Measurement Lab — the two experiments
Read a hidden frequency off a CRO, then map a stationary sound wave with a microphone.
LO (j) · CRO frequencyLO (k) · stationary-wave λv_sound = 340 m s⁻¹
Experiment 1 — CRO: frequency of sound · LO (j)
Find the hidden loudspeaker’s frequency from the trace — the lab grades you.
hidden source: f = ??? · trigger locked, 0 V rising edge at left
Try first: drag cursors A and B onto matching zero-crossings.
TIME-BASE1 ms div⁻¹
VOLTS/DIV1 V div⁻¹
whole cycles between A↔B n
1
Bracket WHOLE cycles only — divide by that count.
Δt = 5.00 div × 1 ms = 5.00 ms
T = Δt / 1 = 5.00 ms
f = 1/T = 200.0 Hz
amplitude read-off — peak-to-peak divisions4.0 div
Vpp = 4.0 div × 1 V div⁻¹ = 4.00 V
Experiment 2 — stationary sound wave: measuring λ · LO (k)
Flag successive mic maxima — the lab computes λ = 2d̄ and v = fλ.
slow motion ≈ ×857 · spatial structure exact
drag the mic in 3D or use the slider · orbit with the mouse to inspect
detector CRO — time-base OFF
85%
line length ∝ pressure amplitude at mic
flag SUCCESSIVE maxima only — skipping one doubles your λ
drop ≥ 2 flags on successive maxima to get d̄, λ = 2d̄ and v = fλ
What this confirms — from “Wave Motion — Everything to Memorise”
- ▸CRO — determining frequency of sound: T = (cursor separation in divisions) × time-base. The trap is real: traces rarely show whole cycles — bracket the largest WHOLE number of cycles and divide by that count.
- ▸Period & frequency: f = 1/T comes out of a measurement, not recall — your cursor-derived T grades against the hidden source to within your reading error.
- ▸Wavelength of sound via stationary waves: Adjacent signal maxima are λ/2 apart, so λ = 2d̄ and v = fλ lands on 340 m s⁻¹ — but only when you divide the flag span by INTERVALS (5 flags → 4 intervals).
- ▸Pressure graphs — two flavours: The mic senses pressure: its maxima sit at displacement NODES (pressure antinodes). Displacement and pressure envelopes are offset λ/4 (π/2), yet both repeat every λ/2.