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How much delayed sound comes through the cone from the inside of the loudspeaker in a closed box ?

Tangband

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One of the reasons Linkwitz didnt like the sound of boxed full range speakers was because of the delayed sound coming through the cone and giving coloration to the sound .
In the lxmini he used a 80 cm long pipe heavily stuffed with damping material at the bottom of the pipe , and with damping material all the way up to the driver . This avoided the reflected sound coming from the pipe and through the cone with about -40 dB .

I wonder how big this problem is with a more normal loudspeaker . If I got a 6,5 inch driver put in a rectangular box of 15 liters, with some light damping material inside , how much of the delayed and out of phase sound from the inside of the box will find its way through the cone, thus coloring the sound?

Is it in the magnitude of -20 dB below the direct sound from the driver , or is it less ?
Can it be heard ?
Is there any investigations done at this subject?

About dB and distortion levels :

Linkwitz investigations :

”When a speaker driver is mounted in a box it radiates as much energy into the space in front of the cone as it does into the much smaller space behind the cone. What happens to the air borne energy inside? At long wavelengths it is common practice to store it in resonant structures to extend the steady-state low frequency response of the speaker. In general, the energy leads to very high sound pressures inside the box. A small amount of the energy is lost as heat in the stuffing material, some in the process of flexing the cabinet walls. Much of it reappears outside the box, because the thin cone presents a weak sound barrier. Just how much is difficult to measure, but it is a contributor to the frequency response. I am of the opinion that the effect is most notable in the low hundreds of Hz region, where stuffing materials are ineffective and the internal dimensions not small enough for the internal air volume to act as a pure compliance. Consequently, enclosures should be either very small (less than 1/16th of a wavelength) or extremely large, both of which are not very practical for different reasons. ”
 
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MaxwellsEq

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My immediate thought is that it must be dependent on the material and construction of the cone and surround.
A possible experiment with a ported speaker would be to place a measurement device inside the speaker and use another speaker to sine sweep the box, e.g. from a metre away. This would give a sense of attenuation at various frequencies for that cone.
 
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Tangband

Tangband

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DanielT

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From the Swedish forum Faktiskt.io, there are some people stating that the sound from the inside of the box trough the membrane is at about -20 dB in a normal speaker. No measurements was shown.
If this is true, - I think -20 dB is a high number and should be clearly audible ?

From that thread, translated via Google Translate what I-or writes in a post:

From a cavity with a well-dimensioned absorber, the sound power transmitted through the cone will not be -20 dB relative to the front, but tens of dB lower. Up to the break-up region, the cone acts as a pure mass (also rigidly connected to the mass of the voice coil), which makes it very unlikely to let through sound waves that are only reflected back into the cavity. In the break-up area, the frequency is so high that there is almost total absorption, which is why the sound effect incident on the cone is close to zero.

For a well-dimensioned speaker construction, one can thus forget about considerations of audible problems from transmission through the cone.

However, the breaks in the speaker cone and resonances in the surround can lead to clearly audible effects for the normal sound radiation from the front, as well as uncontrolled cavity resonances can affect the cone movements with an equally clearly audible effect. It therefore applies, as usual, to focus efforts on the true problem areas.


 

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Tangband

Tangband

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My immediate thought is that it must be dependent on the material and construction of the cone and surround.
A possible experiment with a ported speaker would be to place a measurement device inside the speaker and use another speaker to sine sweep the box, e.g. from a metre away. This would give a sense of attenuation at various frequencies for that cone.
Hmm…A passive radiator inside a closed box has very high levels of reflected sound, mainly at the box and radiators resonance frequency. I wonder how much damping of the active drivers cone the amplifier can do with a low output resistance ?
There is still the very thin cone material in the driver - common sence says that the sound reflected from the inside of the box and going thru the cone would be audible. I wonder why there are almost no measurements to be found about this on the internet ?

An experiment - If a person goes in to a very small room with walls, and another person is in the room on the otherside, and the person in the small room shouts in a very high voice , - wouldnt the sound be heard in the nearby room ?

I guess it depends on how the walls are made for the rooms. A 30 cm thick concrete wall would do a lot of sound damping, but a 2 cm thick mdf wall would be less good ?

Maybe a bad comparison, but anyway…
 
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Tangband

Tangband

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From that thread, translated via Google Translate what I-or writes in a post:

From a cavity with a well-dimensioned absorber, the sound power transmitted through the cone will not be -20 dB relative to the front, but tens of dB lower. Up to the break-up region, the cone acts as a pure mass (also rigidly connected to the mass of the voice coil), which makes it very unlikely to let through sound waves that are only reflected back into the cavity. In the break-up area, the frequency is so high that there is almost total absorption, which is why the sound effect incident on the cone is close to zero.

For a well-dimensioned speaker construction, one can thus forget about considerations of audible problems from transmission through the cone.

However, the breaks in the speaker cone and resonances in the surround can lead to clearly audible effects for the normal sound radiation from the front, as well as uncontrolled cavity resonances can affect the cone movements with an equally clearly audible effect. It therefore applies, as usual, to focus efforts on the true problem areas.


Interesting .

I-or statements differ slightly from I.Ö:s and Bosse Bengtssons.

Bosse Bengtsson, the constructor of Megatrend ( picture) , says in this thread that the sound coming back from the cone in a closed box are about -20 dB at worst ( he investigated it )


Bosse Bengtssons words using Google translate:

”Can only add that I have had the same experiences. About 20 years ago, I amused myself by inserting a 1-liter box inside a 40-liter closed "normally damped" box, equipped with an 8-inch bass/midrange. The small box inside had a 2" wideband element mounted.

Next, I played the small element with the connection to the 8" element a) open, b) shorted and c) with the 8" element connected to its own amplifier in the "on" position but without supplying any signal.

The different loading conditions of the 8-inch driver did produce differences, but in neither case did signals leak through the 8-inch diaphragm greater than -20dB below the SPL I measured with a near-field microphone inside the 40-liter box. The "leakage" was greatest in the area where the 8" driver itself gave rise to diaphragm ruptures. With a dedicated woofer instead, the result would have been even less leakage overall.”

D901CF88-BA58-4BCC-A899-E8D5746A579A.jpeg
 
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DanielT

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Interesting .

I-or statements differ slightly from I.Ö:s and Bosse Bengtssons.

Bosse Bengtsson, the constructor of Megatrend ( picture) , says in this thread that the sound coming back from the cone in a closed box are about -20 dB at worst ( he investigated it )


Bosse Bengtssons words using Google translate:

”Can only add that I have had the same experiences. About 20 years ago, I amused myself by inserting a 1-liter box inside a 40-liter closed "normally damped" box, equipped with an 8-inch bass/midrange. The small box inside had a 2" wideband element mounted.

Next, I played the small element with the connection to the 8" element a) open, b) shorted and c) with the 8" element connected to its own amplifier in the "on" position but without supplying any signal.

The different loading conditions of the 8-inch driver did produce differences, but in neither case did signals leak through the 8-inch diaphragm greater than -20dB below the SPL I measured with a near-field microphone inside the 40-liter box. The "leakage" was greatest in the area where the 8" driver itself gave rise to diaphragm ruptures. With a dedicated woofer instead, the result would have been even less leakage overall.”

View attachment 237318
Okay, but what do you think should be done? Do you think the problems are big enough to skip box speakers and use dipoles, open baffle speakers?
 
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Tangband

Tangband

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Okay, but what do you think should be done? Do you think the problems are big enough to skip box speakers and use dipoles, open baffle speakers?
Well, thats a really good question. I have not enough experience from listening to dynamic open baffle speakers, but you might have even more problems here because you always play the music in a real room, with reflections from the walls and floor. There are for sure some gains to be had, but probably also some drawbacks.
 
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Tangband

Tangband

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I think that -20 dB is a very bad attenuation of the internal box-sound. If it is this bad , its for sure a problem. Here we often demands a SINAD of 110 with dacs , but why should we accept a SINAD of only 20 in a closed box loudspeaker ?

I think its strange that noone have real measurements from investigations of this.:(.
Please link or post If you have some interesting measurement.
 

DanielT

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Well, thats a really good question. I have not enough experience from listening to dynamic open baffle speakers, but you might have even more problems here because you always play the music in a real room, with reflections from the walls and floor. There are for sure some gains to be had, but probably also some drawbacks.
Here in this thread you can see a couple of, as it seems, good dipoles. They most likely have a lot of dynamics::)

Duke:

I think Perry Marshall's controlled-pattern fullrange dipole speaker is BRILLIANT.

Also, page 2 starting #31 where P himself describes them and discusses them with Duke:

Perry Marshall:

You can see extensive design details and measurements for the Bitches Brew Live Edge Dipole speakers here:

(description, pictures and measurements)
Screenshot_2022-10-16_160041.jpg



Link to the thread where Duke and Perry Marshall discuss:

 

olbobcat

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I am guessing that the damping factor of the amp would also play a part in this as well?
 

Kal Rubinson

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One of the reasons Linkwitz didnt like the sound of boxed full range speakers was because of the delayed sound coming thru the cone and giving coloration to the sound .
In the lxmini he used a 80 cm long pipe heavily stuffed with damping material at the bottom of the pipe , and with damping material all the way up to the driver . This avoided the reflected sound coming from the pipe and thru the cone with about -40 dB .
KEF has long been concerned about this and offered a similar loading for the midrange of a design some five decades back.
Also note their current MAAT (Meta) efforts for the HF.
 

Vacceo

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KEF has long been concerned about this and offered a similar loading for the midrange of a design some five decades back.
Also note their current MAAT (Meta) efforts for the HF.
Didn't B&W tried to address that with the long pipes of the Nautilus?
 

puppet

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I've always found this topic interesting and one observation I'd make is the environment inside the enclosure is more pressure than velocity. Could be that makes the difference in audibility.
 

DonH56

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I am not a speaker designer and the coursework I had on the subject was long ago. However, for leakage to occur, it seems like two major things must be considered: the stiffness and mass of the driver (cone), and the driving impedance. Here are my hand-waving thoughts.

A very stiff cone will move as a unit, and the mass and spider/surround compliance will tend to resist movement, but in my mind the biggest resistance to influence (movement) from the internal ("back") wave is the amplifier. The amplifier will attempt to move (or hold) the driver in response to the electrical signal, and resist any external influence like pressure waves from within (or outside) the box that tries to move the voice coil from the intended path. So the suppression of the internal wave would depend mainly upon how low the driving point impedance seen by the speaker, a function of the amplifier, cables, and crossover.

A perfectly stiff cone driven by a zero-impedance amplifier should not be able to move except to follow the signal from the amp. A "floppy" cone driven by a low source impedance might allow the cone material to move even with the voice coil held in place by the amp. I suspect that would happen at higher frequencies where the cone "ripples" in response to applied waves. A "stiff" cone driven by a amp with high output impedance (e.g. tube amp) could move in response to pressure waves hitting it because the amp is not "holding it in place" or forcing it to ideally follow the signal. That could happen over a wide range of frequencies, and of course cone breakup (modes) could still occur.

I imagine many of us have spun a motor or fan shaft with no power applied, or pushed on a speaker cone with the amplifier off. Turn on the motor, or amplifier, and moving the shaft or cone becomes much harder.

Since the goal of most speaker designers (IMO, I am not one) is to use drivers that are stiff enough to not break up (distort) in normal operation, and amplifiers are generally designed to provide a low output impedance, I would expect minimal sensitivity to internal pressure waves. Stiff cone, good amp, no problems. That's my theory, anyway. :) As for the amplitude, I do not know, though -20 dB seems high and I would expect that would show up in measurements, perhaps by a change in amplitude linearity since the back wave during a single-tone test would presumably be the same frequency but different in phase compared to the driving signal.

Passive radiators, or a driver not connected to anything (like the amp), will of course move in response to a signal (pressure) whether front or back, depending upon the compliance of the spider, surround, and any other mounting considerations. That is desirable in a passive radiator, though of course ideally the whole cone would move as a single unit. That requires a stiff cone; the mass (and spider and surround, among other things) will determine its resonant frequency. The cone itself may flex in response to large signals at various frequencies, e.g. exhibit break-up modes, that are undesirable.

Random thoughts - Don
 

Birdy

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If we put in the equation that on top of the sound reflexions in the box returning by the speaker, the added sound of the enclosure it self, ads even more dB to the mixture. I was in the understanding that my PVC pipe enclosure would not add much sound coming from the round shaped pipe material, but it did. I put my ear against the 6 mm / 30 cm diameter pvc pipe at the side of the speaker and the mid band was clearly coming trough the housing. After i stuffed the enclosure and added felt to the walls whitch reduced significantly the output of the housing and the sound was more clear. This is not science but simple empiric trial and error. Probably reflexion in the enclose will find it's way out by the driver, depending on the cone material (polycone, metal ect.) and it's internal damping. Good reason to use lots off damping to avoid returning output. To my subjective hearing, sound / internal reflexions passing the cone is reduced by damping material.
 
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puppet

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If I'm reading DonH56 correctly, the internal pressure may have greater influence on the cone drivers' response (what we hear) vs any "leaking sound contribution". I tend to agree with that.
 

solderdude

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Mount 2 speakers in a box. Connect 1 to an amplifier and short the other one.
Ensure the speaker that does the sweep is not measured (dampen sound or whatever) and measure what comes out of the shorted speaker and you will know how much it is.
Then there is the speed of sound + distance from the back, usage of damping material etc.

I used to build transmission lines and 'directed' the rear sound into the TL under a 45degree angle.
Later on I wasn't that fussed about it as the midrange had its own 'box' and tweeters were sealed anyway.

The problem also exists in closed headphones, the distance is much shorter though resulting in possible issues above 1kHz or so.
 
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