See how much energy a medical ultrasound pulse loses to absorption and scattering as it travels through tissue. Adjust the frequency, depth and attenuation coefficient to watch the one-way loss, round-trip loss, half-value layer and imaging penetration depth update in real time, and feel the resolution-versus-depth trade-off.
Parameters
Ultrasound frequency f
MHz
Higher frequency means finer resolution but more attenuation
Propagation depth d
cm
Distance in tissue to the structure you want to image
Attenuation coefficient α
Tissue coefficient in dB/(cm·MHz). About 0.5 for soft tissue
Incident intensity I₀
mW/cm²
Acoustic intensity the probe sends into the tissue
Results
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One-way attenuation (dB)
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Round-trip attenuation (dB)
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Intensity at depth (mW/cm²)
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Half-value layer HVL (cm)
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Penetration depth (60 dB) (cm)
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Imaging verdict
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Ultrasound beam and attenuation — propagation animation
A pulse travels from the probe at the top down into the tissue, and the beam brightness decays exponentially with depth. The green line marks the half-value layer, the orange line the penetration limit, the white line the current measurement depth.
Attenuation A_dB and the intensity at depth I(d). α is the attenuation coefficient in dB/(cm·MHz), f the frequency in MHz, d the travel distance in cm. Attenuation rises with both frequency and depth.
Half-value layer HVL (the distance over which intensity halves — a 3 dB drop) and imaging penetration depth d_max (where the round-trip loss reaches 60 dB). The factor 2 in d_max comes from the round trip.
What is the Ultrasound Attenuation Simulator?
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An abdominal ultrasound scan works by sending sound from the probe and listening for the echoes, right? Does that sound get weaker as it travels through the body?
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Exactly — and that is the whole point. The ultrasound pulse the probe sends loses energy in two ways as it travels through tissue. One is absorption — the sound energy turns into a tiny amount of heat in the tissue. The other is scattering — the wave hits small structures and is deflected in all directions. Together these two are called attenuation.
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I get that attenuation grows the deeper you go — but when I raise the "frequency" on the left, the one-way attenuation jumps. Why is that?
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Good catch. Expressed in decibels, attenuation is roughly proportional not only to distance but also to frequency. The formula is A_dB = α·f·d, where α is the tissue's attenuation coefficient — about 0.5 dB/(cm·MHz) for soft tissue. So doubling the frequency doubles the loss at the same depth and the echo weakens fast. This is the single most important fact in ultrasound.
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So lowering the frequency means less attenuation and deeper imaging? Then it feels like everyone should just use a low frequency.
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That is exactly the trade-off. Lowering the frequency does reach deeper. But a lower frequency means a longer wavelength, so the resolution — the ability to tell small things apart — drops. A high frequency attenuates heavily and cannot reach deep, but its short wavelength gives fine resolution. So a sonographer uses 10-15 MHz for shallow structures like the thyroid or vessels, and 2-5 MHz for deep ones like the abdomen or the heart.
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There is also a "half-value layer" in the results. What does that one mean?
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The half-value layer HVL is the distance over which the sound intensity falls by half. A 3 dB drop halves the intensity, so HVL = 3/(α·f). At the default 5 MHz the HVL is only 1.2 cm — the intensity halves, then halves again, every 1.2 cm. It is a handy figure for picturing how quickly the sound fades with depth.
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And the "penetration depth (60 dB)"? Why 60 dB specifically?
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The range of echo strengths an ultrasound system can handle — its dynamic range — is roughly 60 dB. The echo makes a round trip, so the attenuation counts twice. The depth where the round-trip loss reaches 60 dB, d_max = 60/(2·α·f), is the limit beyond which the signal sinks below the noise. In practice, if the depth you need exceeds 0.8 of this d_max, you flag the deep region as "near the limit".
Frequently Asked Questions
The amount of attenuation is A_dB = α·f·d, where α is the tissue attenuation coefficient in dB/(cm·MHz), f is the ultrasound frequency in MHz and d is the travel distance in cm. This is the empirical rule that, expressed in decibels, attenuation is roughly proportional to the product of frequency and distance; for soft tissue α is about 0.5 dB/(cm·MHz). The intensity at depth is I(d) = I₀·10^(−A_dB/10), and this tool shows both the one-way loss and the round-trip pulse-echo loss.
The half-value layer HVL is the distance over which the acoustic intensity falls by half — a 3 dB drop. From the attenuation law, HVL = 3/(α·f) cm. For α=0.5 dB/(cm·MHz) and f=5 MHz the HVL is 1.2 cm, so the intensity halves every 1.2 cm travelled. Raising the frequency shortens the HVL, so the sound no longer reaches deep — this is why high-frequency probes are only used for superficial structures.
Attenuation A_dB = α·f·d is proportional to the frequency f. Doubling the frequency doubles the loss over the same distance, so the echoes weaken quickly and deep signals sink into the noise. Higher frequencies do have a short wavelength and fine spatial resolution, so a sonographer uses 10-15 MHz for shallow structures such as the thyroid or vessels and 2-5 MHz for the deep abdomen or the heart. This is the resolution-versus-penetration trade-off of ultrasound imaging.
The imaging penetration depth is the depth at which the round-trip (pulse-echo) attenuation reaches 60 dB, a typical limit of the system dynamic range: d_max = 60/(2·α·f) cm. The factor of 2 appears because the pulse makes a round trip. For α=0.5 and f=5 MHz, d_max = 12 cm. When the actual travel depth exceeds 0.8 times this value the deep region is near the limit, and beyond it the attenuation is too large to image.
Real-World Applications
Probe choice for diagnostic ultrasound: For deep examinations such as the abdomen, heart and obstetrics, a 2-5 MHz low-frequency convex or sector probe is used so attenuation is kept low and echoes still reach a sufficient depth. For superficial structures — thyroid, breast, surface vessels, musculoskeletal tissue — a 10-15 MHz linear probe is chosen, accepting heavier attenuation in exchange for the short wavelength and high resolution. Estimating the penetration depth from α and frequency is the starting point for picking a probe and a frequency.
Designing the depth-gain compensation (TGC): Because deeper echoes are weakened by attenuation, an ultrasound system applies Time/Sensitivity Gain Compensation that amplifies the received signal according to depth. How much amplification is needed is governed by exactly the one-way and round-trip loss this tool computes. Understanding the depth dependence of attenuation lets you correct brightness unevenness with the TGC sliders.
Ultrasound safety assessment (heating, MI): The acoustic energy that is absorbed turns into heat in the tissue. Estimating, from the incident intensity I₀ and the attenuation, how much energy the tissue receives at each depth feeds into understanding the thermal index (TI). Near bone, where attenuation is large, local heating needs care; managing the acoustic output is especially important when scanning fetal bone or the eye.
Application to non-destructive testing (NDT): The same attenuation physics governs ultrasonic flaw detection in metals, concrete and composites. In materials with coarse grains or many voids, scattering attenuation is large, and detecting deep defects requires a low-frequency transducer. Measuring the attenuation coefficient to choose a frequency is a shared way of thinking for medical and industrial ultrasound alike.
Common Misconceptions and Pitfalls
The most common misconception is assuming the attenuation coefficient α is a single fixed value. The default of 0.5 dB/(cm·MHz) in this tool is a representative figure for soft tissue, but the real α varies greatly by tissue. Water and blood attenuate almost nothing (around 0.1), liver and muscle are 0.5-1.0, bone and tendon are larger still, and gas-filled tissue such as lung passes almost no ultrasound at all. The frequency dependence of α is also not strictly linear in every tissue — the power exponent deviates from 1. Use the A_dB = α·f·d of this tool understanding that it is a first-order approximation, accurate enough in practice for soft tissue.
Next, thinking that attenuation equals absorption. Attenuation is the sum of absorption and scattering. Absorption is the component that turns into heat and is tied directly to safety. Scattering is the component that changes direction — and it is in fact the very source of the echoes that form the image; without backscatter from tissue there would be no B-mode image. Lumping everything together as attenuation makes it easy to confuse "the scattering that builds the image" with "the absorption that is simply lost", yet the two play completely different roles.
Finally, confusing round-trip loss with one-way loss. In the pulse-echo method the sound the probe sends reflects off the target and comes back. The signal therefore travels the distance to the target twice — there and back — so the echo loss the system receives is exactly twice the one-way loss. If you allocate the 60 dB budget against the one-way loss when estimating the penetration depth, you overestimate the reachable depth by a factor of two. Always reason about the imaging limit with the round-trip loss. This tool shows one-way and round-trip separately precisely to prevent this mix-up.
How to Use
Set the ultrasound frequency (1–15 MHz) using freqNum; higher frequencies attenuate faster in tissue
Enter tissue depth (0–20 cm) via depthNum to model pulse propagation through muscle, fat, or organ tissue
Adjust the attenuation coefficient (0.5–3.0 dB/cm/MHz) using attenNum based on tissue type (muscle ~1.0, fat ~0.6)
Input initial intensity (10–100 mW/cm²) with i0Num to represent transducer output power
View one-way attenuation (dB), round-trip loss, remaining intensity at depth, HVL (half-value layer), and imaging verdict (feasible/marginal/poor)
Worked Example
Cardiac imaging at 3.5 MHz through 8 cm of chest wall muscle: initial intensity 50 mW/cm², attenuation coefficient 1.0 dB/cm/MHz. One-way attenuation = 3.5 × 8 × 1.0 = 28 dB. Intensity at depth = 50 × 10^(−28/10) ≈ 1.58 mW/cm². Round-trip attenuation = 56 dB. HVL = 0.693/(3.5 × 1.0) ≈ 0.20 cm. Penetration depth (−60 dB) ≈ 1.71 cm. Verdict: marginal (echo recovery requires high-gain amplification).
Practical Notes
Obstetric imaging at 2–3 MHz penetrates deeper (HVL >0.3 cm) than cardiac at 3.5–5 MHz due to frequency-squared attenuation law
Fatty tissue (coeff. 0.6 dB/cm/MHz) transmits better than fibrotic scar; use lower frequency for patients with poor acoustic windows
Round-trip loss doubles one-way dB; a 25 dB outbound path means −50 dB return, requiring 50 dB receiver gain to recover 1 mW/cm² signal
Doppler shifts demand SNR >10 dB; if penetration depth falls below target organ depth, switch to lower frequency or phased-array focusing