Speaker Frequency Response: Reading the Data
A speaker frequency response plot is SPL against frequency for a fixed drive level, usually 1 W or 2.83 V at 1 m. A quoted range means nothing without its tolerance, and for a bass driver the bare plot means less than its Thiele/Small data, because the box sets the low end.
The axes
Frequency runs on a log scale, so each octave takes the same width: 20 to 40 Hz is as wide as 2 to 4 kHz. Level is in dB SPL. The height of the flat region is the driver's sensitivity. +3 dB is double the acoustic power; +6 dB is double the pressure.
Tolerances and limits
| Term | Meaning | Use |
|---|---|---|
| ±3 dB | All output in the stated band within a 6 dB window | A usable measure of flatness |
| −3 dB point (F3) | Output down to half power | The usual low-frequency limit of a box |
| −10 dB point | Output down to a tenth of the power | Makes a range look longer; still useful in-room |
| Breakup | Peaks where the cone stops moving as a rigid piston | Sets the practical upper limit |
"40 Hz to 2 kHz" with no tolerance cannot be compared with "45 Hz to 2 kHz ±3 dB". Ask which it is.
The box sets the bottom end
A driver's published curve is measured on a standard baffle or test box, not in your cab. Below resonance:
- Sealed: rolls off at 12 dB per octave below the box resonance.
- Reflex: extends lower, then rolls off at 24 dB per octave below port tuning. Below tuning the cone is unloaded and excursion rises fast, so it needs a high-pass filter.
- Horn or scoop: raises output across the band the horn loads, then falls away below the horn's cutoff.
That is why bass cabs are designed from Thiele/Small parameters, not curves. With Fs 42.9 Hz, Qts 0.644 and Vas 120.9 L, the WM1850/8 can be modelled in any box before you cut wood.
Why sub curves run to 1 to 2 kHz
An 18" cone still produces output well above where it is used. Two things stop you using it there. First, beaming: a cone narrows its coverage once its circumference passes the wavelength (ka = 1). For Sd 1244 cm², the effective radius is about 19.9 cm, so f = 343 / (2π × 0.199) ≈ 274 Hz. Second, breakup peaks higher up. In a sound system the sub is crossed over long before either, typically between 80 and 150 Hz. See crossover basics.
On-axis is one line of many
A single curve is measured on axis. Off axis the highs drop because drivers and horns get more directional as frequency rises. For tops and horns, ask for off-axis curves or a polar plot: most of the crowd is not on axis. Bass from a single box radiates nearly omnidirectionally, because the wavelengths (8.6 m at 40 Hz) dwarf the cabinet.
In the room
- Each boundary close to a sub (floor, wall) adds up to 6 dB at low frequencies by halving the space it radiates into.
- Subs stacked close together couple: doubling the number of identical boxes, each driven with the same power, adds up to 6 dB at low frequencies, 3 dB of it from the doubled input power.
- Reflections put position-dependent peaks and dips into the response. A flat cab can measure lumpy in a hall, which is why systems are tuned on site with a measurement mic and DSP.
Amplitude is half the story
Every filter and roll-off also shifts phase. Where subs and tops overlap, phase decides whether they sum or cancel, and a flat-looking magnitude plot can hide a hole. See phase and polarity.
Tune at sensible levels and wear hearing protection when testing loud.
Sources: Wavemark WM1850/8 spec sheet. Roll-off slopes, piston directivity and boundary gain: L. L. Beranek, Acoustics. Calculated values rounded, c = 343 m/s.
Questions
What does ±3 dB mean in speaker specs?
All output across the stated range stays within 3 dB above or below the reference level, a 6 dB window. A range quoted without a tolerance cannot be compared with one that has it.
What is a good frequency response for a sub?
One with a stated tolerance covering the band the sub will actually play, typically from the bottom of the music to a crossover between 80 and 150 Hz. The box, not the driver's bare curve, sets the low end.
Does the cabinet change a driver's frequency response?
Yes. Below resonance a sealed box rolls off at 12 dB per octave and a reflex box at 24 dB per octave below port tuning; a horn raises output over its loaded band then falls away below cutoff.
Why does my 18 inch sub's graph go up to 2 kHz?
The cone still produces output there, but it beams above roughly 270 Hz (for Sd around 1244 cm²) and breaks up higher still. Subs are crossed over between about 80 and 150 Hz, so that part of the curve is unused.