"First reflection"...what it means, why it's important, how to find it
- Thread starter tomlinmgt
- Start date
Here ya go, it's addressed in this thread....
http://www.audiokarma.org/forums/showthread.php?t=411360
- Michael
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Thank you and welcome aboard the party boat of superior sound quality! Our motto is simple....no matta' what ya' got, acoustic room treatments will make it mo' betta'!
cool9
Super Member
Can you explain sound behind the speaker? Do the first sound waves behind the speaker come directly from the drivers or is that sound that has bounced off of other walls? I just have trouble picturing sound behind the speaker coming directly from the woofer/tweeter. I've taken enough physics and math and harmonic motion but acoustics is not my strong point.
The first reflecting wave on the ceiling or floor from a speaker should be exactly half the distance between you and the speaker (if your ears are close to same height as mid-speaker or tweeter). Walls are different depending on your position in the room.
Nice thread. Check out acoustic ceiling job below -
The first reflecting wave on the ceiling or floor from a speaker should be exactly half the distance between you and the speaker (if your ears are close to same height as mid-speaker or tweeter). Walls are different depending on your position in the room.
Nice thread. Check out acoustic ceiling job below -
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That ceiling is bad ass. I was recently in a small auditorium that had a ceiling that looked like inverted small rolling hills. I got a huge, shit eatin' grin on my face every time I looked at it.
Just stand behind a speaker sometime and you'll hear output. I've been told the majority of the content is reflected energy and a portion direct from the drivers......LF energy especially.
Thanks for the compliment, BTW.
- Michael
boojum
AK Member
An experiment in creating an echo-free listening room; In my college years, I'd gotten the idea that the best listening environment was one where all reflection had been eliminated by lining all surfaces with sound-absorbing material - maybe from an image of a recording studio or something. So I haunted the grocery store collecting those purple or green divider sheets used for packing apples and other fruits - remember the big squares made from thick paper pulp with pockets like a giant egg carton?
I completely covered the walls and ceiling with the things, edge-to-edge. The floor was carpeted. Furniture consisted of bean bags (in the day, right?) and the cinder block and board shelf where the turntable and receiver sat. It was definitely a strange and unique room to walk into, both visually and audibly, where a cough or a spoken word fell still as a graveyard at midnight. Not only was the sound dead, dead, dead, but the landlady had a very chameleon-like reaction to the decor, turning shades of purple and green herself.
Alas, my experiment had failed. Obviously I should have started removing the packing dividers a little at a time until sonic bliss was achieved. The landlady insisted, however, that they all come down, and that was the end of that.
I completely covered the walls and ceiling with the things, edge-to-edge. The floor was carpeted. Furniture consisted of bean bags (in the day, right?) and the cinder block and board shelf where the turntable and receiver sat. It was definitely a strange and unique room to walk into, both visually and audibly, where a cough or a spoken word fell still as a graveyard at midnight. Not only was the sound dead, dead, dead, but the landlady had a very chameleon-like reaction to the decor, turning shades of purple and green herself.
Alas, my experiment had failed. Obviously I should have started removing the packing dividers a little at a time until sonic bliss was achieved. The landlady insisted, however, that they all come down, and that was the end of that.
RevMen
The Reverend Menacer
The only thing you could do with absorption to address this is make both upper corners very absorptive to remove them from the equation. It's probably not worth doing unless you're specifically hearing a left-right difference due to this.
cogitech
Full Ranger
Mine too. Also, this rule of thumb does not account for toe-in.
Again, first reflections don't move with different dispersion patterns. The shortest path from the speaker to your ear is always a straight line(s) from the driver(s), and the second shortest path is always the same one-bounce path. Sure, it varies a little bit from each driver if you've got a multi-way speaker with wide driver spacing and you're listening from pretty close, but it has absolutely nothing to do with the dispersion pattern / polar response / etc. Those pattern-related characteristics can only effect the magnitude and frequency content of the first reflection, not the location/angle of it.
Another way of stating it: speakers can't throw curve balls.
Another way of stating it: speakers can't throw curve balls.
Thanks dump. Definitely couldn't have said it better myself.
cogitech
Full Ranger
I suppose it's counter-intuitive because dispersion plays such a huge role in interaction with the room, but it doesn't change the angles of reflection. How about a sketch?This is counter-intuitive to me, but I'll take your word for it.
You might realize that the true first reflection might happen off the chair or listener's body, but we're talking room boundaries here.In the second example, the first reflection would have significantly lower treble and upper-midrange magnitude, but it's still in the same place because I haven't moved the speaker.
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1audiohack
Active Member
That's very true. What I am wondering is how do you examine the spectral content of the reflection? Without that information aren't we guessing at for instance, how deep or thick our absorber needs to be?
An example, when measuring my living rooms initial time gap at the general listening position my first significant reflection was off the ceiling. It's a standard eight foot ceiling with two fans and there is not going to be any treating this. The spectral content of the reflection was 7kHz and up. The fix was to replace the JBL 2404's with its 100X100 degree polar pattern with a pair of 2405's with a 90+X 25 degree pattern. That got me a clean ITG of over 6mSeconds.
How are you guys measuring stuff like this?
Thanks and all the best,
Barry.
An example, when measuring my living rooms initial time gap at the general listening position my first significant reflection was off the ceiling. It's a standard eight foot ceiling with two fans and there is not going to be any treating this. The spectral content of the reflection was 7kHz and up. The fix was to replace the JBL 2404's with its 100X100 degree polar pattern with a pair of 2405's with a 90+X 25 degree pattern. That got me a clean ITG of over 6mSeconds.
How are you guys measuring stuff like this?
Thanks and all the best,
Barry.
Good question and my answer is....I'm not. The way I understand it the objective with first reflection absorption is to first surpress as much refected spectral content as is possible from the boundary layers in front of the plane of the speakers. I think I understand you wanting to create an ITG of over 6 ms from my involvement with dipole designs as this is the minimum delay required to realize the sonic benefits of the dipole effect. But I would think there's a big difference between getting proper delay from a reflection originating behind the plane of the speakers as compared to one originating out in front of and above or to the sides of the speakers since the desired result of getting the proper arrival time of the reflection is to embellish sound stage ambiance and dimensionality. Having this reflected information originate primarily from the sound stage and its immediate proximity would, to me, seem much preferred to having it originate from side or overhead boundary layers. But I don't want to come off as preachy as I realize integrating deliberate and proper diffusion provides added benefits....if not a bit more complexity to the scenario. So I think your practice of measuring reflected spectral content is quite valid and useful, if not a bit misplaced.
BTW...if you only have reflected content off your ceiling above 7kHz to tame you could do that fairly effectively with only a few inches of OC703 mounted against the ceiling. Surely your fans aren't standing off the ceiling only 2".
- Michael
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