Sound as pressure variations
Definition
Sound wave
A longitudinal wave that travels through a medium as a series of compressions and rarefactions, which are regions of higher and lower pressure.
- A sound wave travels through air as a series of compressions, where the air pressure is slightly higher than normal, and rarefactions, where it is slightly lower.
- At any one point, the air pressure rises and falls over and over again as the wave passes, and these changes are the pressure variations of the sound.
- The frequency of the pressure variations sets the pitch of the sound, and the size of the variations sets its loudness.
- A microphone turns these pressure variations into matching variations in an electric current, and a loudspeaker turns variations in current back into pressure variations.
How a microphone works
Definition
Microphone
A device that converts the pressure variations of a sound wave into matching variations in an electric current.
Definition
Diaphragm
A thin, flexible sheet in a microphone or headphone that vibrates in step with a sound wave.
- A dynamic microphone contains a thin, flexible diaphragm attached to a small, light coil of wire.
- The coil sits in the narrow gap around a permanent magnet, so it is surrounded by a strong magnetic field.
- When a sound wave reaches the microphone, the pressure variations push and pull on the diaphragm, so the diaphragm vibrates back and forth.
- The coil is fixed to the diaphragm, so it vibrates with it and moves back and forth through the magnetic field.
- As the coil moves, its wire cuts across the magnetic field lines, so a potential difference is induced across the ends of the coil by electromagnetic induction.
- Each time the coil changes direction, the induced potential difference reverses, so the output is an alternating potential difference.
- When the microphone is part of a complete circuit, this potential difference drives an alternating current whose variations match the pressure variations of the sound.
- A higher-pitched sound makes the coil vibrate more times each second, so the current alternates at a higher frequency.
- A louder sound makes the coil vibrate with a larger amplitude and move faster through the field, so the induced current is larger.
- The current from a microphone is very small, so it is passed to an amplifier, which makes it larger without changing its pattern.
Loudspeakers and headphones
Definition
Loudspeaker
A device that uses the motor effect to convert a varying electric current into vibrations of a cone that produce sound.
Definition
Headphones
A pair of small loudspeakers worn on or in the ears, each using the motor effect to turn a varying current into sound.
- A loudspeaker does the reverse job of a microphone, turning variations in current into pressure variations in the air.
- It has a coil attached to a light paper or plastic cone, and the coil sits in the field of a permanent magnet.
- An amplifier sends an alternating current through the coil, with the same pattern of variations as the original sound.
- The current in the coil lies across the magnetic field, so the motor effect produces a force on the coil.
- Each time the current reverses, the force on the coil reverses, so the coil and the cone are pushed out and pulled in again and again.
- As the cone moves outwards it squeezes the air in front of it into a compression, and as it moves back it leaves a rarefaction.
- The vibrating cone therefore sends out a sound wave whose pressure variations match the variations in the current.
- The frequency of the current sets the frequency of the vibrations, and so sets the pitch of the sound.
- A larger current produces a larger force, a larger amplitude of vibration and a louder sound.
- Headphones contain a tiny loudspeaker for each ear that works in exactly the same way, with a small coil and magnet driving a thin diaphragm close to the ear.
- A microphone relies on electromagnetic induction, while a loudspeaker and headphones rely on the motor effect, even though all three use a coil and a magnet.
Practical
Using a loudspeaker both ways
- Aim: to show that an alternating current in a loudspeaker produces sound by the motor effect, and that sound reaching a loudspeaker induces an alternating potential difference.
- Apparatus: a small loudspeaker, a signal generator, an oscilloscope (or a voltage sensor and data logger), connecting leads, a few grains of dry rice and a tuning fork.
- Method, loudspeaker as a speaker:
- Lay the loudspeaker on its back with the cone facing upwards, and sprinkle a few grains of rice onto the cone.
- Connect the loudspeaker to the signal generator, set the frequency to about 5 Hz5\ \text{Hz}5 Hz and the output to a low amplitude, and watch the cone and the rice.
- Raise the frequency slowly to a few hundred hertz and listen to the note, then increase the amplitude and listen again.
- Method, loudspeaker as a microphone:
- Disconnect the signal generator and connect the loudspeaker straight to the oscilloscope input instead.
- Strike the tuning fork and hold it close to the cone, then hum a low note and a high note, and a quiet note and a loud note, close to the cone, watching the trace each time.
- Results:
- At low frequency the cone moves visibly out and in, and the rice bounces in time with the signal.
- A higher frequency gives a higher note, and a larger amplitude gives a louder sound and makes the rice jump higher.
- When the loudspeaker is used as a microphone, the oscilloscope shows an alternating trace whenever sound reaches the cone, with more waves across the screen for a higher note and taller waves for a louder one.
- The same coil and magnet use the motor effect in one direction and electromagnetic induction in the other.
- Watch out: the potential difference produced by a loudspeaker used as a microphone is tiny, so set the oscilloscope to its most sensitive range.
- Safety: keep the signal generator output low, because large amplitudes can damage the loudspeaker cone and very loud sounds can damage your hearing.
Common Mistake
- Do not say that a microphone works by the motor effect, because the moving coil in a microphone has a potential difference induced in it, while the loudspeaker coil has a force acting on it.
- Do not say that the magnet in a loudspeaker vibrates, because the magnet is fixed and it is the coil and cone that move.
Exam technique
Describing the conversions
- For a microphone, link every stage in order: pressure variations, vibrating diaphragm, coil moving through the magnetic field, induced potential difference, varying current.
- For a loudspeaker, run the chain the other way: varying current in the coil, force from the magnetic field, vibrating coil and cone, pressure variations in the air.
- Name the physics at the key step, writing electromagnetic induction for a microphone and motor effect for a loudspeaker.
- When asked how the current changes for a louder or higher sound, link loudness to the size of the current and pitch to its frequency.
Self review
- What makes the diaphragm of a microphone vibrate?
- Explain why a potential difference is induced across the coil of a dynamic microphone.
- Which effect makes the cone of a loudspeaker move?
- How does the current in a loudspeaker change to make a louder sound?
- How are headphones similar to a loudspeaker?
