What is the difference between transfer function and impulse response?

They contain the same information in different domains. The transfer function is the frequency-domain representation (magnitude and phase vs frequency). The impulse response is the time-domain representation (amplitude vs time). They are related by the Fourier transform. SonaVyx can display both from a single measurement.

Why does coherence matter for EQ decisions?

Coherence tells you whether the measured response at each frequency is reliable. Low coherence means the output is contaminated by noise or reflections, making the magnitude value unreliable. EQing at low-coherence frequencies may help at the measurement position but worsen the response elsewhere.

How many averages should I use?

For pink noise transfer function measurement, 16 to 32 linear averages produce stable results. More averaging improves coherence and reduces noise but takes longer. In quiet environments, 8 averages may suffice. In noisy environments or with high reflection energy, 64 or more averages may be needed.

Can I use music instead of pink noise for transfer function?

Yes, but the measurement takes longer to stabilize because music does not excite all frequencies simultaneously. Pink noise reaches every frequency bin consistently, while music may take minutes to adequately excite the full spectrum. Pink noise is always preferred for efficiency and reliability.

What does the phase trace tell me?

The phase trace reveals timing information. A constant slope indicates propagation delay. Phase discontinuities at crossover frequencies indicate driver alignment problems. Wrapping at plus or minus 180 degrees is normal. The group delay display (derivative of phase) converts this to milliseconds, making timing analysis more intuitive.

What is the difference between transfer function and impulse response?

They contain the same information in different domains. The transfer function is the frequency-domain representation (magnitude and phase vs frequency). The impulse response is the time-domain representation (amplitude vs time). They are related by the Fourier transform. SonaVyx can display both from a single measurement.

Why does coherence matter for EQ decisions?

Coherence tells you whether the measured response at each frequency is reliable. Low coherence means the output is contaminated by noise or reflections, making the magnitude value unreliable. EQing at low-coherence frequencies may help at the measurement position but worsen the response elsewhere.

How many averages should I use?

For pink noise transfer function measurement, 16 to 32 linear averages produce stable results. More averaging improves coherence and reduces noise but takes longer. In quiet environments, 8 averages may suffice. In noisy environments or with high reflection energy, 64 or more averages may be needed.

Can I use music instead of pink noise for transfer function?

Yes, but the measurement takes longer to stabilize because music does not excite all frequencies simultaneously. Pink noise reaches every frequency bin consistently, while music may take minutes to adequately excite the full spectrum. Pink noise is always preferred for efficiency and reliability.

What does the phase trace tell me?

The phase trace reveals timing information. A constant slope indicates propagation delay. Phase discontinuities at crossover frequencies indicate driver alignment problems. Wrapping at plus or minus 180 degrees is normal. The group delay display (derivative of phase) converts this to milliseconds, making timing analysis more intuitive.

How to Use Transfer Function for Tuning

6 steps15-20 min readUpdated 2026-03-20

Quick Answer

Using the transfer function for tuning means comparing the electrical input signal to the acoustic output signal using a dual-channel FFT analyzer. The resulting magnitude, phase, and coherence data reveal the true system response independent of the test signal spectrum, enabling precise, confident equalization decisions that single-channel RTA measurement cannot provide.

Try It Now

Open Transfer Function — free, no download required.

Open Tool

Equipment Needed

✓SonaVyx Transfer Function measurement tool
✓Measurement microphone (omnidirectional)
✓Audio interface with reference output and mic input
✓Pink noise source (built into SonaVyx)
✓System processor with parametric EQ

Step-by-Step Guide

Configure Dual-Channel Input

Set up two input channels: the reference (electrical signal feeding the amplifier or processor) and the measurement (microphone capturing the acoustic output). In SonaVyx, the reference can be the internal pink noise generator output, and the measurement is your microphone input. The analyzer divides the measurement FFT by the reference FFT at each frequency bin, producing H(f) = Y(f)/X(f). This division cancels the source spectrum, showing only the transfer function of everything between the reference point and the microphone.

Select Appropriate FFT Settings

Choose an FFT size that balances frequency resolution against time resolution. 4096 points at 48 kHz gives 11.7 Hz resolution and 85 ms time windows. 8192 points gives 5.9 Hz resolution but 170 ms windows. Larger FFTs reveal more low-frequency detail but are slower to update and more susceptible to temporal variations. For system tuning, 4096 is a good starting point. Apply a Blackman-Harris window to minimize spectral leakage. Use linear averaging with 16 to 32 averages for stable results.

Read the Magnitude Trace

The magnitude trace shows the system's frequency response: how much gain or loss occurs at each frequency. A flat trace means the system reproduces the input signal without frequency-dependent coloration. Peaks above the target curve indicate excessive energy that should be reduced with EQ. The magnitude trace is the primary guide for tonal EQ adjustments. Apply 1/6 octave smoothing for EQ decisions. Use SonaVyx's target curve overlay to visualize deviations from your desired response.

Interpret Coherence

The coherence trace (0 to 1) shows how linearly related the input and output are at each frequency. Coherence near 1.0 means the output is entirely determined by the input, confirming reliable measurement data. Coherence below 0.5 indicates contamination from reflections, noise, or nonlinear behavior. The critical rule: never EQ at frequencies with low coherence. If coherence is low, improve it by increasing source level, averaging more, reducing reflections, or moving the microphone closer to the speaker.

Analyze Phase Response

The phase trace shows the timing relationship between input and output at each frequency. A system with pure delay shows a linear phase slope: the steeper the slope, the longer the delay. Phase wrap at 180 degrees is normal and causes the trace to jump. Look for the delay finder or group delay display, which converts the phase slope to propagation time. Phase discontinuities at crossover frequencies indicate driver alignment issues. Subwoofer phase alignment is verified by checking that sub and main phase traces converge at the crossover frequency.

Make EQ Decisions

With magnitude, phase, and coherence visible, apply EQ with confidence. Cut peaks where coherence is above 0.85 using parametric EQ with appropriate bandwidth matching the peak width. Ignore dips where coherence is low, as these represent spatial interference that EQ cannot fix. After each EQ change, re-measure to verify the adjustment produced the expected result. Compare smoothed transfer function against your target curve. Stop when the response is within plus or minus 3 dB of target across the system's passband.

Transfer Function vs RTA: The Critical Difference

An RTA (Real-Time Analyzer) shows only the microphone signal spectrum, which includes both the system response and the source signal spectrum. If you play music through an RTA, you see the music spectrum, not the system response. Even with pink noise, RTA measurement varies moment-to-moment as the noise spectrum fluctuates. Transfer function eliminates this variability by dividing the output by the input, isolating only the system's contribution to the signal.

The H1 Estimator

SonaVyx uses the H1 transfer function estimator, defined as H1(f) = Gxy(f) / Gxx(f), where Gxy is the cross-spectral density between input and output, and Gxx is the auto-spectral density of the input. H1 minimizes the effect of noise at the output (microphone), making it ideal for acoustic measurements where ambient noise contaminates the microphone signal. The companion coherence function quantifies how much of the output is linearly related to the input, providing a built-in reliability indicator.

Practical Workflow

The professional system tuning workflow proceeds as follows: measure transfer function at the primary position, apply corrective EQ, re-measure to verify, store the reference trace, move to the next position, compare, and adjust. SonaVyx's trace store/recall feature supports this workflow by storing up to 10 traces with color coding for comparison. The crosshair cursor provides precise frequency and level readout at any point on any visible trace.

Common Mistakes to Avoid

✗

Confusing transfer function with RTA, which does not remove the source signal spectrum and produces less accurate EQ decisions

✗

Ignoring coherence and EQing at frequencies where the measurement is unreliable due to reflections or noise

✗

Using too small an FFT size for low-frequency analysis, which lacks the resolution to see room modes below 100 Hz

✗

Not averaging enough, causing noisy magnitude and phase traces that fluctuate with each update

✗

Misinterpreting group delay as a frequency response problem, when it actually indicates timing issues

Applicable Standards

Standard	Clause	Relevance
AES-2id:2023	Clause 3	Recommended practice for dual-channel transfer function measurement
IEC 60268-5	Clause 17	Frequency response measurement methodology using transfer function

Try It Now

Open Transfer Function — free in your browser.

Open Tool

Related Guides

How to Tune a PA System

Learn how to tune a PA system with transfer function measurement, system EQ, and delay alignment. Free browser-based tools for live sound engineers.

How to Measure Speaker Frequency Response

Learn how to measure speaker frequency response using swept sine, pink noise, or transfer function. Step-by-step guide with free browser-based tools.

How to Use an FFT Analyzer for Audio

Learn to use an FFT analyzer for audio measurement: spectrum analysis, transfer function, and signal diagnostics. Free browser-based FFT tool.

How to Read a Frequency Response Graph

Learn to read and interpret frequency response graphs for speakers and audio systems. Understand axes, scales, smoothing, and what deviations mean.

Design-Time Companion

Need to design room acoustics before measuring? AcousPlan handles room acoustic design, while SonaVyx handles deploy-time measurement and tuning.