How far is too far a distance for rear ported bookshelf

[lucksin]

Please pardon my ignorance for not doing much of search on the topic. i did tried finding how much is too much for rear ported speakers but i could only found the minimum distance and not maximum distance. if anyone can either clarify or point to a thread where i can get some detailed information i would be thankful.

Recently i have been looking out for Bookshelf and nearly freeze onto one. so i went to buy them but just before making a purchase i asked the speakers to be placed in a different room. since the speakers when auditioned were always in a closed small room with speakers kept close to wall i asked it to auditioned one last time with a different setup.

so 2 tower speakers were moved to a room which were to act like stands(since they dont have any stands for BS) to HT room which was properly treated. however since there were so many speakers were around and wasn't a possibility to move them the FS were placed around may be 5.5 to 6ft from the wall(the room must be good 20 odd ft x 24 odd ft). BS were placed on top of them virtually giving speakers good 6ft approx breathing space. however on the rear it wasn't a wall but home theater screen. since complete room was treated there was no wall to be seen it was all covered with cloth tiles.

and when the audition started, there simply was no bass or rather the lower frequencies sounded like hollow bamboo. no texture no weight nothing in them. it truly turned me off from the purchase and i came back home offcourse disheartened. i started reading about about rear port and bass reflex about cancellation of some frequencies but as i am not a sound engineer all these things were alien to me.

can someone please make me understand how far is too far and if indeed putting a speakers far away from wall will strip it off the lower frequencies. the speakers were suppose to go down till 47 hz which i feel is good enough to produce decent texture for low frequencies.

i am not going to purchase till the time i understand how this things works. also i have seen where ever i went to audition speakers i could always find many floor standers behind the speakers may be around couple of feet from the rear port. does it have something to do with creating proper lower frequencies.

 

[bhagwan]

10 Tips On Speaker Placement | Audiofile Blog

Just to understand;
What Amplifier was used ?
What mode was that amplifier in ?
If it was in 'HT Mode' then nothing less than 80 Htz would be sent to the B/S Speaker - & hence you would feel / hear a 'hallow bamboo' effect.

http://www.crutchfield.com/S-OmRe48KGxoh/learn/learningcenter/home/nonHTspeak.html

http://www.soundfountain.com/amb/speaker.html

 

[asliarun]

I can share what little I know, but I'm not an expert so apologies in advance for mistakes.

The closer you put the speakers to the wall, the more the walls act as the baffle or front panel of your speaker. The baffle and the wall act as a reinforcement to the sound because they reflect sound to the front of the room - the sound pressure you would have lost if you didn't have speaker cabinets or walls. Sound travels in all directions normally.

The interesting thing is that this doesn't happen uniformly for all frequencies. The lower the frequency, the larger the wavelength which means bass frequencies become less reflective. If wavelength is larger than your baffle or even your wall, it will pass through and will not get reflected.

Google audio wavelength chart. You will see that 5 ft is about 200 hz And 2 ft is about 500hz and 1 foot is about 1000hz. This means that if you place your speakers 5 ft from the wall, you will only get half the SPL or db for all frequencies below 200hz. That might be the reason you felt all the bass got sucked out, including mid bass and low bass.

It is a little trickier than this because speaker designers actually compensate for this loss and most speakers will have higher SPL bass than treble. In fact usually people face the opposite problem as a result. They place the speaker too close to the wall and get too much bass as a result and find the bass becoming boomy.

So it really is separate behavior for separate speakers depending on how the speaker designer intended the speakers to be places, how much the room absorbs and reflects the sound, and even how large the room is.

But in your case, maybe the reverse happened?? Others can better recommend what typical or good placement should be.

 

[cyber_cat]

Please tell us which bookshelf you auditioned ?

 

[lucksin]

Bhagwan ji : it was Denon PMA 1510ae linked to sherwood CD player. since the setup has always been on same amp and CDP. it wasn't on AVR. i am aware denon are not considered very good amplifiers but considering many in similar price range i found it to be better.

@asliarun : as you mentioned more closer to wall and it act like baffle, what if there is no wall to reflect instead the complete wall is covered with cloth tile. i am sure cloth tile would trap the wave compared to wall since wall is completely plastered and waves get reflected. can it also be one of the main reason for sound sounding too hollow?

generally people face the issue of too much uncontrolled bass when the speakers are kept in very close proximity however i am not sure if they have tested the same setup with speakers kept much far away like in my case.

cyber_cat : it was Jamo C803.

 

[hydra]

@lucksin, The problem was most likely caused by the fact that the bookshelf speakers were placed on the floorstanders. The same tinny/hollow bass effect can be seen when hollow boxes or lightweight boxes are used as "stands" for bookshelf speakers.

From personal experience (not a lot) and from reading up on the subject (a lot), generally the more space you give behind speakers, the better it is, in terms of depth of sound stage. There is a cost that you will pay in terms of bass reinforcement from the rear wall, but if you prioritize presentation of music more than bass (I do), you should leave at least 30% of the room length behind the speakers, if practicable.

In a recent video shared by an FM on speaker placement (a talk by Jim Smith & Charles Hansen at RMAF), they recommended that in small rooms, it is better to arrange things so the listener is closer to the wall behind him/her, with the speakers pulled into the room towards the listener (and with the maximum possible amount of space behind the speakers). Of course, the wall behind the listener would have to be treated with enough absorption.

 

[Thad E Ginathom]

so 2 tower speakers were moved to a room which were to act like stands

Using speakers as stands?

I would have thought this was unwise. Speaker stands are supposed to be as dead as possible. By putting a speaker on top of a speaker, are you not, in a sense, just adding more speaker cabinet, even though the internal spaces are obviously not connected?

One reason that "bookshelf" speakers have never appealed to me is that I could not be bothered with making two purchases, speakers and stands, instead of one. If I have to improvise a speaker stand, I'd use something like a pile of books. Paper is made out of wood pulp: a pile of books is quite like a solid wood block! Back in the day when living in London for years meant accumulating a considerable hight of telephone directories they served the purpose very well. Except for aesthetics, of course!

I'm talking theory/hunches here. What do others think?

 

[Jay5150]

I think rather than the distance the culprit in this case would be the sheer size of the room! 20'x24' room on top of hollow stands and ?probably acoustically treated walls all contributed to your bad sonic experience

 

[bhagwan]

Jamo C803

System Type 2-way bassreflex
Impedance 6
Tweeter (mm/in) 25 / 1 DTT
Woofer (mm/in) 180 / 7
Product dimensions (mm/in, HxWxD) 380 x 225 x 341 / 15.0 x 8.9 x 13.4
Weight (Kg/lb) 9.9 / 21.8
Frequency Response (Hz, +/-3dB) 45 - 24,000
Power Handling (Watts Long/Short Term) 125 / 250
Sensitivity (dB, 2.8V/1m) 87

These are the specs;

F3 is 45 Hzs
That is good / normal for a B/S Speaker.

This speaker will play well in a room that is 120 to 200 sq. ft.
Larger rooms will need a speaker that can move some more air.

 

[bhagwan]

Generally speaking - not a 100 % rule;

Speaker 'closer' to walls [boundaries] leads to increased 'bass'
& tends to 'cloud' the mid range

Speaker 'further' from walls [boundaries] leads to reduced 'bass'
& tends to 'clean' the mid range

Also, distance from rear wall leads to 1/4th wave length cancellation.

This may be a 'bit' complicated - sorry.
Can try and explain - if we meet in person - my knowledge is 1/5th baked - so will make a lot of mistakes - sorry.
This link may help :-

YourFriendPaul's Sound Wavelength

I do not want to get technical - it will bore people & the fun of music gets 'lost'

 

[chandrakiranraju]

I do not think there is any upper limit in this regard.

 

[cyber_cat]

Dont think speakers are the problem but placement upon FS is what might have caused the problem.

What will be the room size in which these speakers are intended to go ?

You wont be placing your speakers 6 feet away from the wall at your home right (?) , so it wont cause problems.

Cheers.

 

[lucksin]

@hydra : base for the bookshelf can be one of the reason but i believe certainly not the only reason since i have been auditioning them on FS and while in a different room they sounded much better. also i fail to understand since these speakers are already damped from inside will keeping them on another hollow base will actually make much of a difference ? to an extend it does i agree

also dont you think 30% of room length wont be too big incase the room itself is much bigger ? it might not be much worst compared to mine but certainly not better since many frequencies might not be reflected back.

the speakers will be placed in my living room and almost all the walls have some or the other things hanging specially the wall behind the sofa it has a painting. so i guess it will take some care of acoustics.

@thad : well Thad, necessity is mother of all invention good or bad. i belived it might be better hearing the system on ear level than keeping the same on ground and try to judge the output. and i am sure, like i mentioned above, new age speakers are damped from inside(correct me if i am wrong) so keeping it on another speaker should not hurt the sonic much. even if it should, completely ridicule might out of context(again please correct me if i am wrong).

@jay : to think of it now, i agree with you. since the room itself was big with heigh ceiling and completely damped might have played spoiled sport.

bhagwan ji : you are more than welcome to become technical. i enjoy understanding and learning how things work. since placing speakers away from wall has its benefits it does come with disadvantages as well. so understanding the technicalities might just help me and many other like me to place the system at an appropriate distance.

also, indeed Jamo are good on paper and better when played. the tweeter itself was impressive. since i have demoed many speakers now i come to feel that these speakers will not strain ears on extended listening periods.

@chandrakiranraju : i beg to differ, there certainly is. since most of the bookshelf are bassreflex system they do rely on reflected sound wave to fill the sound and room.

@cyber_cat : that might not be case, i agree yet when i am going to put a fortune on a setup i would first understand its shortcomings. like in this case, i will now audition the second set of speakers which were my plan B and see how would they perform when kept far far away from wall. since i will be shifting to another house, a much bigger one with a big living room i have to make sure that i do not regret the purchase later and start looking for upgrades.