Flames stand up for Hallelujah (Part 1)

The song that inspired this post is the popular Hallelujah written by Leonard Cohen and sung here by Jeff Buckley.

Artwork © by Lauren Delora Sears.

One of the great things about staying in post-graduate residential accommodation, here in Dunedin, is undoubtedly meeting some great people from all around the world. I met a lovely Dutch professor here – Maria. We chatted, on and off, as our busy schedules allowed and met up for drinks a few times. On one of those occasions, in fact the last one, we drank a glass or two of wine and played each other some of our favourite music. We both love classical music, and other genres, but at one point she showed me a clip of an Irish priest – Ray Kelly – surprising a couple, at their wedding ceremony, by singing Hallelujah. He did a fantastic job and was happy that I watched because it went viral and several versions have been taken down. Apparently Sony, who own the rights to the original song, is now looking to make a recording with Father Kelly.

It reminded me that Hallelujah is one of those songs which gives me goosebumps. Most people have their favourite rendition of this song. Jeff Buckley’s, seems to be the all-time favourite, with almost 43 million views on YouTube. Although Father Kelly’s managed his 34 million hits in just a few days.

Then there is an amazing version by an eight year old gospel singer! A couple of versions by young women have recently appeared which are worth listening to – Jodi, Alana, Morgan and Hannah Trigwell.

However, the version I usually play is by the talented Michael Henry and Justin Robinett. Interestingly, it’s the two versions with harmonising voices which draws out my goosebumps, as good as the others are. These guys are seriously talented and they have a great sense of humour as their YouTube videos show. But it’s their stunning display, playing Michael Jackson’s Billie Jean, that blows me away. The play simultaneously on twin drum sets and two keyboards while passing the drumsticks back and forth between them. It’s not one of my favourite songs but I love this!

Back to Hallelujah, though.

The lyrics in the first verse got me wondering. It goes:

I’ve heard there was a secret chord

That David played and it pleased the Lord

But you don’t really care for music do you?

It goes like this the fourth, the fifth

The minor fall and the major lift

The baffled king composing Hallelujah.

 But what exactly is the fourth, the fifth, or a minor fall and a major lift?

In order to find the answer to that, we first need a small detour, to explore what standing waves are.

Back in my very first post I explained how sound is energy that moves air molecules. Air is first compressed then extended (rarefaction) and is heard as sound in our ears. The sound is actually the minute variations in air pressure caused by the movement of the molecules. As they bunch up, during compression, the pressure increases and as they spread out again, during extension, the pressure drops. So the sound pressure created by a sound wave may be higher or lower than the surrounding air.

This principle is spectacularly demonstrated with the popular physics experiment using a Ruben’s tube. A Ruben’s tube is a long metal tube, sealed at both ends, with evenly spaced holes along its length. One of the seals is attached to a frequency generator or speaker and the other to a propane gas source which pumps the tube full of gas. The gas is lit as it escapes from the holes.

Particular frequencies for that tube will create standing waves and this is where the beauty of the tube comes in. When the frequencies are just right the flames emanating from the holes will show the pattern of the standing wave. There have been numerous posts lately on the topic since a video of a pyro board was put on YouTube.

So what is a standing wave?

Waves can be generated in the air, for example a woodwind instrument, or along a string such as a guitar string. It’s best explained by the idea of a string which is fixed at one end. Try it by fixing a piece of rope to a tree.

If a single pulse (wave) is sent along the string it will be reflected back along the string, once it bounces off the tree, except it will be upside-down. If another pulse is sent, soon after, the wave travelling in the opposite direction meets the first and they interfere with each other. This interference can be destructive or constructive. If several pulses are made, the result on the string usually looks pretty chaotic as they either add to, or subtract from, the others at different points.

However, if they are well timed, a standing wave can be created. Here, the advancing wave and the reflected wave meet and the whole string oscillates up and down.

Standing waves caused by interference between advancing wave and reflected wave.

Standing waves caused by interference between advancing wave and reflected wave.

The nodes are the places where there is no displacement and the antinodes are where there is maximum displacement. Each frequency has its own pattern.

Now back to the Ruben’s tube.

The height of the flames is proportional to the gas flow which depends on the pressure inside the tube relative to the outside. Since the tube is sealed, the gas has only one way to escape – out through the holes! The pressure it is under at the point where the gas passes under the hole determines how high it will go.

The pressure inside the tube before applying the sound is the same so the flames will all be the same height. When the sound is applied, at one end, the compressed waves produce high flames and the rarefaction produces low flames. The lowest flames correspond to the nodes and the highest flames to the antinodes.

Ruben's Tube standing wave pattern showing flame height relationship to pressure.

Ruben’s Tube standing wave pattern showing flame height relationship to pressure. Credit: Tools for Teaching Science

So now that we have the standing waves sorted, I can look at what this all means for those Hallelujah lyrics in Part 2.

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