Murphy speaker baffle 3 hole cut out

[simpsons]

Whilst I realise my topic may well fall below the standard expected of Members questions, many Murphy radios and, dare I say, TV, have a speaker baffle cut out in the shape of 3 circular holes.

Given Murphy were well known for their engineering prowess, why?

I would have thought putting a "barrier" in the front of a speaker would alter its frequency response negatively.

Can I also mention their console TV with both the 3 circles mentioned and a rectangular port. What was the port for? Can't remember seeing a sealed speaker enclosure mentioned.

Chris

[G6ONEDave]

Don't know the reason but it would be cheaper and easier to put round holes in rather than square, rectangular or slotted holes. It's not just the shape of the holes but also the toolage to create them.

Dave

[simpsons]

Hi Dave

Many thanks for your feedback.

The strange thing is that the "small" round hole cut out is on many but not all Murphy radios.

For example the A424, A272 do but the A 104 doesn't. In the latter case though, increased support for the baffle is designed in by having the circular cut out made in the form of 2 semi eclipses.

Thinking aloud, is this a case of over engineering to stiffen the baffle or is it a cost saving exercise?

Either way, should I make a baffle with only a small circular cut out, I would expect the sound to be quite different to an "open" cut out.

Is this a design to make the radio sound more mellow?

Chris

[Cathovisor]

Quite the opposite - it was to help diffuse the HF content more widely. It seems to be a variation of the patented KB "slot" diffuser.

https://www.diyaudio.com/community/t...ffusor.359705/

https://worldwide.espacenet.com/pate...pn%3DGB659063A

[Hartley118]

ISTR the early BBC monitor speakers used a KB-type slot in front of the main radiator. I understand its function in widening the horizontal radiation pattern, but have never been comfortable with the potential resonant effects within the cavity between the speaker and baffle.

Martin

[simpsons]

Thank you Cathovisor and Hartley 118 for giving some credibility to my Post.

I felt sure that from the design of their baffle radios to TV's, engineers at Murphy didn't stop at the electronics and chassis design but, three circular cut outs to diffuse HF; well, who would have guessed that.

Chris

[Cathovisor]

Originally Posted by Hartley118

ISTR the early BBC monitor speakers used a KB-type slot in front of the main radiator. I understand its function in widening the horizontal radiation pattern, but have never been comfortable with the potential resonant effects within the cavity between the speaker and baffle.

Martin

The prototype of the last large monitor speaker - the LS5/8 - did include a "KB slot" but I am led to believe the sound supervisors didn't like the "sound" and so it was deleted.

I find it quite amusing to remember how a certain Mr. Wells used to deride KB products but they were well in the vanguard of audio design in the 1940s and 1950s - you only have to look at the design of the FG50 radiogram to see this, especially where pick-up equalisation was concerned; if you ever encounter a red Decca XMS head with a blue baseplate, that was a special for KB.

[Cathovisor]

Originally Posted by simpsons

I felt sure that from the design of their baffle radios to TV's, engineers at Murphy didn't stop at the electronics and chassis design but, three circular cut outs to diffuse HF; well, who would have guessed that.

Chris

Murphy were already looking at sources of distortion in loudspeakers in the mid-1930s and the loudspeaker in the A40C was designed to overcome distortion caused by the excursions of the voice coil beyond the pole pieces. Said speaker also had a fabric surround and a rigid cone of "Bakelised paper".

[Edward Huggins]

It would be interesting to see some kind of polar diagram showning the effect on HF dispersion. I would have thought that such an arrangement would have put back-loading on to the cone.

[Cathovisor]

No polar diagrams here, but the design of the LS5/8 is covered here:

https://www.bbc.co.uk/rd/publications/rdreport_1979_22

My "family heirloom" - a KB MG30 radiogram from circa 1956 - uses three loudspeakers, all with slot diffusers and it really does seem to work.

I was somewhat surprised to learn that a commercial manufacturer still makes the LS5/8, albeit with different amplification.

[MurphyNut]

I suppose having the 3 small holes instead of one large would give the front baffle a stronger and more ridged construction. Maybe the designers thought this aspect would be more beneficial to the sound.
Their last Baffle board "winged" consul set had the 5 holes, rather than one big cut out.