Tech experts: Need help with son's Science project!

[Tom Baldwin]

I know there's a bunch of you here, so I'm hoping for some insight. We are trying without success to replicate this experiment: http://www.youtube.com/watch?v=mODqQvlrgIQ

We have it set up just like in the video, and are playing mp3s of Sine waves from a Macbook, through a cable coming out of the headphone jack directly into the rca input on the back of a Polk Audio PSW10. Tried 2 different camcorders.

I have some theories (not in the scientific sense of the word) as to why it's not working.

1. The subwoofer I'm using is rated down to 35Hz, so even though I'm playing a 24Hz Sine wave through it, all I'm really getting is speaker excursions and not that actual frequency.

2. It's not a true Sine wave generator, just an mp3 of the Sine wave.

3. In order to match the frame rate of the Camcorder, I might need something more specific than a whole number frequency. Does such a thing exist - i.e. 24.5 Hz? In the video he has a dial (and probably an expensive Sine wave generator) to tune to exactly the right frequency.

4. The newer camcorder supposedly records at 60 "interlaced" FPS (tried 60Hz, got nothing), so that would negate the effect we are shooting for.

5. I'm not Bill Nye

Any thoughts, help, suggestions will be gladly welcomed!

[fdeck]

Here's an idea to try. Play different notes on your bass -- good strong arco -- until the string seems to stand still in the camcorder. This will tell you what frequency you need to use, and that your setup is generally functioning as expected.

[Tom Baldwin]

That's an interesting thought, but I'm not sure the frequency of the string's excursion has anything to do with the actual frequency (pitch). Plus the bass only goes down to 42 Hz, right?

[jdepriest]

Check the frame rate on the camera and the manual. According to the video info you need to shoot it at 24FPS (frames per second). If you're shooting at "60 "interlaced" FPS" it's way to fast and it the illusion will not be visible. On the camera menu screen my JVC shows it's shooting at 24FPS, but it's actually 23.999 when I import it to my video software, Premiere Pro.

Before HD or IMAX, all movies and TV shows were shoot at around 24 FPS. That's about as slow as you can get and still reproduce very natural looking movement. Now with digital we can shoot much higher frame rates. Most of what I shoot for the university is at 60FPS but if I'm shooting something just for the archives and not for the web site or a DVD project I'll shoot at 24FPS.

[Tom Baldwin]

Thanks, I'll check the frame rate on the older camcorder. It one of those digital Hi-8 jobs, that uses the tapes nobody even carries anymore - you have to order them online!

[fdeck]

You can have a frequency that's a multiple of the frame rate, and still get the same effect. In addition to camcorders, it might be worth looking into a cheap Webcam, or even see if your cellphone camera can take a video.

[Tom Baldwin]

Yeah, that makes sense, sort of like how the octave is twice the frequency of the fundamental. I discovered the frame rate for my particular camcorder is 30 FPS. We were able to achieve some success with 29 and 30Hz Sine waves. Next we will try 59 and 60 Hz. Thanks for the replies!

[fdeck]

Cool. Your A string should be in the ballpark of 60 Hz. If it's not exactly the same, you will see the string appear to move in slow motion. In fact this should be a way to observe the development of the harmonics as you play a note.