Measuring Acoustic Wavelength and Velocity Diva, Tama & Hafiz
Introduction Wave properties: Wavelength ( λ ): parallel displacement in one cycle. Amplitude ( a ): maximum perpendicular displacement. Period ( t ): time taken for one cycle. Frequency ( f ): cycles undergone per unit time. Velocity ( v ): linear displacement per unit time.
Resonance principle Every half a cycle, a wave reaches its amplitude. When the amplitude of a sound wave makes contact with a physical barrier (e.g. the inside of a tube), it amplifies the sound’s intensity. This effect is called resonance.
Hypothesis The relationship between velocity, wavelength and frequency is defined as v = λ f By generating a sound wave with a constant frequency and finding its wavelength through the points of resonance, we might be able to find the speed of sound through a medium using a derivative of the above formula.
Materials Biuret Tuning forks (216 Hz, 288 Hz, 512 Hz) Bucket Retort stand
Procedure 1. Fill the biuret to the brim with tap water. 2. Tap the tuning fork on a hard surface and listen to its vibration as closely to the surface as possible. 3. Open the biuret and let the water flow out. When the water level reaches a point of resonance, the note should be momentarily amplified. 4. Rinse and repeat until no further resonance is heard.
Assuming that the sound wave experiences resonance every half a cycle, l 2 – l 1 = λ /2 Where l = distance of a given resonance point from the surface.
Sources of error Irregular water flow External disturbances Tuning fork vibration frequency not necessarily the same as the sound wave generated Human error False positives Mistiming Zero error/parallax
Results 216 Hz Nodistance from opening (cm) Test 1Test l 2 – l 1 = 2.1 cm, 3.5 cm Calculated length: 4.2 cm, 7.0 cm Mean result: 5.6 cm Calculated velocity: ms -1 Average difference: 3.0 cm, 3.1 cm Avg. calculated length: 6.0 cm, 6.2 cm Mean result: 6.1 cm Calculated velocity: ms -1
288 Hz Nodistance from opening (cm) Test 1Test l 2 – l 1 = 3.1 cm, 2.7 cm Calculated length: 6.2 cm, 5.4 cm Mean result: 5.8 cm Calculated velocity: ms -1 Average difference: 3.7 cm, 3.1 cm Avg. calculated length: 7.4 cm, 6.2 cm Mean result: 6.8 cm Calculated velocity: ms -1
512 Hz Nodistance from opening (cm) Test 1Test l 2 – l 1 = 4.6 cm, 5.7 cm Calculated length: 9.2 cm, 11.4 cm Mean result: 10.3 cm Calculated velocity: ms -1 Average difference: 3.2 cm, 5.4 cm Avg. calculated length: 6.4 cm, 10.8 cm Mean result: 8.1 cm Calculated velocity: ms -1
Observation The values calculated were much lower than the known speed of sound in air (334.2 m/s) This might be due to the experiment method, which depends on human hearing to take readings and is therefore prone to human error.
Conclusion Though the concept is sound, a more reliable method of measurement is required to achieve proper results. Hence, this lab session is inconclusive.
References Brian Arnold et al. International A/AS-Level Physics. London: Hodder Education, “Sound Waves”. Rice University Web Services. Rice University, n.d.