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Shock Response Spectrum (SRS) Analysis Software
ISO 18431-4 spectra, calculated live while the test is running.
A shock response spectrum turns an unreadable transient into a damage-potential curve. Instead of judging a shock by its peak g value — which depends as much on your sensor and mounting as on the event — an SRS predicts how structures of different natural frequencies would respond to the same pulse. It is the language shock specifications are written in, from MIL-STD-810 to spacecraft pyroshock requirements.
DewesoftX calculates SRS according to ISO 18431-4, and it does it online, during acquisition — not afterwards. Shock levels trend live across repeated events, so you can judge a test while it is still running instead of discovering a problem in post-processing. Results display on a log-log pseudo-velocity plot where acceleration, velocity and displacement can be read from the same curve. The SRS module is part of DewesoftX, which ships with every Dewesoft system at no extra cost.
Highlights
Real-time online SRS
Compute shock response spectra live during acquisition, not after. Online processing trends shock levels across a sequence of events and is optimised for high shock rates, so closely spaced shocks are handled without dropping spectra.
Pseudo-velocity shock spectrum
A log-log pseudo-velocity plot with adjustable axes on all four sides, so acceleration, velocity and displacement read from one curve. Cursor values for every quantity appear in the graph header at once.
ISO 18431-4 compliant
SRS is calculated according to ISO 18431-4, so results are comparable with any other compliant analyzer and defensible in a test report.
Automatic shock detection
Fixed and Dynamic detection modes find the event and set the pre-shock, shock and post-shock windows. Dynamic runs online, so repeated shocks are captured without placing cursors by hand.
Nine output channels
Composite (maximax), primary and residual curves, each as absolute maximum, maximum and minimum, available in acceleration, velocity or displacement.
Selectable frequency span and resolution
Set the start and end frequency of the calculation and choose the fractional-octave spacing, putting resolution where the response matters.
Damping and quality factor
Set relative damping or quality factor, and change it offline without re-running the test. The standard 5% damping corresponds to Q = 10; zero damping gives maximum response, while heavy damping flattens the curve.
Any unit system
SRS honours every unit defined in the Physical quantity editor, SI and inch-based alike. Results export to MATLAB, Excel, DIAdem, FlexPro and UFF.
Simple and straightforward setup
The sensors, data acquisition system, and software setup is fast and easy. Sensors auto detection and TEDS support will get you to measure signals in no time.
Advanced Math
All relevant parameters such as composite/maximax, primary, and residual are calculated online in real-time. Results in the frequency domain spectrum can be shown as acceleration, velocity, or displacement.
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What is a shock response spectrum (SRS)?
A shock response spectrum is a plot of the peak response of a set of hypothetical single-degree-of-freedom (SDOF) systems against their natural frequency, when every one of them is driven by the same measured shock at its base.
Picture a row of mass-spring-damper oscillators attached to the same surface, each tuned to a different frequency across the range of interest. Apply the measured acceleration time history to all of them, record the largest response each one reaches, and plot those peaks against frequency. That curve is the SRS. Each SDOF system is assumed to have no mass-loading effect on the base input.
The result tells you what the time history cannot: which frequencies this particular shock excites hardest, and therefore which resonances in the real product are at risk.
Why peak g is not enough
A raw shock trace gives you one number, and that number moves with the sensor, the mounting and the sample rate. It says nothing about frequency content and nothing about whether the energy sits where your product is vulnerable. Two pulses with the same peak g can have entirely different damage potential.
Two properties of the SRS are worth knowing before you rely on one. The transformation is not reversible — many different waveforms produce the same spectrum, because only peak responses are retained. And the curve depends on the damping value assumed in the calculation, which is why an SRS should always be quoted with its damping.
Real-time online SRS analysis
Traditionally, SRS has been a post-processing exercise: run the test, store the data, place cursors around the event, recalculate, then look at the curve. If the shock was wrong, you find out afterwards — and on a destructive test, afterwards means another article consumed.
DewesoftX computes the shock response spectrum online, during acquisition. The spectrum updates as shocks arrive, so you can watch levels trend across a sequence of events and judge a test while it is still running. For repeated-shock programmes — drop sequences, firing series, multi-axis qualification runs — this turns hours of post-processing into a live readout.
Online processing is optimised for high shock rates, so closely spaced events are handled without dropping spectra.
SRS time blocks
Online processing works on triggered time blocks, and you can follow the exact blocks being used for each calculation. That makes it straightforward to set sensible durations for the pre-shock baseline, the shock itself and the post-shock decay — the three windows that determine whether primary and residual results mean anything.
Shock detection modes
Fixed detection uses defined windows, suited to a controlled test where the event arrives on cue.
Dynamic detection adapts the windows to each event and supports online processing, which is what makes unattended capture of a repeated shock sequence practical.
Each mode carries an illustration in the setup showing how its settings change the processing — useful when a test procedure passes to someone who did not write it.
Pseudo-velocity shock spectrum (PVSS)
Acceleration is the natural unit for measuring a shock. It is not always the best unit for judging one.
Pseudo-velocity is proportional to the peak stress a shock induces in a structure, which makes the pseudo-velocity shock spectrum a more direct indicator of damage potential than acceleration alone. Where two shocks with very different frequency content need comparing, PVSS is the view that makes the comparison meaningful.
DewesoftX presents SRS results on a log-log pseudo-velocity plot with axes adjustable on all four sides, so acceleration, velocity and displacement can be read from the same curve. Cursor position values appear in the graph header for every quantity at once — put the cursor on the knee of the curve and read all three without switching views.
The pseudo-velocity plot supports the online Dynamic shock detection mode, so this view is available live as well as in post-processing.
SRS calculation settings
Every parameter that affects the result is exposed, and all of them can be changed offline and recalculated without repeating the test — which matters when a specification turns out to assume a different damping value than the one you measured at.
| Setting | What it does |
|---|---|
| Start / end frequency | Defines the calculation range. The upper limit is bounded by the acquisition sample rate. |
| Frequency division (octave) | Sets frequency-domain resolution. Finer fractional-octave spacing gives more points per octave. |
| Damping / quality factor | Relative damping ratio or Q. 5% damping = Q of 10. Zero damping gives maximum response; very high damping produces a nearly flat line. |
| Remove DC offset | Averages DC over a defined interval at the start of the signal and subtracts it as a constant. |
| Compensate DC drift | Calculates DC at the start and the end of the signal and subtracts a linear interpolation between them. |
| Velocity DC filter | Removes the DC component from the calculated velocity time history. |
| Noise floor | Measures the input noise floor over a defined interval, so the useful dynamic range of the result is known. |
| Shock detection mode | Fixed or Dynamic. Dynamic supports online processing and adapts the time blocks to each event. |
| Units | All units defined in the Physical quantity editor, SI or inch-based. |
Each shock detection mode has an illustration in the setup showing how its settings change the processing — useful when handing a test procedure to someone who did not write it.
Choosing a damping value
5% relative damping — a quality factor Q of 10 — is the usual default and what most specifications assume. The relationship is Q = 1/(2ζ), so halving the damping ratio doubles Q and raises the peaks.
Lower damping produces a more peaked spectrum, because each oscillator rings longer and reaches further. Very high damping flattens the curve toward a straight line. Neither is wrong; they answer different questions. What matters is that the damping value travels with the result, because the same shock produces a different spectrum at a different ratio.
SRS output channels
Results are available as acceleration, velocity or displacement, and each is split into three time sections — nine output channels in total:
| Section | Name | What it represents |
|---|---|---|
| Whole measurement | Composite / maximax | Worst-case extrema across the whole event — absolute max, max and min |
| During the shock | Primary | Forced response, while the input is still being applied |
| After the shock | Residual | Free vibration, after the input has stopped |
Each is available as absolute maximum, maximum (positive) and minimum (negative).
Maximax — the envelope of absolute positive and negative response across the whole measurement — is the standard presentation format and the curve most specifications are written against. If a document says "the SRS" without qualification, it almost always means maximax at 5% damping on a log-log plot.
Primary and residual matter when you need to know where the damage potential comes from. A high primary with a low residual says the structure is being driven hard but settles quickly. A low primary with a high residual says the structure is ringing after the event — which is the usual signature of a resonance problem, and the question that drives isolator and mount design.
Separating positive and negative matters where a structure responds asymmetrically, which is common wherever a joint, a mount or a contact behaves differently in tension than in compression.
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Comparing an SRS against a reference curve
Calculating a spectrum answers what happened. Qualification asks a different question — does it pass? — and that answer comes from comparing the measured SRS against a reference curve: the specification's required spectrum, with tolerance bands either side.
Those bands are written into the standards rather than left to engineering judgement, and they are tighter than most people expect.
What MIL-STD-810H requires
Both shock methods in MIL-STD-810H define their requirements as dB tolerances around a reference maximax acceleration SRS, and both specify how the spectrum must be computed before the comparison is valid.
| Parameter | Method 516.8 — Shock | Method 517.3 — Pyroshock |
|---|---|---|
| Primary band | −1.5 / +3 dB, ≥90% of points | −3 / +6 dB, 80% of points (near-field) or 90% (mid/far-field) |
| Remaining points | −3 / +6 dB | −6 / +9 dB |
| Frequency range | 10 Hz – 2 kHz | 100 Hz – 20 kHz (10 Hz – 3 kHz for shaker-based far-field) |
| Spectrum type | Maximax acceleration | Maximax acceleration, with maximax pseudo-velocity as required secondary display |
| Resolution | 1/12-octave centre frequencies | 1/12-octave centre frequencies (Procedures I–IV) |
| Damping | Q = 10 (≈5% critical) | Q = 10, plus a second pass at Q = 50 where the data drives a lab simulation |
Two requirements catch people out.
An under-test fails too. Method 517.3 requires that at least half the individual SRS values exceed the nominal specification. A spectrum that sits comfortably inside the lower tolerance band across the range has not passed — it has under-tested the article.
Resolution and damping are not free choices. Both methods specify 1/12-octave centre frequencies and Q = 10. A spectrum computed at 1/3-octave, or at a different damping ratio, is not comparable against the reference no matter how good the measurement behind it was.
Positive, negative and pulse symmetry
Method 516.8 uses the maximum and minimum spectra together as a symmetry check: where both the positive and the negative response meet tolerance, the pulse is treated as symmetric. This is where the separate max and min output channels earn their place — the asymmetry between them is a documented result, not a diagnostic curiosity.
Comparing against the reference in DewesoftX
A reference curve is defined in the DewesoftX reference curve module: entered by hand, captured from a measurement with a single button, or pasted straight from a specification document. The measured SRS is compared against it continuously, and a status output channel changes state when the curve crosses a limit.
Because the result is a channel rather than a picture, pass/fail can be logged, displayed, triggered on and exported with everything else from the test — so the qualification verdict lives in the same data file as the evidence behind it.
Explore more at mechanical shock testing — the standards themselves, test types and hardware.
Reporting and export
DewesoftX includes a full-blown reporting tools that enable composition of the reports directly inside our software.
SRS results report directly from DewesoftX alongside the time history, the video and any other channel recorded on the same time base. Export goes to MATLAB, Excel, DIAdem, FlexPro and UFF.
Analysis licences are free and unlimited, so colleagues can open a measurement, change the damping value, recalculate and read the result without buying a seat. For a test report that has to be reviewed by a customer or a certification body, that removes the usual friction of sending someone a data file they cannot open.
DAQ Hardware for SRS measurement
An SRS is only as trustworthy as the measurement behind it. The calculation faithfully reproduces whatever reached the converter — including energy that aliased down from above the sampling limit, which appears in the spectrum as legitimate low-frequency content and cannot be removed afterwards.
Dewesoft data acquisition (DAQ) systems put anti-aliasing filters in the analog signal path, and SIRIUS® XHS covers the bandwidth that pyroshock and ballistic events demand. The SRS module runs on any Dewesoft system, so the same analysis applies whether the front end is a two-channel portable or a 288-channel rack.
Explore mechanical shock testing page — hardware, sensors, standards and test types in full.
What Dewesoft SRS does and does not do
DewesoftX measures and analyzes shock: acquiring the event, calculating the spectrum and reporting the result.
It does not perform SRS synthesis — generating a drive waveform to reproduce a target spectrum on a shaker — and it does not control a shaker or shock machine. Those are controller functions, and Dewesoft systems work alongside whatever controller you have rather than replacing it.
In practice this means Dewesoft measures what actually happened at the test article, independently of the equipment driving the table. For pyroshock, drop, ballistic, blast and transportation shock — where there is often no controller at all — it is the entire measurement and analysis chain.
Frequently Asked Questions
We've gathered all the answers you're looking for, neatly organized just for you.
An SRS plots the peak response of a bank of hypothetical single-degree-of-freedom systems against their natural frequency, all driven by the same measured shock. It shows which frequencies the shock excites hardest, and therefore which resonances in a real product are at risk.
The measured acceleration time history is applied as base input to an array of SDOF oscillators spanning the frequency range of interest. The peak response of each oscillator is retained and plotted against its natural frequency, on logarithmically spaced points. DewesoftX follows ISO 18431-4.
5% relative damping, equivalent to a quality factor Q of 10, is the usual default and what most specifications assume. Lower damping produces higher peaks; very high damping flattens the curve toward a straight line. Always state the damping value alongside any SRS — the same shock gives a different spectrum at a different damping ratio.
Primary is the response during the shock, while the input is still applied. Residual is the free-vibration response after the shock has ended. Maximax is the envelope of the absolute maximum across the whole measurement, and it is the curve most specifications are written against.
Pseudo-velocity is proportional to the peak stress a shock induces, so a pseudo-velocity shock spectrum indicates damage potential more directly than acceleration. It is presented on a log-log plot where acceleration, velocity and displacement can all be read from the same curve, and it is the right view for comparing shocks with very different frequency content.
Yes. DewesoftX computes SRS online during acquisition, with shock trending across repeated events. Online processing is optimised for high shock rates, and the Dynamic shock detection mode handles the time-block setup automatically.
Yes. Frequency range, damping or Q, octave spacing and DC handling can all be changed in post-processing and the spectrum recalculated from the stored time history, without repeating the test.
An FFT decomposes a signal into the frequencies it contains. An SRS does something different: it reports how a set of hypothetical structures would respond to the signal. An FFT describes the shock; an SRS describes its effect. For a transient event, the FFT of a short non-repeating pulse is difficult to interpret, which is why shock specifications use spectra of response rather than of content.
No. DewesoftX measures and analyzes shock; it does not synthesise a drive waveform to match a target spectrum, and it does not control a shaker or shock machine. It works alongside your existing controller, and for drop, pyroshock and blast testing there is usually no controller involved.
SRS is calculated according to ISO 18431-4, the international standard for shock response spectrum calculation, so results are comparable with any other compliant analyzer.
Results export to MATLAB, Excel, DIAdem, FlexPro and UFF, and can be reported directly from DewesoftX. Analysis licences are free and unlimited, so colleagues can open and review the data without buying a seat.
A reference curve is the shock response spectrum a test specification requires, with tolerance bands either side. The measured SRS is compared against it to determine whether the test passed. MIL-STD-810H defines these bands in dB — for example −1.5 dB to +3 dB over at least 90% of points between 10 Hz and 2 kHz for Method 516.8 shock testing.
Method 516.8 requires at least 90% of maximax acceleration SRS points to fall within −1.5 dB to +3 dB of the reference across 10 Hz to 2 kHz, with the remainder within −3 dB to +6 dB. Method 517.3 pyroshock uses −3 dB to +6 dB for 80–90% of points across 100 Hz to 20 kHz, and additionally requires at least half the values to exceed the nominal specification. Both specify 1/12-octave centre frequencies and Q = 10.
MIL-STD-810H specifies 1/12-octave centre frequencies and a quality factor of Q = 10, equivalent to about 5% critical damping, for both shock and pyroshock. Method 517.3 additionally requires a second pass at Q = 50 where the measurement will drive a laboratory simulation. A spectrum computed at different settings cannot be compared against the reference.
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