NewPing Library (Ultrasonic Sensors)
Written by Tim Eckel.
https://bitbucket.org/teckel12/arduino-new-ping/wiki/Home
Modified for Teensy 3.0 & 3.1
https://github.com/PaulStoffregen/NewPing
http://forum.pjrc.com/threads/25907-Multiple-HCSR-04-Ultrasonic-sensors-on-teensy-3
Updated to support ESP8266 (Arduino IDE) by Jordan Shaw
https://github.com/jshaw/NewPingESP8266
Port was influenced by the issue: https://github.com/PaulStoffregen/NewPing/issues/2.
Also, removed non-compatable ping examples using timers due to incompatability with "Timer interrupt ping methods (won't work with non-AVR, ATmega128 and all ATtiny microcontrollers)" (NewPingESP8266.cpp ln. 198)