where C is the speed of the wave; vs  is negative if the source is moving away from the receiver, positive if the source is moving towards the receiver. In our experiment, we will observe the Doppler shifts of sound waves that are governed by the classical Doppler effect. (Doppler shifts of light follow the relativistic Doppler effect. The equations differ slightly, but the principles are the same.)

Procedures

[Preparation]

•    You will need two general-purpose mobile computing devices that are equipped with microphone and speaker (such as cellphone, tablet, laptop, etc.). At least one of the devices should have access to Apple App Store or Google Play Store.

•    Install Phyphox® app on one (or both) device through Apple App Store or Google Play Store.

•    You will need a room with low background noise. If you do not have access to a quiet room, please contact the course instructor to use a lab space at UTM.

•    Note that you will produce some buzzer noise during the experiment. You might want to ask for permission from the people who share the space with you (if applicable).

[Doppler Spectroscopy Experiment]

1.   On the device that has Phyphox app installed, tap on “Doppler Effect” in the Phyphox app. [This device will be the receiver of the sound waves.] For all measurements below, keep the receiver stable and untouched during recording.

2.   If your other device also has Phyphox app installed, tap on “Tone Generator” in the Phyphox app. Otherwise, open this tone generator website: https://onlinetonegenerator.com/using any browser on that device. [This device will be the source of the sound waves.]

3.   (Check background noise) On the receiver, make sure that the “base frequency” is set at  1000Hz (default value), then switch to the “results” tap and press the play (“►”) button to start recording. Keep the receiver stable and untouched during recording. Walk a few steps back and forth. Notice that the receiver should detect your footstep sounds as intermittent random noise. Take a screenshot of the current data screen, then press the trashcan symbol to clear data.

4.   On the source, set the frequency to 1000Hz, and play the sound. Adjust the volume so that the sound is not “annoyingly loud” at close but also clearly audible at about 3 meter distance. Many modern devices have more than one speaker. If your source has more than one speaker, make sure not to keep the source too close to the receiver. (If you use the Phyphox tone generator, note that you must hit “enter” or close the numpad before the new frequency takes effect.)

5.   (Doppler measurement on radial velocity at close distance) Start a new recording on the receiver. Leave the receiver stable and untouched. As you are standing close to the receiver, quietly move the source back-and-forth from the receiver 5 times in total, with a short pause between each stroke of motion. You should see the “frequency” and the “speed” of the source being recorded on the two diagrams of the Phyphox app on the receiver. Stop recording, take a screenshot of the current data screen, then clear the current data. (An example data is shown below. If you see too much noise or variability in this data, clear it and try again with a different source volume or orientation.)

6.   (Doppler measurement on radial velocity at far distance) Start a new recording on the receiver. Take the source in hand and walk three steps (about 3 meters) away from the receiver. As you are standing far from the receiver, quietly move the source back-and-forth from the receiver 5 times in total, with a short pause between each stroke of motion. Stop recording, take a screenshot of the current data screen, then clear the current data. (An example data is shown below. Depending on the actual distance, the cycles should be somewhat less prominent on the data curves but still visible.)

7.   (Doppler measurements  on transverse velocity  at  close distance) Start a new recording on the receiver. As you are standing close to the receiver, quietly move the source left-and-right relative to the receiver 5 times in total, with a short pause between each stroke of motion. Stop recording, take a screenshot of the current data screen, then clear the current data.

8.   (Doppler measurements on transverse velocity at far distance) Start a new recording on the receiver. As you are standing far from the receiver, quietly move the source left-and-right relative to the receiver 5 times in total, with a short pause between each stroke of motion. Stop recording, take a screenshot of the current data screen, then clear the current data.

9.   Before you lose track of which screenshot is which measurement, you might want to transfer the screenshots to your computer now and rename them properly.

[Sound Intensity Control Experiment]

10. On  the  receiver,  exit  the  “Doppler  Effect”  measurement  and  open  the  “Audio  Amplitude” measurement. This measurement simply measures the loudness of the sound. For all measurements  below, keep the receiver stable and untouched during recording.

11. (Intensity measurement at close distance) Start a new recording on the receiver. As you are standing close to the receiver, quietly move the source back-and-forth from the receiver 5 times in total, with a short pause between each stroke of motion. Stop recording, take a screenshot of the current data screen, then clear the current data.

12. (Intensity measurement at far distance) Start a new recording on the receiver. Take the source in hand and walk three steps (about 3 meters) away from the receiver. As you are standing far from the receiver, quietly move the source back-and-forth from the receiver 5 times in total, with a short pause between each stroke of motion. Stop recording, take a screenshot of the current data screen, then clear the current data.

[Tips from former AST201 students]

•    You can use an external microphone (such as one on a headset) to improve sound quality.

•    Echoes from walls nearby may affect your measurements.

•    In Phyphox app, Tap on the data curve and use the “pick data” tool to select data.

Minimal list of tasks

Please complete Tasks i-vi and Tasks vii-xi in separate documents and upload them on Crowdmark.

i.      Crop all your screenshots to keep only the data curves (to save space in your report).

ii.      Attach the data curves ofthe background noise from Step 3 above.