The method suits inspection of slabs, walls, decks and structures suspected of having internal voids or delamination. This article explains the principle, the thickness calculation, the field procedure, how to read the frequency spectrum, and the limitations.

How does Impact Echo work?

Impact Echo relies on the reflection of stress waves. When the concrete surface is tapped with a small impactor, a stress wave travels into the concrete. The wave bounces back and forth between the top and bottom surfaces (or a void/delamination), setting up resonance at a particular frequency.

A sensor placed near the impact point picks up the vibration, which is analyzed into a frequency spectrum. This resonant frequency is related to the depth of the reflector: if the wave reflects from the bottom surface, it gives the thickness; if it reflects from a shallower void, the frequency rises, indicating a defect.

Impact Echo principle: comparing a sound concrete slab with one containing a void or delamination, and the resulting frequency spectra Two panels side by side. The left panel shows sound concrete: the impactor taps the top surface, and the stress wave travels down, reflects off the bottom surface and bounces back and forth through the thickness T. A sensor beside the impact point picks up the signal, and the spectrum below peaks at a low frequency. The right panel has a void or delamination at depth d, shallower than the thickness; the wave reflects from the void at the shallow level, so the spectrum peak shifts to a higher frequency. The bottom strip shows the reflector depth formula T equals beta times Cp divided by 2f, where Cp is the P-wave velocity in concrete, f is the peak frequency, and beta is about 0.96 for slabs per ASTM C1383, and the test needs access from one surface only. Impact Echo: reflector depth ↔ peak freq. Not to scale Sound concrete Tap Sensor T Slab bottom surface Frequency f → Amplitude Low f (deep reflector) Void / delamination Tap Sensor d Void Slab bottom surface Frequency f → Amplitude Higher f (shallower) 1 T = β·Cp / (2f) Cp P-wave velocity · f peak freq. 2 β ≈ 0.96 (slabs) ASTM C1383 3 One-side access only Thickness / voids
The stress wave bounces between the top and bottom surfaces. With a void or delamination, it reflects from a shallower level, changing the resonant frequency — reflector depth is calculated from T = β·Cp / (2f), with β ≈ 0.96 for slabs per ASTM C1383

How does Impact Echo calculate thickness?

Thickness is calculated from the wave velocity in the concrete and the resonant frequency, using the relationship

Thickness T = P-wave velocity (Cp) ÷ (2 × resonant frequency f)The wave velocity Cp of that concrete must be known before frequency can be converted to thickness

For example, a wave velocity of 4,000 m/s and a peak frequency of 10 kHz give a thickness of 0.20 m. So before calculating, you need the wave velocity in that concrete, obtained by calibrating at a point of known thickness or by measuring the surface wave velocity as described in ASTM C1383; in the latter case a factor β ≈ 0.96 is applied for slabs. If the calculated value is less than the actual thickness, a reflector such as a void or delamination lies shallower than the bottom surface.

The figures below simulate an Impact Echo test as a 2D section comparing the frequency spectrum at a sound concrete point with a point over a void, plus a 3D model of scanning a slab on a grid of test points to produce a results map.

2D model (side section) of an Impact Echo test on a concrete slab, comparing a sound concrete point with a point over a void or delamination Section of a reinforced concrete slab on a sub-base. At point A the concrete is sound: a small steel-ball impactor generates a stress wave that bounces between the top and bottom surfaces, a sensor beside the impact point picks up the signal, and the thickness is T. At point B there is a flat void or delamination at depth d, shallower than the bottom surface, so the wave returns sooner. The box on the right shows the frequency spectra of both points: point A peaks at the lower frequency f_T, point B at the higher frequency f_d, together with the thickness formula T equals P-wave velocity divided by twice the frequency and the example velocity 4,000 meters per second, frequency 10 kilohertz, giving a thickness of 0.20 meters. Side section (2D) Not to scale T d A · Sound concrete B · Over void / delam. Impactor Small steel ball Receiving sensor Near the impact point Reflected wave Top ↔ bottom Reflection from void Shorter path → higher f Rebar Dense rebar makes spectra harder to read Sub-base Slab bottom = reflector at point A Best for plate-like members, one-side access Frequency spectrum f_T Pt A f_d Pt B Frequency (kHz) → ↑ Amplitude T = Cp ÷ (2 × f) Cp = P-wave velocity Calibrate at known thickness Example Cp = 4,000 m/s f = 10 kHz → T = 0.20 m High f = shallow reflector Indicates void or delamination ASTM C1383: × β ≈ 0.96 when Cp is measured on surface
2D model (side section) — at point A the wave bounces between the top and bottom surfaces, giving the peak frequency f_T used to calculate thickness. At point B the wave reflects from a shallower void or delamination, so the peak shifts higher to f_d. The example in the right box uses a wave velocity of 4,000 m/s at 10 kHz, giving a thickness of 0.20 m; the actual velocity must be calibrated for the concrete on that job (image: SPN Soil Engineering)
3D model of scanning a slab with Impact Echo on a grid of test points, and a results map pinpointing voids or delamination 3D view of a concrete slab on a sub-base, with the front corner cut away to reveal a flat delamination inside the concrete, shallower than the bottom surface. A grid of test points covers the slab surface: green points are full design thickness, yellow points are minor anomalies, and red points are where a shallow reflector was found, lying above the delaminated area. The tester places an Impact Echo probe point by point, cabled to a data recorder, and there is one confirmation core location within the red area. A box on the right explains the point colors and a 5-step sequence: lay out the grid, tap and read, calibrate the wave velocity, map the results, and core to confirm only at anomalous points. 3D model Not to scale Impact Echo probe Impactor + built-in sensor Recorder Spectrum per point Concrete slab Cutaway view Sub-base Delamination Above slab bottom Red points over void Shallow reflector → high f Confirm by coring Only at anomalies Read results as a map Full design thickness Minor anomaly Shallow reflector, void? 1 Lay out test grid 2 Tap & read each point 3 Calibrate Cp (known T) 4 Map thickness / frequency 5 Core to confirm anomalies
3D model — a grid of test points is laid out on the slab surface and tapped and read point by point. Point colors show the readings; the cluster of red points lies above the delamination visible in the cutaway corner. Impact Echo results are indicative, so anomalous points must be confirmed by coring before drawing conclusions (image: SPN Soil Engineering)

What are the steps of an Impact Echo test on site?

The field test has 5 steps, from laying out the grid to confirmatory coring only at anomalous points.

  1. Lay out a grid of test points on the concrete surface — grid spacing depends on the size of defect to be detected and the total area. The surface must be clean and smooth enough for the sensor to sit firmly.
  2. Tap and read the spectrum point by point — repeat the tap until a clear, repeatable peak frequency is obtained, then record the value.
  3. Calibrate the wave velocity Cp — test at a point of known actual thickness, such as a slab edge, an opening or an existing core hole, to convert the frequency at every point into thickness.
  4. Map thickness/frequency — points whose peak frequency differs from the rest are suspect areas.
  5. Confirm anomalies — drill a Concrete Core Test at selected points before concluding.

How do you read the frequency spectrum?

The spectrum is read from the position of the dominant frequency peak, compared with the peak at points where the concrete is sound.

  • A single clear peak at the full-thickness frequency (fT) — the wave reflects from the bottom surface; the concrete is continuous through its thickness.
  • Peak shifted to a higher frequency (fd) — there is a reflector shallower than the bottom surface; a void or delamination at depth d is suspected.
  • No clear peak or multiple peaks — may be caused by dense reinforcement, member edges or a rough surface; retest and have a specialist interpret the results.

What can Impact Echo detect?

Impact Echo can be used to measure thickness and find internal defects in plate-like members, as follows

  • Measuring the thickness of slabs, walls or decks when only one side is accessible
  • Detecting voids and delamination inside concrete
  • Checking bond between concrete layers (e.g. topping slabs)
  • Assessing grout completeness in post-tensioning ducts (on some jobs)

Note that ASTM C1383 is the standard specifically for measuring concrete plate thickness; finding voids and delamination uses the same principle, with interpretation following ACI PRC-228.2.

How does Impact Echo differ from UPV?

Impact Echo analyzes the frequency of waves reflected from one surface, whereas a UPV Test measures the velocity of waves traveling through the concrete between two transducers.

← Scroll to see the full table →

TopicImpact EchoUPV
AccessOne side is enoughMost accurate with two opposite sides
Key strengthThickness / voidsUniformity / internal quality
PrincipleStress wave + resonant frequencyWave velocity through the concrete
Suitable membersSlabs, walls, decksColumns, beams, walls accessible from both sides
Data needed beforehandWave velocity Cp of that concreteDistance between transducers
StandardASTM C1383ASTM C597

Impact Echo excels at measuring thickness and finding voids in slabs when only one side is accessible, while UPV excels at assessing quality and uniformity. Engineers choose based on the objective and often use the two together.

What are the limitations of Impact Echo?

The main limitation is that interpretation requires a specialist, because frequency spectra in real structures are often complex. Points to be aware of:

  • Dense reinforcement or irregular geometry — produces multiple frequency peaks, making reading harder.
  • The wave velocity of that concrete must be known — if the velocity used is off, the calculated thickness will be off too.
  • Best suited to plate-like structures — members with complex shapes, or columns and beams with small cross-sections, give spectra that are harder to interpret.
  • It is a point test — each reading reflects only the condition below the impact point, so large areas must be tested on a grid.
💡 Combine with other methods for the full picture

In practice, engineers often pair Impact Echo with GPR or UPV to confirm rebar locations and concrete quality, and take core tests at selected points when confirmation with actual values is needed.

SPN Soil Engineering is preparing to offer void detection and concrete thickness measurement by Impact Echo. In the meantime, we carry out non-destructive structural testing (NDT) by UPV, rebound hammer, GPR and core testing, with reports certified by a civil engineer holding a professional engineering license (COE), signed and with registration number. Contact us for a free quotation within 24 hours.