Another option would be to use a spectrum analyzer and input white noise from a PC or something to get the audio frequency response. Alternatively, get an audio spectrum analyzer with a tracking generator.
In my own lab I have an HP 8556A/8552B spectrum analyzer plugin stack for an old 141T mainframe. The 8556A plugins are somewhat uncommon, but have a tracking generator built in. That said, a massive 141T from the 1970's is going to be quite a bit bigger than what you have in mind, so I suggest going the white noise route. given that a PC sound card can be used as a spectrum analyzer, you may have everything you need already.