ENGR10006 Chap.5 Speaker Physics, Sound Levels and Simple Circuits
Speaker Physics, Sound Levels and Simple Circuits
A speaker driver turns a voltage waveform into acoustic pressure waves: current in the coil follows the voltage by Ohm's Law, the coil becomes an electromagnet, and its push and pull against a permanent magnet moves the cone.
This chapter builds the vocabulary of the speaker stream from that chain: frequency, octaves, the woofer, midrange and tweeter ranges across 20 Hz to 20 kHz, and sound pressure level measured in decibels against a 20 micropascal reference.
It works through the coil resistance specification of about 8 ohms, voltage dividers from the student kit using Ohm's Law and Kirchhoff's voltage law, why a driver needs an amplifier and an enclosure, and the half-knob limit that protects the cone collar. The worked example checks a wound coil against its band and designs a divider from kit resistors, and practice items test octaves and decibel reasoning.
What this chapter covers
- 01
Driver, speaker and speaker system
- 02
Ohm's Law and Ampere's Law in the driver
- 03
Frequency and octaves
- 04
Woofer, midrange and tweeter ranges
- 05
Sound pressure level and decibels
- 06
Coil resistance target
- 07
Series voltage divider
- 08
Enclosures and free-air bass cancellation
Worked example · free
Reading sound pressure level from the workshop SPL table
- +1From jackhammer to vuvuzela the pressure rises tenfold, from 2 Pa to 20 Pa, and the level rises by 120 - 100 = 20 dB.
- +1So each tenfold step in pressure adds about 20 dB on this logarithmic scale.
- +1The jackhammer's 2 Pa is 100 times the television's 0.02 Pa, which is two tenfold steps, so the table should show about 40 dB more.
- +1Check against the table: 100 - 60 = 40 dB, and the jackhammer also sits above the 85 dB hearing-damage level while the television sits below it.
Key terms
- Speaker driver
- The electromechanical part that turns a voltage waveform into sound.
- Octave
- A doubling or halving of frequency, such as 1 kHz to 2 kHz.
- Midrange driver
- A driver for roughly 200 Hz to 2000 Hz, the type built in the prototype.
- Decibel
- The logarithmic unit used to express sound pressure level.
- Ohm's Law
- The relation that current through a resistance is proportional to the voltage across it.
- Voltage divider
- Two series resistors that split a source voltage in proportion to their resistances.
- DC resistance
- The coil resistance read with a multimeter, specified as about 8 ohms.
Speaker Physics, Sound Levels and Simple Circuits FAQ
Why does a speaker need an amplifier?
Drivers are passive, and an audio source such as a phone does not supply enough power to move the coil and cone at a useful volume. The handout shows the output is barely audible without amplification, which is why the speaker system includes an amplifier.
How do decibels relate to pressure in the workshop table?
Each tenfold rise in sound pressure adds 20 dB in the table: 2 Pa is 100 dB, 20 Pa is 120 dB and 0.02 Pa is 60 dB. The reference is 20 micropascals, roughly a mosquito three metres away, which is treated as the threshold of hearing.
What resistance should the wound coil have?
The assembly instructions specify about 8 ohms, with up to 8.5 to 9 ohms accepted, using 0.15 mm wire of about 1.12 ohms per metre and around 8 m for 140 turns. Measure it with a multimeter across the ring terminals and record it for Workshop 7.
Why does a bare driver sound thin in the bass?
At low frequencies the long wavelength wraps around the cone, so out-of-phase sound from the front and back cancels. An enclosure separates the two sides, which is why the custom enclosure matters for the final response.
Assessment move
Start with the physical chain and be able to explain each link in one sentence: voltage to current, current to magnetic force, force to cone motion, motion to pressure waves. Then make frequency concrete: place the three driver ranges on a log axis from 20 Hz to 20 kHz and count octaves between familiar frequencies.
Work through the SPL table until the 20 dB per tenfold pressure pattern is automatic, and practise comparing two rows. For circuits, solve divider questions in the order current, then voltage, then a KVL check, using the resistor values in the student kit so the numbers feel familiar. Measure and record your own coil resistance and compare it with the specification band.
Finally, keep the safety and design limits in view: never exceed half the amplifier knob, and remember that early listening tests on the bare driver will sound thin for physical reasons, not because the build failed. Before the end-of-module quiz, redo the practice divider question with fresh numbers until the current-first method takes under two minutes.
Record your measured coil value beside the specification in the test log.
