Seas hexagrid...

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
I have removed the grid on my H1212 tweeter. It is easy, just grab on and pull... it is just glued on. I had to scrape the glue off with a needle, but that was cosmetic. I was having problems with the diffraction in my speaker design and had a post about it a while ago (http://www.diyaudio.com/forums/showthread.php?s=&threadid=85050&perpage=10&highlight=&pagenumber=1) and there are a few plots with and without the cover in there. They are just on axis plots though. I have been recently looking at the contribution of the grid (and phase shield) off-axis, and that is much more compelling. If removed the FR starts a very slight high end droop very low in frequency (it looks to be 1 or 2dB / octave starting at about 3k), with the cover it stays flatter much higher in frequency and farther off axis. I have not been able to really test a stereo pair of speakers yet, so I can not be positive, but my initial impression was that I liked the sound better without the cover. I will be doing more testing on the subject over christmas shut-down. I will keep you posted.
 
Well, for now I am going to err on the side of safety. I will wait and see if there are any more developments and wait for when I can afford an extra tweeter just in case before I go ripping my tweeters apart. =) It seems to me like you should not have to yank it out should you? I was imagining that you could probably just undo those four hex screws that hold onto the driver and from there just pull it out of the back.
 
You can put the cover back on as easily as it came off. All you need is a dab of glue and it will be just like new. You can even just let the magnet hold it in place (thats right, it is ferris :( ) if you want to take it on and off. JM Lab/Focal uses a very similar cover for their tweeters that you can remove and replace that is held on by the tweeters magnetic field.

I did some testing this morning. Here are some detailed off-axis measuremets (from 0-45 degrees in 5 degree increments):

With the hex-cover:
An externally hosted image should be here but it was not working when we last tested it.

Larger version HERE


Without the hex-cover:
An externally hosted image should be here but it was not working when we last tested it.

Larger version HERE

Unfortunately the images on this page did not come out to the same size and it is really hard to scale them without making them look really bad, but the larger versions in the links are the same size so I recomend opening them both in seperate explorer windows and flicking back and forth between the two if you want to really see the differences.


This also shows the effects of baffle diffraction off-axis which I find interesting. This is the same cabinet shown in the post I linked in last post.

I also did some more listening tests. Today I was much less conclusive. Sometimes I felt that it sounded like there was something blocking the sound with the cover on after listening without the cover (psychosomatic perhapse?). But other times I felt that I could definitly hear the lack of high frequency energy without the cover.

Now for some questions for the more experienced members out there. It almost seems like the phase shield flatens the power responce of the tweeter to a higher frequency where it then just falls off into nothing. Am I reading that right? Does it work simply by eliminating phase cancelation from the center of the radiating area? Do driver manufacturers not use phase shields often with cloth tweeters because the center of the dome is not acting like a piston at the frequencies where there would be phase cancelation with the sides, or is there some other reason? I guess I need some more basic knoledge of how and what the shield is doing before I explore further.

Right now I still do not know wether I like the tweeter with or without the cover, but I think I am leaning towards leaving it on... we shall see.

-Chad
 
You largely answered your own question

It almost seems like the phase shield flatens the power responce of the tweeter to a higher frequency where it then just falls off into nothing. Am I reading that right?
It's more the on-axis response. Note from your measurements that the extreme off-axis, though slightly higher for a bit, eventually takes a steep dive.
Does it work simply by eliminating phase cancelation from the center of the radiating area?
Yep. Simple as that.
Do driver manufacturers not use phase shields often with cloth tweeters because the center of the dome is not acting like a piston at the frequencies where there would be phase cancelation with the sides, or is there some other reason?
That and the better ones have closer to optimal damping, not just being no longer pistonic. The best ones work more like a ring radiator as the frequency goes up. The best example is probably the Hiquphon OW1.

dlr
 
Fascinating indeed! I can manage to crawl my way through those technical terms and somehow in the end I usually get a good idea of what is being discussed! =) I think I will definately leave the hexagrid firmly in its place. Who am I to question seasoned acoustical engineers?

As an aside, I have never before posted in such a helpful forum as this for any topic ever! Thanks yall!
 
Just in case anybody was wondering, the hexagrid does not seem to affect distortion very much. I thought that there might be some distortion due to compression behind the grid/shield. I had seen this said elsewhere before, but I dont remember where. Well, I did some distortion sweeps, and they show a slight drop in distortion (about 3db in 2nd and almost 6db in 3rd) from 5k to 10k when the grid is removed. BUT, this comes with a 2-3dB drop in sensitivity in that range when the cover is removed (this is the same problem Zaph runs into, these plots are not relative to output level, they are absolute. So if the tweeter puts out less fundamental and less distortion, they could be at exactly the same persentage, but look like the distortion is rolling off).

With grid
An externally hosted image should be here but it was not working when we last tested it.


Without grid
An externally hosted image should be here but it was not working when we last tested it.


I personaly suspect that this is probably a non-issue as it is pretty high in frequency and only really affects the 3rd harmonic (which would make the actual product VERY high in frequency), but I do not have enough experience to say for sure. I only report the facts ;) .

-Chad
 
I realize this is an older thread, but I've now read in several places people complaining about this hexagrid cover on the H1212. So I'll second ch83575 when he says you can remove it easily (and probably on other seas models). I remember starting to cut the grid around the edges with a pair of small wire cutters. A few seconds later i pulled on it accidentally and the whole thing just came off. So get yourself a pair or small cutters/needle nose pliers, or whatever will grab the cover and just take it out. Be careful and hold a firm grip because the dome (or the grid) is magnetized so the grid will be pulled back towards the dome.. no one likes dented tweeter domes!
 
I removed the grill on my tbfcg and then removed the phase disk, then replaced the grill for protection, The sound is better than stock, But it sound it's best without the grill.

I think this is one disadvantage of metal domes over soft domes, In that metal dome need the grill for protection, So far I have'nt heard a metal dome that did;nt improve in SQ when the grill is removed.

Another solution is to make a big hole on the center of the grill and then solder 2 thick arched shaped copper wire spaced from the center of the holed grill. Then paint. I tested this on my car audio tweeter , And it works, It protects the metal dome while being acoustically transparent.
 
marchel said:
I removed the grill on my tbfcg and then removed the phase disk, then replaced the grill for protection, The sound is better than stock, But it sound it's best without the grill.

I think this is one disadvantage of metal domes over soft domes, In that metal dome need the grill for protection, So far I have'nt heard a metal dome that did;nt improve in SQ when the grill is removed.

Another solution is to make a big hole on the center of the grill and then solder 2 thick arched shaped copper wire spaced from the center of the holed grill. Then paint. I tested this on my car audio tweeter , And it works, It protects the metal dome while being acoustically transparent.
Removing the phase shield is roughly equivalent to adding a slight droop at the top using the crossover. The top end still has the ultra-sonic peak at breakup, but then has the droop in response that had been corrected by the phase shield.

If you were to equalize the response after removing the phase shield to correct for the droop now put back in, the sound would likely be nearly the same as before removal of the shield. The system could easily be designed form the start with a slight slope at the top end without any changes to the tweeter and with similar sonic results.

Dave
 
dlr said:

Removing the phase shield is roughly equivalent to adding a slight droop at the top using the crossover. The top end still has the ultra-sonic peak at breakup, but then has the droop in response that had been corrected by the phase shield.

If you were to equalize the response after removing the phase shield to correct for the droop now put back in, the sound would likely be nearly the same as before removal of the shield. The system could easily be designed form the start with a slight slope at the top end without any changes to the tweeter and with similar sonic results.

Dave


It doesnt matter and I dont care , All I care about is that It sounds better and more transparent without it.

Correct me if I'm wrong, In my understanding, The phase shield was there to reduce the ultrasonic peak. BUt it also affect the SQ, OTOH, some metal tweets dont have it, and they do fine.
 
marchel said:
It doesnt matter and I dont care , All I care about is that It sounds better and more transparent without it.

Correct me if I'm wrong, In my understanding, The phase shield was there to reduce the ultrasonic peak. BUt it also affect the SQ, OTOH, some metal tweets dont have it, and they do fine.

That's a common misunderstanding. The purpose has nothing to do with the peak, although it does alter it.

The phase shield is in an attempt to reduce the droop in response at the top end of any hard domed driver (mid or tweeter) below the peak. Prior to breakup (below the peak), there is an increasing interference effect due to the time delay between the wave launched near the voice coil and that near the tip of the dome. Hard domes, unlike soft ones, have almost no damping to reduce the output near the tip at higher frequencies. At high frequencies, the short distance from former to tip is significant with regard to wavelength. The difference is a phase shift due to the time delay between the areas. As frequency increases, the constant time delay corresponds to increasing phase shift.

The end result is the droop caused by the increasingly destructive interference. The shield partially blocks the output near the tip, reducing the droop, thus the term phase shield.

Much of the perception of hard domes comes from older drivers and/or cheap ones that had a peak below 20K, not ultra-sonic. Removing the phase shield doesn't significantly alter the peak, but it does slightly roll off the top end, differently for each driver, of course. It is essentially adding EQ to the driver response, not much different than simply designing the crossover from the start to roll off or droop, something I often do even with some soft domes. I usually prefer about a 1db drop from 10K-20k. Flat response is not always preferred.

Also, the smaller the cone diameter and/or the more shallow, the less the effect of the phase delta at any given frequency. These domes have less need of a shield for the audible range.

Dave
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.