Modifying my 24 year old B&W P4

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karatestu
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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

Tried my B&W drivers without a filter , just blu tack. Still forward firing. It wasn't quite the magical experience I was hoping for. It was hyper detailed with bags of dynamics but something is amiss. Maybe my passive line level high pass filter needs changing. I can also try it up firing and see what happens.

If that does not deliver the goods then I will try the 6.5" low passed af 500Hz with and without blu tack. If I'm still not satisfied then I am on the look out for a new pair of 6.5" drivers which are designed for sealed enclosures.
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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

A good description of the sound from the unfiltered drivers is honky. This maybe due to the driver beamjng - being highly directional at the top of it's range. Drivers start to become more directional at a frequency whose wavelength is equal to the diameter of thd cone. With a 6.5" that has a cone of 14cm approx then it starts at roughly 2450 Hz ( speed of sound in cm per second divided by cone diameter in cm). It may have started to narrow before that.
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Re: Modifying my 24 year old B&W P4

Unread post by r3xj0hn570n »

Having experimented with blu-tack, can i suggest you aren't getting the best out of it? What you have done there is damp an edge termination. You haven't touched on trying to damp cone breakup modes much at all. Grab an (old free) copy of Martin Colloms' High Performance Loudspeakers and read the relevant chapter.

Did you see the pic i posted months ago of my upstairs centre speaker? The cone on that is boron coated titanium, so it needed resonance damping to be used with a low order crossover. That was the end result of about 60 different positions all measured with a linkwitz style panasonic mic.

IME, honkiness is usually the result of internal box resonances escaping, rather than massive cone resonances. Doping the drivers will tend to mask internal resonances...

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Re: Modifying my 24 year old B&W P4

Unread post by Daniel Quinn »

A disagree , although we may be arguing over the definition of honkiness.

An infinite baffle enclosure won't honk and I
have madee many a driver less at aggressive and ear piercing by damping with contact glue and plastidip.

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Re: Modifying my 24 year old B&W P4

Unread post by r3xj0hn570n »

Daniel Quinn wrote: Sun Oct 31, 2021 8:37 pm
An infinite baffle enclosure won't honk
Any enclosure at all can produce a resonance, by interacting with the air in the room. If the infinite baffle is sufficiently big (like an adjacent room), then that resonance will be very low in frequency indeed. Typically, an infinite baffle is just a closed big box, which will have all the usual internal eigentones.

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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

I was referring to the sound of severe beaming. I maybe wrong. Troel Gravesen describes the sound of beaming as honky. Well what ever is the cause it is mildly annoying. I can play with stuffing.

I listened to the sound of this driver with and without the blutack, it had a marked effect for the better so who knows. Steve agreed earlier in the thread.

Honky maybe backwave coming back through the cone. The enclosure has quite a bit of foam in it exactly the same as it came out the factory plus a bit more where I blocked the transmission line. Fibreglass would probably be better.

I've done doping and I am trying to move away from that. I have more or less decided not to use this driver in my sphere project anyway. I will be low passing it much lower in frequency eventually so hopefully that will help with any break-up being further out of band and attenuated more.

Maybe this effect has come about by blocking the TL which is stuffed loosely along the first half. This soaks up the midrange and let's the bass through. Stuff too densely and you get no bass out of the vent.

I value everybody's contribution to this thread. Thanks for helping me get a better outcome.
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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

Time aligning 1st order requires a stepped or sloping baffle. Many think that aligning the voice coils is enough but apparently that is not accurate. Can it be done by ear with no measurements ?
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Re: Modifying my 24 year old B&W P4

Unread post by Daniel Quinn »

r3xj0hn570n wrote: Sun Oct 31, 2021 8:50 pm
Daniel Quinn wrote: Sun Oct 31, 2021 8:37 pm
An infinite baffle enclosure won't honk
Any enclosure at all can produce a resonance, by interacting with the air in the room. If the infinite baffle is sufficiently big (like an adjacent room), then that resonance will be very low in frequency indeed. Typically, an infinite baffle is just a closed big box, which will have all the usual internal eigentones.
Be honest with yourself this is theoretical bullshit to save your point .

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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

Over a year sitting on this one. Like a distant annoying relative that won't snuff it, it's back :grin:

I managed to cut a 80 mm diameter circle above the mid bass cut out. There was only a mm or two to spare to avoid the top panel and the mid bass cut out. On the back I cut a new hole 60mm in diameter. Into this hole was inserted a tapering hollow plastic table leg (kids ikea table) which was cut to the appropriate length. I knew these legs would come in someday :grin:

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The tapering tube is a very tight fit in the front and back holes - took a bit of working out what size to cut the hole on the back panel but it has all worked out marvellously. So why butcher a perfectly good speaker cabinet ? Why not? These are worth £200-£300 on ebay and I robbed the mid bass drivers out of them plus i don't think I can part with a speaker I bought in 1997 and used almost daily for 20 years.

This tapering tube is the enclosure for the little 2" SB acoustics wide band driver that i like to use as a tweeter in my spherical diy speakers. I like them so much I have bought another pair. Unlike the metal dome 1" tweeter these are replacing, they require a rear enclosure of their own. As these speakers are transmission line then I need to keep the distance between the centre of the mid bass to the top of the speaker (end of the line). Its a very important design aspect for a TL. Putting a round circular tube in from front to back panel achieves this.

The tapering tube was not big enough in internal volume if I terminated at the back panel. Hence why it passes through the back panel. There will be a 90 degree bend here and the tube will continue towards the floor, terminating just above the rear speaker cable binding posts. That will give more than enough volume. This tube is to be progressively stuffed with fibreglass getting more dense as we travel towards ths open end near the floor. This should eat up all the back wave .

I can't decide what to do for the best as regards crossing over between mid bass and mid tweeter. The mid bass driver is good to use without electrical filter if i choose. But as this is a point & squirt then things are a little different. The MB will start tk beam at a certain frequency. In theory the directivity of both drivers at xover frequency should be similar for best results. This won't be the case if I let the MB run fully open. How important this is and if it even makes a difference to my enjoyment is yet to be seen .

I will be trying this with a first order low pass filter on the MB at about 575 Hz . This is the centre (-3dB) of the baffle step transition for this baffle width. Filters will be done passively at line level (power amp input). Gives me options later on. The mid tweeter is big and robust enough (hopefully) to handle a first order high pass this low. Directivity should be much better matched at this frequency.

Another thing to consider is distance between the centre of each driver. This should be kept as small as possible to make the output of both drivers to be seen as coming from the same point. The higher in frequency you xover the smaller this distance gets. If the MB is left to roll off mechanically then this xover is high and we might have problems. At the lower xover frequency of 575 Hz theoretically there should be no problem in that regard.

It will be interesting for me to assess the difference between the two and if these things really matter to me. We shall see. Also will be fun to compare a transmission line and sealed box with the same drivers.

More pics to follow........
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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

I inserted the pipe :shock: but it was a bit too big :shock: One should always have a couple of drums in the background that always sneak into shot :grin:

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As there will be a 90 degree bend straight after it exits ths back panel I chopped it down to size.

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I agonised about what to use for the bend. The best cheap option was some black upvc fall pipe 90 degree bends. That will fit over the yellow pipe.

I had to do some work at the front by chopping off a bit of the tapering pipe that would not fit through the baffle cutout.

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The pipe is pushed in so it's end is 0.5mm behind the baffle's outer surface. This leaves enough room for a decorative cover to sit flush. Also I put another small 1cm long ring of pipe inside the first one which the driver's flange will sit on.

Here is one of the drivers inserted into the pipe. I made them round ages ago by gluing bits of 3mm thick plastic on. This looks crap of course but this and the yellow pipe will be covered by a decorative ring, not sure what yet.

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