Which gate time for frequency response in REQ Wizard?

[Hannes_F]

Hi,

first let me say thank you for this fantastic program!

I would like to know how long actually the gate time is when measuring a frequency response. For example if I do a 1 MB sweep from 30 - 200 Hz (23.8 s) ... how many data points are that and how long is the gate time for each?

The background of this question is that I would like to know whether there is some sort of smoothing involved in the raw data, and how much it approx. would be.

Thank you
Hannes

[JohnM]

The sweep is used to derive the impulse response of the room, the frequency response is then derived by applying an FFT to that impulse response with date (window) settings that you choose via the Impulse Response Windows dialog. The raw impulse response is usually 128k points, spanning 2.73s at 48kHz sample rate. The impulse is located within that period so that there is approx 1s before the peak and 1.7s after.

[Hannes_F]

John,

thank you for your response. May I ask a little further?

JohnM wrote:

The sweep is used to derive the impulse response of the room, the frequency response is then derived by applying an FFT to that impulse response with date (window) settings that you choose via the Impulse Response Windows dialog.

Isn't this a little doubly? The FFT of a FFT should give roughly the same like the original function, so why not use the original sweep response? But maybe it has to do something with data storeage, so what do I know ...

The raw impulse response is usually 128k points, spanning 2.73s at 48kHz sample rate. The impulse is located within that period so that there is approx 1s before the peak and 1.7s after.

So that means that if I want to change the gate time and therefore the smoothing for the frequency response I need to choose a different Impulse Response Window before the measurement because later it cannot be altered any more?

Thank you
Hannes

[JohnM]

Hannes,

The impulse response is the time domain response of the room. Roughly speaking, it is obtained by taking the FFT of the captured sweep, dividing it by the FFT of the generated sweep and then taking the inverse FFT of the result. Once the impulse response has been generated you can generate frequency responses from it using whatever gate times you care to apply. You also have the option of using a long gate time and applying fractional octave smoothing to the frequency response itself.