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This manual is written for beginners, aiming at learning step-by-step how to measure the impulse response. We hope some opinion and advice in the manual would help you to operate the software correctly. Following Q&As and measurement example might be more helpful.

Related Q&A

• Which version of the software is suited for measuring the reverberation time from the impulse response?
• What is the difference of 1ch and 2ch in the impulse response measurement?
• In the impulse response, how can I read the phase delay in each frequency?
• Can I export a raw impulse response data (before normalization) as a .wav format?
• The impulse response measurement does not work. Please give me some advice.
• Can I compensate a PC's internal noise and a frequency response of microphone?
• I can't test or perform any analysis on my impulse response wave file

It would be appreciated if you could let us know your opinion and advice about this manual. Please contact us by email. 

Contents

1. Introduction
2. Features of RA's impulse response measurement
3. Measurement guide
   1. Connecting devices
   2. Input device setting and volume control
   3. Configuration of the measurement conditions
   4. Adjustment before the measurement
   5. Operation test
   6. Loading measurement data, importing the database
   7. Saving measurement data
   8. Deleting saved data
   9. Measurement data backup
4. Tips for the advanced measurement
   1. Using the measurement support functions
   2. Frequency response compensation by the inverse filter
   3. Check of Sound and Multimedia property
5. Measurement examples

1. Introduction

When the sound is emitted from the source in a room, the direct sound reaches to the listening point first. Then, the reflective sounds from the walls and the ceiling arrive. Finally, the sound waves which repeated multiple reflections on the surrounding walls reaches as reverberation. For example, when the impulse (a very short sound with a flat frequency response) is emitted from the stage of a concert hall, the time series of the reflected sound is recorded at the audience seat. This is called "impulse response". The impulse response contains the important information about the acoustical properties of a room. 

Impulse response is widely used for investigating the acoustical properties of sound fields, such as concert halls, recording studios, and listening rooms. Once the impulse response is gained, the transfer function (frequency response function) between the sound source and the listening point is obtained by the Fourier transform. From them, many valuable indices, such as the reverberation time and the absorption coefficient of walls, are calculated. The impulse response is also used for the auralization (virtual acoustic reality) and the variety of sound effects, by using the well established convolution techniques. Realtime analyzer is the ideal software for measuring the impulse response of the concert hall, listening room, and the audio equipment.

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2. Features of RA's impulse response measurement

• MLS and TSP measurement: In this software, the impulse response is measured by using the MLS or the TSP method. Measured data can be analyzed later by the analysis software (Sound Analyzer)  in a same manner. 
• Analysis of the room acoustics parameters (by SA):  From the measured impulse response, the important room acoustics parameters are easily analyzed by SA (Sound Analyzer)^*1. Sound level, initial time delay gap between the direct and the reflected sound, and reverberation time can be calculated from the monaural impulse response for each 1/1 or 1/3 octave band, and IACC can be calculated from the binaural (using dummy head or real head recording) impulse responses. Calculation of the standard room acoustics parameters according to ISO 3382 (strength, T30, EDT, clarity, center time) is also available.
• Measurement support functions: The software is equipped with the measurement supporting functions for beginners. Check the "Auto Retry" and "Auto Level" button to use these functions. In the "Auto Retry" mode, measurement is repeated twice and the accuracy of the measurement is checked by comparing with the test measurement result. When the "Auto Level" is checked, the output volume is adjusted automatically so that the S/N ratio is improved. The output volume is increased within the range that the input signal doesn't become saturated. If you turn "Auto Level" off. You can adjust sound volume (output level) using Windows sound volume control. Also, compensation of the loudspeaker and the microphones' frequency response is possible by using the "inverse filter" function. 
• 96 kHz sample rate supported^*2: If your sound driver supports 96 kHz sample rate, the impulse response is measured with 96 kHz sampling. Even if the sound driver does not support this sample rate, 96 kHz wave file can be loaded in RA and can be analyzed in SA. As a 96 kHz sampling sound device, you can use e.g. Roland UA-5. 
• 32 bit .wav format supported^*2: A .wav format sound file with arbitrary length can be loaded in RA. So the impulse response measured by other software can be analyzed by SA (Sound Analyzer). Now, 32 bit .wav format is supported by the Impulse response and the Running ACF measurement. 

*1. SA (Sound Analyzer) is not included in RAL, DSSF3 Light, and DSSF3 Realtime Analyzer. Use RAD, RAE, DSSF3 Full-system version, and DSSF3 Environmental Noise.  See the function list  to know more about available functions in the products. 

*2. These functions are only available in the latest version of RA (Ver.5.0 or later). Check the version of your program in Help menu > Version Information. If you want to upgrade to the latest version, contact us via e-mail ymec@ymec.com.

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3. Measurement guide

3-1. Connecting devices

Connect the microphones and loudspeaker to the Input and Output of your soundcard as shown in the figure below. The binaural impulse responses can be measured if you use a pair of two microphones, but the monaural impulse responses can be measured by only one microphone. Remember to connect the loudspeaker to the left channel, because the measurement signal is outputted to the left channel. 

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3-2. Input device setting and volume control

First, select the input device (line, microphone, etc) and sound device (soundcard) in the main window of RA. This selection is interlocked with Windows' recording control. Usually, line-in or microphone is used. 

Next, output device has to be set as follows. Open the Windows Volume Control and check Mute except for Wave. Otherwise, input signal from the microphone or the line-in is outputted directly and the howling might occur. Also, do not use the advanced controls, especially the tone controls. If there are excessive sound effects, it cannot be measured correctly. Enlarge the volume as much as possible in the range which is not distorted. Distortion is fatal in a measurement system.

If you still have problems, it is difficult to perform the impulse response measurement by your PC. Please contact us. Perhaps we can give you suggestions depending on your situation. 

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3-3. Configuration of the measurement conditions

Basically, the measurement can be performed correctly by the default setting. Very clean impulse response can be obtained using a standard computer, soundcard, and microphones. Once you confirmed your PC works well, it is better to preserve the setting until the measurement ends. Impulse response measurement requires all the equipments to work well together. If you change the settings, it is difficult to reset them just before the measurement. 

If you find difficulties, follow the instructions below.

The important settings in RA for the impulse response measurement are Sampling Rate, Measure Time, and Summation. It is briefly explained in the following.

Sampling Rate
Select as much high sampling rate as possible. RA works with up to the maximum sampling rate supported by the soundcard. But generally, 44 kHz or 48 kHz is enough. 

Measure Time
To measure the impulse response correctly, measurement time should be set longer than twice of the reverberation time of a room. The longer the Measure Time is set, the more the Signal to Noise (S/N) ratio is improved. 

Summation
Background noise has no regularity in time, so the summation does not increase its sound level, while the level of the impulse response is increased by the synchronized summation. Consequently, the S/N ratio is improved. When the summation is set to 2 or 4, the S/N ratio improves 3 or 6 dB, respectively. However, note that the total measurement time becomes long, when the larger number of summation is executed. If the obtained impulse response is noisy, we recommend to try TSP method rather than M-sequence (MLS). 

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3-4. Adjustment before the measurement

The following adjustment is not necessary if you can measure correctly. If you find difficulties, follow the instructions below.

RA has a function to check the measurement conditions. If it were not good, it can be adjusted automatically. Click the "Adjustment" button on the Impulse response window to open the Adjustment dialog as a figure below. Items to be checked are "Input Output Level", "Mic. Position", and "Measuring Time". They are displayed OK when the settings are correct. 

Input Output Level
It is checked whether the input and output sound level is in the proper range. If it is not displayed OK, check the following problems. 

1. Input volume of the amplifier is too small (or too large). 
2. Output volume of the soundcard or the loudspeaker is too small (or too large). 

First, adjust the volume of each equipment and retry the check. If OK still does not come out, suspect the distortion and lower the reproduction volume in the volume control. When the Auto Level box (on the Impulse response window) is checked, the software outputs the measurement signal as 70 % of the maximum volume. 

Mic. Position
When you perform the stereo/binaural measurement by using the dummy head, the head should be directed to the sound source precisely. If you are using a pair of microphones without the head, two microphones should also be placed at exactly the same distance from the sound source. In this test, the levels from two microphones are checked. Adjust the position and direction of the microphones until it is displayed OK.

Measuring Time
If the necessary measuring time is not clear, it can be checked by this test. The proper measuring time is decided based on the S/N ratio. In the actual measurement, set the longer measuring time than suggested. 

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3-5. Operation test

Select the Channels (1ch or 2ch) and Method (MLS or TSP), and click the Start button to start the measurement. Test signal is generated and the impulse response is measured automatically. 

The example as shown in the figure below is the impulse response of the output signal itself, measured by selecting the input device as Wave. In this case, the signal is inputted without any external device. So the result should include only one pulse and elsewhere should be flat. Otherwise, the settings of the input device and the volume control might be wrong. 

Click the Play button to listen to the measured response. By listening to it, the problem of the measurement which is not known only by seeing a waveform may become clear.

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3-6. Loading the measurement data, importing the database

Click the "Load" button to load the impulse response which has been already measured and saved in the database.  On the "Load Measurement Data" dialog, select the folder and data from the list, and click the Load button.

Also, the WAV file can be loaded by clicking the "Wave File" button.

On the "Load Measurement Data" dialog, click the "Import" button to load the measurement database. When selecting the folder in which the measurement data is saved, a file "FOLDER.DBF" is displayed. Select this file and click the Open button. By this operation, the previous measurement data is added to the current database. It is useful for comparison of different measurements at a time. 

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3-7. Saving measurement data

Once the impulse response is measured, it can be saved to the database. The acoustical parameters can be calculated by using SA (Sound Analyzer). 

*See the SA program manual for how to analyze the impulse response.

Click the "Save" button on the Impulse response window, then the "Save Measurement Data" dialog opens as below. To save data, 1) enter the title and comment on the data, 2) select the folder, and 3) click the "Save" button. If there is no folder, or you want to save data in a new folder, you can create a new folder. See below.

When you make a new folder, click the "Folder..." button on the Save Measurement Data dialog. Folder Management dialog opens as in the figure below. Enter the folder title and click New button.

Also, the measured response can be saved as a WAV format. See the "Output Wave File" on the Impulse response window. Select "Impulse Resp." and "Binary", click the "Output" button. Output signal and input signal used during a measurement can also be saved. Saving data as a text file is also possible. 

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3-8. Deleting measurement data

Click "Load" button. Folder and Measurement data are displayed. To delete a specific data in a specific folder, 1) select the folder and 2) select the data, 3) then click the "Delete" button. 

To delete a whole folder, 1) select the folder and 2) click the "Delete" button. 

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3-9. Measurement data backup

Measurement data of Realtime Analyzer, Sound Analyzer, and Environmental noise Analyzer, and the related data such as the microphone calibration data, the inverse filter, and the noise source template, are stored in the common database. Below, it is explained how to manage the measurement data and delete the stored data.

All of the measured data and the microphone calibration data are stored in the DATA folder in the program folder (e.g. DSSF5E). The database of DSSF3 is 64-bit, and the maximum data size of 2000 GB can be stored. The database of RAE, RAD, and MMLIB is still 32-bit at the time of default installation, but once you install the DSSF3, these software can also use the 64-bit database.

On-line update of the program never erase the data you have stored in the DATA folder. However, when you "re-install" the program, all the data is erased, because the sample data overwrites the existing data. To protect the already stored data, you have to backup the existing data before reinstallation. How to backup data is as below.

1. Rename the DATA folder (for example as DATA1). 
2. Install the program.
3. Delete the automatically created DATA folder and rename the folder DATA1 to DATA.

This backup operation enables you to use the existing data after reinstallation. 

New function of DSSF3 version 5: Setting Utility to specify the data folder

In the RA series (RAD and RAE), measurement data is always stored in the "DATA" folder. To backup the data, this DATA folder has to be moved to the other location or renamed every time. But in DSSF3, this problem has been solved. If you are using the DSSF3 version 5, you can specify the data folder by using the setting utility (DSS.exe). Wherever the measurement data is saved (even in the other computer on the network), DSSF3 can use this data. By this function, management of the measurement data becomes much easier. 

1. Run the DSS.exe and click the Reference button.

2. Specify the measurement data folder, then the file FOLDER.DBF is displayed. Select this file and click the Open button. 

3. Click the OK button. Now the data in the specified folder is used by the DSSF3. 

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4. Tips for the advanced measurement

4-1. Using the measurement support functions

This software is equipped with the measurement support functions, such as Auto Level and Auto Retry. If these functions are used, measurement quality is checked and repeated until a good result is obtained. However, in a normal condition, clean impulse response can be measured with these settings off. 

Auto Level
When using this function, the input and the output level is adjusted by the short test signal. Then the measurement signal is outputted to measure the impulse response. If you do not use this function, adjust the output volume in the Windows volume control. Normal output volume range is around 70% of the maximum. 

Auto Retry
When using this function, the measurement is repeated twice. Cross-correlation of these two measurements is calculated and the time delay of its peak is checked. If the time delay is large (i.e. two measurements are shifted in time), measurements are repeated until it passes the check. 

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4-2. Frequency compensation by the inverse filter

It is ideal if a quality microphone and a loudspeaker are used. However, even if it is difficult to use such superior equipments, the measurement accuracy can be improved in RA by using the correcting function.

Correction of the measurement system by the Inverse filter

The response of the measurement system itself (including microphone, amplifier, loudspeaker, and PC), can be compensated by creating the inverse filter. By removing the response of the measurement system, the correct response of the sound field can be measured. Procedures for creating the inverse filter is described below. 

To create an inverse filter, measure the impulse response close to the loudspeaker. This is to remove the effect of room. Impulse response just after the direct sound represents the response of the measurement system. 

1) To create an inverse filter, place the microphone close to the loudspeaker and measure the impulse response. Then, click the "Save" button for "Correction". The "Save Filter" dialog opens.

2) In the "Save Filter" dialog, responses of the loudspeaker and the microphone is cut. Select only the direct sound from the impulse response. Input the Maximum Level which should be compensated. In this example, this value is set to 30 dB, but there is no limit. However, note that so large peaks or dips in the frequency response could not be compensated. After the adjustment, enter the title and comment, and click the Save button. 

3) Click the "Setup" button for correction. "Setup Filter" dialog opens. Select the filter and click the OK button.

4) Then, start the measurement again. You will see the response of the measurement system is now removed. 

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4-2. Check of Sound and Multimedia property

For the impulse response measurement, more attention is needed for the settings of sound driver than in other measurements, because the impulse response measurement needs the synchronization of input and output. If you have any trouble in the measurement, check the version of the sound driver first. We recommend to use the latest version of the driver. Then check the following settings. 

Open the Sound and Multimedia Properties control panel and click the "Advanced" button. (Sorry that the images below are of Japanese OS)

Open the Performance tab, set the "Hardware acceleration" to None and the "Sample rate conversion quality" to Best. 

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5. Measurement examples

Measurement examples by RA can be seen in the Introduction to Sound Measurement. For the impulse response measurement, see the following:

Measurement of PC's soundboard by RA 5

Audio measurement report 2

Audio measurement report 3

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Y Store.

About this manual  Introduction of RA  Operation Guide  Impulse response  Running ACF  Reference Manual  Appendix

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feel free to contact us by email ymec@ymec.com or by online inquiry form.