Ohmic Audio

9.1 Reading and Interpreting Audio Graphs

Frequency Response Graphs

The frequency response graph is the most important visual in audio. Everything else is secondary.

Illustration in preparation Description: Annotated frequency response graph with: X-axis labels at 20 Hz, 100 Hz, 1 kHz, 10 kHz, 20 kHz; Y-axis labels at -20, -10, 0, +10, +20 dB; callouts pointing to a peak at 80 Hz labeled "cabin resonance +8 dB", a dip at 250 Hz labeled "crossover transition -6 dB", and a broad rolloff above 12 kHz labeled "tweeter natural rolloff"

X-axis (horizontal): Frequency

Always logarithmic — each decade (10× increase) takes the same horizontal space. This matches human hearing, which is logarithmic: we hear equal steps between octaves, not equal steps between Hz values.

Left edge = 20 Hz (lowest audible bass) Right edge = 20,000 Hz (highest audible treble) Middle of graph ≈ 1,000 Hz (midrange)

Y-axis (vertical): Level in dB

0 dB = reference (usually the average level of the curve) Positive dB = louder than reference Negative dB = quieter than reference

Scale matters: A graph with ±30 dB scale looks very different from the same data on a ±10 dB scale. Always check the scale before judging how flat or uneven a response looks.

Illustration in preparation Description: Same frequency response curve plotted on three different Y-axis scales: ±30 dB (looks flat), ±10 dB (looks moderate), ±5 dB (looks extreme) — illustrating why scale matters

Common features to identify:

Feature What it looks like What it means
Peak Sharp upward spike Resonance — a frequency reproduced louder than neighbors
Dip Sharp downward notch Cancellation — two sources opposing at that frequency
Rolloff Gradual slope downward Natural limit of speaker or filter
Shelf Step change, then flat again EQ shelf filter applied
Ripple Regular wavy pattern Interference from reflections or comb filtering
Plateau Flat region Driver reproducing uniformly in that band

Impedance Curves

Illustration in preparation Description: Impedance vs frequency graph with labeled features: Fs peak (high impedance), minimum impedance region, inductive rise at high frequency; separate curves shown for sealed vs ported enclosure

Impedance peaks:

Reading Fs from impedance:

The frequency of the impedance peak in a sealed box = system resonance Fc. In free air = Fs.

Reading Fb from ported impedance:

The frequency at the valley between the two peaks = port tuning frequency Fb. This is a reliable way to confirm box tuning without acoustic measurement.

Minimum impedance:

The lowest point of the impedance curve (usually between resonance and mid-bass) is the true minimum load the amplifier sees. This is what matters for amplifier stability, not the nominal rating.

Inductive rise:

Impedance increases above a few kHz due to voice coil inductance. For a nominal 4Ω speaker, impedance at 10 kHz may be 10–20Ω. This affects power delivery at high frequencies and passive crossover design.

Waterfall / CSD Plots

Illustration in preparation Description: 3D CSD waterfall with annotations: time axis (0 to 500 ms), frequency axis (20 Hz to 5 kHz), level axis; arrows pointing to a resonance ridge at 95 Hz labeled "panel resonance — dampening needed" and a clean decay region labeled "good transient response"

How to read time axis:

Left face of waterfall = time zero (immediately after signal stops) Going back (depth) = increasing time after signal ends A frequency that appears only at time zero has decayed instantly — excellent. A frequency that extends far back has lingered — a resonance.

Practical threshold:

Any frequency taking more than 100–150 ms to decay 30 dB is problematic for most music. Bass frequencies (below 80 Hz) naturally take longer due to longer wavelengths — allow 200–300 ms there.

What ridges indicate:

Phase Response Graphs

Illustration in preparation Description: Phase vs frequency graph showing: gradual linear phase rotation (good), phase wrapping (normal at extremes), abrupt phase jump at a crossover (potential issue), smooth through-band behavior

Phase is displayed in degrees, wrapping between +180° and -180°. Don't be alarmed by wrapping — it's normal. The shape and rate of change matter more than the absolute value.

Smooth phase: Gradual, continuous rotation. Indicates minimum-phase system with no anomalies.

Abrupt phase jumps: Sudden 90–180° discontinuities at specific frequencies. Often indicates cancellation, a problem crossover setting, or driver reversal.

Phase at crossover: Expect 90° shift per crossover order. An LR4 (24 dB/octave) crossover produces 360° total shift — both drivers in same effective polarity at crossover.

Group Delay Graphs

Illustration in preparation Description: Group delay (ms) vs frequency graph showing: peaked group delay at subwoofer resonance (~2 ms), flat group delay in midrange, slight rise at tweeter crossover; dashed line at 10 ms labeled "audibility threshold for most listeners"

Group delay is the time delay as a function of frequency. Constant group delay = no time distortion. Frequency-dependent group delay = different frequencies arrive at different times.

Audibility thresholds (from research):

Bass frequencies (below 100 Hz) tolerate much more group delay — the long wavelength prevents temporal resolution. High frequencies are most sensitive.

Causes of high group delay: