Surprising). Verified with my recording mic and that seems to be about right.

I tried the test again louder with ATP50 horn tweeters. Set at 110dB at the mic. Similar results.

[weltersys]

Originally Posted by Pano

I did toe in the HiVi done tweeters. From what I can find, the RS meter is within a decibel or so at 18-20Khz. (Surprising). Verified with my recording mic and that seems to be about right.

I tried the test again louder with ATP50 horn tweeters. Set at 110dB at the mic. Similar results.

Most likely the close distances you used preclude your ear from getting in to the convergence zone.

Seems the RS meter does not adhere to the A or C scale weighting in the HF, it should be 9 or 10 dB down at 20K.

I'll compare my old RS analog SPL meter to Smaart and the Terasonde Audio Toolbox response on "A" scale next time I do some HF testing.

With all the ringing in my ears the recent compression driver and VHF tests have caused, that may be some time from now, must give the ears a rest...

[speaker dave]

Originally Posted by weltersys

At any rate, neither my ears or the B&K microphone were distorting during the recording of VHF tones .

Art

I'm not sure you understand what Earl and I have been saying (over and over). The fact that you hear and record the difference tones are proof that something was distorting. Those difference tones can only occur via nonlinearity in the system. They are not beats, they are intermod products.

Tis the nature of nonlinear systems that harmonics are generated for single tones and new products at other mathematical frequencies are created for multiple tone stimulus.

Note that the order of the nonlinearity is important. For HF two tone tests you will get adjacent sidebands from 3rd order nonlinearity and sub tones from second order. It goes like this: (1) x 20k - (1) x 19k = 1k There will also be a 39k component. The 1 and the 1 add to 2 for 2nd order nonlinearity required. 19kHz and 20kHz give 18 and 21 from third order nonlinearity because (2) X 19K - (1) X 20K = 18K, and (2) x 20k - (1) X 19k = 21K The 2 and the 1 add to 3 and are evidence of 3rd order nonlinearity.

Not proof but at least a good way to remember it.

I remember the bullet tweeters on traffic signals. That was a little before my time at JBL, but they still talked about that as a lucrative contract. Imagine doubling your business due to widespread theft of your product!

David S.

[Pano]

If I mix the 18K and 20K in software, there is only* the two tones in the mixed result. If I mix them in the air, there is the the 2K spike. Is that the air, or microphone? How can I tell?
I did move the mic to double the distance and saw a 3dB relative drop in the 2K spike. The mic? Or something else?

*Below -110dB there were a lot of artifacts, I guess not even a software mixer is perfect.

[weltersys]

Originally Posted by Pano

If I mix the 18K and 20K in software, there is only* the two tones in the mixed result. If I mix them in the air, there is the the 2K spike. Is that the air, or microphone? How can I tell?
I did move the mic to double the distance and saw a 3dB relative drop in the 2K spike. The mic? Or something else?

*Below -110dB there were a lot of artifacts, I guess not even a software mixer is perfect.

The nature of the air non-linearity is explained quite well here:
Sound from ultrasound - Wikipedia, the free encyclopedia

As Earl and David have been saying (over and over:^)." The fact that you hear and record the difference tones are proof that something was distorting".

The somewhat surprising detail is that the distortion is in the air itself (not our ears or a 150 dB low differential distortion capable microphone recording a 104 dB event) non-linear enough to cause the audible intermodulation products at a much lower SPL than I would have thought possible.

Several companies have exploited the audible intermodulation products to make various commercial devices for targeted communication in a single point in space.

I was a bit surprised to find the basics of that technology can be duplicated with standard audio gear.
Art

[1audiohack]

Looking back at SpeakerDaves post 191, (took me this long to set up and check this out) I set up two tone generators and wye'd them with a 4.7K Ohm resistor on each posative lead and tied the grounds together and used this to single endedly drive one channel of a Crown D-45 amp in turn driving a Jbl 2451 compression driver.

Generator one was monitored on channel one of my scope, gen two- ch two and the summed signal on ch 3, the amp output on ch4.

Channel three and four show the expected "motor boating" of the amplitude of the single sinewave. With gen one set at 2kHz and gen two set at 2.2kHz the sound is modulated as one would expect. Gen one at 20,000 Hz and gen two at 22,000 Hz ch 3 and 4 show amplitude modulation, no audible sound beyond the noise floor of the set up, as you would expect in a linear system.

Just my 2 cents.

Barry.

[weltersys]

Originally Posted by 1audiohack

Gen one at 20,000 Hz and gen two at 22,000 Hz ch 3 and 4 show amplitude modulation, no audible sound beyond the noise floor of the set up, as you would expect in a linear system.

Barry.

Barry,

That is exactly what I found in my tests of single drivers.

The sub tone audible air IM distortion only happens in the convergence zone of two drivers each driven with a discrete tone, if the discrete tones are above audibility, the IM tone is more obviously audible.

If you want to hear the audible IM effect created by air, simply put each tone in to a pair of drivers, the more "beamy" they are at 20 kHz, the easier it will be to hear. I set the drivers in my tests to 104 dB output.

Be forwarned, high SPL 20 kHz can still cause hearing damage even though you don't hear it.
OSHA sets a limit of one hour exposure to 105 dBA "slow" per day, a lot more SPL than I like to expose my ears to.

Art

[Pano]

Originally Posted by weltersys

Several companies have exploited the audible intermodulation products to make various commercial devices for targeted communication in a single point in space.

I was a bit surprised to find the basics of that technology can be duplicated with standard audio gear.
Art

Me too, as I know how these devices work. The Ultrasonic levels they use are very high, thus we expect the non-linearity. My levels were much lower, yours a bit above.
Perhaps it's as Earl says, the non-linearity is progressive. I certainly don't know how much at what levels.