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Discussion: IB or reflex?

 
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audiolabtower
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PostPosted: Fri Dec 20, 2013 11:00 am    Post subject: Discussion: IB or reflex? Reply with quote

Reading the LS50 design notes, I marvelled at how much effort had gone into making the reflex port work properly, getting rid of unwanted resonances and turbulences that upset the midrange.

It struck me how Kef had relatively few reflex designs in the Cooke days, as opposed to the BBC practice, and in the larger designs tended to ABRs in comparison before embarking on coupled cavity. Perhaps ABR was a Laurie Fincham personal preference?

It seemed the wisdom was that reflex worked best in mid/large enclosures where you could get a resonance 50 Hz and below, and once you got to mini-monitors closed box was better, cf LS3/5a, original Spencer Hughes SA1, original Codas up to Coda 3 etc. I don't remember many tiny reflex boxes in those days.

This then changed in the days of Coda7 and Crestas etc where small boxes sprouted reflex ports. Does a reflex port really do much on such a small box and is the reduction in woofer excursion worth the extra cutoff, or is it just the trend for louder output in the digital era that has driven this?
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speakerguru
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PostPosted: Mon Dec 23, 2013 10:35 am    Post subject: Reply with quote

As posted before, Laurie Fincham introduced the ABR at Celestion and brought it to KEF, initially with Model 104. The efficacy of the ABR as opposed to a vent at suppressing midrange box resonances can be seen in the coupled cavity paper available on this site. In the trial coupled cavity where only the ABR or vent output is used, the midrange output from the ABR/vent is not masked by the direct driver output and can be clearly seen in the graphs as huge peaks. Low mid colouration was a holy grail in many reviewer's eyes in the 70s, hence the use of ABRs instead of holes by KEF in that era.

However, the ABR is normally more expensive, cannot always be fitted into some cabinet configurations and has does have an excursion limit. On the other hand, a port/vent is more difficult to tune and goes non-linear , making chuffing noises if not designed for smooth air flow.

In both cases, to realize the full gain in driver excursion reduction, a reflex system should ideally have no box or driver losses, i.e. no surround damping, box lining or leaks. ... so as per usual it's all a fine balancing act....


Last edited by speakerguru on Wed Dec 25, 2013 9:32 am; edited 1 time in total
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mattcambs
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PostPosted: Mon Dec 23, 2013 3:27 pm    Post subject: Reply with quote

I wonder if the fashion for small speakers to be sealed was mainly due to them typically being located on shelves and hard against a wall. A reflex design can often sound too boomy in such locations. The later fashion for stand mounting and the ever increasing search for efficiency would suit and more often than not dictate vented boxes.
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SaSi
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PostPosted: Tue Dec 24, 2013 11:17 pm    Post subject: Reply with quote

I had the same question made to myself about a year ago.

As I understand it, you are comparing sealed against ported, (IB=sealed). I think that since the AR3, IB became obsolete Very Happy

The ported design is a bit more difficult to tune compared to a sealed version. You can err with the dimensions "as much as you like" with a sealed enclosure and the Q won't suffer much.

I have built a 180lt enclosure housing a B300. It was initially built to study enclosure resonances and the effect of bitumen sheets covering the inside. Comparing this to 105, initially it appears that bass is lacking but give a bit more power and the performance is good - less boomy - and the effect of 9mm of bitumen all over the inside is also beneficial.

I was planning to alter this into using a pair of B139 PR on an alternate faceplate. Never completed this but I do intend to try it. Only problem is I don't know how to calculate the tuning of them. I will need to experiment.
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