Automotive Vibration FFT Resolution: Check the Analyzed Time Before the Peak

Start with the interval behind the spectrum

An automotive vibration spectrum can look precise while leaving its time context unclear. A long recording does not establish that the FFT analyzed the entire file. Before comparing two peaks, identify the selected interval, how the software formed the spectrum and whether the operating condition supports that comparison.

This is an interpretation and documentation workflow for professional shops. It gives no universal NVH acquisition setting, road-test maneuver or component acceptance threshold. Follow the equipment and vehicle procedures for sensor mounting and any permitted capture; perform file review when it is safe to do so.

Keep three lengths separate

Write down the total saved history, the selected analysis interval and the acquired data used by each FFT block. They can be different. A selected span may contain several blocks whose spectra are averaged; extending that span can add averages without lengthening an individual block. Virtins' manufacturer explanation, section 6, describes this distinction between record length, FFT segments and averaging. It is an example of a possible analysis method, not a claim about how a particular NVH application processes a file. Preserve a view that identifies the analyzed interval within the original recording, rather than saving only the final frequency plot.

In Pico staff's May 24, 2019 explanation of NVH data, the highlighted history interval supplies data for the frequency view. The discussion also connects interpretation to the engine and road-speed conditions within that interval. This is a dated explanation of that software workflow, not a promise about a currently installed version's defaults.

For a current job, verify the installed software and its documentation. If the application does not expose the interval or processing detail, record it as unavailable. Do not infer it from the length of the file or the spacing of labels on the axis.

Calculate unpadded bin spacing, then state its limits

NI's LabVIEW Analysis Concepts, March 2004 edition, printed pages 4-14 to 4-15, relates FFT spacing to sample rate and record length. For one continuous block of N uniformly sampled acquired points at sample rate fs, the unpadded spacing is fs / N, equivalently 1 / T where T = N / fs is that block's duration. Identify segmentation and overlap if the application uses them; do not substitute the total selected span for T.

For an invented arithmetic example using one continuous FFT block, a 1-second acquired block gives 1 Hz unpadded spacing. A 4-second block at the same sample rate gives 0.25 Hz. These are calculation examples, not vehicle captures, recommended recording times or proof that two particular vibration sources can be separated.

A longer block must also represent the question being asked. Record engine speed, road speed and the event's behavior across that interval. If those conditions change, preserve that change in the interpretation instead of treating the spectrum as an unexplained snapshot of one steady condition.

More displayed points do not mean a longer observation

The same NI manual distinguishes zero padding from acquiring a longer block of measured data. Adding zeros can create a more closely spaced, interpolated spectrum display without improving the underlying frequency resolution of the measured time record. Save the acquired point count per block separately from any padded FFT length.

In the fictional 1-second block example, padding the calculation to four times as many points would yield a 0.25 Hz display grid. The acquired block duration would still be 1 second. Calling that identical to the 4-second acquired block would hide a meaningful difference in the evidence.

Display zoom is another separate entry. A closer view can make a peak easier to inspect, but the screenshot should still identify the analysis that produced it. Keep the original record so a colleague can inspect the interval and processing.

Window settings also affect what a peak means

NI's Window and FFT documentation lists different spectral widths and uses for different window functions. Bin spacing alone is not a complete measure of the ability to distinguish nearby components. Preserve the window and any averaging or filtering settings with the spectrum; do not compare peak shapes as though those settings were interchangeable.

The PicoDiagnostics guide pd.en-19, published in 2020, printed page 54, separately describes windowing, FFT bin count and a responsiveness setting that changes scan duration. It notes the frequency-resolution tradeoff when reducing that duration. Use this as an example of separate controls to identify, not as instructions to change unsupported advanced settings in another version or tool.

Build a spectrum handoff record

Use one entry for each analyzed interval:

• Complaint and acquisition context; original filename and recording time range.
• Sensor identity, location, orientation, attachment and applicable setup instructions.
• Software/version, selected start and end, FFT block duration and sample rate if exposed.
• Acquired points per block, segmentation, overlap, padded FFT length and spacing where applicable.
• Window, averaging, filters, amplitude convention and frequency units.
• Engine and road-speed history; event stability or changes inside the interval.
• Saved spectrum, comparison question, unresolved limit and next verification step.

If two records have matching peak labels but different block durations or processing, resolve those differences before treating the plots as equivalent evidence. A peak by itself does not identify the failed part. The useful next step might be a documented reanalysis, a comparison under matching conditions or a further test from the applicable diagnostic procedure.

The oscilloscope timebase and zoom guide addresses acquisition context, while the waveform averaging guide addresses processed comparisons. Keep those records with this FFT-specific interval record when relevant. Request STEP Early Access to discuss clearer evidence handoffs in your shop.