How a loudspeaker uses the motor effect to turn a changing current into sound, how frequency and current size change the note, and how a microphone does the reverse. Higher tier.
Objectives
Describe how a loudspeaker uses the motor effect to produce sound.
Explain why a loudspeaker needs an alternating current.
State how the frequency and the size of the current affect the pitch and loudness of the sound.
Describe how a microphone changes sound into an electrical signal using the generator effect.
Loudspeakers and microphones explained
Start here
Put your hand on a speaker that is playing loud music and you can feel it shaking. That shaking is the whole secret. The front of the speaker moves backwards and forwards very quickly, pushes the air, and the air carries the sound to your ears. The bigger the shake, the louder the sound you hear.
Think of pushing a flat hand backwards and forwards in a bath of water. Each push makes a ripple, and a fast steady rhythm makes a neat pattern of ripples. A speaker does the same job to the air, but it does it hundreds or thousands of times every second, which is far too quick for your hand to copy.
So the real question is what makes the front of a speaker move so fast and so exactly. The answer is a force on a wire that carries a current inside a magnetic field. This is the motor effect, and a loudspeaker is really a motor that wobbles to and fro instead of spinning round.
This lesson is Higher tier, and it ties together magnets, currents and sound waves. If you have met the motor effect and the generator effect before, they are both about to be useful, and if not, the steps below explain each idea in plain words. Take it one step at a time and the whole device will make sense.
By the end you will be able to do three things. First, describe how a loudspeaker turns an electrical signal into sound. Second, say how the frequency and the size of the current change the pitch and the loudness. Third, explain how a microphone does the job in reverse.
A longitudinal sound wave drawn as vertical lines with squashed compressions and stretched rarefactions, spreading out along an arrow showing the direction the sound travels.
Parts of a loudspeaker
A loudspeaker changes an electrical signal into sound, which is why it is found in phones, televisions and music systems. It has just three main parts that you need to know. These are a permanent magnet, a coil of wire that is free to move, and a paper cone fixed to the coil.
A permanent magnet is a magnet that keeps its magnetism all the time. In a speaker it is fixed in place and does not move. It gives a steady magnetic field in the space around the coil, and that field is what the current in the coil pushes against.
The coil is a long piece of wire wound round and round, so that a current in it acts like many wires side by side. It sits in the field of the magnet and is free to slide backwards and forwards. The cone is a stiff, light cone of paper or plastic fixed to the coil, and it is the part that pushes on the air. A big cone moves more air, which is why bass speakers are large.
The electrical signal comes from an amplifier, which is a device that makes a small signal bigger. The signal is an alternating current. That is a current that keeps changing direction, and the next step explains it fully, so do not worry if the words are new.
Remember the job of each part. The magnet gives the field, the coil carries the current and moves, and the cone pushes the air. If you can say that in one breath, you have the basis for every exam question on this topic. Magnet gives the field, coil carries the current and moves, and cone pushes the air.
Alternating current
A battery gives a direct current. That means the charge flows round the circuit in one direction only, and it never turns round, so the current always has the same sign on a graph. A lamp on a direct current just glows steadily, and a torch or a phone battery gives this kind of current.
An alternating current keeps changing direction. First the current flows one way, then it flows the other way, and it repeats this over and over. The graph shows it as a wave that goes above the zero line and then below it, and above the line means one direction while below the line means the opposite direction.
The frequency of an alternating current is the number of complete cycles each second, and it is measured in hertz in the same way as for a wave. One cycle is one trip forwards and one trip backwards. A frequency of 50 Hz means the current goes through 50 full cycles every second, and the mains electricity in a UK home does exactly that.
The signal sent to a speaker is also an alternating current, but its pattern is not a plain, regular wave. The pattern is a copy of the sound that was recorded, and it changes in size and frequency all the time. Music and speech have many different frequencies mixed together, which is why a voice sounds so different from a flute.
Why does a speaker need an alternating current? A direct current would give a steady force that pushes the coil one way and holds it there, so there would be no vibration. Only a current that keeps reversing can make the coil vibrate, which is why the amplifier gives the speaker an alternating signal.
A graph of an alternating current against time. The current rises above zero, falls below zero, and repeats, which shows the current changing direction again and again.
The motor effect in the coil (Higher tier)
This step is Higher tier. When a wire carries a current and sits in a magnetic field, the wire feels a force, as long as the wire is not lined up with the field. This is the motor effect, and it is the same force that spins the motor in a toy car.
The direction of the force depends on the direction of the current and the direction of the field. If you reverse the current, you reverse the force, and a larger current gives a larger force. A stronger magnet also gives a larger force, but that is not needed here. These two facts are all that a loudspeaker needs, because one gives the vibration and the other gives the loudness.
In the speaker, the coil carries the current and sits in the field of the permanent magnet. So there is a force on the coil, and the force pushes it along its length, in or out of the magnet. A coil that is free to slide will move, and because it is stuck to the cone, the cone moves with it.
Now the current is alternating, so it reverses again and again. Each time it reverses, the force on the coil reverses too. The coil is pushed one way, then the other, backwards and forwards, which is a vibration.
So the answer to the question why the coil moves backwards and forwards is a short one. The current keeps reversing, so the force on the coil keeps reversing. Write it that way in an exam and you will have the key mark. Do not say that the magnet moves, because it does not.
A free body diagram of the coil in a loudspeaker, with an arrow labelled force pointing to the right. The note says that when the current reverses, the force reverses.
From movement to sound, and pitch
The coil is fixed to the cone, so the cone moves backwards and forwards with it. When the cone moves out, it pushes the air in front of it and squashes it. When the cone moves back, it leaves a gap and the air spreads out. This happens again and again, so the air near the speaker is squashed and stretched in turn.
Those squashes and stretches are compressions and rarefactions, and they travel away from the speaker. This is a sound wave. Sound is a longitudinal wave, and the cone is the thing that starts it. The air itself does not travel across the room, only the vibration does.
The cone vibrates at the same frequency as the alternating current. So the sound wave has the same frequency as the current in the coil, and the speaker cannot change the frequency of the signal it is given. A current of 500 Hz makes a sound of 500 Hz, and the cone moves back and forth 500 times each second, or 1000 times in 2 seconds.
Pitch means how high or low a note sounds, and it depends on frequency. A higher frequency current makes the cone vibrate faster, so the note sounds higher. Doubling the frequency from 500 Hz to 1000 Hz makes a higher note, which a singer would call going up an octave.
Loudness is separate from pitch, and we will meet it in the next step, so keep the two ideas apart. For now remember one rule. Frequency of the sound = frequency of the current, and a higher frequency means a higher pitch.
Bars showing the frequency of the current in hertz for a low note and a high note. The high note has the larger frequency of 800 hertz compared with 200 hertz, so the cone vibrates faster and the pitch is higher.
Loudness, and changing both
A larger current gives a larger force on the coil. So the coil and the cone move further each time, and the air vibrates with a larger amplitude. The sound is louder. You can see this on a big speaker playing a deep bass note, where the cone moves a long way.
Amplitude is the biggest movement from the rest position. Loudness depends on the amplitude of the sound wave. So loudness depends on the size of the current, and pitch depends on its frequency. One does not change the other.
That means loudness and pitch are separate. Turn up the volume on your phone and the amplifier makes the current bigger, but the notes do not change, and they just get louder or quieter. Press a different key on a keyboard and the frequency changes, but the loudness can stay the same if you press with the same force.
A real signal changes in size and in frequency at the same time. Music does this all the time, with notes rising and falling and getting louder and softer. The loudspeaker follows both changes, and the sound it makes copies the signal, which is how a recording of a band can be played back so faithfully.
If only the size of the current changes, the pitch stays the same. If only the frequency changes, the loudness stays the same. Use those two sentences to answer any question that asks you what changes, and remember to name the quantity, either size of current or frequency.
Microphones do the reverse
A microphone changes sound into an electrical signal. It does the opposite job to a loudspeaker. Think of it as a loudspeaker run backwards, with sound going in and a signal coming out. Phones, voice recorders and singers' microphones all work in this way.
In one common type of microphone, sound waves hit a thin sheet called a diaphragm, which is just a very light skin, and make it vibrate. A coil fixed to the diaphragm moves backwards and forwards in the field of a magnet. This is the same arrangement as a loudspeaker, except that now the sound is pushing the coil.
A coil moving in a magnetic field has a potential difference induced across it. Induced means created by the movement. This is the generator effect, and it is the effect used in a power station to make electricity, where the moving part is much bigger and moves much faster.
The coil moves in time with the sound, so the potential difference changes in time with the sound, and it changes direction as the coil changes direction. A louder sound moves the coil further and gives a bigger potential difference, which is why you can see the signal get taller on a screen when you shout. A higher note moves the coil faster and gives a higher frequency, so the signal is a copy of the sound.
This is why a loudspeaker can work as a simple microphone. Speak into it and the cone and coil vibrate in the magnet, so a small potential difference is induced. It is weak, but an amplifier can make it bigger. That is why an old intercom can use one speaker for both listening and talking.
A flow of four boxes showing how a microphone works. Sound makes the diaphragm vibrate, the coil moves in the magnetic field, a potential difference is induced, and the electrical signal copies the sound.
Recap
This lesson is Higher tier. A loudspeaker uses the motor effect to turn an alternating current into sound. The current in the coil sits in the field of a fixed permanent magnet, so there is a force on the coil, and the force reverses as the current reverses. The coil is pushed one way and then the other, over and over.
The coil and the cone vibrate, and the cone makes the air vibrate, which makes a sound wave. The cone vibrates at the same frequency as the current, so frequency of the sound = frequency of the current. A larger current gives a louder sound, and a higher frequency gives a higher pitch.
A microphone does the reverse. Sound makes a diaphragm and a coil vibrate in the field of a magnet, and the generator effect induces a potential difference that copies the sound. A loudspeaker is therefore a motor effect device, and a microphone is a generator effect device.
Compare the two in one line. Loudspeaker: electrical signal in, sound out, by the motor effect. Microphone: sound in, electrical signal out, by the generator effect. Learning this as a pair stops you mixing the two effects up under exam pressure.
How this is asked in an exam: you may be asked to describe how a loudspeaker produces sound, which is a four mark answer with the steps in order, starting from the alternating current and ending with the sound wave. You may also be given a frequency and asked for the sound frequency, or asked what changes loudness and what changes pitch. Write short linked sentences, and use the words alternating, force, coil, cone, frequency and amplitude.
Key points
A loudspeaker uses the motor effect to turn an alternating current into sound.
The magnet is fixed. The coil and the cone move.
The cone vibrates at the same frequency as the current.
A larger current gives a louder sound. A higher frequency gives a higher pitch.
A microphone uses the generator effect to turn sound into an electrical signal.
A loudspeaker can be used as a simple microphone.
Worked examples
Example 1
Explain how a loudspeaker produces sound from an electrical signal.
An alternating current flows in the coil, which sits in the magnetic field of the permanent magnet.
The motor effect puts a force on the coil, and the force reverses each time the current reverses.
The coil and cone vibrate and make the air vibrate, which produces a sound wave.
Answer: A changing current gives a changing force on the coil, so the cone vibrates and makes sound.
Common slips
Slip: The magnet spins and pushes the air The magnet is fixed. The coil is the part that moves.
Slip: A direct current makes the coil vibrate A direct current gives a steady force, so there is no vibration.
Slip: The cone makes electricity from the air In a loudspeaker the cone makes sound. It does not make electricity.
Example 2
A loudspeaker current has a frequency of 250 Hz. State the frequency of the sound. Then say what happens to the pitch when the frequency is doubled.
The cone vibrates at the same frequency as the current, so the sound is 250 Hz.
Doubling the frequency gives 2 × 250 = 500 Hz.
A higher frequency means a higher pitch, so the note is higher.
Answer: 250 Hz, and when the frequency is doubled to 500 Hz the pitch is higher.
Common slips
Slip: The sound is half of 250 Hz The two frequencies are equal.
Slip: Doubling gives 252 Hz Doubling means multiply by 2, not add 2.
Slip: A higher frequency makes the sound louder Frequency changes the pitch. The size of the current changes the loudness.
Example 3
A loudspeaker is driven by a current of frequency 200 Hz. How many times does the cone move back and forth in 3 seconds?
The cone vibrates at the same frequency as the current, so it makes 200 vibrations each second.
Multiply the number of vibrations each second by the number of seconds: 200 × 3.
Work out 600 times.
Answer: 600 times
Common slips
Slip: The cone moves once for each second A frequency of 200 Hz means 200 vibrations every second.
Slip: 200 ÷ 3 = 66.7 To find the total, multiply by the time. Do not divide.
Slip: 203 times Do not add the numbers. Multiply 200 by 3.
Questions and answers
Loudspeakers and microphones practice questions for GCSE Combined Science Physics. Try each one, then open the answer. Easy questions come first.
Question 1 Easy
Name the effect that makes the coil in a loudspeaker move.
Show answer
Answer: the motor effect
A current in a magnetic field feels a force.
This is the motor effect.
Question 2 Easy
What type of current is supplied to a loudspeaker?
Show answer
Answer: alternating current
The cone needs to vibrate.
The current must keep changing direction.
Question 3 Easy
What is fixed to the coil to push the air?
Show answer
Answer: a paper cone
The coil moves.
The cone fixed to it pushes the air.
Question 4 Easy
What does a loudspeaker change electrical signals into?
Show answer
Answer: sound
The cone vibrates.
The vibrating air is a sound wave.
Question 5 Medium
A loudspeaker current has a frequency of 300 Hz. State the frequency of the sound.
Show answer
Answer: 300 Hz
The cone vibrates at the same frequency as the current.
So the sound is 300 Hz.
Question 6 Medium
What happens to the loudness when the current in a loudspeaker gets larger?
Show answer
Answer: the sound gets louder
A larger current gives a bigger force.
The cone moves further, so the sound is louder.
Question 7 Medium
What happens to the pitch when the frequency of the current increases?
Show answer
Answer: the pitch gets higher
The cone vibrates faster.
A higher frequency gives a higher pitch.
Question 8 Medium
Name the effect a microphone uses to make its signal.
Show answer
Answer: the generator effect
The coil moves in a magnetic field.
A potential difference is induced. This is the generator effect.
Exam-style questions
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
Exam-style question 14 marks
Describe how a loudspeaker produces sound. [4 marks]
Show answer and marking points
Answer: An alternating current flows in the coil in the field of a magnet. The motor effect gives a force on the coil that reverses with the current. The coil and cone vibrate. The cone makes the air vibrate, producing sound.
alternating current in the coil (1)
the coil is in the magnetic field so there is a force (motor effect) (1)
the force reverses with the current so the coil and cone vibrate (1)
the cone makes the air vibrate and produces sound (1)
Exam-style question 23 marks
A microphone changes sound into an electrical signal. Explain how. [3 marks]
Show answer and marking points
Answer: Sound makes the diaphragm and coil vibrate in the field of a magnet, so a potential difference is induced which changes with the sound.
sound makes the diaphragm and coil move (1)
the coil moves in a magnetic field so a potential difference is induced (1)
the potential difference changes with the sound (1)
Common mistakes
Saying the magnet in a loudspeaker moves
The magnet is fixed. The coil and the cone are the parts that move.
Saying a loudspeaker works with a direct current
It needs an alternating current so that the force on the coil keeps reversing.
Mixing up loudness and pitch
A larger current makes the sound louder. A higher frequency makes the pitch higher.
Saying a microphone uses the motor effect
A microphone uses the generator effect. The motor effect is used in a loudspeaker.
Saying the sound has a different frequency from the current
The cone vibrates at the same frequency as the current, so the two frequencies are equal.
Questions students ask
Is Loudspeakers and microphones on the Foundation or Higher tier?
Loudspeakers and microphones is Higher tier content for GCSE Combined Science Physics, aimed at around grade 7.
What is a common mistake on Loudspeakers and microphones?
Saying the magnet in a loudspeaker moves. The magnet is fixed. The coil and the cone are the parts that move.
Where are the answers to the Loudspeakers and microphones questions?
Open Show answer under each question on this page. The questions are Yumio practice questions, not past exam questions.