SPRDSenior Physics Research DatabaseDickinson College

All projectsDirectional speaker

Measurement

Measurement setup

Get to know the department's directional speaker, set up the microphone and audio interface, and make sure you measure the beam and not the microphone.

Week 2 of 4; the paper is due at the end of week 4

Goals

  • Find the department speaker's carrier frequency, input and safe operating level from its manual.
  • Set up the measurement microphone and audio interface, and know their sample rate and frequency range.
  • Calibrate the recording chain so a number in your file becomes a pressure in pascals.
  • Show that the audio you record is made in the air, not inside the microphone by the ultrasound.

Why this matters

This step is the Methods section of your paper. Measuring a parametric speaker has a well-known trap: a microphone hit by strong ultrasound can generate audible sound inside itself, so you would be measuring the microphone’s nonlinearity, not the air’s. A reader will want to see that you checked.

The physics

Three facts about the recording chain decide what your numbers mean.

Sample rate. An audio interface running at fsf_s records frequencies up to fs/2f_s/2 and filters out everything above. At 48 kHz that is fine for a 1 kHz tone and its harmonics, but the ultrasonic carrier is invisible. To record the carrier as well you need a 192 kHz interface and a microphone that responds that high; most measurement microphones roll off above about 20 kHz.

Calibration. The interface stores numbers, not pressures. A sound calibrator produces 94 dB SPL at 1 kHz, which is exactly 1 Pa RMS, so recording it once gives the conversion factor for that microphone, preamp gain and interface setting.

Microphone nonlinearity. Any nonlinear receiver demodulates AM the same way air does. The difference is where it happens: air demodulates along the whole beam, the microphone only at its diaphragm. A thin film that blocks ultrasound but passes audio, placed in front of the microphone, separates the two. If the audio reading barely changes with the film in place, the sound came from the air.

Hints

Hint 1Read the speaker’s manual first

Find the carrier frequency, the input it expects (line level, USB, Bluetooth), the maximum volume and any warnings. The carrier frequency goes straight into your simulation as --fc.

Hint 2Calibrate at the start of every session

Gain knobs get bumped. A 30-second calibration recording at the start of each session makes every later file trustworthy. If the department has no calibrator, compare against a sound level meter at 1 kHz.

Hint 3Test the artifact on purpose

Record the 1 kHz tone on axis with and without an ultrasound-blocking film in front of the microphone, then with two different film materials. If the readings disagree badly, work out why before collecting any beam data.

Hint 4Mind the ultrasound

Intense airborne ultrasound can cause discomfort and headaches even though you cannot hear it. Look up airborne-ultrasound exposure guidelines, agree on a volume limit with your advisor, and don’t stand close on axis at full volume.

Hint 5Make positions repeatable

Tape a measuring tape to the floor along the axis and mark angles on an arc at a fixed distance. Ten minutes of setup makes every later measurement faster and easier to repeat.

What past teams did

2026–27 team

First team on the project. Add the speaker model, the microphone and interface you used, how you calibrated, and a photo of the setup.