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Mystery of the Disappearing Frequency

I have only been involved in the pursuit for about 3 years(since my retirement) but have tried to read and research extensively. In this work I have not yet found a satisfactory answer to my dilemma. I have a 10 Db dip at about 50 Hz!!  This alone of course is not that unusual, what seems to be unusual is no matter where I take the measuring microphone(calibrated Omnimic) the dip never goes away!!  I have walked all around my somewhat small listening room(15 by 18 by 8) and the dip never get significantly lower. I have certainly read about dips but always thought they were specific to certain areas due to nulls and cancellation effects. My room is a little unique but I do not believe the following factors account for this dip. The long sides of the room have a 10 foot long bank of windows covered by thick drapes. It has a wood floor on a pier and beam foundation and the wall behind the speakers is the wall where the main house met the attached slab garage before the garage was converted to a few small rooms. The small amount of wall material is not sheetrock but rather a very thick, ribbed solid wood paneling. I followed expert advice as far as acoustic treatments(room tunes pillows at the ceiling corners,panels on the ceiling to reduce echo near the speakers, a rug with thick under pad to cover about 80% of the wood floor. I have corner bass traps on the speaker wall. The room is overflowing with records and CDs. This phenomenon was present with my previous speakers(NHT 3.3s) as well as my current BBC monitor style speakers. I have tried a variety of experiments to see if I can change the issue. The room has 2 doors but opening doors,windows and/or opening the drapes did not change the situation. Also tried removing the bass traps to no avail. Have run out of theories and experiments at this point, outside of wondering just what materials there may be in the room that absorb this frequency. My equipment has also mostly changed since I first measured a few months back, have mostly Allnic gear at this point. Your theories and suggestions are kindly solicited. Many thanks, LS
trytone
shadorne's avatar
shadorne
7,477 posts
 
50Hz has a wavelength of more than 20 feet (longer than your room). You are hearing room mode bass response. Since the effect is everywhere then it is also likely you have quarter wave cancellation. 

http://www.mh-audio.nl/CancellationFreq.asp

The only only thing you can do is move the speakers. 

Quarter wave cancellation occurs around 50 Hz when the speakers are 5 to 6 feet out from the back wall behind them. Try placing the monitors right up against the back wall or try placing the speakers far out into the room in a nearfield listening setup.
trytone OP
113 posts
 
Thanks for replies. Microphone was calibrated upon receipt and the phenomenon shows up on an Ipad app as well. Old speakers and current speakers have been in about 6 different positions(the NHT's are designed to be only a few inches from back wall)-the front edge of the current monitors are about 4 feet from back wall and as much as I hope that moving the speakers might do the trick I think that past history would rule it out but I can always check.
randy-11
1,913 posts
 
see if you can rent some tube traps
erik_squires
16,655 posts
 
I think your problem in part may be your measurements.

Are you using a steady state signal? Try Room EQ Wizard or OmniMic instead.

Next, if this is real, you'll need bass traps. Tube traps are good, but ASC is pricey, GIK Acoustics Soffit Traps are as good and better value.

Best,

E
trytone OP
113 posts
 
Am using Omnimic, think it was mentioned in original post, if not my apologies, have floor to ceiling corner bass traps from GIK in corners on wall behind speakers. Most would probably think my room has too much rather than too,little room treatment, am curious as to the theory that adding even more absorptive treatment will cancelthis phenomenon but am willing to listen. Thanks all so far for the reasoned if ineffective suggestions, please keep them coming. Even though the speaker position is probably not a factor will try to move them around soon.
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