pinkmouse said:Or you could always build your box as a terminated TL, no chance of standing waves then!
And so as not to be accused of being off topic, it is a divider, and it is vented at the bottom
I don't think you'd want to vent a terminated TL (since that unterminates it), but the pic pm shows is a good trick done in midrange enclosures and i don't see why it wouldn't work here. Doubling the path length halves the resonance and tapering it moves it down some more. Progressive damping to absorb all the energy and you have something like the B&W Nautilus.
dave
In a closed box, the standing wave problem could be solved with damping material - end of story. However, isn't one of the purposes of this discussion to explore alternatives to damping, as damping can cause a "deadening" effect on the sound?
One suggestion had been to minimize the damping material by using less but placing it where it can do the most good, ie. half way down the cabinet. This is clearly a sensible approach but it can still be fun to consider others.
The TL and Mass Loaded TL solutions are interesting but do they solve the damping problem or is damping material still required?
In any case, I seem to remember from A. Bailey's original articles on TLs that he said that the line could as well be open or closed. If we do nothing with a tall, narrow enclosure we could claim that it was a terminated TL (perhaps a closed version of one of John Cockroft's "Short-Lines").
Maybe the Helmholtz resonator approach (which I'm enjoying learning about) could also be used as an alternative to filling in a TL.
Finally, at the beginning of this thread I suggested that a wedge-shaped filler added to the bottom of the cabinet could dilute and spread the standing wave resonance. Is there any reason why glueing little pieces of wood at random to the inside of the cabinet should do a better job?
Steve
PS: I have become a huge fan of this forum. The depth and breadth of the knowledge advanced by its members is enough to cause considerable standing waves of its own.
One suggestion had been to minimize the damping material by using less but placing it where it can do the most good, ie. half way down the cabinet. This is clearly a sensible approach but it can still be fun to consider others.
The TL and Mass Loaded TL solutions are interesting but do they solve the damping problem or is damping material still required?
In any case, I seem to remember from A. Bailey's original articles on TLs that he said that the line could as well be open or closed. If we do nothing with a tall, narrow enclosure we could claim that it was a terminated TL (perhaps a closed version of one of John Cockroft's "Short-Lines").
Maybe the Helmholtz resonator approach (which I'm enjoying learning about) could also be used as an alternative to filling in a TL.
Finally, at the beginning of this thread I suggested that a wedge-shaped filler added to the bottom of the cabinet could dilute and spread the standing wave resonance. Is there any reason why glueing little pieces of wood at random to the inside of the cabinet should do a better job?
Steve
PS: I have become a huge fan of this forum. The depth and breadth of the knowledge advanced by its members is enough to cause considerable standing waves of its own.
I can also see advantages in doubling the box length as pinkmouse and planet10 have pointed out.
One concern I do have, bearing in mind that I tend to use full-range drivers, is that the upper mids and high frequencies are reflected straight back at the drivers rather than down the cabinet to be absorbed by whatever damping is down there. Is more damping required to fix that?
Steve
One concern I do have, bearing in mind that I tend to use full-range drivers, is that the upper mids and high frequencies are reflected straight back at the drivers rather than down the cabinet to be absorbed by whatever damping is down there. Is more damping required to fix that?
Steve
7V said:
Yes, but less
Yes, but ideally for a closed line we would like it to be a 1/2 wl of the lowest notes, and for an open ended TL 1/2 wl.
pm's drawing makes the line longer, it eliminates a pr of parallel walls, and it acts as an effective trapezoidal brace for the cabinet walls it is between.
dave
Zozo:
Thank you for sharing your experience regarding standing waves with the 39" tall enclosure you built.
For other members reading this, the graph Zozo provided showing the standing wave appears earlier in this thread-just click on this link to take you to it:
http://www.diyaudio.com/forums/attachment.php?postid=134967
Zozo, can you tell us what the other dimensions of the enclosure were? If you can't remember offhand, just tell us the approximate internal cubic volume, in either Metric or standard measurements. We can take it from there.
The reason for this is that your graph seems to show the "spike" as being approximately (+ or-) 3 dB. As I recall the Pioneer graph from those years ago, the "spike" seemed to be more like 10 dB or higher-but again, very narrow band.
The Pioneer tower speakers were really very pipe-like. Their veritical dimension, as I recall, seemed to be about 5 times any the width or the depth dimension.
I am wondering if the greater the vertical dimension is, compared to the height and width dimensions, the greater the "spike" at the standing wave frequency. In other words, the skinnier the speaker, the greater the "spike".
If you could give us the either the internal width and depth dimensions, or even the approximate cubic volume, (no need to be fussy with exactness here, LOL), it would help a great deal.
Thank you for sharing your experience regarding standing waves with the 39" tall enclosure you built.
For other members reading this, the graph Zozo provided showing the standing wave appears earlier in this thread-just click on this link to take you to it:
http://www.diyaudio.com/forums/attachment.php?postid=134967
Zozo, can you tell us what the other dimensions of the enclosure were? If you can't remember offhand, just tell us the approximate internal cubic volume, in either Metric or standard measurements. We can take it from there.
The reason for this is that your graph seems to show the "spike" as being approximately (+ or-) 3 dB. As I recall the Pioneer graph from those years ago, the "spike" seemed to be more like 10 dB or higher-but again, very narrow band.
The Pioneer tower speakers were really very pipe-like. Their veritical dimension, as I recall, seemed to be about 5 times any the width or the depth dimension.
I am wondering if the greater the vertical dimension is, compared to the height and width dimensions, the greater the "spike" at the standing wave frequency. In other words, the skinnier the speaker, the greater the "spike".
If you could give us the either the internal width and depth dimensions, or even the approximate cubic volume, (no need to be fussy with exactness here, LOL), it would help a great deal.
kelticwizard said:
Referring to my trusty classic "Acoustics" by Leo L Beranek I note that he examines wave propogation in a hollow cylindrical tube, closed at both ends (in other words our column speaker).
Prior to his pages of calculations involving "ros" and "gammas" and other hideous symbols that I find a bit heavy going to be honest, I did manage to dig out this snippet:
"We shall assume that the diameter of the tube is sufficiently small so that the waves travel down the tube with plane wave fronts. In order that this be true, the ratio of the wavelength of sound to the diameter of tube must be greater than about 6." (Acoustics by Leo L Beranek)
So, for a resonant frequency of 140Hz with a wavelength of 92", we could expect the effect to be more pronounced when the "equivalent diameter" of the speaker is less than 15". Over 15" would still give standing waves however (take rooms as an example).
This could be a partial answer to your most recent question kelticwizard.
I've just taken delivery of "Master Handbook of Acoustics" by F. Alton Everest (about 2 days from Amazon) and I would recommend it to anyone on this forum. One point that he makes strongly is that this phenomenum is totally dependant on the ends of the column being reflective surfaces. So, absorbant material at the top and bottom could accompany our damping material in the middle. He also says that we must watch out for the harmonics (in this case at 280Hz and 420Hz).
On the Helmholtz Resonator front, I'm currently building a prototype column so that I can test the effects of different sized resonators, I suspect that, if I go this way, the harmonics would have to be dealt with too.
Incidentally, someone on "that other forum" has suggested something for room resonances that may work in this application. It's to insert a sonotube of half the cabinet height with the mouth close to the top or bottom.
Steve
Looks like we are reaching something of a consensus here.
An important thing to establish is:
A) Is the volume of the internal resonator subtracted from the total volume of the enclosure?
B) If so, can we make the internal resonator of sufficiently small volume to make the tradeoff worthwhile?
An important thing to establish is:
A) Is the volume of the internal resonator subtracted from the total volume of the enclosure?
B) If so, can we make the internal resonator of sufficiently small volume to make the tradeoff worthwhile?
The above agreement, encouraging as it is, has to do with resonators of a certain wavelength. Volumes might have something to do with it, just as volumes have something to with full scale Transmission Line enclosures. But length of the line is the critical factor.
It would seem that the idea is to build a mini-Transmission Line in your enclosure. This is welcome news.
However, it would be nicer if we could deal with a pure Helmholtz resonator. Helmholtz resonators would be the the equivalent of building a mini-Vented Box in your enclosure. The advantage would be simplicity. As the picture in the link below shows, all we would have to do would be to cover one of those braces with MDF, put a port or maybe just cut a hole, and we have broken up our standing wave. Length would be unimportant-only volume. It really would amount to little more than a brace with a quick modification.
Here is the link to the earlier picture:
http://www.diyaudio.com/forums/attachment.php?postid=133916
Having either method work would be nice. If both can be made to work, I would lean toward the Helmholtz resonator.
Favoring the mini-Transmission Line idea is the fact that a Sonotube affixed near the top, in an enclosure where the woofer is near the top, would have the advantage of preventing the back waves from slamming against the back of the enclosure and rebounding against the woofer. This rebound reportedly makes the midrange peaky.
It would seem that the idea is to build a mini-Transmission Line in your enclosure. This is welcome news.
However, it would be nicer if we could deal with a pure Helmholtz resonator. Helmholtz resonators would be the the equivalent of building a mini-Vented Box in your enclosure. The advantage would be simplicity. As the picture in the link below shows, all we would have to do would be to cover one of those braces with MDF, put a port or maybe just cut a hole, and we have broken up our standing wave. Length would be unimportant-only volume. It really would amount to little more than a brace with a quick modification.
Here is the link to the earlier picture:
http://www.diyaudio.com/forums/attachment.php?postid=133916
Having either method work would be nice. If both can be made to work, I would lean toward the Helmholtz resonator.
Favoring the mini-Transmission Line idea is the fact that a Sonotube affixed near the top, in an enclosure where the woofer is near the top, would have the advantage of preventing the back waves from slamming against the back of the enclosure and rebounding against the woofer. This rebound reportedly makes the midrange peaky.
I have not read all the posts on this topic but I can offer 3 solutions that have worked for me for thin tall vented boxes. It is combination of these 3 that i find works best.
1. horizontal braces usually I just use MDF and cut holes in them to make the braces.
2. I use Palter of Paris and construct a wavy pattern on the tall walls (sides and rear). This wavy pattern mens the waves are as much as 4" high at peaks and about 0.5" at the valeys.
3. Open cell foam (1.5-2") on the walls on top of these waves.
I have used this to build 2 ported boxes in the past (read as 80s). (a) a MTM using 2 Focal 8" 8N515 and a Morel MDT33 and (b) a 3 way using Dynaudio 24W100, D52af adn D21af.
Today I'd use fiberglass to constrct the waves. The simplest way wouldbe to use fiberglass roving to create lumps using resin to make this hard. then thae fiberglass matting and cover the rovings. This has to be done before the sides are joined or you can work on one side at a time.
the braces attach at the valleys in the waves.
hope this helps.
1. horizontal braces usually I just use MDF and cut holes in them to make the braces.
2. I use Palter of Paris and construct a wavy pattern on the tall walls (sides and rear). This wavy pattern mens the waves are as much as 4" high at peaks and about 0.5" at the valeys.
3. Open cell foam (1.5-2") on the walls on top of these waves.
I have used this to build 2 ported boxes in the past (read as 80s). (a) a MTM using 2 Focal 8" 8N515 and a Morel MDT33 and (b) a 3 way using Dynaudio 24W100, D52af adn D21af.
Today I'd use fiberglass to constrct the waves. The simplest way wouldbe to use fiberglass roving to create lumps using resin to make this hard. then thae fiberglass matting and cover the rovings. This has to be done before the sides are joined or you can work on one side at a time.
the braces attach at the valleys in the waves.
hope this helps.
kelticwizard said:
Yes, I'm sure it is.
I don't yet know. This is the live or die question for the concept - whether an effective resonator can be made small enough.
Yes, this is the approach that I'm prototyping. I believe that the HR with filling will be more absorbant than the tube per given volume. I don't know this for sure though.
With the protoype, I am going for an HR which will be larger than I would use in the speaker. The volume will be varied by inserting blocks of wood. I will be able to vary the amount of stuffing to tune the Q and further tune the frequency. Also I will either make a number of holes in the top, more than required, and plug them with wine bottle corks or I will have a variable flap.
Steve
Kelticwizard,
The internal dimensions of the box I experimented with (in meter) : 0.97H x 0.2W x 0.17D.
I attach here two layouts wich worked the best for me. The first is rather a TL like approach, while the second is a combination of the TL and Helmholtz chambers. Both can reduce the spike, but also the Q of the vent, therefore I think it is important to tune the chamber(s) exactly to the frequency of the standing wave as you and others already pointed out.
Zozo
The internal dimensions of the box I experimented with (in meter) : 0.97H x 0.2W x 0.17D.
I attach here two layouts wich worked the best for me. The first is rather a TL like approach, while the second is a combination of the TL and Helmholtz chambers. Both can reduce the spike, but also the Q of the vent, therefore I think it is important to tune the chamber(s) exactly to the frequency of the standing wave as you and others already pointed out.
Zozo
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