Speaker, Cabinet and Air

Week 10 followed the signal to the amplifier’s electrical output. That output still isn’t sound — it’s a voltage across a speaker terminal. Turning that voltage into acoustic pressure in the air is its own physical process, with its own way of reshaping the signal, and it’s the last stage in the chain before a listener, or a microphone, hears anything at all. By the end of this week you should be able to explain how a loudspeaker converts an electrical signal into mechanical motion; explain why guitar speakers are deliberately not flat, transparent transducers; describe resonance conceptually as part of why a speaker’s response isn’t flat; compare open-back and closed-back cabinets conceptually; explain, briefly, that radiated sound depends on frequency and direction; explain that microphone type, position, angle, and distance all affect a recorded tone; and connect a cabinet’s impulse response back to Week 9’s delay/reverb ideas.

1. Voice coil + magnet: force→
2. Cone: mechanical motion→
3. Cabinet: shapes radiation→
4. Mic position: shapes what's captured

A studio monitor is designed to reproduce a signal as transparently as possible across the audible range. A guitar speaker is not designed that way, and isn’t trying to be: its cone, surround, and magnet structure have their own mechanical resonances, and its frequency response rolls off and peaks in ways a “flat” transducer deliberately avoids. That coloration isn’t a flaw to be corrected — for a guitar amplifier, it’s a major, expected part of the final tone.

A bright small speaker rolls off early at the low end but extends further at the top; a darker cabinet rolls off the highs much sooner; a resonant cabinet peaks near both ends instead of rolling off smoothly. None of these is “correct” — they’re different, deliberate colorations, the same way different clipping topologies in Week 6 were different, deliberate distortions rather than one being more accurate than another. A speaker cone, its surround, and the air trapped inside a cabinet all have natural mechanical or acoustic resonances — the same general idea Week 7 introduced for electrical filters, just realised here mechanically and acoustically instead of electrically. This course treats resonance here only conceptually, not as detailed mechanical or room acoustics.

The box a speaker is mounted in matters too. An open-back cabinet leaves the rear of the cone exposed, so sound radiates from both the front and the back; a closed-back cabinet seals the rear, so only the front radiates. The cabinet’s internal volume, and how many speakers share it, also shape the result. This course doesn’t require advanced enclosure-design theory — only that the enclosure is a deliberate design choice, not incidental packaging.

How sound radiates from a cone also depends on frequency: lower frequencies tend to radiate in most directions fairly evenly, while higher frequencies become more directional, radiating more narrowly along the speaker’s axis. That’s one reason a guitar cabinet can sound different depending on where, relative to the speaker, a listener or a microphone stands. When a cabinet is recorded rather than heard directly in the room, the microphone becomes part of the signal path too: different microphone types respond differently, and where a microphone is placed relative to the cone (on-axis versus off-axis, near the center versus near the edge), the angle it’s aimed at, and the distance from the cabinet, all affect what’s actually captured. This course introduces that these factors matter conceptually — it does not turn into a recording-engineering course, and doesn’t claim any one placement is universally “correct.”

Week 9 introduced the impulse response as a way to describe a linear system’s behaviour: excite it with a single impulse, and its output over time tells you everything about how it will treat any input. A recorded speaker-cabinet-microphone combination has its own impulse response too — it captures, in one linear description, the combined effect of the speaker, the cabinet, the microphone, and its position, all at once. That’s exactly why a “cabinet impulse response” is a genuinely useful, reusable recording: convolving a signal with it (Week 9’s convolution reverb, again) applies that whole combined coloration in one step.

1. 01

   A loudspeaker's voice coil sits in a magnet's field; current through it creates a force, converting the amplifier's electrical signal directly into the cone's mechanical motion.

2. 02

   Guitar speakers are deliberately not flat, transparent transducers — their frequency-response coloration is a major, expected part of guitar tone, not a flaw.

3. 03

   Resonance in a cone, surround, or cabinet volume shows up as a peak in frequency response, the mechanical/acoustic counterpart to a high-Q electrical filter.

4. 04

   Open-back cabinets radiate from both the front and back of the cone; closed-back cabinets radiate only from the front; internal volume and speaker count also matter — none of this requires advanced enclosure engineering to understand conceptually.

5. 05

   Radiated sound depends on frequency and direction, and microphone type, position, angle, and distance all further shape what a recording captures.

6. 06

   A cabinet impulse response captures the combined linear behaviour of speaker, cabinet, microphone, and position in one reusable recording — directly connecting back to Week 9's impulse-response and convolution ideas.