audio-talk

[cressy]

ive laid my hands on a set of fostex fe127e's for the bargin price of 37 quid the pair, i was going to make a bigger pair of metronomes based on the plans on the fugelhorn site and leave it at that. the only problem is that my desire to complicate matters got the better of me and ive ended up with 2, 8 inch rel bass brivers for 25 quid.

now methinks ive got a quandry. do i make a pair of metros and 2 passive subs, or do i go the whole hog and create a hybrid based on the metro cabs with some extra height/width. the problem is that the fostex is and 8ohm driver and the rels are 4 ohms so im not sure what would be best on the end of my el34. the vendor of the rels reckons they are better in a sealed box of 11 litres though hes only had them in subs not complete cabs and that experimenting with ported sub cabs made the bass sound flabby and by his account abit slow. the sealed box sub wouldnt be very efficient so the op trannies on my amp might struggle. on the other hand in the metro style ported cab at the bottom with the 127 a third of the way up with a height of about 5 feet as opposed to 4 might give better efficiency but flabby bass.

my room is about 12 foot square with a large cavity next to the right hand speaker and a door between the speakers so alot of bass gets muddled from that side. any help would be greatly apreciated before i buy the wood and start sawing! cheers guys

[Scottmoose]

Separate subs would probably be the better option in this particular case. Volume / length / driver position etc is all critical with the Metronomes as they're QW, nor reflex, cabs.

Without the parameters for the RELs it's a crapshoot, although I suspect if you went with a vented EBS or a trad alignment (you don't want to know how big the latter would be, trust me) they'd gain some of the lost speed. Thing is, in a room that small, you ain't going to be plunging the depths anyway.

[Greg]

Hi Scott,

This is a little off topic but hopefully useful to Anthony so will post here. Your comment on room size vs depth (I presume you mean audible bass extention) is interesting. I've read comments like this elsewhere. My room is 4m x 4m so similar in size to Anthony's. Within my own listening capabilities, without a sub woofer, I get on the relevent recordings, thunderous bass. Obviously this is enhanced on film DVD but I have it on Pink Floyd vinyl albums (for example) as well. In addition to what I hear, I'm talking about feeling the sound in my chest and behind me from my chair. It is a seriously physical experience. Is there some long accepted mythology going on here which denies that a small room has the potential to reproduce very deep sound? I've considered that there may be more below what I hear but have discounted it because I don't find anything lower in the cathedral or in the cinema.

I'll be interested in what you think on this.

Best wishes,

Greg

[cressy]

in truth greg ive never really experienced a 'loss' per se of bass in this room, infact as a room its quite neutral. i actually got lovely tuneful and quite deep bass out of my old quad 11ls.
the problem i have is with speaker positioning with the kef q55s. the speakers are along the wall with the kitchen door in. the left hand speaker is alright as it is about 18 inches to the right of the corner, back up to the wall. the right hand one however is smack next to the door jamb with the sofa going up the right hand wall. the problem is that the sofa is close enough to touch the speaker and it leaves a cavity about 2 feet long going into the corner and as high as the sofa arm. there is a honk from this cavity then a loss through the door to the kitchen so as youd expect the bass gets rather muddled, infact its possible to 'tune' the bass by opening and closing the kitchen door!!!! the quads were on open stands higher than the sofa so this was never an issue. the positioning is unfortunately all i can get due to space and child constraints.
i think ill take your advice scott as the resident guru on these matters,and build simple metronomes and a pair of subs. dependent on the performance of the speaker on its own ill add the subs in and have a listen. if theyre too much theres plenty of young car kids with tommee tippee corsas at work who'd buy them off me for their cars!

on the subject of the room, i reckon sometimes the bass is better in a smaller room. just look at owston or eggborough, the little metros always sound tinny in a huge room wheras at the old mans without the subs in i never thought they lacked any bass. but then i suppose theres the question of driver size, positioning and whatever other factor are present (ie soft furnishings).and its the one thing you cant change in a system unless of course you move! (could that be the most expensive upgrade ever?!!!)

[Scottmoose]

Hmm. Room acoustics. A thorny area if ever there was one.

There's a lot of misunderstanding about rooms & their effects & I don't pretend to be an expert. However, FWIW, I can give a few general thoughts.

You're probably right Greg with what you say about mythology. I suspect it comes from a partial misunderstanding of some of the physics, particularly WRT room resonant modes. The first mode is taken as one-half wavelength, so a room 22.6ft long will have a primary resonant frequency of ~25Hz, depending on air-pressure, humidity, temperature etc. Assuming a speaker can produce sufficient acoustic power at this frequency to activate this mode, it will then provide considerable gain. That doesn't automatically mean that a smaller room cannot support LF, but the ratio of indirect to direct radiation will be higher, and more acoustic power will be needed from the speaker, although this latter can be tuned on its head (to an extent) due to the likelihood of a closer listening position etc. ROT: big rooms require big speakers. Little ones are lost.

Then there's build materials, decay time... this is why I like horns & dipoles (and properly executed arrays) -for different reasons, the room has less of an influence.

[Ali Tait]

Interesting post Scott.Is the room less of an issue with 'statics than with box speakers? assuming of course that you can place them far enough away from the rear wall.I'd say my room is about 6 by 5m,and a Victorian house so a high ceiling.I'd guess this helps?.I seem to get quite an even response,though maybe I've just got used to it! Sorry for nicking the thread.

[Clive]

Hi Greg,

My room is quite square too, 5m x 5m with a bay window at one end so it's not fully square, the ceiling is low (about 7ft - it's a cellar room). I have strong room modes at 34Hz and 59Hz - about 12db. These add to the bass considerably or remove most of it depending on listening and speaker position. It may be that the thundering bass is room mode related or just your great speakers.

I have 15" inch subs in sealed boxes and 18" as dipoles. The dipoles definitely work better, their positioning requirements are "unusual" too. I've be using some room analysing s/w with a calibrated mic, my measurements stack up with my subjective experience. The dipoles I find are less influenced by the room. I still have work to do to reduce the room modes.

I doubt this helps anyone much....just my experience.

Clive

[cressy]

just on the subject of the subs, im wondering how sensitive to positioning they would be. the consensus on several interweb pages seems to be stick it in a corner and tweak the controls til it sounds right. these planned boxes would be a different kettle of fish as they'd be passive. hmmmm, thinking back on my experience with sealed box speakers i wouldnt think they'd be that fussy, but i still cant get my head around sticking it in a corner. i would have thought that using a corner to excite the room would miss the point of having the speed of sealed box sub by muddying its transient response. or how about sticking the 2 boxes together, adding a doylie or 2 and plonking it in the middle of the room diguised as a coffee table? youd get a lovely foot massage every time you put your feet up!
now for a stupid question, as bass frequencies are omnidirectional, (eh?) does it really matter where the hell they go? im assuming that left and right need to be the same l/r as the corresponding speaker, but could i put them at the other end of the room.? like i said, a stupid question.

[Clive]

I've found with sealed and reflex that 1 sub in a corner and the other a little away from the opposite corner works. I do find I can detect a single sub that is off-centre but a lot depends on the roll-off of the crossover.

[cressy]

i was planning to roll these off between 40 and 50hz dependant on the cutoff point of the metronomes. with the smaller ones i seem to remember them being built to roll off at about 50 hz so 45 to 50 seems about right to me. care to elaborate scott?? give me a turmtable to build and ill do it and do it well, anything with any maths or suchlike involved and i'll go with my gut feeling rather than any proven data. silly way to go about things if you ask me !

[Clive]

BTW, don't forget that the rollover f is one thing, the slope is another.

[Scottmoose]

[Scottmoose]

[cressy]

im just wondering what material may be best for building the cabs out of. mdf is a no no as i dont like the stuff (just me being pedantic), the obvious option is birch ply, not a problem as i can get hold of it fairly easily. another option is to use oak as ive found a supplier who can get me a load of recaimed oak floorboards that are easily wide enough to make the parts. i like to use oak as i like its machining/finishing properties and id have thought its density would be beneficial in this application. the cost isnt a problem so any other suggestions are welcome. id love to build some perspex ones as again ive found a supplier who can supply all the bits ready cut. in not sure about its properties for speakers though although james's quasars made from this are superb. any advances on these three?

[Toppsy]

Hi Cressy,

from ease of working, I would recommend 18mm birch ply. Oak is very nice to work with but unless you have a planer-cum-thicknesser machine, a biscuit cutter and all the woodworking tools to work with solid Oak you'll find it hard work, even using old oak floorboards. Unless a single Oak floor board is wide enough to be used for the sides then you'll have to butt join boards and for this a buscuit cutter is essential.

If you want a oak finish then I'd recommend the birch ply skinned with veneer. You can still use some solid oak for the top, and some solid down the vertical corners if you want a routed edge finish. This is what I'm doing for a new pair of oak MLTL's for my Son and also a pair finished in American Cherry for Andrew Lehane.

Just thought I'd comment on sheet acrylic. This is a right pain to work with and you'll most likely end up as James did and have them professionally built. Cannot comment on the acoustic qualities this material as James speakers are open baffle and your are box speakers, yes? I doubt very much the two designs share similar resonance charactoristics.