Why is the 1st spike at zero?

[asylum]

I am just wondering why is it that the initial spike is at 0, would it not
make it easier to read the impulse response if the 1st spike was at the distance
the mic is at, that is measured in ms, so if my mic is 10 feet away, the 1st
spike is at 10ms, then reflections from that say floor 3 feet - next peak 13ms?

if the speeed of sound is 1130 approx: 1 foot per ms, why does REW move
the whole plot back to 0? would it not be better to have that distance show up
in the graph for mic to speaker measurments?

Maybe there is a fundamental reason for this, but i,m confused...


but getting there.

[brucek]

Maybe there is a fundamental reason for this

How would you establish when the signal left the speaker?

The reference is the signal reaching the microphone, so that is used as time zero.

There is a system delay feature in REW that can be used if the "Use Left channel as calibration reference" is employed. The timing difference of the loopback cable on the left channel in comparison to the right channel is the reference.

brucek

[asylum]

Thanks again, i still have a niggling problem i'd like to post, i hope i,m not
driving you mad,

[asylum]

Hi Brucek - How do i incorporate the system delay into my reading as below?
system delay 5.6 ms
system distance 6.25 feet = 6ms (approx)

here is a ETC
mic 6' (approx) from speaker
4' from floor
4'4'' from ceiling

Should there be a reflection at 10ms 6'+4'?

When i try and predict the reflection point, it
doesn't seem to tally with the actual measurment?

thanks

[brucek]

Should there be a reflection at 10ms 6'+4'?

Mmm, I don't think so.

At time=0, you will have the biggest spike since that is the direct sound from the speaker. It is attenuated the least and travels the shortest distance. That's time zero.

If there is a floor or a side wall 4 feet from the speaker, then I would say the next spike would show at ~ 8ms (4ms+4ms). And indeed I see you have a spike at 8msec.

How do i incorporate the system delay into my reading as below?

That's not really the purpose. The system delay gives a fairly true reading of a speaker from the listening position (mic), so you can set the time trims in your receiver. Be sure time is set to zero on the channel you are using for testing and run a full range sweep.

brucek