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IR Length

Wolfenstein98k

Power User
How would you stop the reflections from the ground?

Keep in mind the speed of sound and the distance involved....

A good studio and an aeroplane hanger would yield similar results due to the speed of sound.

One is cheaper and more accessible though
That's why I said "an oval + pointed to the sky". Unless the volume has to be particularly loud?
 

jon

Fractal Fanatic
Sound travels in all directions.....it's like waves emanating from a source.....there WILL be reflections from the back unless the speaker is somehow only unidirectional which is clearly not the case. A speaker vibrates MOST of its energy to the front but there will also be sound generated to the back and sides as well, which will inevitably cause reflections.

These reflections happen so quickly (at the speed sound) that it's VERY difficult to avoid them.....sound travels so quickly it covers huge distances in milliseconds.
 

Wolfenstein98k

Power User
Sound travels in all directions.....it's like waves emanating from a source.....there WILL be reflections from the back unless the speaker is somehow only unidirectional which is clearly not the case. A speaker vibrates MOST of its energy to the front but there will also be sound generated to the back and sides as well, which will inevitably cause reflections.

These reflections happen so quickly (at the speed sound) that it's VERY difficult to avoid them.....sound travels so quickly it covers huge distances in milliseconds.
Yeah, I get that. I'm assuming that sound waves travel directionally though - ie the only reflections that hit the mic are those that bounced off a surface and then into the mic, which would be almost none of the emanating waves.

On top of that, surely the waves caught in any modern IR are orders of magnitude worse - they'd be bouncing off the floor and into a mic set an inch away within what, less than 2ms?

EDIT
Okay, so my idea is to suspend the amp and the mic away from each other by the requisite distance (say 20 feet), and off the floor and walls by (say) 20 feet.

This would leave a gap between the "direct" signal and the earliest possible reflection of >17ms.
 
Last edited:

Repartee41

Inspired
Sorry for the bump,
I read the first 7 pages of comments so sorry if some of this has been mentioned after, but I had a couple questions.

It sounds like the optimal IR length is just before the early reflections, so have all the IRs in the III/FM3 been adjusted to this optimal value on a per-IR basis?

Also, could recording outdoors be feasible in any way? How could one dampen as much noise as possible (wind, bugs, etc), and is this a superior environment to say a warehouse?
 

Justincase

Power User
"Anechoic" is only an approximation. All rooms have reflections, even anechoic ones.
I was just reading the Wiki
"Anechoic chambers are commonly used in acoustics to conduct experiments in nominally "free field" conditions, free-field meaning that there are no reflected signals."
They went on to say:
"In 2015, an anechoic chamber on the campus of Microsoft broke the world record with a measurement of −20.6 dBA."
So bill gates probably has the best place to do IRs

Lol
 

Flaggschiff

Member
I was just reading the Wiki
"Anechoic chambers are commonly used in acoustics to conduct experiments in nominally "free field" conditions, free-field meaning that there are no reflected signals."
They went on to say:
"In 2015, an anechoic chamber on the campus of Microsoft broke the world record with a measurement of −20.6 dBA."
So bill gates probably has the best place to do IRs

Lol
Who would have thought of this!?

@topic: very interesting. I will start to play around with IR lengths. Seems pretty reasonable to me. Thanks for sharing this information.
 

Repartee41

Inspired
For stock IR's, there's no easy way to determine where the first reflection is.
It looks like Cliff already has
For example, this is the plot of the "4x12 Brit 70s GB 313 A" IR:
View attachment 59676

IME that blip around 570 samples in or so is a reflection.

Plotting it using log magnitude is perhaps more illuminating:
View attachment 59677

You can see the energy decay and then pop up at around sample 570.

Here's another. This is an AC-15 cab:
View attachment 59681

Looks like a major reflection starting in the mid 300s!
Time consuming to go through each one for sure, but I was curious as to what Cliff was going to do with his findings here. I doubt he'd share them and then not act on it..
 

Jason Scott

Fractal Fanatic
It looks like Cliff already has



Time consuming to go through each one for sure, but I was curious as to what Cliff was going to do with his findings here. I doubt he'd share them and then not act on it..
The problem is, there's no easy way to get a stock IR into REW in order to analyze it.
 
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