All stimuli were calibrated using two ITC hydrophones, one serving as sound source and the other as receiver. The stimulus hydrophone was placed at one side of the tank close to the wall (see figure below for details) while the receiving hydrophone was placed at different locations in the tank. Six recording positions were used. 1) Directly beside the sending hydrophone. 2) On the opposite side close to the wall (longest distance). 3) In line with the stimulus hydrophone but directly before center tube. 4) In line with the stimulus hydrophone but directly behind the center tube. 5) A quarter turn left from the stimulus source close to the wall. 6) A quarter turn right from the wall.
The sound levels recorded at position 2 were used to set the stimulus amplitude. The recorded signals were amplified (Stewart VBF-7), sampled at a rate of 1 MHz (IoTech Wavebook 512), and stored on a PC. These signals were analyzed using Matlab (Mathworks) and Igor Pro.
The final results were shown on the graphs below. During experiments, the calibration software only delivered the sound pressure level (SPL) in real time. Frequency analyses (FFT and spectral analyses) such as those shown below were computed off-line.
There were nearly no amplitude differences in the tank (only position 1 compared to 2 showed a 1 to 2 dB difference). Thus the amplitude of the transient sound was the same throughout the tank. Due to the short wavelength of ultrasound signals (velocity (c) = frequency (f)* wavelength (l)) in water (c = 1500 m/s) between 7.5 (20 kHz) and 0.75 cm (200 kHz), local fields of intense signal amplitude caused by reflections in the tank are possible, but were not found in our tank. A comparison of the two 80 kHz samples (below) shows that there are harmonics of the stimulus frequency in the water but the signals themselves are stable and sharp in their spectral shape. The FFT in each graph demonstrates the power of the stimulus relative to the ambient noise in the tank.
The noise was measured differently even though the graph on the bottom of this page is kept in the same style as the ultrasound measurements for comparison. Noise levels were measured as described in chapter 1 of Whitlow W.L. Au's book "The Sonar of Dolphins" (Springer 1993). Click on the figures below for enlarged graphs.

	Record from hydrophone at position 3 stimulus frequency was 80 kHz peak amplitude measured was 185 dB SPL

	Record from hydrophone at position 2 stimulus frequency was 80 kHz peak amplitude measured was 185 dB SPL

	Record from hydrophone at position 2 White noise was band pass filtered between 0.2 and 20 kHz