Geotech

No, and a whole lotta single frequency detectors (and PI as well) use square wave audio. Again, it's less to do with the audio waveform and more to do with how the target signal is processed. The steel bottle cap gives the unusual response of iron-coin-iron as the coil passes over it. A detector that processes target ID on the peak of the response will usually call it a coin. A detector that processes all the way through the response will be confused and should present confusing information. In audio, that will be a combination of ferrous and non-ferrous sounds. For VDI it will be numbers that bounce around a lot. My favorite visual response system is the White's SignaGraph and the even better SpectraGraph. They showed the real-time phase responses and bottle caps stood out. My favorite audio (for bottle caps in particular) was in the old 4-filter detectors in which bottle caps very clearly rang out. And it had nothing to do with sine, square, or PWM. In modern designs I much prefer mixed-mode audio which (I think) has much farther to go.

None whatsoever. If you're talking about whether the audio is produced using sine/square/pwm then it depends more on how the detector is processing the target signal into audio than on the actual audio waveform. Ferinstance, you can do identical processing and use either sine or square and end up with pretty much the same audible information. However, the sine audio will sound smoother and maybe more pleasant, while the square audio is full of harmonics which sound harsher but might help reveal some nuances. PWM adds another element, that of pulse width, which can add some more detail but can also sound even more harsher. Add to this that square & PWM can be smoothed (either in hardware or software) to temper the harshness. There's no simple answer which is probably why we're seeing multiple audio mode selections to let the user pick what they like.

Yes, that's a magnetic response, not an eddy response.

Data came from the CRC Handbook of Chemistry & Physics. I've looked all over for similar data for copper-silver alloys but haven't found any.

What Pimento said. All we're talking about here are eddy current responses which depend on electrical conductivity. Magnetic responses (soil & iron) involve susceptibility and BH curve hysteresis. Most iron also have eddy responses which complicate things even more.

I just played that for my partner. She looked confused and said I sound like that every time I talk.

Good point... here is a graph I recently did for ITMD3 showing the conductivity of copper-nickel alloys, from pure copper to pure nickel. As you can see, practically the whole alloy range is worse than pure nickel despite the addition of copper. This quirk is true of many simple alloys.

The Gemini (and TW6) is primarily a utility locator although it does get sold to occasional treasure hunters (the TM808 is a better choice for that). The best way to use the Gemini is to split it apart, connect the TX to the line or pipe, and then use the RX box to trace it out. At my previous home I traced the power line out to the well house, about 150 feet. In utility locating there are 2 popular frequencies used: 8.2kHz and 82kHz. 82kHz is the better choice for tracing metal pipes because it does a better job of jumping across the poorly conductive joints. Probably why the TW6 runs at 82kHz, and the Gemini is just a copy of the TW6. As a unified 2-box detector the Gemini is not especially sensitive. I know that it failed my 24" silver cache test, but so did the TM808 and everything else I tried.

Nice post Steve. US classic coinage provides an excellent example. We have 4 coins (dime, quarter, half, dollar) that are all the exact same alloy: 90% silver, 10% copper. Yet have different responses due to size & thickness. And a nickel that is larger and thicker than a dime yet has a much lower phase response, showing that metal type can also play a strong role. But, yeah, any alloy can end up just about anywhere. I'm reminded of the big aluminum token I once found that had the exact same TID as a silver dollar.

Phrunt, that post is probably less sarcastic than you think. For 2022, if I'm lucky then my walk-thru detector will get released. I don't know of anything else even close, and I can easily see the walk-thru getting pushed to 2023. On a good note, I'm told the 12" coil will be hitting Amazon soon. "Soon" is a broadly relative term here, but I think it meant this year.

The use of the term "-3dB frequency" is misleading if you know what that term usually means in electronics. It's not a -3db frequency at all, rather it's just the frequency where you get a maximum R-response. A picture is worth 900 words so... Suppose we're talking about a copper coin of some kind. At 1kHz it has a small response magnitude at a phase just past 90°. Increase the frequency to 2kHz and the magnitude and phase both increase. Again for 5kHz, 10kHz, and 20kHz. The magnitude and phase always increase with increasing frequency. Seems like the target will be easier to detect at 20kHz, that's a big response. Except... we don't look at the target magnitude. Instead, we only look at the R-response. The reason is that ground is phase-adjusted to lie along the 0° axis which means the X-response may have a lot of ground signal in it while the R-response has none. This means that only the R-channel is clean and useful for initial target indication. (When you listen to a threshold-based detector you are listening to the R-channel. If you hear ground noise then it's because the ground phase isn't properly adjusted and some of it is getting into the R-channel.) In this example the peak R-response is at 5kHz. This also corresponds to one-half of an eventual maximum X-response the target could possibly generate. Payne calls it the half-power/-3dB frequency which I don't care for because it doesn't correspond with the traditional use of those terms. Some people call it the resonant frequency which is even worse, there is no resonance at all. It's just a result of only using one-half of quadrature demodulation. In any case, it is true that (everything else being equal) a 5kHz detector will detect this particular coin better than at other frequencies.

As frequency increases the response strength of salt increases so running at 17kHz will be very noisy. To run a single frequency machine in salt you want to go the other way. The Fisher 1280x (2.4kHz) is actually usable in salt water because of its low frequency. However, it sucks on gold jewelry but would be a good choice for finding Atocha bars. This is why MF detectors are so valuable in salt. You can notch out the salt and still get a lot of the jewelry. Nothing's perfect though, and often when you get underwater with swells overhead the salt response phase can move around a little and cause noise. If you have a speed/SAT setting then speeding it up might help, although you gotta be careful not to make it too fast. You can't swing a coil very fast in the water and for this you usually want a slower SAT speed.

This is correct. As you go up in frequency the R response increases, then decreases. The X response always increases. And the composite vector response always increases. The reason the R response matters is because in VLF detectors the R response is what we look at to determine when a target is under the coil. When the detector is ground balanced the R channel has no ground signal at all but the X channel does. A target shows up in both the R channel and the X channel but it is the R channel that is target-only. High conductors tend to have a fairly weak R signal at higher frequencies but a very strong X signal. If you have really good differentiators then you can probably get enough ground suppression in the X channel to separate out big silver. Back when VLF was first introduced there was only one demodulator. In TR-Disc mode you would tune the demod phase to discriminate. In VLF mode you would tune it to ground balance. There was no separate X channel which is why the early VLFs ran at such low frequencies: 1-3kHz. It was the only way to get the R response of silver out of the mud.

The waveforms look the same but there is quite a bit of difference between them. The Deus2 actually looks a lot more like the DFX than the Equinox. It's always better to look at the coil current when figuring out MF waveforms. That can be done with a pick-up coil if you know how to do it.

If you've ever had your luggage "swabbed" in airport security they are doing the same thing. It would be amusing if TSA used rats instead.