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Building Acoustics
Anechoic Chamber Low-Frequency Cutoff Simulator
A design calculator for anechoic chambers: enter the chamber dimensions, wedge length, absorber density and target cutoff to see the wedge cutoff frequency, usable free-field volume, minimum measurement distance and maximum measurement radius update in real time — and check whether the room is suitable for loudspeaker measurement, microphone calibration or HRTF capture.
Parameters
Chamber length L_room
m
Chamber width W_room
m
Chamber height H_room
m
Wedge length L_wedge
cm
Base-to-tip length of a pyramidal absorber. Sets the cutoff via the quarter-wavelength rule.
Absorber density ρ
kg/m³
Bulk density of the fibrous absorber (glass wool, rock wool, etc.).
Target cutoff f_target
Hz
Lowest frequency at which free-field conditions must be guaranteed.
Use semi-anechoic floor (rigid)
Leaves the floor as a reflective slab; standard for automotive and large-machine tests.
Results
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Wedge cutoff frequency (Hz)
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Cutoff wavelength (m)
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Free-field volume (m³)
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Required wedge length (cm)
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Min. measurement distance (m)
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Max. measurement radius (m)
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Anechoic chamber cross-section — wedges and measurement layout
Pyramidal absorber wedges on walls, ceiling and floor (no floor wedges in semi-anechoic mode). A speaker and microphone sit at the centre, and wavefronts spread out and are absorbed at the wedges.
L_wedge: wedge length (m), c = 343 m/s (speed of sound in air at 20 °C). A wedge longer than one quarter wavelength absorbs more than 90% of the incident sound at that frequency, so the lowest usable frequency is set by the wedge length.
Required wedge length to reach the target cutoff, and the free-field volume after subtracting the wedge depth from each wall. L'_room etc. are the interior clear dimensions.
If the smallest interior dimension is larger than one wavelength, no strong standing waves remain. Maximum measurement radius leaves a 0.5 m margin from each wedge tip.
Anechoic chamber low-frequency cutoff and wedge design
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An "anechoic chamber" is one of those pitch-black rooms where you can hear your own heartbeat, right? Why isn't a normal sound-proof room good enough?
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Exactly. A normal sound-proof room is built to stop sound from leaking in or out — its walls are heavy, but the inside is bare concrete and sound bounces around like crazy. An anechoic chamber is the opposite: its job is to suppress every reflection inside, so the interior behaves like open air, a true free field. To do that, the walls, floor and ceiling are covered with huge pyramid-shaped wedges 0.5 m to 2 m long that trap incoming sound and turn it into heat. You need that free field whenever you want to measure how a loudspeaker really radiates, or to calibrate a microphone.
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What does the wedge length actually decide? When I shorten the "wedge length" slider on the left, the cutoff frequency keeps climbing.
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Great observation — that is the core of anechoic-chamber design. Sound is a wave, so the absorption depends on the wedge length L_wedge relative to the wavelength λ. The rule of thumb is that a wedge longer than a quarter wavelength absorbs about 90% or more of that frequency. Writing it as an equation, f_cutoff ≈ c/(4·L_wedge). With c = 343 m/s, a 1 m wedge gives 86 Hz and a 0.5 m wedge gives 172 Hz. Conversely, if the spec says "free-field down to 100 Hz", then L_wedge = c/(4×100) = 0.86 m — you need wedges of at least 86 cm. Anything shorter and the 100 Hz wave just slips past the wedge, reflects off the wall behind and you have lost your free field.
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So if I make the wedges crazy long, I can measure down to ridiculously low frequencies?
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In principle yes, but the wedges live inside the room — every centimetre of wedge eats into the usable interior on both sides. Take a 10×8×6 m room with 1 m wedges on all six faces: the usable volume drops to 8×6×4 = 192 m³ (the full-anechoic case). Push the wedges to 2 m and the usable volume becomes 6×4×2 = 48 m³, almost an order of magnitude smaller. So in practice you trade "lowest usable frequency" against "interior volume you keep". Microsoft's anechoic chamber in Redmond — one of the world's largest — uses metre-class wedges and a noise floor of −20.6 dB(A), below the human hearing threshold.
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There's also a "semi-anechoic" check box — what is that for?
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That stands for a half-anechoic, or semi-anechoic, chamber. The floor is left as bare concrete, so it acts as a perfect reflector, but the upper hemisphere is still a true free field. You use it for cars, washing machines, outdoor units of air conditioners — any product that is supposed to sit on the ground when it operates. You can drive the device in, anchor it to the slab, and you save the volume that the floor wedges would have eaten. Tick the box and watch the usable volume jump up. Both ISO 3744 / ISO 3745 and the Japanese JIS Z 8732 explicitly include full- and semi-anechoic methods for sound-power testing.
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One more — why is there a "minimum measurement distance"? It feels like you should be able to stick the microphone right against the speaker.
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Good catch. One of the conditions for a free field is that the inverse-square law holds — sound pressure drops by 6 dB for every doubling of distance. That only holds in the "far field"; closer in, you are in the "near field" where the relation between distance and pressure is messy and your data gets contaminated by the speaker's directivity. Empirically, you need to be more than about a quarter wavelength of the lowest frequency away. At 100 Hz (λ = 3.43 m), d_min ≈ 0.86 m. And if the room is small, the maximum measurement radius shrinks (subtract a 0.5 m margin from the wedge tip). Always compare the two values from the tool: if d_min is bigger than r_max, your room cannot actually deliver the free-field measurement you want.
Frequently Asked Questions
It is set by the length of the pyramidal absorber wedge L_wedge through f_cutoff ≈ c/(4·L_wedge). The rule of thumb is that a wedge longer than a quarter wavelength absorbs more than 90% of the sound at that frequency. With c = 343 m/s, a 1.0 m wedge gives a 86 Hz cutoff and a 0.5 m wedge gives 172 Hz. A chamber that needs free-field behaviour down to 100 Hz therefore requires wedges of at least 86 cm.
A full anechoic chamber covers all six surfaces with wedges and recreates true free-field conditions (no reflections). It is used for microphone calibration, HRTF measurement and small-loudspeaker radiation. A semi-anechoic chamber leaves the floor as a rigid reflecting plane and uses only five absorbing walls. It is the standard for cars, large machines and other devices that must sit on a floor; cost, space and access are far better. Toggling "Use ground" in this tool removes the floor-wedge volume loss and shows the increased usable volume.
The minimum distance d_min is roughly one quarter of the wavelength at the lowest frequency of interest. Below that, the near-field of the source breaks the inverse-square law (−6 dB per doubling of distance) and the data is no longer free-field. At 100 Hz (λ = 3.43 m) this means d_min ≈ 0.86 m. The tool computes d_min from the wedge cutoff and shows the maximum measurement radius the chamber can offer (wedge tip minus a 0.5 m margin).
Acoustic wedges are typically made of glass wool or rock wool at 16–32 kg/m³. If the density is too high, the flow resistance becomes too large at low frequency and sound is reflected at the surface instead of penetrating into the wedge; too low and absorption drops. Empirically 16–24 kg/m³ delivers absorption coefficients of 0.85–0.95 near cutoff. The tool uses a small linear correction around 16 kg/m³ to visualise this sensitivity.
Real-World Applications
Loudspeaker and microphone characterisation: Audio manufacturers and acoustics labs measure loudspeaker frequency response, directivity and distortion inside anechoic chambers. With no reflections, the true radiated power and beam pattern can be separated cleanly. Microphone reference calibration (IEC 61094, JIS Z 8732) also requires a full anechoic chamber to obtain free-field sensitivity. Going below 100 Hz needs metre-class wedges and a correspondingly large room — facility cost climbs steeply.
Head-Related Transfer Function (HRTF) measurement: VR, games and spatial audio rely on personalised HRTFs that capture how each listener's head and ears shape incoming sound. A subject sits in the centre while loudspeakers placed on a 1–2 m radius around them play sequential signals, and microphones in the ears record the response. Reflections corrupt the HRTF, so an anechoic chamber is the standard tool. Apple and Sony's spatial-audio products use averaged HRTFs measured this way.
Sound power for cars and industrial equipment (semi-anechoic): Vehicle engine noise, EV motor whine and home-appliance running noise are measured in semi-anechoic rooms per ISO 3744 / ISO 3745. The device sits on the floor and microphones on a hemispherical array integrate sound pressure into a sound-power level. Modern noise targets push designers towards high-end semi-anechoic chambers with background noise below −10 dB(A), driven by quiet EV motors and low-frequency heat-pump rumble.
Active Noise Control and machine-sound source identification: Developing ANC algorithms for headphones or car cabins requires independent measurement of source and reference signals in a free field. Beamforming microphone arrays in anechoic rooms also localise noise sources in compressors and machine tools, mapping the radiation pattern to find which component is failing. The world's quietest anechoic chamber — at Microsoft in Redmond — reaches −20.6 dB(A), well below the 0 dB SPL human hearing threshold.
Common Misconceptions and Pitfalls
The most common misunderstanding is that "an anechoic chamber works at any frequency". The low-frequency limit is entirely set by the wedges. Inject a 50 Hz signal into a lab with 50 cm wedges and the 6.86 m wavelength flies past the wedges, reflects off the wall behind and forms standing waves. The "low-frequency data" you record is just the room's modes in disguise. Treat the wedge cutoff value from this tool as a hard floor — only frequencies above it are trustworthy. Above that, the wedges only get more efficient as frequency increases.
Second pitfall: assuming the semi-anechoic floor is automatically a perfect reflector. Vertical-incidence reflection is indeed above 99.98% because the impedance ratio between concrete and air spans four orders of magnitude. But at grazing incidence, floor flatness, joints, drains and surface coatings start to absorb or scatter, distorting the assumed hemispherical pattern. ISO 3745 specifies floor flatness of ±1 mm/m and reflectivity within 1 dB measurement error; new facilities should run a calibrated sweep to confirm. Old facilities often drift because of dirt, peeling paint or surface wear.
Finally, "higher wedge density always means better absorption" is wrong. Fibrous absorbers behave non-monotonically with bulk density. As density rises, flow resistance climbs and eventually sound is reflected at the surface instead of entering the wedge. For glass wool, low-frequency optimum sits around 16–24 kg/m³; pushing past 32 kg/m³ actually drops the low-frequency absorption. Mid- and high-frequency design (above 500 Hz) can use denser material, but at low frequency staying near 16 kg/m³ is safer. Ageing also matters — fibres sag and the chamber's free-field quality degrades over 10–20 years; periodic acoustic re-measurement (reverberation time, deviation from the inverse-square law) is how you know when to replace the wedges.
How to Use
Enter chamber dimensions in meters: length, width, and height (e.g., 6m × 4m × 3.5m for a typical automotive test cell).
Input wedge absorber length in centimeters (typically 30–100 cm depending on target low-frequency cutoff).
The simulator calculates the fundamental acoustic mode and wedge cutoff frequency, determining the lowest frequency at which anechoic conditions are guaranteed.
Review the free-field volume and minimum measurement distance to ensure compliance with ISO 3745 or similar standards.
Worked Example
For a 5m × 3m × 2.8m semi-anechoic chamber with 60 cm melamine foam wedges: the fundamental mode is approximately 23 Hz (limited by chamber length). Wedge cutoff frequency calculates to ~57 Hz. Free-field volume available is 31.2 m³. Minimum measurement distance is 0.65 m (2.4 × longest wedge). For sound power measurements of HVAC units or small machinery, cutoff of 57 Hz means uncertainty below this frequency; measurements at 63 Hz 1/3-octave band and above are valid.
Practical Notes
Wedge length directly controls cutoff: 40 cm wedges yield ~90 Hz cutoff; 80 cm wedges reduce it to ~40 Hz. Choose length based on target frequency range (e.g., 50 Hz for electric motor testing).
Chamber aspect ratio matters: square cross-sections (L ≈ W) create degenerate modes that degrade anechoicity; use L:W ratio of 1.2–1.5 to separate modes.
Free-field volume must exceed source–microphone test sphere volume by at least 3×; verify measurement distance stays within the calculated max radius to avoid boundary reflections.
Low-frequency cutoff increases with room temperature (sound speed rises ~0.6 m/s per °C); temperature-compensate for precise compliance audits.