How to Read a Frequency Response Graph
Quick Answer
Reading a frequency response graph means interpreting a plot that shows sound pressure level in decibels on the vertical axis against frequency in Hertz on the horizontal axis. The curve reveals how a speaker, room, or system reproduces different frequencies, with deviations from flat indicating coloration, resonances, or rolloff that affect perceived sound quality.
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Equipment Needed
- ✓SonaVyx Transfer Function or RTA tool for live measurement
- ✓Measurement microphone for accurate capture
- ✓Alternatively, a published frequency response graph to interpret
Step-by-Step Guide
Understand the Axes
The horizontal axis represents frequency, almost always plotted on a logarithmic scale from 20 Hz to 20 kHz. Logarithmic spacing means each octave (doubling of frequency) occupies equal visual width, matching how the ear perceives pitch. The vertical axis shows level in decibels (dB SPL or dB relative). A 10 dB range typically spans a significant portion of the vertical scale. Most speaker response plots use a range of 40 to 60 dB, while system measurement plots may use a tighter 20 to 30 dB range.
Identify the Frequency Regions
Divide the spectrum into regions: sub-bass (20 to 60 Hz, felt more than heard), bass (60 to 250 Hz, kick drum and bass guitar fundamentals), lower midrange (250 to 500 Hz, warmth and body), midrange (500 Hz to 2 kHz, vocal presence), upper midrange (2 to 4 kHz, speech consonants and detail), presence (4 to 8 kHz, brightness and air), and brilliance (8 to 20 kHz, sparkle and sibilance). Understanding these regions helps you interpret what a peak or dip at any frequency sounds like.
Assess Overall Shape
Before examining details, note the overall shape of the curve. A studio monitor should be nearly flat from 80 Hz to 15 kHz. A PA system may target a gentle downward slope above 1 kHz. Consumer speakers often have boosted bass and presence regions (the "smiley face" curve). Compare the measured response against the manufacturer's published specification and your target curve. SonaVyx overlays target curves directly on the measurement for easy visual comparison.
Evaluate Smoothness and Peaks
Look for peaks that rise more than 3 dB above the average level at adjacent frequencies. Peaks indicate resonances, room modes, or driver behavior problems that add coloration to the sound. A peak at 3 kHz makes the system sound harsh and aggressive. A peak at 100 Hz causes boominess. Broad peaks spanning an octave or more are usually room interaction effects. Narrow peaks spanning less than 1/3 octave often indicate mechanical resonances or room modes.
Interpret Dips and Nulls
Dips in the frequency response have different causes. A smooth, broad dip may indicate insufficient driver output at that frequency and can potentially be corrected with EQ. A deep, narrow null (10 dB or more) usually results from acoustic cancellation due to reflections, comb filtering, or polarity problems, and cannot be fixed with EQ. SonaVyx's coherence display helps distinguish: high coherence dips are system response issues (fixable), while low coherence dips indicate interference (fix the cause instead).
Consider Smoothing Level
The same measurement looks different at different smoothing levels. Raw (no smoothing) data shows every fine detail including measurement noise and non-repeatable variations. One-third octave smoothing approximates the ear's critical band resolution and is appropriate for overall system assessment. One-sixth octave reveals crossover details and narrower issues. When comparing measurements or evaluating specifications, always note the smoothing applied. SonaVyx lets you toggle smoothing in real time.
Deep Dive: What Frequency Response Tells You
Frequency response is the most fundamental performance measurement for any audio component. It quantifies the device's ability to reproduce all frequencies at their correct relative levels. While no single measurement tells the complete story (phase response, distortion, and directivity also matter), frequency response remains the primary indicator of tonal accuracy and the first thing to evaluate in any audio system.
Comparing Specifications
Manufacturers specify frequency response with a tolerance window, such as "45 Hz to 20 kHz plus or minus 3 dB." The tighter the tolerance, the more accurate the response. However, specifications can be misleading without knowing the measurement conditions (anechoic, ground plane, or room environment), smoothing applied, and reference level. Always compare specifications measured under similar conditions. A claimed plus or minus 1.5 dB response with 1/1 octave smoothing may actually be plus or minus 6 dB at 1/12 octave resolution.
Audibility of Response Variations
Not all measured deviations are audible. Research suggests that broadband level changes of less than 1 dB are imperceptible to most listeners, while narrow-band variations of 3 dB or less at a single frequency blend into the overall sound. However, peaks are generally more audible than dips because the ear is more sensitive to added energy than missing energy. Group multiple small peaks can create an overall tonal shift that is clearly audible even though no individual peak is large.
Common Mistakes to Avoid
Judging a frequency response as "bad" based on raw unsmoothed data that shows normal measurement variation
Comparing measurements taken at different distances, mic positions, or environmental conditions
Focusing only on on-axis response while ignoring off-axis behavior that affects room interaction
Expecting a perfectly flat line from any real-world loudspeaker in a room with reflections
Interpreting comb filtering nulls as speaker defects when they are caused by nearby reflective surfaces
Applicable Standards
| Standard | Clause | Relevance |
|---|---|---|
| IEC 60268-5:2003 | Clause 17 | Standardized method for measuring and presenting loudspeaker frequency response |
| AES-2id:2023 | Clause 6 | Data presentation and smoothing recommendations for frequency response |
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