The Logsweep recorder plays a sine wave throught he loudspeakers which gradually increase in frequency from the low limit specified to the high limit. The sound is simultaneously captured by the measurement microphone and recorded. The computer is then able to analyze the recording and determine how the system would have responded to an impulse, which mathematically consists of all the frequencies in the logsweep played simultaneously.
In a theoretical ideal case, the transmitted signal would be the same as the recorded, and the impulse response would be very sharp and short. But all devices involved in the sound generation and the space itself distort the signal, all of which are factors in the impulse response.