An electroglottograph (EGG) is a non-invasive device that indexes the contact area between the two vocal folds. A small, high-frequency current is passed between two electrodes that are secured around the neck at the level of the larynx. The opening and closing of the vocal folds causes variation in the electrical resistance of the current. These changes in resistance are then displayed onscreen.
How an EGG works
While human tissue is a fairly good conductor of electricity, air is not. During phonation, the vocal folds (human tissue) are, at times, separated by the glottis (air). As the vocal folds move apart, the glottis opens, thereby increasing the electrical impedance across the larynx. When the vocal folds come closer together, the size of the glottis decreases, thereby decreasing the electrical impedance across the larynx. These changes in impedance are then displayed in a signal onscreen.
How to use an EGG
The lab is equipped with a two-channel electroglottograph (Model EG2, Glottal Enterprises). The instructions given below are based on instructions prepared by Glottal Enterprises, but with additional steps and helpful hints.
Here is what the front panel of the EGG looks like:
Figure 1: Two-channel EGG
Selecting the signal
Select one of the four signals, described below, using the “Output Signal” (knob #4 in Figure 1).
a. VFCA (Vocal Fold Contact Area)
b. IVFCA (Inverse Vocal Fold Contact Area)
The VFCA and IVFCA are the most common EGG signals.
c. DEGG (Differentiated EGG) – This output represents the differentiated EGG signal, in VFCA polarity.
d. Ext LF (Extended Low Frequency response) - This output allows you to observe the lower frequency components of the laryngeal movement.
In addition, you can also manipulate the low frequency limit (#3). According to Glottal Enterprises “the higher cutoff frequencies (10 Hz to 40 Hz) will allow the display to be better stabilized [but].... a slight waveform distortion...” will occur.
Setting up the EGG
Insert the electrodes into the port labeled “ELECTRODES” (#2) on the EGG. Connect the EGG box to the A/D device - usually the XAudio box (connected to the computer), but sometimes in the field a laptop sound card.
Turn the EGG on by turning the battery knob (#1) to either “A” or “B”, depending on which battery is currently charged. (If you select battery “A”, then “B” is being charged, or vice versa.) The light above the battery knob should now be green. If it is red, the battery you selected is not fully charged; in that case, just select the other battery. Even if the unit is plugged in, you must select a battery.
Cover the gold surface of the electrodes with a thin layer of electrode gel. A Q-tip works well for this. (If the gel is not available, a saline solution can be used to wet the electrodes.)
Place the electrodes on the subject’s neck at the level of the thyroid cartilage. (According to the directions from Glottal Enterprises, the left electrode (from the subject’s point of view) is the one with the black band of wires, though we found that it does not matter.) The white gap separating the gold-plated areas on the electrodes should be roughly horizontal, not vertical.
Make sure that the electrodes are positioned at the correct distance apart (on either side of the thyroid cartilage) for your subject.
Now that the electrodes are attached, you will need to make sure that they are in the correct positions. While having the subject produce a constant, prolonged vowel sound, in the pitch range to be tested, adjust the vertical position of the electrodes, so that the center green light on the “Electrode Placement/Laryngeal Movement” (#5) indicator is lit. If the electrodes are not in the right place, the bar will indicate whether you should move up or down.
Note: The indicator is very sensitive, and small changes in movement cause large changes on the indicator. I found that one (slightly) easier way to center the indicator was to start at an extreme point (e.g., the base of the neck), and then SLOWLY move upwards until the center green light is lit. I also found that positioning the electrodes was much easier to do on myself than on a subject. Remember that changes in pitch will change the position of the larynx, so it is important to keep this factor constant.
The subject should try to produce a strong enough signal so that one of the green lights on the “Signal” indicator (#7) is lit.
Using PCQuirer for the EGG
PCQuirerX, usually used to record from the EGG, has a very helpful feature - Scope Mode. You can find this under Record/Play > Scope Mode. Scope Mode will allow you to adjust the settings of the hardware, as well as the software settings (found under Options > Record/Play), before you actually begin the recording. Select Scope Mode and have the subject produce a constant prolonged vowel. You should see the signal in Channel 1. In this mode, you can manipulate the gain – either on the EGG itself (#6), or in PCQuirer under Options > Record/Play.
To record the EGG session, select Record under Record/Play (not in Scope Mode). If possible, have your subject begin the vowel before you actually start the recording. This produces a better recording.
To make an audio recording along with the EGG recording, use a microphone linked to the EGG via the interface box. You can change the gain in PCQuirer under Options> Record/Play. This option can be manipulated in Scope Mode.
Note about the version of PCQuirer for use with the EGG:
If you change any settings (such as gain, recording time, bandwidth, etc.), the EGG version of PCQuirer will save the changes you made, even after you exit the program. So, it is important to check most of the major settings before you begin recording. For example, if the person who used the EGG last made a recording that was 1 minute long, and you make a recording without checking the settings, your recording will have to be exactly 1 minute long, since there is no way to end the recording early.
Cleaning the equipment
When you are finished using the EGG, you should clean off the electrodes with an alcohol-wipe.
Understanding the EGG Signal
The EGG signal in the VFCA mode is an indicator of vocal fold contact area. The top portion of the signal reflects closing of the vocal folds, while the bottom portion reflects opening. The reverse is true in the IVFCA mode.
This webpage was created by Christina Esposito, with help from the EGG class (Fall, 2003).
New in 2010: EggWorks: A free program by Henry Tehrani, created for the NSF Voice project to analyze EGG signals (closing quotients, peak increase in contact) -- download EggWorksSetup.exe (current version is from July 2011), then run it to install EggWorks on your own Windows computer; documentation is included under the Help menu. Note that files to be analyzed should generally be under 5 min. long.