Dirac is a sophisticated tool to calculate a very wide range of acoustical parameters from impulse response measurements of an enclosure. Through many years of development and customer feedback, Dirac has grown to support many different applications. Due to its user friendly interface it is suitable for field measurements, but it also contains many advanced features that make it useful in a laboratory or research setting.

To capture an impulse response, a stimulus signal is generated by Dirac or an external source. This stimulus signal is played in the enclosure through a sound source, and recorded through one or more microphones. Dirac then deconvolves the recorded response with the original stimulus to arrive at the impulse response.

Dirac will work with most Windows compatible sound devices. External USB based sound devices, such as the Triton, are recommended. For wireless (Wi-Fi) measurements, Dirac can work with the HBK 2255 Sound Level Meter and the HBK 2755 Smart Power Amplifier. The FAQ lists of some of the other sound devices that can be used with Dirac. Dirac is very easy to use, yet provides all the features you may need for your measurements.

Dirac comes with an extensive context sensitive help system and a comprehensive manual that contain many practical hints and tips that will help you to use the tool successfully. Qualified support is available through the worldwide network of Hottinger Brüel & Kjær backed by Acoustics Engineering specialists.


Download the Dirac datasheet including all specifications as a pdf.



  • Multi-channel measurements
  • Real Time Analyzer
  • Calculates a large number of acoustical parameters.
  • Conforms to ISO 3382 and IEC 60268-16 standards and supports ISO 3741 and EN 1793 standards.
  • Open loop (asynchronous) measurements using a CD or MP3 player to play back MLS or sweep stimuli.
  • Intermittent stimulus for background noise capture.
  • Single measurement per position for ISO 3382-3 open plan office measurements.
  • The included 1/1 and 1/3 octave band filters are fully IEC 61260 compliant.
  • Scale model measurements with air absorption compensation, and auralisation.
  • Realtime pre-averaging for up to 1000 recordings. Post averaging for direct impulse measurements.
  • Automated measurements reduce the total time required for a measurement session.
  • Reverse filtering technique enables accurate short reverberation time measurement, such as in studios and building structures (loss factor).
  • Multiple impulse response views with zoom: waveform, ETC, Decay, Frequency and Phase response.
  • Waveform editing with unlimited undo.
  • Uses standard WAV and ASCII files as well as MLSSA TIM files. Multi-channel import from B&K WAV files (PULSE).
  • Calculates and graphically displays statistics over multiple measurements (mean, standard deviation, min-max).
  • Plot parameter values versus frequency or versus source-receiver distance.
  • Directivity, waterfall and spectrogram plots.
  • Parameter graphs can be printed, saved as text files or copied to the clipboard for use in reports.
  • Organize measurements in projects and export all parameters for all files in a single operation.
  • Automatic notification and download of software updates.
  • Runs on Windows Vista, 7, 8, 10 and 11
  • Support for CoreAudio (WASAPI) and ASIO audio.
  • Support for HBK 2255 SLM measurements via Wi-Fi.
  • Support for stimulus playback on HBK 2755 Smart Power Amplifier via Wi-Fi.


New in Dirac 7.0

New in Dirac 7

Dirac 6 users can read about the new features in Dirac 7 in this blog post.


Energy ratios:

  • Bass Ratio (based on level), BR(L)
  • Treble Ratio (based on level), TR(L)
  • Centre Time, Ts
  • Clarities, C30, C50, C80
  • Clarity (user defined integration interval), Cx
  • Definition (Deutlichkeit), D50
  • Definition (Deutlichkeit, user defined integration interval), Dx
  • Echo Criterion (for music or speech), EC music, EC speech
  • Echo Criterion (user defined), ECn,τ
  • Hallmass, H
  • User-defined Energy Ratio, ER
  • Early Support, STearly
  • Late Support, STlate
  • Total Support, STtotal
  • User-defined Support, STX,Y


  • Early Lateral Energy Fraction, LF
  • Early Lateral Energy Fraction, LFC
  • Early lateral sound level, GEL
  • Late lateral sound level, LG (GLL)
  • Inter-Aural Cross-correlation Coefficient, IACC80
  • Inter-Aural Cross-correlation Coefficient (user defined interval), IACCx

Speech intelligibility:

  • Modulation Transfer Function, MTF
  • Modulation Transfer Index, MTI
  • Speech Transmission Index (male, female), STI
  • STI for PA systems, STIPA
  • Room Acoustics STI, RASTI
  • STI for TELecommunication Systems, STITEL
  • Percentage Loss of Consonants, % ALC
  • A-weighted SPL of speech, Lp,A,S
  • A-weighted SPL of speech at 4m, Lp,A,S,4
  • Spatial decay rate of A-weighted SPL of speech, D2,S
  • Comfort distance, rC
  • Distraction distance, rD
  • Privacy distance, rP
  • Speech level
  • Noise level
  • A-weighted background noise level, Lp,A,B



  • Early Decay Time, EDT
  • Reverberation Times, T10, T20, T30
  • Reverberation Time (user defined decay range), Tx
  • Reverberation Time (from best decay sections), RT
  • Bass Ratio (based on reverberation time), BR(RT)
  • Treble Ratio (based on reverberation time), TR(RT)


  • Impulse response to Noise Ratio, INR
  • Peak to Noise Ratio, PNR
  • Signal to Noise Ratio, SNR
  • Strength (Level relative to 10 m free-field), G
  • Strength (user defined integration interval), GX,Y
  • Early Strength, G80
  • Late Strength, GL
  • Relative Strength, Grel
  • Magnitude Spectrum
  • Magnitude Spectrum Pink (-3 dB/octave offset)
  • Equivalent Sound Level, Leq
  • Equivalent A- and C-weighted Sound Level, LAeq, LCeq
  • Sound Intensity, I
  • Level difference between channels, D
  • Minimum sound level (A/C/Z and F/S weighting), Lmin
  • Maximum sound level (A/C/Z and F/S weighting), Lmax
  • Peak sound level (A/C/Z weighting), Lpeak
  • Percentile sound level (A/C/Z and F/S weighting), LNx

Reflection & absorption:

  • Reflection Index, RI
  • Sound Insulation Index, SI
  • Sound Power Reflection Factor, Qw
  • Reduction Factor, Rsub
  • Gain Correction Factor, Cgain


  • Total Harmonic Distortion, THD
  • Total Harmonic Distortion plus Noise, THD+N
  • Spurious Free Dynamic Range, SFDR
  • Effective Number of Bits, ENOB
  • Intermodulation Distortion, IMD
  • Crest Factor, CF


  • Source-receiver distance




Get the Dirac 7 demo now


Download the Echo signals

The Echo signals can be injected into a PA system for speech intelligibility measurements


Frequently Asked Questions

Will Dirac work with my sound device?

In general: Yes. Dirac should work with any sound device that supports CoreAudio (WASAPI) or ASIO. Dirac requires that your sound device is full duplex, meaning it will playback and record at the same time. Almost all current sound devices, notebook sound systems and external sound devices are full duplex. The unregistered (demo) version of Dirac, which is available for download, can be used to perform a sound device test.

Which sound devices do you recommend for use with Dirac?

The Triton sound device was designed specifically for use with Dirac, so we can highly recommend this device. Lower cost devices that work well are for instance the Behringer UCA202, the ESI MAYA44 USB+ and the Focusrite Scarlett 2i2. For scale model measurements the E-MU 0202 USB can be used. For multi-channel measurements the RME Babyface Pro FS and the Behringer XR18 are good choices. Please note that there are many more devices that work perfectly with Dirac. Use the demo version to test Dirac with your sound device.

What are the PC requirements to run Dirac?

The minimum system requirements are a 1 GHz CPU, Microsoft® Windows 7, 8, 10 or 11, Microsoft .NET framework 4.5, 500 MB of available disk space, a WSVGA resolution 1024 x 600 (WXGA 1280 x 768 or higher recommended), A full duplex sound device with support for CoreAudio (WASAPI) or ASIO. A typical current laptop would therefore be more than adequate.

Which hardware system components can be used or are required?

To perform single channel parameter measurements according to ISO 3382 or IEC 60268-16, you can use a type 1 sound level meter, meeting the IEC 651 requirements, and equipped with a line output. Normally, single channel parameters can also be approximated using lower cost omnidirectional electret microphones or sound level meters. The measurement of LF requires either an additional bidirectional microphone, a.k.a. a “pure pressure gradient” or “figure-of-eight” type, or a switchable omni-bi-directional type. The latter is particularly useful if you have only one measurement channel available. The measurement of LFC or Sound Intensity requires a matched omnidirectional microphone pair at a fixed distance, such as a sound intensity probe. The measurement of IACC requires a head simulator.

Can I use a sound intensity microphone probe as omni-omni-directional receiver?

Yes (actually, they are equivalent).

Which brands of microphones can I use for the measurement of LF, LFC and IACC?

Bidirectional microphones (for LF or LFC measurements) are supplied by Schoeps, AKG, Neumann and Sennheiser. A switchable omni-bi-directional microphone is supplied by Neumann. An example of a sound intensity microphone probe (for LF, LFC and Sound Intensity measurements) is Type 3519 from B&K. Examples of usable head simulators (for IACC measurements) are the HMS III Artificial Head from Head Acoustics, the KU 100 from Neumann and Type 4100 from B&K.

Is it useful to choose a head simulator with on-board A/D converters?

The onboard A/D converters in a digital head simulator will provide a THD+N that is probably superior over any analog head simulator with external sound device, because the analog microphone signal path is highly optimized. This very low noise configuration is useful for music recording purposes. However, for room acoustic measurements, where minimum SNR values are normally much higher and controllable, it is not really necessary to use a digital head simulator.

Is it possible to use Dirac with a wireless transmission channel?

For wireless measurements we recommend the use the HBK 2255 Sound Level Meter. You can use wireless transmission of analogue input or output signals under certain conditions. The transmission channel may be equipped with a compander, i.e. analog level compression at the transmitter and complementary level expansion at the receiver. This can for instance be found in some wireless microphones and may cause allowable noise.

What is the typical dynamic range of a measured impulse response?

The loopback dynamic range or Impulse response to Noise Ratio INR, averaged over the octave bands from 125 Hz through 4 kHz, is typically about 60 dB. With a good sound device, the loopback INR is about 50 dB at 31.5 Hz, increasing up to 96 dB at 4 kHz and up, and 80 dB on average. Sound studios typically show 60 dB, and concert halls 50 dB.

What are the differences between impulse responses from MLS and sweep?

MLS and sweep will normally result in the same impulse response, but the methods differ in the effect of system irregularities, such as click noise, system variations during a measurement, distortion in the measurement chain, etc. With MLS these effects result in parasitic energy, time-distributed as noise over the impulse response. With sweeps, these effects result in parasitic energy, time-lumped as e.g. small sweeps in the impulse response. Unlike noise, the energy packages can often be removed very easily or have hardly any impact on the derived acoustical parameters. On the other hand, recognizing impulse response details may be easier if the impulse response is affected by random noise rather than by a nonrandom but unknown parasitic signal.

Why is the INR from sweeps often higher than from MLS?

One cause is mentioned above, and has to do with the way parasitic energy is converted into noise using MLS rather than sweeps. Another cause is that a filtered MLS signal has a higher peak to rms ratio than a filtered sweep signal. This allows the power amplifier to produce a higher rms level from a sweep than from an MLS signal.

How does Dirac process recorded data during import?

The imported file is processed as if it were measured at the actual settings. Therefore, the imported file MLS/Sweep/Capture length and Pre-Average value should match the corresponding values in the Measurement window. Also, the imported file sample rate should match the one in the sound device setup window.

Which results can be exported to Excel?

The following data can be exported:

  • The original impulse response (or any other opened .wav file), by saving it in Dirac as a .txt file and opening it as such in Excel.
  • Any parameter table in the Parameter window, including statistical data over several impulse responses, by saving the table in Dirac, and opening it as .txt file in Excel, or through copy & paste using the clipboard.
  • All (or selected) parameter values for all measurements in a project, using the 'Save Project Data' menu item of the Parameter window.
  • Any single impulse response parameter table in the Parameter menu, by saving the table in Dirac and opening it as .txt file in Excel.
  • Any of the impulse response views: Energy-Time Curve, the Decay Curve, the Magnitude Spectrum or the Phase Spectrum.
How to interpret the noise level?

With an External Impulse measurement, the graphically displayed signal and noise simply reflect the signal produced and the system noise present during the measurement. Therefore, the displayed energy ratio equals the real ratio. System noise includes acoustical and electrical noise.

With a non-Pink+Blue filtered MLS or lin-Sweep measurement, under certain conditions the graphically displayed noise relates to the real noise as follows. The ratio of the total energy of the file and the total noise energy (with the same file length) equals the ratio of the received signal energy produced (plus system noise energy) and the system noise energy present during the measurement. The mentioned conditions are: Pre-Average = 1, time invariant system and no significant signal distortion. The displayed ratio is basically proportional to the Pre-Average value, but practical acoustical systems may vary slowly in time.

In any other case, there is no one-to-one relation between the displayed and the real noise.

Can the impulse response starting position be moved?

You can move the total impulse response rotation-wise using the Rotate command in the Edit menu. This will in most cases not affect the calculated parameters. For some parameters such as the Strength (G) and related parameters, the source-receiver distance is essential, and rotate should not be used.

The red line indicates the start of the impulse response, as used in parameter calculations and determined in conformance with ISO 3382. You can influence this starting point, hence the calculated parameters, by setting the 'Minimum Source-Receiver distance' in the Measurement window. Normally this option is used to skip response peaks caused by crosstalk between output and input lines.

Is it possible to determine other energy ratios?

It is possible to get any energy ratio, by setting the appropriate ER integration intervals in the Parameter Setup window.

What are the differences between Dirac and competitors?

The most important difference probably stems from the fact that Dirac is developed by users wanting a user-friendly tool. The best way to experience the differences, is by trying out demo versions.

Does Dirac contain feature X?

Dirac may not yet contain your favorite feature. Given enough interest, and provided the feature fits within the 'philosophy' of Dirac, we will implement it. Just send us a description of your favorite feature, and we will consider it for inclusion in one of the next versions of Dirac.

Do you have localized versions available?

No, currently Dirac is only available in an English-language version.

How much does Dirac cost and how to order?

Please contact your local HBK representative for purchasing information.


Worldwide sales and marketing are handled exclusively by Hottinger Brüel & Kjær. To order Dirac you must contact your local HBK representative. If you have technical questions regarding Dirac, you may contact us directly.