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Polyrhythms
Posted May 21, 2001 as a Math Forum EPoW

Introduction: In this problem, students investigated different rhythms, both aurally and visually, using the applet. They then quantified these rhythms based on the accented beats.


Where’s the Math:
The first few questions and the investigation deal mainly with ratios and their properties. To find the phrase length of a complicated rhythm, however, the concept of the LCM is needed, since the total phrase length must be evenly divisible by each of its constituent ratios.

Standards: Algebra, number & operations

Role of Components: AgentSheets is used to create the sounds and visuals for the different rhythms. A button panel is used to allow students to play the rhythms and select different patterns, and a swing slider allows the selection of different tempos.

Try the applet!

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Sample submitted solutions:

From:  Mark, age 13
School:  Issaquah Middle School, Issaquah, Washington

1. What is the ratio of the mystery polyrhythm? Explain how you know.
The ratio of the mystery polyrhythm is 3:4. When I built the same song on the composite panel, the notes match with the two ratios at 1:3 and 1:4. To find a ratio of two speeds, you take the smallest number on the right side of the ratio and add the bigger number on that side from the other number. Therefore, 1:3 and 1:4 equal 3:4.

2. Create a polyrhythm of your own using two of the rhythms in the applet. What is the ratio of the polyrhythm and its phrase length? Explain how you know.
My composite rhythm has a ratio of 2:6.(One bar is 1:2, the other is 1:6). The phrase length of this rhythm is 6. This is because you count how many note slots it takes before the rhythm of sound repeats itself.

3. A complicated polyrhythm has the ratio 2:3:4:5:6:7. What is its phrase length? How do you know?
To find the phrase length of this rhythm, you must find the least common denominator of all the numbers. Since 5 was one of the numbers, the number would have to end in a 5 or 0. Since 7 was the biggest number, I decided to check by sets of 70's. I went up till I got to 420, which all the numbers go into. So the phrase length of that song is 420.


From:  Andy, age 14
School:  Issaquah Middle School, Issaquah, Washington

1. What is the ratio of the mystery polyrhythm? Explain how you know.
The ratio of the mystery polyrhythm is 3:4. I found this by positioning the squares on the beat boxes of the composite polyrhythm so they coincided with the beat box X's of the mystery. I them took into account the number of beats for each polyrhythm and noticed it took 12 beats in order for the beats to be simultaneous, or 3 sets of the 4 original beats. 12 is also the LCM of the latter numbers in the two combinations of two(1:3, 1:4), to which we could conclude produced the phrase length in other cases.

2. Create a polyrythm of your own using two of the rhythms in the applet. What is the ratio of the polyrhythm and its phrase length? Explain how you know.
I made a polyrhythm with the beats 1:3 and 1:5. The result was a polyrhythm that has a phrase length of 15 and a ratio of 3:5. The ratio was the smaller of the latter of the two beat ratios in direct relation to the other beat. The phrase length was the least common multiple of the digits of that ratio

3. A complicated polyrhythm has the ratio 2:3:4:5:6:7. What is its phrase length? How do you know?
The phrase length is 420. I found this by obtaining the least common multiple of the six digits contained in the ratio.

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Reflections: The definition of the ratio and phrase length were not understood by all of the students who submitted solutions. Some simply followed the pattern given in the instructions and when they had to develop their own polyrhythm, they found that the pattern didn't work for all ratios. Some had trouble finding the ratio. Instead of thinking about the lowest common multiple (LCM) and a ratio as a fraction, they tried to use the pattern in the intro. This resulted in added numerators and the keeping of the highest denominator, and it didn't always work out.

The last question was the hardest for those who didn't understand the LCM and how it intertwined with the ratios. Some chose the phrase length of 1 or 7 either because they went with the highest denominator or the lowest numerator in the ratio as being equal to the phrase length. If there were no problems in the first two problems, then there was generally no problems with the third. The best solutions were able to use the logic that was needed in the third problem to be expressed throughout the entire problem (LCM, common denominators, fractions like ratios).

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