Sound Measurement Practicals

Overview

This page focuses on practical methods used to measure properties of sound waves, especially the speed of sound in air, frequency, and wavelength.

Common H2 Physics approaches include:

• microphone + oscilloscope (CRO) methods
• stationary wave methods
• resonance tube / air column methods
• analysis of experimental uncertainty

Definition

Most sound practicals use:

$$v=f\lambda$$

where:

• $v$ = speed of sound
• $f$ = frequency
• $\lambda$ = wavelength

If two quantities are measured, the third can be calculated.

Why It Matters

Sound practicals test whether students can connect wave equations to real measurements. They also test graph reading, uncertainty reduction, stationary waves, resonance, and correct SI units.

Key Representations

Method 1: Microphone + Oscilloscope (CRO)

Purpose

Measure frequency of a sound wave.

Apparatus

• loudspeaker / signal generator
• microphone
• oscilloscope (CRO)

Principle

The microphone converts sound pressure oscillations into an electrical signal.

The CRO displays voltage against time.

From the waveform, period $T$ can be measured.

Then:

$$f=\frac{1}{T}$$

Procedure

1. Produce a steady tone.
2. Connect microphone to CRO input.
3. Adjust time-base until clear cycles appear.
4. Measure time for one or more cycles.
5. Determine period and frequency.

Example

If 5 cycles occupy $10\text{ ms}$:

$$T=\frac{10\text{ ms}}{5}=2.0\text{ ms}$$ $$f=\frac{1}{2.0\times 10^{-3}}=500\text{ Hz}$$

Method 2: Stationary Wave with Loudspeaker and Reflector

Purpose

Measure wavelength, then speed of sound.

Apparatus

• loudspeaker connected to signal generator
• flat reflector
• microphone + CRO
• metre rule

Principle

Incident and reflected sound waves superpose to form a stationary wave.

This creates:

• nodes (minimum signal)
• antinodes (maximum signal)

Move microphone along the line of travel.

Key Relationship

Distance between adjacent nodes or adjacent antinodes:

$$\frac{\lambda }{2}$$

So if spacing is $d$:

$$\lambda =2d$$

Then:

$$v=f\lambda$$

Example

Given:

• frequency $f=400\text{ Hz}$
• node spacing $0.42\text{ m}$

Then:

$$\lambda =2(0.42)=0.84\text{ m}$$ $$v=400(0.84)=336{\text{ m s}}^{-1}$$

Method 3: Resonance Tube / Air Column

Purpose

Determine wavelength using resonance lengths.

Apparatus

• tuning fork or speaker
• resonance tube with adjustable air column
• water reservoir (if applicable)
• ruler

Principle

When air-column length matches a natural mode, sound becomes loudest.

Standing waves form inside the tube.

Closed-End Tube Conditions

Closed end = node Open end = antinode

Resonance lengths:

$$L=\frac{\lambda }{4},\frac{3\lambda }{4},\frac{5\lambda }{4},\dots$$

Difference between successive resonances:

$$\frac{\lambda }{2}$$

So if two adjacent resonances differ by $\Delta L$:

$$\lambda =2\Delta L$$

Example

Successive resonances at:

• $0.18\text{ m}$
• $0.51\text{ m}$

Difference:

$$\Delta L=0.33\text{ m}$$ $$\lambda =0.66\text{ m}$$

If tuning fork frequency is $512\text{ Hz}$:

$$v=512(0.66)=338{\text{ m s}}^{-1}$$

Apparatus Interpretation Skills

Students should identify:

Loudspeaker

Produces steady sound source.

Signal Generator

Sets frequency.

Microphone

Detects sound pressure variation.

CRO

Measures waveform and period.

Reflector

Creates reflected wave for stationary pattern.

Metre Rule

Measures node spacing or air-column length.

Sources of Error and Uncertainty

CRO Method

• unclear waveform
• counting wrong number of cycles
• poor time-base reading

Stationary Wave Method

• node/antinode positions not sharp
• background reflections
• ruler parallax

Resonance Tube

• judging loudest point by ear
• end correction
• room noise

How to Improve Accuracy

• measure across many cycles
• measure several node spacings
• average repeated readings
• keep room quiet
• align ruler at eye level
• use successive resonances

Typical Speed of Sound

At room temperature:

$$v\approx 340{\text{ m s}}^{-1}$$

Common Exam Pitfalls

• using node spacing as full wavelength
• forgetting adjacent nodes are $\lambda /2$ apart
• mixing frequency and period
• forgetting unit conversion (ms to s, cm to m)
• using first resonance incorrectly in closed tube

Quick Revision Checklist

Ask:

• What quantity is directly measured?
• Is this node spacing or wavelength?
• Is the tube open or closed?
• Did I convert units?
• Should I average multiple readings?
• Final answer reasonable near $340{\text{ m s}}^{-1}$?

Formula Summary

$$v=f\lambda$$ $$f=\frac{1}{T}$$ $$\text{Adjacent nodes}=\frac{\lambda }{2}$$ $$L=\frac{\lambda }{4},\frac{3\lambda }{4},\frac{5\lambda }{4},\dots$$

Related Links

• Waves
• Stationary Waves
• Interference and Diffraction
• Superposition of Waves

Links

• Main topic: Waves
• Related concept: Stationary Waves
• Related concept: Interference and Diffraction
• Related topic: Superposition of Waves

Summary

Most sound practicals measure frequency or wavelength first, then use:

$$v=f\lambda$$

Success depends on recognising the apparatus, using correct stationary-wave relationships, and handling uncertainties carefully.