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I use a PC to measure frequency response, timing, etc... People have always told me that I need to gate my measurements. Nobody has ever taken the time to explain gating to me in terms I can understand. Can someone put this into laymans terms?

Ge0
 

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Can someone put this into laymans terms?
Once the impulse response has been captured following a sweep measurement, you have the option of processing different portions of the impulse by adjusting a variable gate.

You can see the gate by examining the impulse response graph using the windowed checkboxes. They will show the width size and shape of the gate you have selected. The defaults gate type and widths are usually acceptable for most measurements. The reference position for the gate is the impulses peak at time zero. For a full range measurement, you would want the impulse window to cover the signal until it disappears into the noise. Using the IR Windows pop-up you can adjust the gates left and right window.

REW remains the same for taking the measurement, but once taken, you can work with the impulse response to set an appropriate gate time depending on the frequencies you're measuring. It can be as simple as fine tuning the gate to ensure you're not windowing any noise into your measurement. The impulse will change whether you're in a small or large room, so you may want to adjust the gate to capture the correct amount of signal before it enters the noise.

Let me give a quick example how you could use the gate if you wanted to measure a tweeters response without the effects of the room. A normal measurement in the room would include the speakers response, but in addition you would be measuring all the reflections to that response in the room - (You've seen what high frequency response graphs look like with the comb filtering).

Gating puts a limit on the lowest frequency and the resolution of the response. For example, if you were trying to limit reflections from surfaces 1 meter away, you would use a 6msec gate (d=(time*speed)/2), but this would limit the lowest frequency of usable response information to ~167Hz (1/gate time).

Simply open the IR Windows pop-up and select a gate that you want and watch the impulse response to ensure it's appropriate (with respect to reflections), then select Apply Windows and your response graph will adjust to that gate time response plot. The popup will show the frequency resolution for you, so you don't need to do the math. You're trying to exclude reflections from any surfaces nearby. It could be as simple as the mic stand, but predominantly hard surfaces such as the floor.

So, perhaps you select a left gate of 1msec and a right gate of 5 msec. A gate time of 6 msec would give you a reflection free time of about 1 meter. Remember the sound has to hit the boundary and bounce back. So, distance=(time*speed)/2 = (6msec*344m/sec)/2. Remember that a short gate time limits the lowest frequency you can get useful response information for. So 6 msec limits your lowest frequency to 166.67Hz (freq=1/period).

So, for reflection free measurements where your closest boundary is 1 meter away, you could use a 5 or 6 msec gate and check your response fairly accurately without smoothing for a measure from maybe 500Hz-20000Hz. Then for the bandwidth below 500Hz, you could measure from 0-500Hz and extend your gate out.

If you wanted an overall full spectrum measure, you could use a wide gate and use smoothing. A bit less accurate, but suitable to take a wide general look.

That should give you enough info to play around with it.......

brucek
 

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That should give you enough info to play around with it....... brucek
It most certainly does! Thank you so much for taking the time to craft that response.

The reason I asked to begin with is that I am trying to take somewhat accurate measurements in a highly reflective listening environment. I was trying to get a handle on what gating really was and how to properly set it up.

You have my gerbil wheel a churning now.

Now, can you point me to a post / link that describes in detail how to interpret what I am seeing in an impulse response plot? How do I distinguish real signal from probable reflection, or is there even a way to do so?

Thanks again...

Ge0
 

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how to interpret what I am seeing in an impulse response plot?
Well, the impulse response itself isn't the easiest of plots to interpret. That's why all the other plots that are derived from it are so handy. Again, remember that the gate will determine and limit your lowest frequency (the IR Window pop-up displays this frequency), so to get the best full range frequency response graph you can adjust the left window to include the signal just as it enters the noise. You certainly don't want to include any of the distortion harmonic peaks which are located to the left of the main peak. To the right of the main peak, if you are measuring full range, set the window as the signal enters the noise.

Otherwise, if you're interested in the response at the higher end, then you can back the right gate so you don't include any reflections (while you watch your IR Window frequency resolution display of the lowest frequency).

For reflections (which you can also see in the impulse response) are easier to see in the ETC plot. That will tell you where they are.
ETC plots shows the large direct pulse and then any reflections after that. Measuring the distance of the reflections from the main bang can be used to establish how much further the signal had to travel to reach the microphone. This knowledge allows you to determine where the reflection point might be. It could be a side wall, or a back wall, etc.

Use the REW measuring tool to tell you the distance. To use the tool simply place the cursor over the main pulse and hold down the Ctrl key and the right mouse button and drag to the pulse you want to measure. It will display the distance.

Here's an example of an ETC (showing the measure tool) where the extra distance the signal had to travel was 2.5 feet. This could be a back wall that was 1.25 feet away (2.5/2), or perhaps a side wall that required the signal to go an extra 2.5 feet to get to the mic. This would be a good spot to test treatment on to reduce the reflection.

ETC
ETC measurement reflection.jpg

The reflections cause the comb filtering in the response plots. It's quite easy to watch the response plot and adjust your right gate and see the comb filtering go away. You have to be aware of the IR Windows frequency resolution display of the lowest frequency while you adjust though.

brucek
 
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