could someone explain this please? - Home Theater Forum and Systems - HomeTheaterShack.com

Old 11-09-08, 06:08 PM Thread Starter
Shackster
asylum

Join Date: Nov 2007
Posts: 53

In the rough diagram, just to show what i am actually measuring:
do i take the distance from the speaker to the wall,then add that to
distance back to mic, to calculate reflection point, or do i just, measure
the distance the mic is from the wall and calculate that, e.g

speaker from mic 4 feet
speaker from wall 5 feet

mic from wall 3 feet
1st reflection point = 8ms (5 feet to wall- then 3 feet to mic @ 1130)

or is it 3 ms (the distance the mic is from wall)

None of the measurments i take tally with reflection points in ETC?

i use the calibration on the channel/ and have the sound card calibrated,

help much appreciated.
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Old 11-09-08, 06:38 PM
Elite Shackster
brucek

Join Date: Apr 2006
Posts: 7,514
Re: could someone explain this please?

The shortest distance from the speaker to the mic offers the largest signal, and will be time = 0

Any reflection will be further than the shortest distance from the speaker to the mic, and will be time = (0 + extra travel).

brucek
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Old 11-09-08, 06:52 PM Thread Starter
Shackster
asylum

Join Date: Nov 2007
Posts: 53
Re: could someone explain this please?

so am i right in assuming i take the distance the speaker is to the wall and add that
to the distance mic is from the wall, so sound has to bounce off the wall and travel back into the mic?

or do i calculate just the mic from the wall, the reason for all these qustions on this, is
that i set up an experiment and was moving my mic in 1 foot increments and taking
measurments, but none of the results would give me the predicted distance, it was
always out by 1 to 2 ms (2 feet),
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Old 11-09-08, 07:17 PM
Elite Shackster
brucek

Join Date: Apr 2006
Posts: 7,514
Re: could someone explain this please?

Quote:
The shortest distance from the speaker to the mic offers the largest signal, and will be time = 0.

Any reflection will be further than the shortest distance from the speaker to the mic, and will be time = (0 + extra travel).
Just use the statement above.

So, in this case, the distance from the speaker to the mic will be time = 0.
The distance extra it has to travel in addition to the time = 0, is from the mic to the wall behind it and then back to the speaker.

brucek
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Old 11-10-08, 05:02 PM Thread Starter
Shackster
asylum

Join Date: Nov 2007
Posts: 53

Hi Brucek:
i have re-run all my tests again and still the reflection readout is wrong,
here are the specs it would be great if you could take a look,

i put my mic exactly 5 feet from the speaker, and the mic exactly 2 feet from
the wall, i was expecting a 7ms reflection?

mic to wall back to speaker = 7ms or do i include the out journey 2 ms = 9ms?

either way there is no reflection point at these times, the main one i have is

3.5ms?

this is definitly the wall but why is the time so out?

all help really appreciated.
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Old 11-10-08, 05:24 PM Thread Starter
Shackster
asylum

Join Date: Nov 2007
Posts: 53
ETC graph

this is the ETC.
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Old 11-10-08, 05:59 PM
Elite Shackster
brucek

Join Date: Apr 2006
Posts: 7,514
Re: could someone explain this please?

You're still missing the point I'm afraid.

As I wrote above, the reflection will show up as the extra travel distance in addition to the time it takes to travel the shortest distance from speaker to mic (which is time = 0).

Question: What is the extra travel in addition to the time it takes to travel the shortest distance from speaker to the mic?

Answer: The extra travel in this case is from the mic to the wall and back to the mic.

You can see I have been hesitant to simply give you the answer, since it will serve no purpose.

But, I have to assume I'm a lousy teacher, so the extra distance is 2 feet to the wall and then back, which equals around 4msec.

You have a peak at 3.5msec. That's your reflection from the wall directly behind the mic.

brucek

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Old 11-10-08, 06:16 PM Thread Starter
Shackster
asylum

Join Date: Nov 2007
Posts: 53
Re: could someone explain this please?

you have the patiance of a saint, just a lousy student, i got it now, so i just teke the the mic position from a boundry and double it and this will give me reflection measurment? why would it have been out by .5ms, is this usual,
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Old 11-10-08, 07:15 PM
Elite Shackster
brucek

Join Date: Apr 2006
Posts: 7,514
Re: could someone explain this please?

Quote:
why would it have been out by .5ms, is this usual
Don't really know. You could do some tests with known distances to see if it's because the relection point is so close, but it's usually fairly accurate. Remember the 1msec per foot is a rough approximation. You can get out your calculator (or REW measuring tool).

You do have to measure from the peak, so extend you graph axis out a bit and use the REW measuring tool to tell you how far it is in feet.

Place the cursor on the first peak, then hold down the CTRL button and sweep the cursor with the right mouse button and pull down a bit and see the nice calculation. Actually, I believe 3.5ms = 4feet. The speed of sound is 343.37 m/s at 20 degrees (or 1.1265 feet/msec).

See the example below of the measuring tool..... I pulled it out to 3.5msec as an example for you...

brucek

edit: here's another way of thinking about the distance you've been having trouble with. Think of stretching a string from the speaker to the mic. Now stretch another string from the speaker to the reflection point (on a side wall for example), and continue to the mic. The difference between the two strings will be the reflection distance on the ETC graph.
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Old 11-10-08, 07:39 PM Thread Starter
Shackster
asylum

Join Date: Nov 2007
Posts: 53
Re: could someone explain this please?

WOW, thank you so much for taking the time to get the point accross, really appreciate it, now off to measure some more measurments, this software is really amazing, i just cant belive its free, when others like it are in the hundreds of dollers region, also the forum great, i,m learning alot,
thanks.
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