You know, there are some computer programs that act as a sweep generator and then record the output using a condenser mic. Some can compensate for room reverberations and they provide a nice graphical plot.
You need a good electric condenser mic like
Behringer ECM8000 from Parts Express.
You need a preamp that supports 48 VDC phantom power and a sound card in your computer. Software should be available free on line.
The whole process can be done in under 10 seconds using the PC. I have a similar setup. The Behringer mic is very good and cheap.