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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 28th May 2020, 8:06 pm   #1
stevehertz
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Default Curtain inside hifi speakers

In around 1972 as part of my four year apprenticeship at Thorn EMI Electronics, I spent a few months in the service department. A few of the guys there were into hifi and one of them had fitted curtains inside his hifi speakers. Apparently it was a DIY project in either a hifi mag or an electronics mag. The curtain was hung behind the drive units towards the back of the cabinet and presumably its purpose was to dampen sound waves from the back of the drive units. The one guy said he had tried it and couldn't hear any difference which didn't surprise me really, especially if the cabinet was already damped to some extent.

Apart from being a quirky thing to share, I wondered if anyone else had heard about this 'mod' and especially if they recall the source of it in a magazine as a DIY project.
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Old 28th May 2020, 8:09 pm   #2
paulsherwin
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Default Re: Curtain inside hifi speakers

I vaguely remember this idea. I think it was to reduce standing waves, though the fact that no commercial manufacturer adopted it speaks volumes.
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Old 28th May 2020, 9:02 pm   #3
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Default Re: Curtain inside hifi speakers

I think commercial designers found that BAF wadding was more effective and didn't flap about in the same way. I remember reading the article in the dim and distant past but can't recall the publication responsible.

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Old 28th May 2020, 10:53 pm   #4
Ted Kendall
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Default Re: Curtain inside hifi speakers

It's quite an effective idea - hanging blankets were used in BBC studios in earlier times and are still used in bass traps for commercial studios. Problem with using them in speakers is that it depends on the user keeping them the right way up.
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Old 29th May 2020, 7:30 am   #5
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Default Re: Curtain inside hifi speakers

Absorbent materials work by turning sound wave air movement into heat, so are most effective in damping standing wave resonances if positioned at points of maximum air ‘particle’ velocity. Unfortunately, the walls of an enclosure are inevitably points of minimum velocity (and maximum sound pressure). So if the designer wants to damp any low-mid frequency resonances, a lining has to be pretty thick to have much effect. An absorbent ‘curtain’ hanging in the regions of maximum particle velocity within the cabinet is much more effective.

However, as has been pointed out, a curtain doesn’t tend to travel well, so tends not to be commercially attractive. There’s also the question of how many of the internal resonances actually trouble the listener if the cabinet is substantial and the speaker drive unit has a relatively heavy cone, thus reducing sound transmission from the cabinet interior to the exterior.

Martin
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Old 29th May 2020, 9:13 am   #6
rontech
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Default Re: Curtain inside hifi speakers

I think in the late 1970s long staple wool was recommended for that purpose.

At one time I had some large Lentek transmission line speakers which a friend gave me.

One speaker seemed to have rather "rattly" bass and lower mid range problems. On opening up the cabinet I found a large roll of long stable wool sitting at the bottom of the cabinet. I fixed it back in the right position ( checked from the other speaker ) and all was well.

I had brought the speakers all the way from Cornwall to Southport so the comment about not travelling well clearly applied.
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Old 29th May 2020, 11:12 pm   #7
Ted Kendall
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Default Re: Curtain inside hifi speakers

References to "Dr Bailey's long fibre wool" peppered loudspeaker adverts during the 70s. Bailey had used it in a speaker design for its effective broad band absorption, and the self-build sector particularly adopted it with relish.
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Old 29th May 2020, 11:30 pm   #8
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Default Re: Curtain inside hifi speakers

Bailey's wool also reduced the velocity a little, so transmission lines were effectively a little longer. Chris Rogers' design put the wool in in different densities along the line to selectively damp harmonic energy. Seems to work well.

David
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Old 29th May 2020, 11:56 pm   #9
m0cemdave
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Default Re: Curtain inside hifi speakers

Quote:
Originally Posted by Ted Kendall View Post
Bailey had used it in a speaker design for its effective broad band absorption,
Ready availability too - he was based in Bradford which, at that time, was still the centre of the wool trade.
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Old 30th May 2020, 9:08 am   #10
rontech
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Default Re: Curtain inside hifi speakers

Quote:
Originally Posted by Ted Kendall View Post
References to "Dr Bailey's long fibre wool" peppered loudspeaker adverts during the 70s. Bailey had used it in a speaker design for its effective broad band absorption, and the self-build sector particularly adopted it with relish.
Is that the same Dr. Bailey who worked with Arthur Radford on the excellent Radford valve amplifiers? About 1962 to 1964 I believe.
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Old 30th May 2020, 9:33 am   #11
Craig Sawyers
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Default Re: Curtain inside hifi speakers

Long fibre wool is very much still available

https://www.woollyshepherd.co.uk/pro...dspeaker-wool/
https://www.qtasystems.co.uk/loudspe...-materials.htm

And others too. Google is your freind

I built the Bailey transmission lines back in the day, and used them in the mid 70's to early 80's. I seem to recall that he gave an address from which you could get the long fibre wool.

Craig
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Old 31st May 2020, 3:46 pm   #12
sp10mk11
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Default Re: Curtain inside hifi speakers

The Boffle
A friend at schools grandad did a sort of curtain arrangement, where by squares (the cabinets where square) of carpet felt with a round hole were fitted each one a different distance from the next and so on and with a reducing in size hole these were also open back so to speak.
Gary
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