I want to obtain the impulse response of a space to calculate reverberation time using the Schroeder method. This space is characterized by the short reverberation time within 0.5 seconds due to the properly installed sound absorption materials.
Theoretically, I know that impulse response can be obtained using the inverse FFT of the frequency response function which is usually obtained from a dual channel FFT analyzer. This is theoretically very simple. I thought that there would be many factors that could cause errors experimentally, but there are not many references in the literature.
Anyway, I tried to measure the frequency response function (FRF) using a volume acceleration source (which can measure volume acceleration m3/s2) and a diffuse field microphone. These signals are measured by a Siemens LMS mobile scadas data acquisition system.
Test setting are given below:
- Sampling frequency: 40960 Hz
- Resolution: 0.3125 Hz (=Frame size is about 3.2 s), which is much longer than expected reverberation time.
Input signal for volume acceleration source:
- Burst random (burst time 70% is set)
- Exponential sine sweep (sweep time 70% is set.)
For both signals, a uniform window is applied and 15 averaging is performed to calculate the FRF.
As in attached Fig.1, it was confirmed that both FRFs measured with the two signals were very similar in magnitude and phase. Finally, the impulse response measured using the inverse FFT is shown in Fig.2. Of the total 3.2s of data, only the data corresponding to the initial 0.27s is indicated.
To summarize my question:
Is it reasonable to calculate the impulse response by the inverse FFT of the frequency response function whose unit corresponds to Pascal/volume acceleration? I have seen several papers that use an exponential sine sweep or maximum length sequences to obtain impulse response, but none of the papers mention units, so I am very confused. For example, is it okay to measure the FRF and find the impulse response by assuming the input signal as the voltage level of the signal transmitted to the power amp or speakers?
If there is something I have overlooked in the above method, please let me know. I'm not familiar with digital processing, so I'd appreciate it if you could explain.
If the method of obtaining impulse response using the inverse FFT is effective, is there any other reason to use an exponential sine sweep or maximum length sequences to obtain the impulse response?