Speech Intelligibility

In order to obtain quantitative data on the quality of our drive through system, we needed to make measurements of speech intelligibility. There are two often used speech intelligibility standards: the speech intelligibility index (SII) and the speech transmission index (STI). The SII is an American standard, while the STI is international. Both take some form input with noise and signal, and output a number between 0 and 1, with 0 being extremely poor intelligibility and 1 being perfect intelligibility.

The speech intelligibility index and ways to measure it is found in the ANSI S3.5-1997 standard. There are three methods given in the standard for calculating the SII. We will be using option two, which takes a noise and signal that may be correlated (any electronic noise would be correlated with the signal) to determine the "equivalent speech spectrum." This method finds the signal to noise ratio using a modulated transfer function, and finds the (noise)+(signal) spectrum level in several frequency bands from part of the calculation of the signal to noise ratio. These two are combined to calculate the equivalent speech spectrum and the noise spectrum levels. These values are then manipulated to obtain an SII value from 0 to 1.

It should be noted that we will not be able to measure the intelligibility of current drive through systems to compare to ours in the foreseeable future. Therefore, we cannot definitively say that our system is better than existing ones. We will, however, be able to determine what kiosk setups (microphone placement, speaker placement, foam insulations, etc.) give better results and make a recommendation for the ideal setup. Most of the work done on Speech Intelligibility was carried out by Kevin Gullikson.