This video is section 4.6 in the volume of Physclips about light. www.animations.physics.unsw.edu.au/light/interference/index.html 4.6 is a demonstration and its explanation is in section 4.5. You may also need to go back to Chapter 1: The Nature of Light www.animations.physics.unsw.edu.au/light/nature-of-light/index.html

You are correct in saying that pitch references are to some extent arbitrary and there are arguments about which is best. Humans have different voice ranges. They come in different sizes and shapes so the range of their vocal resonances also vary. So, in principle, we'd expect that different pitch references might suit some singers better than others. However, the difference between 432 and 440 is not great, even compared with the day-to-day (or morning to evening) variation in singing ranges or vocal resonance ranges. A larger variation comes from 'original instrument' performances of baroque works. Often these use A = 415 Hz. This can make life easier for sopranos and tenors: their 'high C's in baroque tuning have the same frequency as a high B in modern tuning. Conversely, baroque pitch makes very low notes harder to sing. I don't know whether anyone has studied resonance tuning in baroque pitch. Woodwind instruments provide a practical constraint on changing pitch references, because the tuning slide doesn't affect all notes in the same way: 'long tube' notes are affected less that 'short tube' notes. Modern woodwinds play well in tune at roughly A = 440 Hz or a bit higher. Recreation baroque instruments are usually made to play at 415 Hz.

@@JoeWolfe Indeed, I didn't quite consider the in-between pitch reference of Baroque tunings. Your response was a valuable assessment of pitch-reference challenges throughout history. Thank you! In addition to meditation on the textural differences of A432 vs. A440 vs. A444hz (Shcumann Resonance, Earth's Atmosphere), I am observing the "primal" pitches as called or cried by different animals. Birds are bit more a chore to isolate and table a specific frequency on my tuners. I am convinced that not only is our hearing spectral (C/"red" to F/"lime-green as the warmer colors and F#/"green" to B/"magenta are the cooler notes. Meaning each note has a specific textual quality. Fore example F# always as a buzzy sort of twangy sound to it. Marking it as green reminds that F# is a pivotal note is the vocal register-shift from F to F#. Also, just as C is the opposite of F#, so is C/red the tritone of F#. Further, the color green divides or "splits" the 12 tone color wheel just as F# splits the octave of Middle C to a higher C. There is clearly a REASON that Middle C is associated as the _center_ of the keyboard. In my work I have confirmed thus far that animals do not sing in A440 concert pitch. Which is not surprise. Ha. I have found that in general they DO tend to pitch lower, rather than higher. And, yes, closer to a432hz than A440! At the moment my focus is to prove a general psychology of the keys (by analyzing song-lyric themes and thematic choices of Classical works. I also am working to prove that the reason people have perfect pitch at all is because they have learned or have always been able to identify at note by it's intrinsics texture (coloring). Which can be seen parallel to the color spectrum. Right down to the try-tone complimentaris.. And I'm bringing attention to my channel by creating stunning Sia music videos!

Thanks for the kind words. This has been a theme in our lab's research, so we thought we should make a video explanation for singers..

It just hit me that this videos systematically explains how overtone throat-sining works.

@@Acoustic-Rabbit-Hole Yes, overtone or harmonic singing is an application of resonance tuning. I spoke a little about harmonic singing in this talk, starting at 11:30 and showed a measurement, but I didn't discuss it in any detail.

If the object is close (e.g. less than a metre) you can use parallax - the slightly different views you get from each eye.

Depending on how you measure, it's either theta or tan theta or sin theta. For small angles (as in this case), these are all approximately equal.

It might take time to find an appropriate file because John and I are now both retired. And you might be disappointed: R2:fo doesn’t sound much different from R1:fo - after all, a resonance is a resonance. The difference in sound is mainly that R2:fo usually, though not always, is used at higher pitch.

Sorry for the delay. I posted a sound file on this page: www.phys.unsw.edu.au/jw/broadband.html

i) The system we used would be hard to reproduce at home and we have no current plans to develop a do-it-yourself version. (We are physicists, not software developers.) ii) We've never had a soprano overtone singer in the lab. However, I'd expect that such a singer would have a big advantage in learning R2:fo tuning.

Thank you for your quick reply :-) @@JoeWolfe

There's a calibration I should have mentioned. Photomultiplier tubes (PMTs) have efficiencies of tens of percent, so it's not hard to satisfy the condition of only one photo or fewer in the volume at a time. (And even if the efficiency were low, we'd not expect it to change over the time of the experiment so we'd still have the paradox that, at some angles, more photons in produce a weaker signal at the PMT.)

Your question goes straight to the truly astonishing point: we have a beam of photons with certain phases. Then we add to that beam another beam with opposite phases. The two beams cancel out. Adding more photons gives (at this point in the interference pattern) fewer photons. In the wave picture, this is no surprise. Several chapters of Physclips before this one lead us to expect destructive interference of waves that are π out of phase. It's when we use the particle picture that it seems strange: by opening the second slit, we've doubled the number of photons entering the apparatus but, at this particular point in the interference pattern, we suddenly get near zero detection. (Elsewhere, of course, the interference pattern has become four times as intense and, on average, the number of photons in the pattern has doubled.) As Abhishekshar11 below seems to say: It's not hard to understand, it's just hard to believe! Other experiments? I know of no other table-top experiments that are as clear as this one. And this gear is well designed for just one purpose. However, there are many other cases of destructive interference that raise the same issue, of course and these are good to think about. For example, consider the anti-reflection coating on camera lenses or some spectacles. As discussed in the Thin Films section of Physclips, these give greater transmission of light into the camera and less light reflected back. No surprise. But consider this from the point of a photon: how does the photon entering the MgF layer 'know' that it will soon encounter glass? what if it were just a thick medium of MgF? How does it 'know' not to be reflected? Have fun!

Urban legend.

Underrated comment.

If you know trigonometry or angles(mathematicians measure angles in radians) in general you will realise it's all ratios to begin with you may be more familiar with degrees which can be converted to radians using this this 360=2pi

If you don't know _some_ dimension of the object you're ranging, you can't estimate how far away it is. But many things are standard sizes such as: animal or human height/width, doors/windows, shipping containers, building floors, traffic signs, electric poles, oil drums, etc..

If you can't estimate the size of anything, you're sunk. There's no visual difference between a 1m-tall dropbear that's 100m away from you and a 10m-tall dropbear that's 1km away.

Um. The technique in the video doesn't require measuring any angles, so units of angles are irrelevant.