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Old 3rd September 2012, 07:13 PM   #11
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Originally Posted by weiyan View Post
Usually diy box maker us 25mm mdf face and 18mm mdf for the rest. There are hard wood box. Are these good material for speaker?
You'll get lots of differeing opinion, but i don't think MDF is a good material for making loudspeakers.

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Old 3rd September 2012, 07:54 PM   #12
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Hi,

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Originally Posted by twest820 View Post
Any chance you could post the data somewhere? Good accelerometer measurements are rather hard to find, particularly on three ways.
no, I'm sorry, I accidentally hit the wrong button and all data was lost. But I will do some more tests in this case, and then it will be easy to repeat the measurements.

Quote:
Originally Posted by planet10
The opposite is more typically true. Raising the frequency of a (potential) panel resonance higher will (usually) make it less likely to get excited (as will increasing its Q).
It will shift the frequency higher, increase the Q, and as the input force is constant, it will be excited like the lower resonances, but with at least equal amplitude. That's what all publicly available and my own measurements show.

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Old 3rd September 2012, 08:11 PM   #13
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and as the input force is constant
But it isn't. Input force decreases by the square of frequency

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Old 3rd September 2012, 08:18 PM   #14
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What I have not seen discussed much here is the advantages of forcing materials away from a slab. I have an idea for a low diffraction box of only simple curves. Think about a wide oval cylinder on end with the top a simple curve leading away from the baffle. ( my use case requires it to sit on a shelf, so a round bottom is not viable.) The two edges basically perpendicular to the wavefront. Yes, MDF looks like it will be the material, but laminating several layers of thin hardboard may be better. This is a couple of projects away. Can't say 3/4" MDF is a great material, but I have not had better luck in the 10 to 20L size with other materials. I have no idea where to get that super-duper CF reinforced 9-ply 1/2 inch spruce grown from only north side trees in Norway. I have to deal with what Home Despot sells. I can BUY quarter sheets of MDF which automatically makes it a suitable material.
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Old 3rd September 2012, 08:31 PM   #15
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hi dave,

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Originally Posted by planet10 View Post
But it isn't. Input force decreases by the square of frequency
you are arguing against Newton's third law.

The acceleration of the driver's cone/membran/anything is constant above its resonance frequency*. Otherwise, you wouldn't have a linear frequency response. Thus, the opposing force on the cabinet is constant. There is no mysterious 1/f, only for the energy, but energy is not equal to amplitude (a 1h signal of level 1 has the same energy as a 1 Minute signal of level 60).

* In real applications there will be usually a 6 dB down step in acceleration due to the baffle step, so this might be beneficial.

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Old 3rd September 2012, 10:02 PM   #16
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Originally Posted by Baseballbat View Post
There is no mysterious 1/f, only for the energy
Ues i should have said energy. It is energy that excites the resonance.

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Old 4th September 2012, 03:08 AM   #17
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Quote:
Originally Posted by planet10 View Post
The opposite is more typically true. Raising the frequency of a (potential) panel resonance higher will (usually) make it less likely to get excited (as will increasing its Q).

dave
Still waiting to see your data on this since all the plots I've seen show fairly even distribution of resonant peak level across the frequency band. Harwood and Shorter showed that greater wall stiffness actually raised peak level (made damping less effective).

There is no simple relationship between Q and likelyhood of excitation. With a mechanical system, if Q is increased (resistance reduced) then peak height goes up. Area under the curve increases and likelyhood of excitation goes up. The only way to reduce resonant bandwidth while keeping the same resonant frequency would be to increase mass reactance and stiffness reactance in step, hard to do.

The fact that peak heights are fairly constant is proof that the resonances are equally likely to be stimulated.

David S
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Old 4th September 2012, 03:26 AM   #18
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Harwood and Shorter showed that greater wall stiffness actually raised peak level (made damping less effective).
The BBC study is what it was. It begs more research.

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Old 4th September 2012, 04:15 AM   #19
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Quote:
Originally Posted by planet10 View Post
But it isn't. Input force decreases by the square of frequency

dave
Internal SPL does tend to roll downwards with frequency, but accelerative force from the woofer chassis (reaction force) is constant. Baseballbat is correct that all published data shows a fairly broad trend to resonant output, rather than any 1/f^2 trend.

David S
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Old 4th September 2012, 06:10 AM   #20
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The BBC study is what it was. It begs more research.

dave
Here is some more.

http://downloads.bbc.co.uk/rd/pubs/reports/1958-31.pdf

Page 20 (25 of the PDF) shows the output of a cabinet with and without a damping layer. Strong sequence of resonances from 200 to 3000 Hz.

This was a fully sealed cabinet with a buried internal driver.

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