I do like his conclusions. I just have a bit of trouble thinking of a standardized method. What it looks like to me is a shape of fiberglass suspended right in the center of the box..?
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The half wave and odd multiples just have pressure antinodes in the middle in common. Moving the driver around the nodes also changes damping.
CSD presentation of impulse response done in nearfield (less than 2cm from cone) will tell about driver resonances and box's internal "backwave leakage"
Frequency response measurement will tell some of that too - both and more come easily with modern FFT-based measurement programs.
If anyone is interested to see how her/his speaker performs, first measure the driver in the box and then take it out, place on a large baffle and measure again (use same crossover or measure both without it). The difference that you will see comes from what happens inside the box!
Who will do this first?
Frequency response measurement will tell some of that too - both and more come easily with modern FFT-based measurement programs.
If anyone is interested to see how her/his speaker performs, first measure the driver in the box and then take it out, place on a large baffle and measure again (use same crossover or measure both without it). The difference that you will see comes from what happens inside the box!
Who will do this first?
Regarding the effect of stuffing on effective cabinet volume, just to clarify: the stuffing converts converts near-adiabatic expansion/compression to near-isothermal. This is down to the thermal capacity of the stuffing material. The end result is that the effective volume of the enclosure is increased (in practice, by up to about 20%).
This can be taken into account when designing for a given bass response, as the system Q depends upon effective enclosure volume.
This can be taken into account when designing for a given bass response, as the system Q depends upon effective enclosure volume.
I know where you are going here. Well, at least you are trying. Those nasty evil boxes! Those no good rotten boxes should be banned forever !!
Let's just think about this for a moment. If there was no "box" there would not even be a need for discussing this first back wave reflection nonsense.
Free yourself from the box ! Rejoice and live for wrap-around cancellation and increased modulation distortion ! yes, that is the way !!
Of course, as an alternate, one could just design and build a proper and confident speaker enclosure. It's a tedious and time consuming adventure, to be sure. All the answers have already been given.
Happy New Year to all !
Well, I've banned them from my house! I've seen the light and multi-way OBs have replaced them - there's no looking back. (No boxy colouration, plus the benefits of dipole radiation pattern.)
As long as the speakers don't need to be close to a wall behind, I heartily recommend OBs.
(Sorry to be totally off-topic!)
Keith, I appreciate your enthusiasm !
However, I don't share it !
OB's seem to have quite the following, but I submit, they are not for everyone.
One way I've seen of doing it is to send clicks to the driver, and watch the reflections on a scope. The first click back is straight from the driver, and then there's usually basket, side walls, back wall and some composite. By checking the timebase and figuring out the time-of-flight, you can find exactly where the reflections are comng from.
The tallest peaks show the loudest reflections, but later reflections should be prioritised, since IIRC audibility increases with time delay.
Chris
Maybe because of the qualities of the stereo imaging you get with dipoles...
Back on-topic though, I was struck by the contents of this thread:
http://www.diyaudio.com/forums/multi-way/264621-maxs-behringers.html
In particular, the use of "acoustic pinboard" to seemingly completely solve the problem of back-radiation.
Thanks. Very informative. Derek already offered very similar to that as a solution in a different thread. I use something similar as well on any conventional back panel that I encounter, plus I use pink panther to fill the enclosure. It's getting to where this has been repeated several times now.
I've been exchanging on this subject with Martin J King, most known for his work with transmission lines. Below I'm copy/pasting his answers. I think he's basically agreeing with Speaker Dave, and going counter to apparently widely held paradigms here.
1st e-mail:
Hi Horacio
A 10 liter box sounds pretty small for a 10" woofer. Assuming 10 liters is correct then this is a cube with 0.215 m sides. The first standing wave in this enclosure would be at the following frequency.
344 / 2 / 0.215 = 800 Hz
You would excite a standing wave from front to back at 800 Hz, if the driver is centered on the baffle the first standing wave side to side or top to bottom would occur at 1600 hz.
If your max usable frequency is in the 400 to 500 Hz range then I think a simple cube would work real well, the box would act as a nice air spring without any standing waves messing up the response.
Martin
2nd e-mail:
Hi Horacio,
"Thanks for the quick reply. The piece I'm not tracking is why you focus on the first standing wave and above frequencies? All frequencies in the passband, namely 80 to 500Hz, will be reflected back and potentially bounce back from the back wall. Right?"
(MJK) No, I do not believe that is correct.
Going back to your 10 liter box. If it is a cube then it has half wavelength standing waves at 800 Hz, 1600 Hz, 2400 Hz, ... and integer multiples of 800 Hz in each direction (front to back, side to side, and top to bottom). For now let's assume that the box is empty, no fiber damping, and the driver can be treated as a source at the center of the front baffle.
Working in the frequency domain, each excitation frequency can be thought of as the driver's cone oscillating in and out of the box and the air compressing and uncompressing all happening at the same frequency. At the low frequency tuning fc, the air in the box acts as a second spring (the spider is the first spring) pushing on the back of the driver's cone raising the driver's resonant frequency from fs to the closed box speaker's fc and the driver's Qts to the closed box speaker's Qtc of 0.71 as you described. If I remember correctly the acoustic impedance of the sealed box at low frequencies is
Z_ab = C_ab = V_b / (rho x c^2)
Once the frequency is above fc the SPL output is ideally a function of only the cone mass and the stiffness and damping represented by the box and the driver's suspension are no longer important. This is true until the frequency rises to the point of exciting a standing wave in the box.
At 800 Hz the first standing waves in all three directions are excited. Two of the standing waves, top to bottom and side to side, have a velocity maximum and pressure zero at the driver location in the center of the baffle. The front to back standing wave has a pressure maximum and zero velocity at the front baffle pushing against the back of the driver's cone. Acoustic impedance, Z_ab, is the ratio of pressure over volume velocity. So in two directions, top to bottom and side to side, Z_ab is zero and in the front to back direction it is approaching infinite. The front to back standing wave's acoustic impedance will almost stop the driver's cone motion leading to a deep sharp null in the SPL response at 800 Hz. The top to bottom and side to side standing waves will not be excited because of the driver location with respect to the standing wave.
At the higher harmonics a similar situation occurs where an infinite acoustic impedance is seen by the back of the driver's cone, stopping the motion, and creating deep sharp nulls in the SPL response. If we now factor in fiber damping in the box then the standing waves will be significantly attenuated and the deep nulls become much broader and shallower. If enough damping is used in the right location the standing waves are completely damped and the SPL response is smooth again.
Based on this description of the dynamics of the air in a sealed box two of the create audio myths should be called into question.
1) Sound waves do not reflect or bounce off the back wall of a box and exit through the cone. Discrete standing waves can be excited leading to nulls in the SPL response but fiber damping can be used to suppress this problem. I find it hard to believe that sound waves in a subwoofer box can be transmitted through a structural driver cone and be audible in the room.
2) Parallel or non parallel walls in a box will both have standing waves. The frequencies and mode shapes will be different but they will exist. The exact same problem is common in both arrangements and the same solutions can be applied.
So in your case, where you want a sealed 10 liter subwoofer operating from fc to a maximum of 500 Hz, I think building a cube and adding a light amount of fiber stuffing is all you need. Don't over think it.
Martin
3rd:
Hi again Horacio,
Continued from last night's response.
What I described below was an ideal closed box SPL frequency response (like one calculated by some of the freeware simulation programs) with a series of sharp deep nulls at 800 Hz, 1600 Hz, 2400 Hz, ... and so on superimposed. Assuming no crossover, the frequency response looks somewhat like a comb, it is flat in the pass band with a series of "teeth" sticking down. Remember, this is all in the frequency domain.
If we convert the SPL back to pressure (SPL is an indication of normalized sound pressure and remembering to include the SPL's phase) and then inverse FFT the pressure frequency response to get the pressure's time domain response we will see a large pulse from the driver followed by a string of smaller spaced pulses. Some might mistakenly claim that these are due to reflections of the sound off the walls of the box coming back out through the cone.
What really causes these peaks are the sharp nulls created in the SPL plot by the standing waves. The driver's motion is significantly attenuating at these discrete frequencies, nothing is being transmitted through the driver cone. The time domain plot is just strongly influenced by the displacement of the driver's cone as a function of frequency. Each sharp null in the SPL plot contributes to producing peaks in the pressure's time domain plot.
So if you crossover at 500 Hz, well below the first standing wave at 800 Hz, you will eliminate the string of additional pulses in the time domain. No more "reflections".
Martin
4th message:
Hi again Horacio,
Continued from this morning's response.
After thinking some more about the standing waves in a sealed box I remembered an example problem I had run recently for a closed ended TL. In the curves you can see the nulls in the frequency domain and the "reflections" in the time domain. There is no capability for modeling sound coming through the driver cone in this simulation.
This is only a 1D model, including the other two directions would yield more nulls in the frequency plot and thus more peaks in the time plot.
Martin
1st e-mail:
Hi Horacio
A 10 liter box sounds pretty small for a 10" woofer. Assuming 10 liters is correct then this is a cube with 0.215 m sides. The first standing wave in this enclosure would be at the following frequency.
344 / 2 / 0.215 = 800 Hz
You would excite a standing wave from front to back at 800 Hz, if the driver is centered on the baffle the first standing wave side to side or top to bottom would occur at 1600 hz.
If your max usable frequency is in the 400 to 500 Hz range then I think a simple cube would work real well, the box would act as a nice air spring without any standing waves messing up the response.
Martin
2nd e-mail:
Hi Horacio,
"Thanks for the quick reply. The piece I'm not tracking is why you focus on the first standing wave and above frequencies? All frequencies in the passband, namely 80 to 500Hz, will be reflected back and potentially bounce back from the back wall. Right?"
(MJK) No, I do not believe that is correct.
Going back to your 10 liter box. If it is a cube then it has half wavelength standing waves at 800 Hz, 1600 Hz, 2400 Hz, ... and integer multiples of 800 Hz in each direction (front to back, side to side, and top to bottom). For now let's assume that the box is empty, no fiber damping, and the driver can be treated as a source at the center of the front baffle.
Working in the frequency domain, each excitation frequency can be thought of as the driver's cone oscillating in and out of the box and the air compressing and uncompressing all happening at the same frequency. At the low frequency tuning fc, the air in the box acts as a second spring (the spider is the first spring) pushing on the back of the driver's cone raising the driver's resonant frequency from fs to the closed box speaker's fc and the driver's Qts to the closed box speaker's Qtc of 0.71 as you described. If I remember correctly the acoustic impedance of the sealed box at low frequencies is
Z_ab = C_ab = V_b / (rho x c^2)
Once the frequency is above fc the SPL output is ideally a function of only the cone mass and the stiffness and damping represented by the box and the driver's suspension are no longer important. This is true until the frequency rises to the point of exciting a standing wave in the box.
At 800 Hz the first standing waves in all three directions are excited. Two of the standing waves, top to bottom and side to side, have a velocity maximum and pressure zero at the driver location in the center of the baffle. The front to back standing wave has a pressure maximum and zero velocity at the front baffle pushing against the back of the driver's cone. Acoustic impedance, Z_ab, is the ratio of pressure over volume velocity. So in two directions, top to bottom and side to side, Z_ab is zero and in the front to back direction it is approaching infinite. The front to back standing wave's acoustic impedance will almost stop the driver's cone motion leading to a deep sharp null in the SPL response at 800 Hz. The top to bottom and side to side standing waves will not be excited because of the driver location with respect to the standing wave.
At the higher harmonics a similar situation occurs where an infinite acoustic impedance is seen by the back of the driver's cone, stopping the motion, and creating deep sharp nulls in the SPL response. If we now factor in fiber damping in the box then the standing waves will be significantly attenuated and the deep nulls become much broader and shallower. If enough damping is used in the right location the standing waves are completely damped and the SPL response is smooth again.
Based on this description of the dynamics of the air in a sealed box two of the create audio myths should be called into question.
1) Sound waves do not reflect or bounce off the back wall of a box and exit through the cone. Discrete standing waves can be excited leading to nulls in the SPL response but fiber damping can be used to suppress this problem. I find it hard to believe that sound waves in a subwoofer box can be transmitted through a structural driver cone and be audible in the room.
2) Parallel or non parallel walls in a box will both have standing waves. The frequencies and mode shapes will be different but they will exist. The exact same problem is common in both arrangements and the same solutions can be applied.
So in your case, where you want a sealed 10 liter subwoofer operating from fc to a maximum of 500 Hz, I think building a cube and adding a light amount of fiber stuffing is all you need. Don't over think it.
Martin
3rd:
Hi again Horacio,
Continued from last night's response.
What I described below was an ideal closed box SPL frequency response (like one calculated by some of the freeware simulation programs) with a series of sharp deep nulls at 800 Hz, 1600 Hz, 2400 Hz, ... and so on superimposed. Assuming no crossover, the frequency response looks somewhat like a comb, it is flat in the pass band with a series of "teeth" sticking down. Remember, this is all in the frequency domain.
If we convert the SPL back to pressure (SPL is an indication of normalized sound pressure and remembering to include the SPL's phase) and then inverse FFT the pressure frequency response to get the pressure's time domain response we will see a large pulse from the driver followed by a string of smaller spaced pulses. Some might mistakenly claim that these are due to reflections of the sound off the walls of the box coming back out through the cone.
What really causes these peaks are the sharp nulls created in the SPL plot by the standing waves. The driver's motion is significantly attenuating at these discrete frequencies, nothing is being transmitted through the driver cone. The time domain plot is just strongly influenced by the displacement of the driver's cone as a function of frequency. Each sharp null in the SPL plot contributes to producing peaks in the pressure's time domain plot.
So if you crossover at 500 Hz, well below the first standing wave at 800 Hz, you will eliminate the string of additional pulses in the time domain. No more "reflections".
Martin
4th message:
Hi again Horacio,
Continued from this morning's response.
After thinking some more about the standing waves in a sealed box I remembered an example problem I had run recently for a closed ended TL. In the curves you can see the nulls in the frequency domain and the "reflections" in the time domain. There is no capability for modeling sound coming through the driver cone in this simulation.
This is only a 1D model, including the other two directions would yield more nulls in the frequency plot and thus more peaks in the time plot.
Martin
As far we can stop the enclosure to sing from its external walls, maybe the way to go is finally the strongest BL and stiffer/heavier cone with the lowest Le to get rid (or at least to get lower) of the interactions with the sealed air load and its spring effect. Btw, I often read than the back damped cone have more sound qualities than a simple front damped one despite the price to pay in Mms ! The trade off seems to weight towards more mms for sealed loads....as far as you don't need a very light one to your SET amp !
Is the wood brassing a myth for those frequencies involved in the Martin's answers ? Or just here to get the cabinet rigid ?
More fiber damping (100%) is equal to a more muffled sound in a sealed load or is it a myth ? Or is it just the result of a lowest spring effect by the volume load rising allowed by an important fiber stuffing ? To rephrase it : do we need to decrease the volume of the box to allow 100% fiber stuffing to fight against standing waves but to stay on the ideal planed Qtc ?
It coulb be a way for the poster to tune its load ! if 100% filled with fiber, add around 10 tp 20% of plain "brick" to stay on the same volume and controll if it works at ears ?... If the cabinet is rigid from the beginning and the upper drivers are playing to allow the transcient?
Is the wood brassing a myth for those frequencies involved in the Martin's answers ? Or just here to get the cabinet rigid ?
More fiber damping (100%) is equal to a more muffled sound in a sealed load or is it a myth ? Or is it just the result of a lowest spring effect by the volume load rising allowed by an important fiber stuffing ? To rephrase it : do we need to decrease the volume of the box to allow 100% fiber stuffing to fight against standing waves but to stay on the ideal planed Qtc ?
It coulb be a way for the poster to tune its load ! if 100% filled with fiber, add around 10 tp 20% of plain "brick" to stay on the same volume and controll if it works at ears ?... If the cabinet is rigid from the beginning and the upper drivers are playing to allow the transcient?
Yeah, when I say it, it's meh... When MJK says it, it's golden 🙄
Kidding! I would listen to MJK too. But really guys, this is a non-issue. The science is well established on what should be done.
So if the cabinet is well made, the OP just doesn't like this driver ! Or it needs more XO works to integrate it...
Frankly, the cabinet construction is the nightmare of the normal diyer ! (We don't want to destroy a forest to find just the good enclosure !)
Frankly, the cabinet construction is the nightmare of the normal diyer ! (We don't want to destroy a forest to find just the good enclosure !)
Instinct and experience kicking in here ?
By the way, I can always tell who the members of the OB cult are, because they always say, "box"
A 10 liter box sounds pretty small for a 10" woofer.
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I doubt Martin King is an OB fan given his work with transmission lines, which of course involve an enclosure.
BTW, a 10 liter enclosure 😉 is indeed pretty small for a 10". You might recall from other threads people telling me this was not a good driver choice if I was going to build a sealed enclosure. The Beyma 10G40 is recommended for vented boxes.
Why not inputt this driver's parameters in SoundEasy or Clio if the choice of sealed design is choosed ? As the OP is not using DSP it could be interresting to look at the results of the filter with linearisation function and box size ?
You could install this driver in a .7 cubic foot enclosure to try it vented, and then stuff the ports to compare the sound quality. Plus, you can always make the volume smaller by installing displacers, but you can never make it larger once it's built.
http://www.quarter-wave.com/OBs/OB_Design.pdf
Designing a Passive Two Way
Open Baffle Speaker System
Martin J. King
40 Dorsman Dr.
Clifton Park, NY 12065
MJKing57@aol.com
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