Sound device

1. Will Dirac work with my sound device?
2. Which sound devices do you recommend for use with Dirac?
3. Do I need to calibrate my sound device?
4. How does sound device calibration affect measurement accuracy?
5. What if the sound device calibration fails?
6. How can I see whether a sound device is calibrated or not?
7. Why do all measured impulse responses look the same on a Dell Latitude 600?  

1. What are the PC requirements to run Dirac?
2. Which hardware system components can be used or are required?
3. Can I use a sound intensity microphone probe as an "omni-omni-directional receiver"?
4. Which brands of microphones can I use for the measurement of LF, LFC and IACC?
5. Is it useful to choose a head simulator with on-board A/D convertors?
6. Is it possible to use Dirac with a wireless transmission channel?
7. What to do if my new laptop PC has no parallel port for my dongle?

Measurement and analysis

1. What is the typical dynamic range of a measured impulse response?
2. What are the differences between impulse responses from MLS and sweep?
3. Why is the INR from sweeps higher than from MLS?
4. How does Dirac process recorded data upon import?
5. Which results can be exported to Excel?
6. How to interpret the noise level?
7. Can the impulse response starting position be moved?
8. Is it possible to determine other energy ratios?

1. What are the differences between Dirac and competitors?
2. Does Dirac contain feature X?
3. Do you have localised versions available?
4. How much does Dirac cost and how to order?

Sound device

1. Will Dirac work with my sound device?
In general: Yes. Dirac should work with any sound device that supports the Windows MME/wave API. Dirac requires that your sound device is full duplex, meaning it will playback and record at the same time. Almost all current soundcards, 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 soundcard calibration. A successful sound device calibration implies that Dirac will work with your sound device.

2. 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. Another fine device is the Edirol UA-25 which provides a phantom power supply. Lower cost devices that work well are for instance the Behringer UCA202 and the Terratec Aureon 5.1 USB MKII.
Please note that there are many more devices that work perfectly with Dirac. Use the demo version to test Dirac with your sound device.

3. Do I need to calibrate my sound device?
No, you do not need to calibrate your sound device in order to use Dirac. However, working with Dirac becomes a lot easier once your sound device has been calibrated. During the sound device calibration procedure, Dirac figures out how to control your sound device hardware. It also finds the optimum settings for the volume controls. Finally, the calibration procedure will correct the loopback frequency response of your sound device if required. 
You could of course set the volume controls yourself before every recording. However, it is almost impossible to reproduce the optimum settings for each measurement session.

4. How does sound device calibration affect measurement accuracy?
The sound device calibration has no impact on the measurement accuracy. However, because the THD+N (relative level of total harmonic distortion + noise) is minimised, the reproducibility will generally improve.

5. What if the sound device calibration fails?
There are a number of possible causes for a sound device calibration to fail:
1. The problem could be as simple as an incorrect connection between the selected input and output. Also make sure to use a (short) shielded cable, to minimize interference.
2. Not all sound devices support all sample frequencies. Many current sound devices have the hardware laid out for a single sample frequency (often 48000 Hz for AC'97 codecs), and use sample rate conversion algorithms for other sample frequencies. Sometimes these algorithms produce results that are unusable for Dirac.
3. Some sound device manufacturers make near perfect hardware, but manage to write drivers that make the sound device almost useless. Switching drivers may help in this case.
4. The load on your processor may be too high. The calibration procedure uses a lot of processing power. Running compute intensive programs in the background may prevent Dirac from completing the calibration procedure. In particular, we have seen problems caused by virus scanners running in the background.

6. How can I see whether a sound device is calibrated or not?
You can see whether a sound device calibration was performed in the Sound Device Setup window. If there is an entry in the combobox underneath the 'Use calibration' checkbox, then a calibration was performed. Note that this calibration will only be used if the option 'Use calibration' is checked. To see if a sound device calibration is active, you can open the Measurement window: the window caption will display the name of the active calibration.

7. Why do all measured impulse responses look the same on a Dell Latitude 600?
I have a Dell Latitude D600, which apparently calibrates the sound device correctly, but subsequently cannot do real measurements. The input seems to be shorted to the output, so no microphone signal comes in, and the calibration works fine even without loop-back cable! What to do? 
To circumvent this (sound device driver) problem, do the following (similar to a hint in the user manual for the case when there is no input): In the Sound Device Setup dialog window, change the Input Line from "Stereo Mix" to "Microphone" and then click Calibrate.

Notes
- The microphone input is dual channel at microphone level sensitivity, i.e. 50 mV max.
- Using a line input signal like from a sound level meter, turn the sound level meter gain down by 30 dB to avoid overload of the PC input.

Other hardware

1. What are the PC requirements to run Dirac?
The minimum system requirements are a 300 MHz Pentium® processor, Microsoft® Windows 2000/XP/Vista, 200 MB of available disk space, a SVGA resolution (1024 x 768 recommended), a CD-ROM drive and a full duplex sound device with support for the Windows MME/wave API.  A typical current laptop would therefore be more than adequate.

2. 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, e.g. when using a random noise source, which will occupy channel 2. 
The measurement of LFC requires a matched omnidirectional microphone pair at a fixed distance, such as a sound intensity probe. 
The measurement of IACC requires a head simulator.

3. Can I use a sound intensity microphone probe as omni-omni-directional receiver?
Yes (actually, they are equivalent).

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

5. 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 optimised. 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. 

6. Is it possible to use Dirac with a wireless transmission channel?
You can use wireless transmission of 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.

7. What to do if my new laptop PC has no parallel port for my dongle?
Instead of a parallel port dongle, you can also use a USB dongle. Please ask your local Brüel & Kjær representative.

Measurement and analysis

1. 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 upto 96 dB at 4 kHz and up, and 80 dB on average. Sound studios typically show 60 dB, and concert halls 50 dB.

2. 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, recognising impulse response details may be easier if the impulse response is affected by random noise rather than by a nonrandom but unknown parasitic signal.  

3. Why is the INR from sweeps 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.

4. 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 Options dialog box (Setup menu).

5. 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.
• Any single impulse response parameter table in the Parameter menu, by saving the table in Dirac and opening it as .txt file in Excel.

You cannot export the Enery-Time Curve, the Decay Curve, the Lin Spectrum or the Log Spectrum.

6. 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. 

7. 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 not affect the calculated parameters.

The red line indicates the start of the impuls 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 Options window. Normally this option is used to skip response peaks caused by crosstalk between output and input lines. 

8. Is it possible to determine other energy ratios?
It is possible to get any energy ratio, by setting the appropriate integration time in the Options dialog under the Custom Parameters tab.

General

1. 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.

2. 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.

3. Do you have localised versions available?
No, currently Dirac is only available in an English-language version.

4. How much does Dirac cost and how to order?
Please contact your local Brüel & Kjær representative for purchasing information.