Why does amplitude of a mechanical wave increase and then decrease with increasing driving frequency?

I was doing an experiment where I explore how the frequency of the oscillation of a vibrating water hose (the end of the water hose), affects the amplitude created by the water path (a wave-like motion is created).

Here are the results:

Frequency (Hz) - Amplitude (cm)

• 5 - 1.93
• 10 - 3.7
• 15 - 4.7
• 20 - 5.5
• 25 - 7.9
• 30 - 8.33 (this is where the amplitude was the greatest)
• 35 - 5.6
• 40 - 2
• 45 - 1.43
• 50 - 0.73

From the results, it was clear that at 30/31 Hz, the amplitude is at its greatest, and then it decrease after it surpasses that certain frequency.

Why is it that at approximately 30 Hz, the amplitude is at its maximum, and why does it decrease back down again? Why does the results (amplitude) go up and back down?

Why doesn't a larger frequency give a larger amplitude?

• youtube.com/watch?v=uENITui5_jU Hi Gert, this link should give an idea for how the experiment looks like. In my experiment however, I attached a vibration generator on the end of the water hose and controlled the frequency with a signal generator. This created a wave like motion shown in the video. (At about 1:39 of the video, this is an example of what I was investigating. Note that I had the water path going vertically straight down, so that I could measure the amplitude of the wave created at a certain point, which I did for all other frequencies) Commented Dec 8, 2015 at 3:55
• Have you repeated the experiment several times and got the same answer? I mean have you tried different sound generators and everything? Remember you cannot call something "experimental fact" unless it is reproducible.
– Ari
Commented Dec 8, 2015 at 4:21
• Yes I did, the values I gave are the average from three trials. Commented Dec 8, 2015 at 4:23
• I've tried two different signal generators, and they all produced the same effect. Commented Dec 8, 2015 at 4:24