Discussion arising from Geddes loudspeaker

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frugal-phile™
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tinitus said:
I can relate to a horn sounding different with different "wood", sure that will be the case...but I think its a quite different matter with 2- or 3-way box
Well, in that acoustic respect(also) a closed box may even be better than BR

Everything matters... the idea is to not get the box to sound... and the low level stuff that oozes out of most MDF boxes does a lot to kill low level detail.

dave
 
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planet10 said:


Sorry John, but i think you missed a whole lot :)

dave



Oh, and here I thought the only real "data" in thread was the results of my testing.

Personal preference, listening impressions and learned opinions are not data.

MDF is just fine for building speakers. You don't need to feel you have lowered your standards when you use it, as it is just as good, if not better than the other choice's.
 
From the FR tech archives:

Absorbation of energy in a given medium:

I keep on seeing the debates on what constitutes a good build material. Well like anything there are positives and neagtives to any material selection and application.Its covered in engineering materials.

Absorbtion of mechanical energy depends on density and elasticity of a medium given that the medium is a near constant in mass. This is more like MDF. It has a reflective index based on mass and impedance value and an abosrbation based on thickness. Many do not like the sound of MDF but that is due to the lower resonance frequency of the material for a given wavelength. In a strictly BR or TL alingment it will perform in an adequate manner as even a TL may be a totally resonante device, its dependant primarly on a given pressure and phase point and not on harmonics at a given frequency.

Horns are different as they depend primarly on harmonics to blend in with different actions. A good material for BLHs is a multi layered composit as the harmonic range is greater than a constant mass/density material which is more monocromatic in resonance. On the opposite side is the fact that a composit material can abosrbe more energy due to the fact that as any energy that traveles thru the mass is faced with different impedance values and the energy loss is greater due to the many interfaces of different impedance values.
ron
 
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ronc said:
A good material for BLHs is a multi layered composit as the harmonic range is greater than a constant mass/density material


Hi Ron,
What is a horn? Isn't a horn the interface between the driver and the rest of the world? Isn't the purpose of the horn to provide a low impedance transition for the loudspeakers energy?
How does the building material relate to this process, unless it is allowed to resonate without control?
 
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ronc said:
On the opposite side is the fact that a composit material can abosrbe more energy due to the fact that as any energy that traveles thru the mass is faced with different impedance values and the energy loss is greater due to the many interfaces of different impedance values.
ron


You lost me there...I obviously dont understand english good enough:D
 
Any acoustical energy that encounters an interface of a different medium losses energy. If the energy is encountering several interfaces there is more energy loss than a monocromatic medium.

The best abosrber is an ever increasing impedance value with several interfaces.

Example! Your car strikes the water barrels ( God forbid) at the exit of a freeway. The first barrels are filled with 25% water, the second are filled with 50% water,the third are filled with 75% water the final are 100% filled. This is a progressive index of energy absorbation and allows for less reflected energy. Now the barrles are normally V shaped so you are striking an ever increasing resistance. If all the barrles were filled 100% the absorbation rate would more near linear. If its progressive the absorbation rate is more near a log function.

I have studied HF sound waves for many years in the range of .5 to 20 Mhz. Simply stated the total abosorbation of energy thru several interfaces is greater than the sum total of the impedance values of the different mediums.

ron
 
Tinitus,

Think of a muffler in a car exhaust -- the varying impedances due to cross section changes etc present partial reflection points for the soundwave, and in this "hall of mirrors" it looses it's energy in a well damped and distributed manner, instead of exiting a narrow-bandwith high Q ringing.

A good paper on cabinet issues etc:
http://www.bwspeakers.com/downloadFile\technicalFeature\800_Development_Paper.pdf

Personally I think for smaller CB/BR enclosure MDF or HDF is fine in a well executed design with a lot of bracing. Bigger boxes and especially horns might profit from more rigid stuff if one manages to get the resonances above (or below) the passband. Big MDF horns built from flat panels often sound lame IME, from the few MDF horns I've heard.

- Klaus
 
How does the building material relate to this process, unless it is allowed to resonate without control

Thats the point, design in the resonance value. All materials resonate, and the frequency/value changes with the mass/medium and design.

I have a poster in my office which shows the Tacoma Narrows bridge ( before it gave up and fell into Puget Sound) doing its snake dance, the poster reads " sometimes we make mistakes". You have to consider any and all variables ( as much as you can anyway) in any design.

ron
 
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KSTR said:


Personally I think for smaller CB/BR enclosure MDF or HDF is fine in a well executed design with a lot of bracing. Bigger boxes and especially horns might profit from more rigid stuff if one manages to get the resonances above (or below) the passband. Big MDF horns built from flat panels often sound lame IME, from the few MDF horns I've heard.

- Klaus


Most of the "horns" I've seen here don't qualify as such. Take a contraption that has large, unbraced panels and a low frequency roll off of ~200Hz and you are bound to hear some problems.
Regardless of what they are made from.
 
frugal-phile™
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MJL21193 said:
Oh, and here I thought the only real "data" in thread was the results of my testing.

You must have missed the attached real "data" :) LF resonances much worse than HF ones...


MDF is just fine for building speakers. You don't need to feel you have lowered your standards when you use it, as it is just as good, if not better than the other choice's.

I'm sure if you really believed that MDF was better you wouldn't have said this...

MJL21193 said:
I love working with BB, and to be honest, I'm not going to be using MDF anymore. I will use BB and cover that with HDF if I want to paint.

dave
 

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planet10 said:


I'm sure if you really believed that MDF was better you wouldn't have said this...


Ah, doing some digging.

I will not be using MDF for a few reasons, completely unrelated to it sonic quality. I won't bother going into those reasons here.

The tests I ran tell the story. No mumbo-jumbo. The mic will "hear" a difference, if there is one.
 
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ronc said:


Thats the point, design in the resonance value. All materials resonate, and the frequency/value changes with the mass/medium and design.


Do you have those figures - the resonant frequency of MDF and Baltic birch plywood? How far apart are they?
Resonances can be controlled by bracing and damping, so why would your design need to accommodate it?

I have seen many pictures of single driver "horns" sitting in the middle of a tastefully decorated living room, producing next to nothing below 200Hz, and resonating quite happily with large unbraced, undamped (heaven forbid) panels.

To build these from BB plywood is an all out affront to nature. A pure waste of a good building material.
 
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