ENGR10006 Chap.6 Filters, Equalisers, Crossovers and Speaker Testing
Filters, Equalisers, Crossovers and Speaker Testing
Workshops 4 to 8 add three circuits to the prototype midrange driver: Circuit 1, a high-pass filter with an op-amp buffer; Circuit 2, an equaliser that boosts or cuts chosen bands; and Circuit 3, a crossover that splits the signal between drivers. Only Circuits 1 and 2 go inside the custom enclosure for the demonstration.
This chapter explains what each circuit does, why cascading filter stages gives a sharper roll-off than a single first-order RC filter, why buffers prevent loading between stages, and how the sound test wires source, circuits, amplifier and driver. It then covers Workshop 7 testing with the Analog Discovery 1 and REW, and how the report expects every cut-off choice to be justified from a measured frequency response.
A worked example argues a filter corner from measured data.
What this chapter covers
- 01
Circuit 1, high-pass filter and buffer
- 02
Circuit 2, equaliser
- 03
Circuit 3, crossover
- 04
Loading effects and buffering
- 05
Cascaded filter stages
- 06
Demonstration signal chain
- 07
Impedance and REW response testing
- 08
Justifying a cut-off frequency
Worked example · free
Reading a frequency response to justify an equaliser band
- +1Between 2 kHz and 10 kHz the level falls from 103 dB to 76 dB, a drop of 27 dB, while 100 Hz to 5 kHz stays within 98 to 103 dB.
- +1The electrical goal in the report requirements is a response close to flat across the range, so the falling treble region is a clear target for correction.
- +1A treble-lifting band is therefore justified for this driver; set it from the measured fall and confirm with a second REW sweep.
Key terms
- High-pass filter
- A circuit that passes frequencies above its corner and reduces those below it.
- Op-amp buffer
- An op-amp stage that isolates one circuit from the next to prevent loading effects.
- Equaliser
- A circuit that lets chosen frequency bands be boosted or cut with adjustable gain.
- Crossover
- A circuit that splits an audio signal into ranges matched to different drivers.
- Frequency response
- A plot of sound pressure level against frequency, measured here with REW.
- Loading effect
- A change in a circuit's behaviour caused by connecting another stage to its output.
- Modular design
- Designing and testing subsystems separately before integrating them.
Filters, Equalisers, Crossovers and Speaker Testing FAQ
Which circuits must be inside the custom enclosure?
The custom speaker specification requires Circuit 1, the high-pass filter and buffer, and Circuit 2, the equaliser, to be integrated within the enclosure. The crossover, Circuit 3, is built in Workshop 8 but is not part of the final enclosed system.
Why add a buffer after the high-pass filter?
Workshop 5 shows that connecting one stage directly to the next can change its behaviour, a loading effect. An op-amp buffer between stages isolates them, so the equaliser that follows does not alter the filter's response. The report asks you to justify any buffering you used.
Will our system reach a perfectly flat response?
No. The report requirements say plainly that the ideal flat response cannot be reached with the built driver and simple circuits. What is assessed is a well-justified design, so explain each decision with your measured frequency response, quantitatively where possible.
How is a speaker driver tested in Workshop 7?
Two measurements are made. The impedance of the midrange driver is measured with the Analog Discovery 1 and its Waveforms software, and the frequency response is measured with REW for both a commercial tweeter and the driver you built. Check that the driver moves on a brief battery touch first.
Assessment move
Learn the three circuits by job before you learn them by component: removing a band, shaping bands, and splitting the signal between drivers. Draw the demonstration signal chain from memory, including where the op-amps get their power, and say in one sentence why each block sits where it does.
Build every circuit as a module and test it alone before connecting the next, keeping a debugging log of tests, results and changes, because the report asks for it. Measure your own driver with REW early and save the curves with labels for date, enclosure and connected circuits. When you choose a cut-off or an equaliser band, write the justification immediately using two numbers from your curve.
Before the demonstration, make sure every member can answer theory questions on the filter, the buffer and the equaliser, and submit the demonstration template on time. After each workshop, write two sentences on what the latest curve showed and what you will change next, so the report's design story writes itself.
