johnt
New member
Hi all, I've lurked but not properly introduced myself. As a harmonica player I like old tube amps, Rochelle-salt mics, and have done some mods.
Lately, I've been ear-birding. In the spring and fall of this and last year, friends and I recorded sounds of spring and fall migrants in the Texas night sky, their nocturnal flight calls. NFC may last a fraction of a second, repeat every few seconds, have a frequency of 8 (5-10) kHz and a loudness of 60 (50-90) dB at 1 meter. Doppler radar results show that Birds migrate over Texas at a median altitude of about 1,000 meters (up to 3,000 meters), so as I figure by the inverse square law a calling bird at that altitude will result in a sound pressure of only about -60 dB at the ground. Yikes. I'd like NFC signals to be at minimum 10X over mic/preamp/ADC self-noise, so it seems our listening stations should at worse have -80 dB self noise in the relevant frequency range.
Competing noise (highways, HVAC units, insects, resident birds) must also be quite low. A shotgun mic helped reduce noise. So did a DIY beamforming condenser array -- a revised one is on the bench. I'm dubious about these approaches and favor acoustic concentration, geometric amplification, as gotten for instance by parabolic dish and exponential and linear horn microphones.
So, my DIY interest is now mostly about acoustic concentrators, very low noise preamp/ADC electronics, weather/water/theft proofing, and how we can maintain a few listening stations remotely, without evening and morning visits to swap mem cards, batteries, etc.
Thanks,
-John Thaden
Lately, I've been ear-birding. In the spring and fall of this and last year, friends and I recorded sounds of spring and fall migrants in the Texas night sky, their nocturnal flight calls. NFC may last a fraction of a second, repeat every few seconds, have a frequency of 8 (5-10) kHz and a loudness of 60 (50-90) dB at 1 meter. Doppler radar results show that Birds migrate over Texas at a median altitude of about 1,000 meters (up to 3,000 meters), so as I figure by the inverse square law a calling bird at that altitude will result in a sound pressure of only about -60 dB at the ground. Yikes. I'd like NFC signals to be at minimum 10X over mic/preamp/ADC self-noise, so it seems our listening stations should at worse have -80 dB self noise in the relevant frequency range.
Competing noise (highways, HVAC units, insects, resident birds) must also be quite low. A shotgun mic helped reduce noise. So did a DIY beamforming condenser array -- a revised one is on the bench. I'm dubious about these approaches and favor acoustic concentration, geometric amplification, as gotten for instance by parabolic dish and exponential and linear horn microphones.
So, my DIY interest is now mostly about acoustic concentrators, very low noise preamp/ADC electronics, weather/water/theft proofing, and how we can maintain a few listening stations remotely, without evening and morning visits to swap mem cards, batteries, etc.
Thanks,
-John Thaden
