How a dynamic speaker works
- Source
- Amplifier
- Speaker
- Room
The last link in the chain. It turns the amplifier’s electrical signal into sound: current through the voice coil, sitting in the magnet’s field, pushes the cone back and forth, and the cone pushes the air.
One cone can’t cover the whole range well: a cone big enough to move bass is too heavy to keep up with treble. So a crossover splits the signal, sends the low band to a large woofer and the high band to a small tweeter, and the box behind them holds the air the woofer works against. Nearly every speaker in the catalogue works this way; planar, electrostatic and horn designs do it differently.
The sum line is what reaches you. Flat means every pitch plays at the same level. At the left end the bass falls away, and the box decides where and how fast.
When pairing: how low the sum line reaches is how much bass the speaker makes on its own, and how much a subwoofer would add.
The amplifier sees this load. Dips toward 4 Ω ask the amp for more current. The humps come from the woofer’s resonance and the crossover, not from anything the amplifier does.
When pairing: an amplifier rated into 4 Ω handles the dips. The nominal 8 Ω is an average; the lowest point is what counts.
Each doubling of power adds 3 dB. This generic 87 dB speaker reaches a loud 95 dB at 1 m on about 6 W; an 84 dB speaker needs 13 W for the same.
When pairing: lower sensitivity needs a more powerful amplifier for the same loudness, and its headroom goes sooner.
Ported: the port’s air resonates at 45 Hz and carries the deepest bass, so the bass holds to about 57 Hz, then falls steeply, at 24 dB per octave.
When pairing: a ported box gets more bass from a small cabinet, which suits a room without a subwoofer.
Sealed: the same box with the port plugged. The air inside is a spring behind the cone, so the bass starts to fall earlier, near 96 Hz, but gently, at 12 dB per octave. The impedance keeps one bass hump. The dotted line is the ported box.
When pairing: sealed boxes trade deep bass for a tighter, earlier roll-off, and often pair well with a subwoofer, whose output blends into that gentle slope.
Drivers
Crossover
Cabinet
How the numbers work
Every number on the page comes from one small model of a generic speaker. The drawing, the chart and the readout read from the same copy.
- Crossover: a 2nd-order Linkwitz-Riley at 2.5 kHz into 8 Ω. The coils are 1.02 mH and the capacitors 3.98 µF in both legs. The low-pass is
1/(1 + jx)²and the high-passx²/(1 + jx)²withx = f/fc. Each driver carries half the signal at 2.5 kHz. The tweeter is wired in reverse polarity, which makes the two sum flat; a 2.2 Ω resistor levels its output to the woofer’s. - Bass: the vented-box (ported) circuit (Thiele/Small). The 165 mm woofer has
fs 50 Hz, Qms 3, Qes 0.44, Vas 20 Lin a 15 L box tuned to 45 Hz with leakageQL 7. The cone’s and the port’s outputs come from that circuit. At the tuning the cone nearly stops and the port carries the bass; below it the two cancel and the output falls at 24 dB per octave. - Sealed: the same circuit with the port’s mass removed, so the box is only a spring of air behind the cone. The woofer then resonates at
fc = fs·√(1 + Vas/Vb)= 76 Hz withQtc0.59, so the bass is 3 dB down at 96 Hz and falls at 12 dB per octave. Ported, it is 3 dB down at 57 Hz. The impedance loses the port’s dip and keeps one hump, at 76 Hz. The cone’s travel uses one scale for both boxes, so at 45 Hz it moves about twice as far sealed. - The chart: the woofer line is the cone through the low-pass, the port line the port through the low-pass, the tweeter line the high-pass, and the sum is their complex sum, in dB against the woofer’s passband.
- Impedance: the woofer’s electrical impedance in the box (voice coil 6 Ω, 0.4 mH, plus the motional part) and a tweeter with one resonance (4.8 Ω, fs 750 Hz), each behind its crossover leg, in parallel. The readout’s ohms are what the amplifier sees.
- Sensitivity: 87 dB at 1 W and 1 m is a typical figure for a standmount, chosen for the chart, not measured. Each doubling of power adds 10·log10(2) = 3 dB. The 84 dB line is the same sum with a lower starting point.
- The sound waves: each ring’s strength is that source’s share from the chart (cone and port through the low-pass, tweeter through the high-pass). The ring spacing is a log map of 20 Hz to 20 kHz onto 26 to 7 units of the drawing; the real wavelength is 343/f metres, 17 m at 20 Hz, far too big to draw. The cone’s travel is its volume velocity over omega from the box circuit, scaled to its travel at 20 Hz; it nearly stops at the port tuning. The rings move at a constant speed, so bass rings pass slowly and treble rings quickly.
- Simplifications: the crossover curves are the network’s targets into a resistive load, not its response into the woofer’s rising impedance. The tweeter’s own roll-off is ignored. The room is not in the model.