Korben said:
I am requesting help to construct an ultrasonic device that
emits sound at about 25Khz. Unlike most ultrasonic devices,
I want the device to carry sound over 100 feet (or 30 meters)
and still be over 140 decibels at 100 feet (or 30 meters).
I will assume you mean dBA SPL, not dB. The Decibel is a ratio,
not an absolute quantity. What you wrote above translates to
"I want the device to ... be 30,000 times louder than at 100
feet." 30,000 times louder than what?
All I know of this is that the sirens on fire engines can
do this for miles.
In that case, you have a big problem. The one thing that you think
you know is completely false. A typical 100 watt fire engine siren
puts out 130 dBA at 1 meter on axis. To get that up to 140 dBA at
1 meter would take a 10,000 watt speaker (+10 dBa is x10 power and
x2 perceived loudness).
Sound pressure levels in a free field from a point source decrease
6 dB for each distance doubling. So to increase that 10 kW 140
dBA enough to reach that level at 30 meters instead of 1 meter,
you would need these levels:
1 meter 170 dBA
2 meters 164 dBA
4 meters 158 dBA
8 meters 152 dBA
16 meters 146 dBA
32 meters 140 dBA
....and your 140 dBA 10,000 Watt speaker would have to be
replaced with a 170dBa 10,000,000,000 Watt speaker.
So, do you happen to have a 10 billion watt power amplifier
laying about that you can use to drive this speaker?
Now let's talk abhout the fence you will need to build.
Federal maximum exposure standards are:
90 dBA Sound Pressure Level for eight hours, or
95 dBA Sound Pressure Level for four hours, or
100 dBA Sound Pressure Level for two hours, or
105 dBA Sound Pressure Level for one hour, or
110 dBA Sound Pressure Level for 30 minutes, or
115 dBA Sound Pressure Level for 15 minutes, or
120 dBA Sound Pressure Level for 7 minutes
You need to move the fence twice as far to get each
6 dB of noise reduction, so the fence will be:
32 meters away from the speaker for 140 dBa exposure.
64 meters away from the speaker for 132 dBa exposure.
128 meters away from the speaker for 128 dBa exposure.
256 meters away from the speaker for 122 dBa exposure.
512 meters away from the speaker for 116 dBa exposure.
1 kilometer away from the speaker for 104 dBa exposure.
2 kilometers from the speaker for 98 dBa exposure.
4 kilometers from the speaker for 92 dBa exposure.
8 kilometers from the speaker for 86 dBa exposure.
That's a circle 16 km across.
I have a suggestion. Why don't you tell us what you are
trying to accomplish rather than asking us to help you to
implement the "solution" that you came up with on your own?