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In Reply to: RE: Yes, Ethan and David have different viewpoints... posted by naimie2000@yahoo.com on December 25, 2007 at 14:22:40
"My head ends up roughly 2 feet off the back wall-- so that's awfully close to any absorption stuff".
Awfully close to any absorption stuff? Perhaps in terms of physical measurement but there's not going to be issues with going closer as far as getting the benefit of absorption goes.
But the page on the Audio Physic setup at the Immedia site says "The best listening position is 1 to 3 feet from the rear wall" and a little later "The close proximity of the head to the rear wall has two effects. At the room boundaries (walls) the room nodes are suppressed - because the sound pressure is high and the velocity is low. Sitting in the maximum pressure area gives the best perception of deep bass. Secondly, the reflections are shorter than the circumference of the head, so the brain cannot measure the time delay between the ears. When the brain cannot localize reflections it ignores them."
Now what they're talking about here is bass response and localisation issues. Ethan's concern is with comb filtering, a different problem. If you've followed a lot of the debate below between Ethan, Klaus R and I about reflections you will see some strong differences of opinion between Ethan and Klaus about the audibility of comb filter effects. The one text I have, Everest's 'Master Handbook of Acoustics" indicates that the thresholds for audibility of comb filter effects are those associated with the audibility of spaciousness effects and those effects are strongly associated with lateral (side wall) reflections. Everest also indicates that comb filter effects often are very troublesome in small spaces due to short delays so Everest definitely provides support for Ethan's position on comb filters rather than Klaus'.
Moving closer to the wall behind you should reduce the reflection delay and increase the intensity of any comb filter effects making them more clearly audible but moving closer to the wall may also put the reflection point close enough to the head so that the reflection is blocked and absorbed to some degree by the back of the ear which may block the straight line path between the wall and the entry to the ear canal.
Still, you're 2' from the wall and the Audio Physic method suggests a distance between 1' and 3' is ideal. You have room to play. Try moving your seat back and forward a little and see what the effect of that is. Find the location that pleases you most and then try that position with and without a pillow placed behind your head to absorb wall reflections. Try things with and without the pillow at your current distance as well. See which distance from the wall you prefer and whether or not you prefer things with and without the pillow. That will give you an idea about whether or not absorption on the wall will help and may result in a better listening position as well.
Note: if you prefer things with absorption behind you and you decide to treat the wall itself, you will need to treat an area wide enough to cover the reflection points and 6"-12" wider on each side in order to get the best results. If the absorption is placed directly behind your head you won't need to treat as wide an area because the reflections are converging towards the ear. You will need a smaller area of absorption to get the same benefit if the absorption is placed closer to the head and further away from the wall or, conversely if the reflection is placed on the wall and the listening position—and your head— is moved closer to the wall. The smaller the distance between head and absorption, the smaller the area of absorption required. The larger the distance between head and absorption, the larger the area of absorption required.
Doing this experiment will tell you whether or not you will prefer the sound with or without treatment behind you. If you do, you then have options about how you instal that treatment, on the wall or behind the head. If you go for behind the head then you want to insure that the treatment does not press on the back of the ears and change the contours of the ear 'flaps' because that will change the way they reflect sound into the ear canal and will change the tonality of what you hear.
One of the things I often advise when someone is considering room treatment is to try and find a way of experimenting with it using materials already available in your house. In this case you can experiment using a pillow behind your head or propping a pillow or two against the wall. Pillows won't be as effective as an absorbent panel but they should be effective enough to show an effect large enough to show you whether or not you want to do something permanent about treatment at this location and you can do the test easily, quickly, and at no cost. You don't have to guess about treatment at this location when it's that easy to confirm whether or not you want to actually go to the expense of buying an acoustic panel.
I can't do this experiment in my setup because of the L-shape of my room. The bend in the L is slightly behind and to the right of my right shoulder. I'm physically close to the wall but the bend results in a situation in which I'm unlikely to actually receive a direct reflection from the wall. The bend is far enough away that any reflections from the wall before the bend are going to pass in front of me and the wall behind me on the left side is the rear wall 10' behind me. The 20' or so greater length of the reflection path from that wall means that the level of that reflection is much lower than the level of the direct signal from the left speaker and comb effects will be minimised. An awkward room shape in my case certainly causes some problems but it probably works to minimise comb filter effects from reflections off a surface behind me so that may well be why I don't notice the issues that disturb Ethan.
David Aiken
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