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'Slap Echo' please help !

Discussion in 'Home Cinema Buying & Building' started by 0B1-1, Apr 19, 2005.

  1. 0B1-1

    0B1-1
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    Guys,

    I have a lot of slap echo at the screen and front speaker end of my home theatre garage conversion. Would a sheet of say 2" thick x 72" wide x 40" tall (approx) foam (ulphostery type?) behind the screen make any difference? (basically, the foam would be hidden behind my 72" wide 16x9 manual pull down screen)

    I don't want to put any stuff on the side walls if I can help it as the room is only about 8 feet wide. the rear of the room is quite deadk as this is where I have a large sofa. The room is carpeted and there is a deep pile rug near the screen running the width of the room
     
  2. mattym

    mattym
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    the slap echo is caused by the walls that you dont want to treat, you would need to do something on them to cure it though, you could hang carpet or rugs on the wall, that would do it(in the correct place of course)

    If you wanted to buy treatments to stop it, you could try our Profoam or Absorbor..

    if your interested, i have a review of profoam from way back in 99!

    *edit*
    our absorbors are only 25m thick, so you wouldnt lose too much room space...
     
  3. Gary Lightfoot

    Gary Lightfoot
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    I used ordinary cheap rubber backed black carpet from Carpet Right on my screen wall and grey carpet (to match the wall colour) on the lower half of my side walls. I used thick wallpaper paste to glue it so that it could be removed easily at a later date if required. It cured my room of the slap echo, but isn't scientific in application unlike Matts products which will treat a wider range of frequencies I would think. Pics via my link at the bottom.

    You can use a mirror to see where the first reflection points from the speakers would be and treat those parts of the walls only. Matts products would be ideal for that I would think.

    If I remember correctly, you use the mirror by sitting in your viewing position. and placing the mirror on the wall - move it until you can see the speaker and that's where the first reflective point will be, so that's the part of the wall that should need treating. If that's not correct, hopefully someone will jump in and tell me otherwise. :)

    Gary.
     
  4. 0B1-1

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    Gary, it all sounds a bit complicated. Are you saying that the foam behind the screen would make NO difference to the room acoutics at all? Like I said, I dont really want to put anything on the wall if I can help it, the problem is the costs (and the wife won't be happy!). I've just spend over £10k on the garage conversion (including the AV kit) and I would not know how much foam to buy or where it should go!
     
  5. Killahertz

    Killahertz
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    Yes, that's spot on (nice room by the way :) ). With the mirror on the left wall, come towards the listening/viewing position until you see the lefthand speaker reflected in the mirror, and mark the point. This is a primary point of reflection. You can continue towards the listening position until you see the reflection of the righthand speaker, and mark that point. This would be considered a secondary reflection point (in real terms, and in terms of treatment priority). Do the same for the righthand wall, as well as the floor and the ceiling. As far as the latter goes, if there is modest height, and/or distance from the listener, this is an ideal location to consider diffusion over absorption.

    That said, there is (I believe) a different philosophy to AV acoustics than Audio (as in domestic hifi). In treating mirror point reflection (primarily with absorption) it will (assuming the room otherwise has a modest amount of soft furnishing) help reduce reverberation, and hence slap echo. However, whether it needs to be so 'measured' is debateable. The use of carpetting on walls in the area of a screen is a common and effective approach. With less in the way of qualitive markers - in audio we know how a double bass should sound, but in AV how do we know what a passing spacecraft really sounds like? - it's almost certainly enough to concentrate on ironing out major anomalies. Rather than a measured and derivable acoustic response, ameliorated with tuned devices. There is certainly plenty of lattitude for experiment.
     
  6. 0B1-1

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    Killahertz,

    It's all getting far too technical and anally retentive for my liking. I know nothing about acoustics other than soft furnishings seem to deaden the echo in the rear of garage conversion room.

    If I could refer you back to my original thread starter, I have asked whether the foam behind the screen would make any noticeable improvement! any reponses would be appreciated.

    Going on from original question, if I was to buy some cheap bog standard upholstery type foam (say 2" thick x 12"w x 36"h) to stick on my side walls, how is one supposed to know how much quantity to buy and would it be any good in reducing this slap echo and 'liveliness' of the room?
     
  7. mattym

    mattym
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    If you treat the wall behind the viewing postition, it may reduce the slap echo a little, but the problem is that the sound from the speakers is hitting the side walls and reflecting back to the other side wall and back again, so treating the wall behind the screen would have minimal effect.

    If you use 2" upholstery foam it will only absorb certain frequencies depending on density, so yes, it may work, but how effective is another question. How much would that foam cost? would it be cheaper to buy a couple of sheets of foam designed for acoustic purposes? If you bought a slab of foam 1200x600 for each wall to place, as Gary correctly stated, at the reflection point, it will make a difference.

    If you want a professional looking foam, you could probably get you a couple of sheets of profoam for around £15 per sheet.....grey or purple...
     
  8. Gary Lightfoot

    Gary Lightfoot
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    Hi Killahertz,

    Thanks for the clarification and extra info - very informative. :)

    I've heard of two different methods employed by two proffessional AV room designers - one is to make the wall behind the screen dead, and the reamaining walls dead below ear height (and the floor). The rest is to be reflective, as a completely dead room may sound dull and lifeless. The other method is that all walls from floor to ceiling should be dead (and the floor), as you don't have these reflections in real life.

    I hadn't heard of the second approach at the time, so employed the first using carpet. It cured the slap echo and sounds fine to me, but it's a movies only room and I'm no audiophile. :)

    One problem I did have was that the rear speakers (of a 6.1 configuration) sounded as if they were coming from the front. I thought it may be a sound reflection coming from the screen, so tried angling them down a little with minimal effect, but it was only fully cured with bi-poles. I don't know if it was the screen being reflective or if it was just the effect that two speakers behind you can have at fooling you. Toeing them in may have worked but I didn't think to try that at the time :rolleyes:

    Obi,

    As I undertsnad it, for a multi-channel AV rom you should completely treat the entire screen wall. I can't remember the reasons for this though. It's up to you if you treat the remaining walls below ear height or completely, and I haven't heard a room that was completely treated so have nothing to compare to. If my room is anything to go by though, you should be able to eliminate the echo using either method. Using proffessional products or foam backed carpet may look nicer than the foam you mention, unless you intend on covering it with accoustic cloth which will give it a more finished look, but it involves more work.

    I have no real knowledge of accoustics, but after reading other peoples accounts of them using various methods including carpet (some of which looked very nice IMHO), I decided to try it myself. Like Matt says though, I've no idea what frequencies it was attenuating, so it was lucky that it seemed to work. :)

    I guess you could try the cheap carpety approach and that may be easier to velcro other treatments to if further attenuation is needed. Another poster called Godfather has a treated room using products like those Matt supplies, and it looks very good I think:

    http://www.avforums.com/forums/showthread.php?t=142256&highlight=pictures

    There are some better pictures of the room in other threads, but I think you can get a reasonable idea of what the products will look like once installed from those above.

    Gary.

    Gary
     
  9. mattym

    mattym
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  10. 0B1-1

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    Many thanks for all the replies guys. Gary, I took a look at godfathers jpegs - those grey sheets he has in his theater look superb, but pricey I bet. I wonder how many I would need to stop the echos! Also like his grey colour scheme in the room. :)

    I have a few jpegs of my room but I'm not sure how to upload them to the forum, they are big hi-res images about 1mb each jpeg!

    I think I need to decide how much of a sound improvement will be gained and balance this against the cost of the materials.
     
  11. Londondecca

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    The acoustics of the room will probably be the most important part of how your system sounds. It is worth every penny to get it right :thumbsup:
     
  12. mattym

    mattym
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    Have a look at the powerbuy section, we have our Profaom acoustic foam in there, acoustic absorption data is available through the links i put in there.
     
  13. mattym

    mattym
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    If you want to see a sample of the foam, i can get you a small piece in the post, or i can get you some high res piccies. let me know! :smashin:
     
  14. Gary Lightfoot

    Gary Lightfoot
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    Ob1,

    If you like, you can email the pics to me, and I'll post them here for you.

    My room is grey too - I colour matched a Kodak 18% grey card so any reflections will be neutral and not effect the screen image. I used black for the screen wall. I have some pics on my website if you're interested, and the grey paint code is Duluxe N6000.

    If you knwo where the main reflection points are, you can use a few absorber pads or absorber foam so the cost will not be too great.

    Gary.
     

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