Speakers and microphones

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Lesson notes

Speaker Construction

A speaker consists of a **coil of wire**, a **permanent magnet**, and a **cone**.
The coil is attached to the cone and sits in the magnetic field of the permanent magnet.
When a current flows through the coil, a magnetic field forms around it.

How a Speaker Produces Sound

The coil's magnetic field interacts with the permanent magnet's field, producing a **force** on the coil.
This force causes the cone to move.
If the current is **alternating** (constantly changes direction), the force direction reverses each time.
The cone **vibrates** back and forth, creating sound waves.
The **frequency** of the sound equals the frequency of the cone's vibration.

Making Sound Louder

Increase the **current** flowing through the coil.
Increase the **number of turns** in the coil.
Increase the **strength** of the permanent magnet's magnetic field.

Microphone Construction

A microphone is built similarly to a speaker but works in reverse.
It contains a **diaphragm** (equivalent to the cone) and a coil of wire in a magnetic field.

How a Microphone Works

Sound waves cause the diaphragm to **vibrate**.
The vibrating diaphragm moves the coil within the magnetic field.
The changing magnetic field induces a **current** in the coil.
This current can be sent to a speaker or recorded.

Cross-section of a speaker showing permanent magnet, coil, and cone.

Microphone construction showing diaphragm, coil, and magnet.

Slides

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Practice questions

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1.Which of the following is NOT a part of a speaker?
Easy
- ACoil of wire
- BPermanent magnet
- CCone
- DDiaphragm
2.What is an alternating current?
Easy
- AA current that always flows in one direction
- BA current that constantly changes direction
- CA current that flows in a loop
- DA current that is very strong
3.Which part of a microphone vibrates when sound reaches it?
Easy
- ACoil of wire
- BPermanent magnet
- CDiaphragm
- DCone
4.If the current in the coil of a speaker is decreased, what happens to the sound made?
Medium
- AIt becomes louder
- BIt becomes quieter
- CIt stays the same
- DIt stops completely
5.Which of the following would NOT make a speaker produce a louder sound?
Medium
- AIncrease the current flowing through the coil
- BIncrease the number of turns in the coil
- CIncrease the strength of the permanent magnet
- DDecrease the current flowing through the coil
6.A microphone works in the opposite way to a speaker. What does a microphone convert sound into?
Medium
- ALight
- BHeat
- CAn electrical current
- DMagnetic field
7.In a speaker, what causes the cone to move?
Medium
- AThe magnetic field of the coil interacting with the permanent magnet
- BThe sound waves hitting the cone
- CThe current in the coil heating up the cone
- DThe permanent magnet pushing the cone directly
8.If the frequency of the alternating current in a speaker is increased, what happens to the sound produced?
Hard
- AIt becomes louder
- BIt becomes quieter
- CIt becomes higher in pitch
- DIt becomes lower in pitch

More questions

9.A student says: 'A microphone works by converting sound into a changing current because the coil moves in a magnetic field.' Is this statement correct?
Hard
- AYes, it is correct
- BNo, because the coil does not move
- CNo, because the magnetic field is not changing
- DNo, because the current is constant
10.Which of the following correctly describes the energy conversion in a speaker?
Hard
- AElectrical energy → sound energy
- BSound energy → electrical energy
- CElectrical energy → light energy
- DSound energy → kinetic energy
11.What is the role of the permanent magnet in a speaker?
Medium
- ATo produce a magnetic field that interacts with the coil's field
- BTo attract the cone directly
- CTo generate electricity
- DTo amplify the sound
12.A student makes a speaker with a coil of 20 turns. To make the sound louder, she doubles the number of turns to 40. Which other change would also make the sound louder?
Medium
- AHalving the current in the coil
- BUsing a weaker permanent magnet
- CIncreasing the current in the coil
- DDecreasing the frequency of the alternating current
13.A microphone has a coil of wire that moves in a magnetic field. The coil moves 100 times per second. What is the frequency of the alternating current produced?
Hard
- A50 Hz
- B100 Hz
- C200 Hz
- D400 Hz
14.A student connects a microphone to an oscilloscope. When she speaks into the microphone, the oscilloscope shows a waveform with a large amplitude. What does this indicate about the sound?
Medium
- AThe sound has a high pitch
- BThe sound is loud
- CThe sound is quiet
- DThe sound has a low frequency
15.In a speaker, the cone vibrates at a frequency of 440 Hz. What is the frequency of the sound wave produced?
Medium
- A220 Hz
- B440 Hz
- C880 Hz
- DIt depends on the loudness
16.Which of the following is true about a microphone?
Easy
- AIt converts electrical energy into sound energy
- BIt converts sound energy into electrical energy
- CIt converts light energy into sound energy
- DIt converts sound energy into light energy
17.A student has a speaker with a coil of 50 turns. She wants to make it produce a sound that is twice as loud. Which combination of changes would achieve this?
Hard
- ADouble the current and halve the number of turns
- BDouble the current and double the number of turns
- CHalve the current and double the number of turns
- DDouble the current and use a weaker magnet
18.A microphone diaphragm vibrates due to a sound wave. The vibration causes a coil to move in a magnetic field. What is produced in the coil?
Medium
- AA constant direct current
- BAn alternating current
- CA magnetic field only
- DNo current
19.Which part of a speaker vibrates to produce sound?
Easy
- AThe permanent magnet
- BThe coil of wire
- CThe cone
- DThe casing
20.A student sets up an experiment with a coil of wire connected to a sensitive ammeter. She moves a bar magnet in and out of the coil. The ammeter shows a reading that changes direction. What does this demonstrate?
Hard
- AHow a speaker works
- BHow a microphone works
- CHow a permanent magnet works
- DHow a battery works
21.A sound wave causes a microphone diaphragm to vibrate with an amplitude of 2 mm. If the sound becomes louder, what happens to the amplitude of vibration?
Medium
- AIt decreases
- BIt stays the same
- CIt increases
- DIt becomes zero
22.What type of current is used in a speaker to make the cone vibrate?
Easy
- ADirect current
- BAlternating current
- CStatic electricity
- DNo current
23.A student has two identical speakers. Speaker A has a coil with 100 turns, speaker B has a coil with 200 turns. Both are connected to the same alternating current. Which speaker produces a louder sound?
Medium
- ASpeaker A
- BSpeaker B
- CBoth produce the same loudness
- DIt depends on the frequency
24.A student connects a coil of wire to a sensitive ammeter and moves a bar magnet in and out of the coil. The ammeter shows a reading. Which statement explains this observation?
Easy
- AMoving the magnet changes the magnetic field through the coil, inducing a current.
- BThe magnet creates a permanent current in the coil.
- CThe coil creates a magnetic field that attracts the magnet.
- DThe ammeter is affected by the magnet's field directly.
25.A student has a speaker with a coil of 50 turns. She wants to make the sound louder. Which of the following changes would NOT make the sound louder?
Medium
- AIncrease the current in the coil.
- BIncrease the number of turns in the coil.
- CIncrease the strength of the permanent magnet.
- DIncrease the frequency of the alternating current.
26.In an experiment, a microphone is connected to an oscilloscope. When a sound wave of constant frequency but increasing amplitude reaches the microphone, what change is observed on the oscilloscope?
Hard
- AThe wavelength of the waveform increases.
- BThe amplitude of the waveform increases.
- CThe frequency of the waveform increases.
- DThe waveform becomes a straight line.
27.A student builds a simple speaker using a coil, a magnet, and a paper cone. She connects the coil to a battery. What will happen?
Medium
- AThe cone will move once and then stop.
- BThe cone will vibrate continuously.
- CThe cone will not move at all.
- DThe cone will move but no sound is produced.
28.Which of the following correctly describes the energy conversion in a microphone?
Medium
- ASound energy → electrical energy
- BElectrical energy → sound energy
- CSound energy → kinetic energy
- DKinetic energy → electrical energy
29.A student investigates how the number of turns in a speaker coil affects the loudness. She uses a constant alternating current and measures the sound level with a decibel meter. Which graph shows the expected relationship?
Hard
- ALoudness increases linearly with number of turns.
- BLoudness decreases as number of turns increases.
- CLoudness is independent of number of turns.
- DLoudness initially increases then decreases.
30.A student connects a microphone to an oscilloscope. When she speaks into the microphone, the oscilloscope shows a waveform with a large amplitude. What does this indicate?
Hard
- AThe sound is loud.
- BThe sound has a high pitch.
- CThe sound is quiet.
- DThe sound has a low pitch.
31.A student builds a simple speaker using a coil of 100 turns, a permanent magnet, and a paper cone. She connects the coil to an alternating current supply. She then replaces the permanent magnet with a stronger one. What effect does this have on the sound produced?
Medium
- AThe sound becomes louder.
- BThe sound becomes quieter.
- CThe pitch of the sound increases.
- DThe pitch of the sound decreases.
32.A microphone is connected to an oscilloscope. A sound wave of frequency 256 Hz and amplitude 1.0 mm causes the diaphragm to vibrate. The diaphragm is attached to a coil of 50 turns in a magnetic field of 0.2 T. What is the frequency of the alternating current induced in the coil?
Hard
- A256 Hz
- B128 Hz
- C512 Hz
- D50 Hz
33.A student investigates how the loudness of sound from a speaker depends on the current in the coil. She uses a constant alternating current and measures the sound level with a sound meter. She repeats the experiment with different currents. Which graph shows the correct relationship between current and sound level?
Medium
- AA straight line through the origin with positive slope.
- BA curve that flattens off at high currents.
- CA straight line with negative slope.
- DA horizontal line.
34.A student has two identical speakers. Speaker A has a coil with 100 turns, speaker B has a coil with 200 turns. Both are connected to the same alternating current supply. What is the ratio of the force on the coil of speaker B to the force on the coil of speaker A?
Medium
- A2:1
- B1:2
- C4:1
- D1:1
35.A student sets up an experiment with a coil of wire connected to a sensitive ammeter. She moves a bar magnet in and out of the coil. The ammeter shows a reading. She then moves the magnet faster. What happens to the reading on the ammeter?
Hard
- AThe reading increases.
- BThe reading decreases.
- CThe reading stays the same.
- DThe reading becomes zero.
36.Which of the following is NOT a way to make a speaker produce a louder sound?
Easy
- ADecrease the number of turns in the coil.
- BIncrease the current flowing through the coil.
- CIncrease the strength of the permanent magnet.
- DIncrease the number of turns in the coil.
37.A microphone diaphragm vibrates 440 times per second due to a sound wave. The diaphragm is attached to a coil in a magnetic field. What is the frequency of the alternating current produced in the coil?
Medium
- A440 Hz
- B220 Hz
- C880 Hz
- D4400 Hz
38.A student connects a microphone to an oscilloscope. She speaks into the microphone with a constant pitch but varying loudness. Which of the following describes what she sees on the oscilloscope?
Hard
- AThe amplitude of the waveform changes, but the frequency stays constant.
- BThe frequency of the waveform changes, but the amplitude stays constant.
- CBoth amplitude and frequency change.
- DNeither amplitude nor frequency changes.
39.In a speaker, the cone vibrates at a frequency of 1000 Hz. What is the period of the sound wave produced?
Medium
- A0.001 s
- B0.01 s
- C0.1 s
- D1 s
40.A student builds a speaker with a coil of 50 turns. She wants to make the sound twice as loud. She increases the current to twice its original value. What else should she do to ensure the sound is exactly twice as loud?
Hard
- ANothing, doubling the current alone makes the sound twice as loud.
- BDouble the number of turns as well.
- CHalve the number of turns.
- DDouble the strength of the magnet.

Revision notes

## Speakers and Microphones ### Speaker - **Parts**: coil of wire, permanent magnet, cone. - **How it works**: - Current flows through coil → magnetic field forms around coil. - This field interacts with permanent magnet’s field → force on coil → cone moves. - Alternating current (AC) changes direction → force reverses → cone vibrates → sound. - Sound frequency = vibration frequency. - **Making sound louder**: - Increase current in coil. - Increase number of turns in coil. - Increase strength of permanent magnet. ### Microphone - **Same construction as speaker but works in reverse**. - **Parts**: diaphragm (cone), coil, permanent magnet. - **How it works**: - Sound waves vibrate diaphragm → coil moves in magnetic field. - Changing magnetic field induces current in coil (electromagnetic induction). - Current pattern can be sent to speaker or recorded. ### Key Points - Any current in a wire creates a magnetic field. - AC constantly changes direction. - Moving a coil in a magnetic field generates current. - Decreasing current in a speaker makes sound quieter (not louder).

Alternating current: A current that constantly changes direction.
Electromagnetic induction: The process of generating a current in a coil when it experiences a change in magnetic field.
Diaphragm: The cone in a microphone that vibrates when sound reaches it.

Flashcards

What are the three main parts of a speaker?

A coil of wire, a permanent magnet, and a cone.

What happens when a current flows through a wire?

A magnetic field is formed around the wire.

How does a speaker produce a force on the cone?

The magnetic field from the coil interacts with the permanent magnet's field, creating a force that moves the cone.

What is an alternating current?

A current that constantly changes direction.

How does alternating current cause the cone to vibrate?

The changing current reverses the coil's magnetic field, reversing the force direction, so the cone vibrates back and forth.

What is the relationship between cone vibration and sound?

The vibration of the cone creates sound waves.

How does the frequency of vibration affect the sound?

The frequency of the sound equals the frequency of vibration; faster vibration gives higher frequency sound.

Name three ways to make a speaker louder.

Increase the current, increase the number of turns in the coil, or increase the strength of the permanent magnet.

How is a microphone similar to a speaker?

It is built in the same way but works in the opposite direction.

What is the diaphragm in a microphone?

The cone in a microphone, which vibrates when sound waves hit it.

How does a microphone generate a current?

Sound vibrates the diaphragm, moving the coil in the magnetic field, which induces a current in the coil.

What happens to the current produced by a microphone?

It can be passed to a speaker for playing or saved as a pattern for later playback.

True or false: Any time a current passes through a wire, a magnetic field is formed.

True.

True or false: A microphone contains a diaphragm that vibrates when sound reaches it.

True.

True or false: An alternating current always flows in the same direction.

False.

True or false: If a coil moves in a magnetic field, a current is made.

True.

True or false: Decreasing the current in a speaker coil makes the sound louder.

False.

What is the function of the permanent magnet in a speaker?

It provides a magnetic field that interacts with the coil's field to produce force.

What is the function of the cone in a speaker?

It moves back and forth to create sound waves.

What causes the force on the coil in a speaker to reverse direction?

The current changing direction, which reverses the coil's magnetic field.

What is the relationship between the coil's movement in a microphone and current?

The coil moving in a magnetic field induces a changing current in the coil.

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