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Low Frequency Sweep - Problem at 50-55Hz

Discussion in 'Home Cinema Speakers' started by Apocalypse, Oct 24, 2002.

  1. Apocalypse

    Apocalypse
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    Owners of Avia will know that the disc contains low frequency sweeps that are labelled onscreen so you know what frequency you are hearing. Thanks to the helpfull sweep I now know where my low frequency sweep really peaks, it's from 55hz to 50hz. It can go from 70db to 88db during that drop of 5hz, the same happens if I do the sweep on the front mains (if set to large) so I'm assuming the problem is with room acoustics.

    1, Can I cure/lessen the anomaly with things like more soft furnishings/bass traps?

    2, If it's not room acoustics then what other possible cause could there be and resultant cure?

    Incidently from 49Hz to 20Hz the db level is constant with maybe a 1db variation. The sweep was played at my normal listening volume of -15db on the Marantz AV9000 (which works out to about 75db on a spl), the readings were taken at the position my ears would normally be.

    The gear :

    Power Amp - Rotel RMB 1075
    Pre Amp - Marantz AV9000
    Sub - HGS18
    Front Mains - Paradigm Reference Esprit (flat to 40hz), these are normally set to "Small" though anyway I just wanted to see if the bass anomaly was present in these as well as the sub.


    Room furnishings are 50/50 re soft furnishings, there is a bed, a rug, curtains and a small rug under the sub. If needs be I can place a large curtain behind the front soundstage (if this will help)

    Any advice will be appreciated or if anyone had similar problems but cured them please post away :)

    Regards
    Phil
     
  2. GaryG

    GaryG
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    Phil

    The following link (you might have seen it already in another thread) will help you position you sub/speakers. There is also a spreadsheet on the site which will calculate the room modes for you.

    www.harman.com/wp/pdf/Loudspeakers&RoomsPt3.pdf
     
  3. Couch Potato

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    I had =/- 12db in a 16-80hz sweep with the main culprit being a huge peak at 35hz, I played around with position etc but it became fairly apparent that it was a room issue. I bought a Behringer Feedback Destroyer from here and I've now got 16-80hz =/- 3db :D

    It took me a couple of hours of playing with the filters but it's an awesome piece of kit.

    Steve
     
  4. Apocalypse

    Apocalypse
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    A Parametric Equalizer does seem my best option short of massive expenses on sound absorbers. There are 3 films where even if the bass is below normal volume it booms on these certain scenes (Godzilla - Chapter 12, when Godzilla ducks and the missiles hit the building behind him, the explosions sound boomy and unclean) (Armageddon - Chapter 26, when the shuttle blasts off from the asteroid and mission specialists in Houston say "Freedom has Max Thrust" the sound from the rockets is just boomy) (Titan AE - Chapter 16, when the first ice shard collides and heads towards the ship the sound again is boomy and unclean).
    I'm guessing/hoping that these sounds are in the 55-50hz range which makes them sound boomy so if a parametric equalizer can reduce those peaks at 55-50hz I'll defo get one. Can anyone else confirm if the above sounds are simlar on your setups in those scenes or is the bass normal (not boomy) which will confirm that it is indeed my problem only. Those are the only scenes that make this boomy sound, everything else is clean and tight as it should be with a HGS18.

    Couch Potato - I assume the Parametric Equalizer connects to the pre amp via XLR cables, can you state how please as I'm more than likely going to purchase it.
     
  5. Couch Potato

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    You can use XLR or phono, I connected the sub out from my AV32R to the unbalanced input in the BFD and output to my sub cable. The connectors on the back of the BFD are 1/4" I think so I used a couple of phono-1/4" gold plated converters.

    To accurately measure and setup you're best using the comprehensive setup guide foundhere and use the test tones and plot your graph as suggested in the setup. I followed those instructions and it took me a couple of hours. It was well worth it.

    For £100 it is extremely good value!!

    Good luck.

    Steve
     
  6. Apocalypse

    Apocalypse
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    I placed lots of spare plasterboard and foam behind and around the sub and did the sinewave tests. Here are the results :

    [​IMG]


    I swore I'd never do another one of these after the stick I got last time but I needed to see what results I got. So as to answer your inevitable criticism beforehand I'll explain how I did the tests.

    The sinewaves were burned onto a CDR which was played in my DVD player, the pre/power was set to the Stereo mode and at a volume of -18db which is equivalent to a pink noise of 75db. I used the digital version of the Radioshack spl which can be set to record the average sound (preferred for low frequencies), the spl was held at the position my head would normally be at in an upright position but angled slightly forward.

    The speakers are set to the "Small" mode.

    The gear : Rotel 1075 Power amp, Marantz AV9000 Pre, HGS18 and Paradigm Reference Esprit speakers.

    Conclusion

    If anything the graph has showed that my setup performs well below 60hz but weakens above 60hz, I'm at a loss to explain this but maybe someone else could step in. In the stereo mode and with the speakers set to small is the crossover still 80hz or is that only in THX mode because I've noticed that for music in stereo mode the HGS18 performs better using it's own crossover set to say 40Hz and the front speakers set to Large but for movies it's far better setting the front speakers to small and using THX crossovers on the pre amp. I guess I'm trying to get the best of both worlds here, music and movies :rolleyes:

    P.S - I know the graph isn't an accurate portrayal but it's helpfull for showing how your room behaves IMO.
     
  7. Couch Potato

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    102dB @ 16Hz :eek: :D :eek:

    I would have said that above 60hz is where the HGS is starting to rolloff to the mains if your crossover is at 80?

    It doesn't look too bad, you could definately flatten out the 16hz and 50 hz peaks with the BFD giving you pretty much flat, but you could also tweak forever!!

    Steve
     
  8. Apocalypse

    Apocalypse
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    Yeah how does 91db translate into 102db, silly graph :)

    When the sub does a sweep from 100-20hz the same thing happens, it suddenly increases it's volume below 60hz peaking at 55-50 then remaining constant right down to 20hz. So the question is, how does one increase the bass response above 60hz?

    Thanks for your help C Potato.

    Regards
    Phil
     
  9. Couch Potato

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    91 - 102 is because of the inacuracies of SPL meters at low hz, theres an uplift in the 2nd column in the spreadsheet. 1st column is raw SPL 2nd column "real" SPL.

    I noticed a rise in "sound" it actually turned out to be door surround and the windows vibrating. Take a walk around and be liberal with decorators caulk on door surrounds/skirting. Check for furniture rattles and possibly pipes rattling in wall spaces :eek: :eek:

    Lifting the SPL is more difficult than taming it, have you played around with the crossover?

    Steve
     
  10. Apocalypse

    Apocalypse
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    Well I use a THX system with the speakers all set to Small so I turn off the sub's crossover and use the Pre amp's crossover which is 80hz. Correct me if I'm wrong but I can't set the speakers to small and also use the crossover on the sub as having 2 crossovers enabled creates sound problems all it's own. Small setting yields better results for movies but it's not ideal for music, having a small room means standing waves create a boomy sound on some musical material.

    I should make it clear that I'm more than happy with movies, it's trying to sort out the music side that's tricky, from reading many sources on the net it seems clear that with a small room you MUST have an absorbtive material behind the front soundstage to deaden standing waves which will give that tighter sound. If I stand right next to the sub you can appreciate how tight the sound is at the source, it gets less tight the further away you get which more than likely means standing waves are the culprit. I still have another ace up my sleeve, tomorrow I'll fit a large curtain behind the front soundstage and see what results if any that brings. The annoying thing is I have a large rectangular room downstairs which is empty and would be perfect for a dedicated room, the problem is it has an old lady living one floor below and windows on one side. Both would need some serious soundproofing before it could be used for anything and the prices for proper soundproofing make me quiver with fear. The room would be a dream for movies I bet, it measures 6m x 3.5m with a ceiling of 2.5m rising to 3m gradually, I need to know more about soundproofing and it's effectiveness before I'd commit any resources to this project though.

    Regards
    Phil
     
  11. Couch Potato

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    Sorry Phil, what I meant was changing the crossover in your pre/pro to maybe 50hz rather than 80 and setting your mains to full range for music?

    I just played...... it took a while but then I use a diffrent room for Audio :rolleyes:

    Soundprofing the other room sounds like a long term project ;)

    Good luck

    Steve
     
  12. Apocalypse

    Apocalypse
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    AFAIK the Marantz AV9000 has no option to change the crossover point, you either set the speakers to Small/THX, Large or None.

    Am I missing something in the manual :blush: :D
     
  13. Couch Potato

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    No not necessarily, some pre/pro's allows different crossover settings, sorry for leading you up the garden path :(

    If you are willing I'd seriously consider Parametric EQ The BFD does a sterling job and after investing such a lot on Speakers/sub it would be criminal to let it down due to poor room effects, it's very good at reducing peaks not increasing troughs, so in theory select the right filters to reduce everything to a norm and then increase the SPL, simple.....ish!!

    I'd get F Alton Everest, Master Handbook of Acoustics also, see if you can get the BFD SOR then it's a no brainer. The book is an excellent easy'ish read on acoustics and a reference piece.

    Welcome to the world of the intangiable!!

    Steve
     
  14. Apocalypse

    Apocalypse
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    For the 30th time I tried the HGS18 in a slightly diff position, just 1 foot forward from it's previous position. All of a sudden the boomy sound dissapeared and I found myself having to turn the sub's volume up :eek:

    Still the bass sounded weak so I put Titan AE's ice field chapter DTS in the mix to see what the bass was like, heh, the bass wasn't weak, it was simply the first time I've heard clean bass. When the ship flew by, the room and me shook like never before but even though it was too powerfull as it was clean you could keep that setting if you were mad enough, I chickened out and turned it down by 2db. I'll keep this setting and see if it's fine for all movies tomorrow but I'm well chuffed so far. I guess I thought that the HGS18 with it's no distortion tag could be placed anywhere but standing waves have nothing to do with distortion. Now the music is very tight, the thump from the sub matches the beat from the speaker perfectly, the muddy sound has gone and been replaced with a warm altogether pleasing sound. The HGS18 reminds you that it's no toy very effectively at times, playing Massive Attack - Angel, the beats at the start of the song were shaking any loose coins or floorboards but if you were blindfolded you would be hard pressed to locate where the sub was. It's too late at night now to play some more but I'm very excited by the results and can't wait to try more material tomorrow.

    I'm amazed that such a small move for the sub could radically alter it's performance, 5 years I've been into Home Cinema and I'm still learning valuable lessons :blush:

    C Potato - Sorry if I've made a meal of this, I tried diff positions before for the sub but none really changed anything so it was my last throw of the dice this time and it worked. I guess the added soft furnishings and absorbtive material I placed around the walls near the sub changed everything. I'm caught between feeling rather silly and jubilant also at the results, I do regret that I've wasted a rather long thread on a simple cure though :blush: :clown:

    Regards
    Phil
     
  15. JSW

    JSW
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    Your thread hasn't been wasted m8 :)

    I've been following it as I have the same peak but with my lesser CHT 15.

    Ive been busy so havn't had time to play around with it so instead I just kept looking in on your thread and later today when i get up I will pull my CHT 15 forward a bit and see if i can get rid of the peak too :)

    Cheers
    Bry
     
  16. Couch Potato

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    Why consider it a waste? Some of our estemed members have forgotten more than us mere mortals will ever know about HC and especially acoustics, I've learnt quite a bit also thro' the life of this thread.

    Steve
     
  17. Apocalypse

    Apocalypse
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    JSW - I moved it forward a foot and angled it so the driver was facing 45o towards the middle of the room, in the corner behind the sub I placed a lot of plasterboard and foam to soak up standing waves. It's worked but I need a permanent solution to the ugly mass of foam in the corner of the room, proper bass traps are about £120 just for a 2 foot corner item. This site has prices and guides if you're interested http://www.customaudiodesigns.co.uk/soundproofing.htm

    Thanks to both you guys for the kind words, nice to know it hasn't been wasted.
     
  18. JSW

    JSW
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    Thanks for the link the missus has given me approval to soundproof the the floors upstairs :)
     
  19. Phil Hinton

    Phil Hinton
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    I get more pronounced problems than most in the 40-60hz area because of my room layout. I am seriously thinking anout a feedback destroyer but will it cure my probs?
     
  20. Couch Potato

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    The BFD will tame peaks but would find it difficult to boost troughs, it just takes some playing with the filters, every change affects more than just the wavelenth you're trying to adjust. Use the link I posted earlier in this thread and give it a read.

    Steve
     
  21. dunkyboy

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    I've found that sticking a rolled-up futon mattress in the corner of my room behind my speakers tames unruly bass and harsh treble to a surprising degree - and it doesn't look *too* hideous or bizarre... ;)

    'Course a new max. thickness futon mattress from Ikea costs a fair bit... but if you have one lying around, or if you can find one 2nd-hand somewhere, then it may prove to be a very cost effective solution. :)

    Dunc
     

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