Antenna arrays are used to greatly increase s/n and directivity. Radio telescopes and military radars use multi-antenna, electronically steered, synthetic aperature techniques. The same thing can be done with arrays of microphones.
The power supplies need not be isolated. One supply could power a hundred electret mikes.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
The resonance of those tubes is rather low-Q, the frequency bands mainly overlap. Acoustic noise, reduced by bandpass limitation, at the tube ends, is not coherent, but sound from ONE AND ONLY ONE direction IS coherent: that 'same exact signal' condition is why the microphone is directional.
Power supply rejection is an issue, probably not hard to solve; pi filters are not pricey, and you might be safe in using one filter to power two or three of the pickups, provided their bandpasses are not overlapping.
_Is_ done, if you accept that a SONAR hydrophone is a "microphone".
It'd be interesting to see what you could do with this.
-- Tim Wescott Control systems, embedded software and circuit design I'm looking for work! See my website if you're interested http://www.wescottdesign.com
He said he would connect the Electrets in series. No two would generate the same signal, due to different tube lengths.
As far as stacking antennas, I was doing that in the mid '60s. Like I said above, they have to be the same antenna and aimed at the same transmitter t get the gain. Most of the ones I worked on were aimed at two or more cities, or they stacked a VHF and a UHF antenna. Neither of these methods gave you any gain.
One Ch 4 antenna I used at a CATV headend weighed a little over 1400 pounds. It was the largest load on the self supporting 130' tower. With it's gain, and the HSP input gain set to maximum, we could get a usable, but not a great signal from a station that was in the next state. I thought it was a waste of resources, since most of their programming had to be blocked.
If you have any real interest in stacked antennas you need to look at the FM and TV broadcast antennas. They are arrays that are pahsed to shape the radiation pattern.
My suggestion was to separately digitize the signal from each microphone and apply massive DSP. That wouldn't be all that difficult nowadays. The data rate from microphones is tiny compared to radar.
We're working on a system that uses square miles of antennas. We're not supposed to know what it's for, but it's hard to hide anything that big.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
In fact, such microphone arrays are done all the time. That's how telephone headsets (the ones without boom/mic in front of the mouth) work and many voice recognition systems.
Finding Dory?
Coherent and in-phase.
I don't know about microphones, but when you build a phased array antenna out of dozens of antennas, amplifiers, and phase shifters, and then combine the received signals, there's no decrease in base line noise level.
I was digging around with Google and found: "Application of a microphone phased array to identify noise sources on a roof bolting machine" That's 42 microphones in a beam forming array. See Fig 6 with the results on Fig 11 and 12. (I didn't think that kind of resolution was possible at audio frequencies). Anyway, if there was a increase in base line noise level for 42 phase shifted and combined during post processing, such a system would not be possible.
I have seen an additive noise problem in multi-track analog magnetic tape recorders, where garbage from adjacent tracks increases the noise levels. That's why one never saw analog mag tape recorders with more than 8 tracks. I once attended an AES convention (1968?) where someone demonstrated a
16 track recorder. The meters hovered between 1/8th to 1/4th scale during playback and made a rather nice but expensive white noise generator.-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
In the multi-barrel form of shotgun microphone, all the microphones are picking up the exact same signal, with almost exactly the same phase, before being combined. Here's how it works (see the sketch): "How A Shotgun Mic Works" Notice the path lengths for sound arriving from dead ahead are all the same.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Oops.
That should read "no increase in base line noise level".
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Except they aren't -- as I noted below, the (acoustic) path lengths aren't equal because differing velocities versus frequency in the dispersive pipe.
It also doesn't help that the array is arithmetic, not geometric, giving disproportionate weight number of elements) to very low frequencies, and presumably giving very poor directionality to high frequencies.
The elements also don't have geometric diameters (i.e., the longest is also the thickest), which exacerbates the velocity problem.
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
I said microphones, not electret microphones.
I said _same_ audio.
If you insist electret microphones, let me restate:
Put two microphones in _parallel_ and feed them with the same audio. The _current_ will be doubled (+6 dB), while the non-coherent noise _power_ will also be doubled (+3 dB), Thus, there is a 3 dB net advantage in SNR.
Now you can DC feed multiple electrets in parallel.
Do you even need tubes ? Just multiple microphones in different directions and use DSP to combine multiple directional signal just as in MIMO (Multiple Input Multiple Output)
In Ambisonics
Except that analog multitrack recorders got up to at least 24 tracks before digital took over. The problem with multitrack analog tape was that adding tracks was originally done by making each track narrower, which resulted in lower SNR on each track. The fix was to leave the tracks the same width and just make the tape wider, which solved the SNR problem, but made the transport design more difficult.
Computer tapes were 1/2 inch with either 7 or 9 tracks.
(Non-audio) instrumentation recorders could have a large number of tracks with 1 or 2 inch tapes
For a single project? At what cost? For the components, the design time and learning to write the software?
A replacement for retired HARP project, with all of the modified high power solid state broadcast transmitters?
Try turning on every input on a 64 channel mixing board, with the unused inputs set to at least mid range. You'll quickly discover why unused inputs are switched off the active output buses. The reason for so many inputs is to allow for multiple setups without needing to rearrange the cabling between sets. You don't have this with a digital mixing board, and that is all that companies like Wheatstone are making. They no longer make analog distribution amps, as well.
Tape recorders in 1968 used a lot lower grade of tape in 1968 than even a few years later. Scully made four and higher track versions of their workhorse 280 studio machines. Better tape and lower noise electronics caused most of the machines to be retired for recording studios, but they continued to be used in radio stations for decades.
I worked in radio & TV stations in the early '70s and saw the state of the art, first hand. Compact Cassette recorders were not used, until better tape was available, along with Dolby noise reduction. Most stations even refused to use them for news stories, recorded by their staff. The venerable Uher portable reel to reel machines were thee gold standard, at that time. The same recorders used by film studios for their pre production audio.
If two microphones are fed to the same input, the extra current won't matter, because they are fed into a fixed termination.
I mentioned Electret, because the comment was made of a microphone for each tube. That would be a lot more than two. Even with dynamic cartridges, putting them in series would make the array susceptible to picking up stray electrical noise since only one would be connected to common.
Electrets are higher impedance than dynamic, due to the internal FET amplifier.
-- Subject: Spelling Lesson The last four letters in American.........I Can The last four letters in Republican.......I Can The last four letters in Democrats.........Rats End of lesson. Test to follow in November, 2016 Remember, November is to be set aside as rodent extermination month.
Calculating a direction is only possible if you can hear it. The shotgun mike is supposed to give significant gain so you can hear things you couldn't before.
-- Rick C
Most of this stuff is pretty much cook-book now. Array microphones are used all over the place and DSPs are cheap.
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