Frequency Response
Canonical Definition
Frequency response describes the magnitude and, when referenced, phase relationship between a system's output and input as a function of frequency under stated measurement conditions.
It is the response of a defined source-to-sensor path at a defined position, level, time window, smoothing, and operating state.
Spectrum Versus Transfer Response
| Method | What is measured | Typical use | Main limit |
|---|---|---|---|
| RTA or spectrum analyser | Frequency content arriving at the input | Live pink-noise tuning, background-noise checks, quick changes | No independent input reference; source spectrum and room/system response are combined |
| Log swept sine | Referenced system response derived from stimulus and recorded output | Magnitude, impulse, phase, group delay, distortion, documentation | Requires stable routing, level, timing, and valid analysis settings |
| Stepped sine | Level or distortion at discrete frequencies and often discrete levels | Low-distortion electronics, compression, output sweeps | Slow and potentially stressful to drivers |
| Nearfield acoustic measurement | Local cone, diaphragm, port, or source pressure | Low-frequency source behaviour and enclosure diagnosis | Not the same as listening-position response and may require level/area correction when merged |
What The Axes And Scales Can Hide
- Frequency axis: audio plots are normally logarithmic; equal screen distance represents a ratio, not equal hertz.
- Vertical scale: a 30 dB-tall graph can make a response look dramatic; a 100 dB-tall graph can hide the same variation.
- Smoothing: changes visible detail without changing the raw measurement.
- Normalisation: moves or reshapes the displayed reference and must be stated.
- Windowing: changes how much direct and reflected sound contributes and sets a low-frequency validity limit.
Classify Shapes Before Correcting Them
| Observed feature | Possible causes | Useful next check |
|---|---|---|
| Broad peak | Source balance, cabin loading, resonance, target mismatch | Nearby positions, individual sources, level and EQ state |
| Narrow peak | Resonance, reflection, filter interaction, measurement noise | Repeatability, decay, Q, nearby positions |
| Deep dip | Interference, polarity, crossover, geometry, reflection, local position | Individual sources, phase, polarity, delay, nearby positions |
| Rolloff | Driver bandwidth, crossover, microphone or interface response, window limit | Calibration, nearfield/farfield comparison, raw response and filter state |
| Fine ripple | Reflections, comb filtering, noise, excessive display resolution | Smoothing comparison, mic movement, time-domain view |
Do not call every dip a cancellation null or every peak a resonance until the diagnostic measurements support that label.
What Frequency Response Does Not Prove
- low distortion or absence of compression
- correct polarity or crossover phase at every frequency
- good impulse, decay, group-delay, or transient behaviour
- good directivity or passenger-seat coverage
- safe excursion, thermal margin, or hearing exposure
- that the source, microphone, interface, or ADC remained clean
Minimum Reporting Set
- measurement method and stimulus
- source path, system state, level, microphone, calibration, orientation, and position
- frequency range, vertical scale, smoothing, averaging, and windowing
- timing reference when phase or impulse data is shown
- individual, combined, nearby-position, and repeat traces used to reach the conclusion
Standards Context
Standards apply to defined devices, environments, signals, and procedures. IEC 60268-21 covers acoustical output-based measurements for passive and active sound systems, including automotive systems and small- or large-signal operation. Older IEC 60268-5 loudspeaker editions should be cited by exact edition and lifecycle status rather than described generically as the current standard.