Overview and Background

The speaker bracket holds the speaker and, with a 10-24 machine screw (or thumbscrew) and nut allows the bracket to clamp on to a support rod (up to 3/4 inch, 19 mm). There is a feature inside the cylinder for the 10-24 nut (gluing the nut into the feature is helpful, but not necessary). Loctite 40140

The center bracket is the fixture for connecting a string to the speaker. I use BOSS BRS35 replacement speakers ($7.19 on amazon.com as of 4/18/2018), and remove the center plastic with a utility knife, then glue in the center bracket with Loctite 40140. I have tried red Loctite (Loctite 272), but that does not bond strongly enough for this application. Expect to replace the speakers fairly often: they are not designed for this application.

To generate the waves, I use the website http://www.szynalski.com/tone-generator/ on a laptop, with a 3.5mm cable into a cheap audio amplifier (about $20USD on amazon), then speaker wire from the amplifier to the speaker. I soldered the speaker wire onto the speakers, mostly for convenience.

This set up works very well. There are several possible experiments that can be done with this set-up; I have included the student handout with the directions, data tables, formulas, and questions with the 3D printer files and images.

Lesson Plan and Activity

Waves on strings

This lab can be done in one 45-minute class period, if the equipment is set up for the students. If the students have to set up and tear down the equipment, it might take two class periods.

I start the period by demonstrating the lab, especially how to determine the harmonic number of a standing wave (count the anti-nodes, and/or refer to the diagram in the handout), collecting data for a couple of harmonics. Along the way, I explain the learning target and the equipment, and how the lab relates to the other labs about waves they have already done. This usually takes between five and ten minutes, then the students begin working.

Materials Needed

3D printed brackets
Loctite 40104
10-24 machine screw and nut
3.5" replacement speaker
speaker wire (0.5 to 1 m or 18 to 36 inches), any gauge.
3.5 mm male/male audio cable
signal http://www.szynalski.com/tone-generator/ on any online device
string
small masses (200-500 g) to put tension in the string
pulley
support rods and clamps

Skills Learned

Wave equation
Waves
Harmonics

Duration of Lesson

One 45-minute class period if set-up is ready for students.

Rubric and Assessment

There is a checklist/rubric for grading in the Word (.docx) file.

3.5" Speaker Bracket Set