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

Jim Taylor

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Perhaps I'm missing something here.
Let's take a 7" (frame) driver. The actual cone and surround is, of course, less. For example, the SEAS Excel W18NX is a 7" frame, but the cone is only 5.7" in diameter.


This driver is used in the SEAS Bifrost DIY speaker, which has a cabinet front of 200mm x 500mm. This results in a cone-plus-surround area that has a ratio to the baffle of roughly 16%, or 6.5 (baffle) to 1 (cone).

Will ALL back wave from the driver exit the cone? Or will only about 16% of the back wave actually hit the cone and surround? How much hits the rear of the motor? How much of that back wave will be absorbed in the box, bouncing around at angles that do NOT lead to an intersection with the rear of the cone and eventually being absorbed?

Not trying to muddy the waters, but I don't know how much of the energy generated by the cone actually returns to hit it, and returns at both an angle and a percentage that is significant.

Does anyone know how to find out?

Jim
 
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Tangband

Tangband

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Perhaps I'm missing something here.
Let's take a 7" (frame) driver. The actual cone and surround is, of course, less. For example, the SEAS Excel W18NX is a 7" frame, but the cone is only 5.7" in diameter.


This driver is used in the SEAS Bifrost DIY speaker, which has a cabinet front of 200mm x 500mm. This results in a cone-plus-surround area that has a ratio to the baffle of roughly 16%, or 6.5 (baffle) to 1 (cone).

Will ALL back wave from the driver exit the cone? Or will only about 16% of the back wave actually hit the cone and surround? How much hits the rear of the motor? How much of that back wave will be absorbed in the box, bouncing around at angles that do NOT lead to an intersection with the rear of the cone and eventually being absorbed?

Not trying to muddy the waters, but I don't know how much of the energy generated by the cone actually returns to hit it, and returns at both an angle and a percentage that is significant.

Does anyone know how to find out?

Jim
A guess is that without any damping material in the box, and the walls made of hard material, it should be near 100 % reflected sound that hits the back of the cone ?

The inverse square law states that with every doubling of distance away from the sound source, the sound will be four times less intense.

Said in another way: In the realm of acoustics, the inverse square law states that the intensity of sound decreases by approximately 6 dB for each doubling of distance from the sound source.

But here we are talking very short distances inside a loudspeaker box…

…and then there is damping material in all real loudspeaker boxes and depending on what material used, its gonna attenuate reflections more or less good. This seems very complex to me.
 
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ctrl

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But:
In real life, we use bigger boxes. A standmount loudspeaker can easily be 34 cm big on the inside , and you get a reflection in this case thats 2 ms delayed. Can one hear this ?

A floorstander can be 1 meter high , delayed reflection through the cone would then be more than 5 ms delayed - clearly audible.
Not trying to muddy the waters, but I don't know how much of the energy generated by the cone actually returns to hit it, and returns at both an angle and a percentage that is significant.

Does anyone know how to find out?

So with a CB floorstanding loudspeaker (we'll ignore BR concepts, since in most cases the port will be the main source of the unwanted, delayed sound) of 1m height, the damping material must be packed pretty tightly to avoid a standing wave inside the cabinet - so even low frequencies (100-500Hz) should be heavily damped.

In post#29 @No. 5 described how the reflected sound would affect the frequency response of the speaker. If the reflected sound would be attenuated only by -20dB, then in the frequency response for, for example, the reflection from the inner back of the cabinet should show a ripple in the FR (about +-0.8dB).

Since the path length always remains the same for each frequency, there is a frequency at which the phase shift of the delayed sound that passes through the drive cone causes the two sound components (delayed sound and main sound) to add up optimally, and a frequency at which both sound components cancel each other out optimally.
Therefore, the range around these frequencies (and their multiples) results in a rippled FR.

This is exactly what I tried to show in post#33 with two real examples. In the two nearfield measurements of the 15'' subwoofer and 4'' bass-midrange driver you can see a slightly wavy frequency response around the two frequencies for optimal addtion and cancellation.
There can be many reasons for this, of course, but if we were to assume that the cause of the less than ideal frequency response is solely due to the radiation of reflected sound through the cone of the driver, then the ripple would have the result shown there.

For a 4'' bass-midrange driver with a cabinet depth of about 0.2m and dense damping I would assume an average of +-0.3dB (see enlarged image) over a little more than an octave.
1667150067482.png 1667150204519.png

The 15'' subwoofer (cabinet depth of about 0.5m, with "normal" damping) unfortunately starts to break up at 300Hz (so we have to ignore frequencies above 300Hz). There I would assume an average of +-0.5dB ripple over a little more than an octave.
1667150654818.png 1667150707055.png
As said, if we would attribute the frequency response deviations completely to the influence of the reflected and time-delayed sound radiated through the cone.

I can't remember the sound pressure level in the near-field measurements, but assumed 95dB SPL should apply quite well.
Thus, the delayed sound radiated by the driver cone would cause a sound pressure change, in the worst case, of +-0.3dB to +-0.5dB at 95dB when added coherently - according to the examples presented.

Converting the difference from 95dB to 95.5dB (and 95.3dB), the SPL of the sound radiated through the cone would be 70dB and 66dB, respectively, an attenuation of -25dB and -29dB.

For a well damped loudspeaker, I would assume from the measurements shown that the delayed sound radiated through the driver cone is damped by significantly more than -20dB - in the shown examples shown rather -25dB to -30dB.
The -20dB attenuation quoted by @Tangband in post#3 seems a bit low, perhaps with lightly packed damping material.

And as @No. 5 had already pointed out, the assumed -20dB attenuation and the attenuation from the measured examples are in an inaudible range as far as sound image and spaciousness are concerned. Sound delayed by 5ms with -20dB or more attenuation does not change the perceived sound event.
1667154810288.png

Theoretically, it would be possible to perceive a slight change in tonality in an A-B listening test if the slight octave wide ripple shown in the two examples were due to the reflected radiated sound through the driver cone.
But something like this, of course, will be triggered by any edge or transition of the speaker (due to edge diffraction) - with more impact on the frequency response.
 
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Jim Taylor

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This seems very complex to me.

Me, too!
If this is a big concern, why do we not hear about it more? Doesn't it seem to be a non-issue? At least, it seems to not be an audible issue. Maybe I'm wrong, though.

Jim

p.s. - just read post by @ctrl. Makes a lot of sense. So it seems I was correct in surmising that it seems not to be an audible issue. :)
 
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fluid

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Its true that we humans cant hear 1 ms delay, we hear this as only one sound.

A floorstander can be 1 meter high , delayed reflection through the cone would then be more than 5 ms delayed - clearly audible.
I think you are mixing up how these different things would be perceived. 5ms is still generally in the fusion zone, where it creates a timber modification rather than being heard as a discrete reflection and processed differently in the brain.

I really don't understand why anyone would want to build a bad box where any of these effects would start to become a significant concern. In a good box sound coming back through the cone is extremely small as there are so many obstacles for that to happen. On the other hand sound can be re-radiated from the cabinet walls and standing wave resonances can create audible resonances if the size and shape of the box and how it is constructed and braced isn't paid much attention.

All of the things that you stand a good chance of hearing in the form of resonances can be seen easily in an impedance response. If there are no blips that don't appear in the drivers free air response then the job is done and it's time to move on.
 

fineMen

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Hmm…

Maybe Im calculating this wrong, but…

1ms soundtravel is 34,3 cm in 20 degree celsius. ... delayed reflection through the cone would then be more than 5 ms delayed - clearly audible.
Exactly as I said: You somehow ignore the frequency when considering--is it monophonic, delay of parts of the total output. I don't expect You to argue with group delay, do You? What is this all about, if I may ask?
 

fineMen

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So with a CB floorstanding loudspeaker (we'll ignore BR concepts, since in most cases the port will be the main source of the unwanted, delayed sound) of 1m height, the damping material ...
Synopsis: nothing to talk about. Regarding the deviation from (theoretically) totally flat, it can be addressed by the cross-over. Big speculations, heavy doubts, bad feelings from fantasy. High end is fear. Don't even dare to enjoy the music.
 
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