After the previous thread, I picked up a couple of these, they seem quite well designed overall, the tri-tone is about 500hz and my 85yo mother can hear them quite well - BUT she cannot hear the voice, which is still at very high frequency probably averaging over 2khz.
The unit can be tested and muted by any (?) tv IR remote, and does not seem too sensitive to accidental activation.
$35 at Lowes.
The one feature I did not get is the "OneLink" that lets one alarm in your house set off all the others via 900mhz link. Nice feature, but doubles the price, and the house isn't all that large.