The Echo files
The Brüel & Kjær Echo Speech Source Type 4720 is a small active speaker used for speech intelligibility measurements where the voice spectrum, level and directivity are important. It is typically used in situations where no PA installation is used (f.i. in classrooms), or where the acoustics of a room or the background noise influence the speech intelligibility of a person speaking into a microphone. The Echo contains several preprogrammed stimulus signals that are recognized automatically by Dirac. The Echo also has an output connector for playing these signals over a PA system. Here you can download the same stimulus signals as used by the Echo. These signals can be played from an MP3 or CD player through a PA system to perform accurate speech intelligibility measurements.
The speech intelligibility stimulus signals can be written to an audio CD, copied to a smartphone or to any other device that can play them. The following signals are available:
| # | Name | Sequence [s] | Duration [m:s] | Description |
|---|---|---|---|---|
| 1 | intermittent | 10.92 | 10:00 | Intermittent Echo signal |
| 2 | continuous | 10.92 | 10:00 | Continuous Echo signal |
| 3 | voice | 10.92 | 10:00 | Male voice |
| 4 | stipa | 100 | 10:00 | STIPA signal |
Using the signals
Before starting a measurement the PA system's sound level needs to be set. The voice file (#3) can be used for this purpose. Play the voice file and adjust the PA system to a normal speech level. This is the sound level which would normally be used for speech. The speech level in the voice file is related to the level of the Echo and STIPA signals, so once the PA system is set as desired using the voice file, the stimulus files can be used for the actual measurements.
In Dirac, perform a level-calibration of your microphone or SLM and select the Echo measurement setup for IR measurements using the Echo stimulus files. No further options need to be set. Play the intermittent (#1) file over the PA system and start a measurement. The resulting window will show 2 channels. The top channel is the IR while the bottom channel contains the background noise.
Open the Speech table from the Parameters menu and verify that the INR values for all octave bands from 125 to 8 kHz are 20 dB or higher. If this is not the case then use pre-averaging to improve the INR. For the outer bands (125 Hz and 8 kHz) an INR somewhat lower than 20 dB may be acceptable because these bands do not weigh heavily in the final STI value. Particularly in the 125 Hz band an INR of 20 dB is not always easy to achieve due to background noise.
If pre-averaging is not enough to increase the INR to over 20 dB, you may be able to increase the PA system gain. Any gain increase of the PA system should also be entered as input gain on the Gain tab of the measurement window. In this case you should also consider whether the initial setup of the PA system was loud enough.
Verify that the impulse response fits within the 5 s window. If the impulse response (or the reverberation time) is much longer, you need to use the continuous MLS signal (#2). The background noise is not acquired automatically with this signal, so you will have to measure the background noise separately and enter these values in the measurement properties under the Speech tab. As a rule of thumb you should select an intermittent signal when SNR*RT < 120 dBs.
For measurements with PA systems that use compression, the STIPA signal (#4) may be a better choice. Measurements using the STIPA signal are Signal measurements in Dirac as opposed to IR measurements.
Note that you should not use the Echo files with a small speaker to simulate an actual mouth. Both the spectrum and level will be incorrect. Such a setup requires a careful equalization and calibration of the speaker, which has been done in the Echo speech source.
Dirac 6
Although support for the Echo was first added to Dirac 6, using the Echo with Dirac 6 requires the installation of the Echo support files.