The tone frequency in hz of the systembeep.
The frequence must be an integer between 37 through 32,767. By default, the frequency is 750 hz.
The following trial presents a beep at 2000 hz.
<trial beeptrial>
/ ontrialbegin = [systembeep.frequence = 2000]
/ stimulustimes = [1=systembeep]
/ validresponse = (anyresponse)
</trial>