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Due to the particular position of my tweeter relative to the midrange horn's prominent upper lip (see picture) I'm using "acoustic foam" around it.
It does help tame some diffractions, resulting in more focused stereo image and more disappearing speakers.
However, I still have a nasty peak centered around 8KHz which I feel might be caused by the foam being ineffective at some frequencies?....
I'm thinking replacing it with a suitably thick layer of flet. Or something else. At least on the upper lip.
What are you guys thinking?
Edits: 04/10/17Follow Ups:
That's an Electro-Voice Sentry IV tweeter and midrange horn. Given what you've already done to it, I have to ask: Is it a stock system, is the crossover stock or custom, and are they parts scavenged to build a different speaker system?
The original Sentry IV didn't have a "nasty" peak at 8 KHz, at least not that I remember.
Only the midrange horn is from an EV Sentry.
Tweeter is a Beyma CP25, mid driver is old Klipsch K55V, bass horn is Peavey FH1 with Klipsch K33 woofer. Network is ALK "universal".
They are the evolution of a pair of LaScala I used to own!
Ah, yes - I just read your reply to tomservo regarding the tweeter. Sure looks like the one from the Sentry III/IV though. That's why I was puzzled about the 8K bump. Anyway,...
Not being sarcastic or anything, but, what you have there is what might be called a Rube Goldberg or Frankenstein speaker system - an assemblage of components which don't necessarily work well together, and which you are trying to "fix" by adding band-aids here and there. I think what you're going to find is that the band-aids are introducing their own problems while mitigating some other problems. I'm not so sure that the "notch filter" is going to make the system sound "better", but it certainly will make it sound different.
Still, I AM sure that it's great fun to play around with it and see what happens! Good luck!
My speakers are in fact the result of modding a pair of LaScalas and are all (tweeter, crossover, midrange horn) "recommended" upgrades -or in the case of the crossover, built for the task.
As such I can assure you they are a success as they sound 1000x better than vintage stock LaScalas, and have provided me with great sound for years.
The thing is 1) as an audiophile I keep wanting them to be "better" and 2) I have now dofferent acoustics and it makes treble "issues" stand out more. But they still sound great :)
If the notch filters aren't satisfying I will just remove them, and maybe try to locate a pair of 2404 from JBL in the future.
Hey, don't worry about it! You like them and you're having fun. That's all that matters.
I would try to figure out what the 8k peak is from first, move the speaker to a neutral location like outside or the middle of large quiet room.
If it's still there when you move around in front,its not reflected sound then consider a series RLC tank with an 8 k notch.
If it's a lobe / directional then try;
If you have an electronic crossover handy, set up a band pass filter from say 6 to 12Khz then play pink noise through it (leaving the 8k much more audible).
Then walk around it, see if you can localize it, take a chunk of foam and see if you can find it blocking things off or absorbing. do not put foam anywhere you could see if you were looking through the driver mounting hole, if anything on your horn, continue the horn progression at the top and bottom with foam, that is the likely diffraction point on that shape.
If you measure, set up and rta when you do this and then you have a objective indicator when you hit gold with the foam or moving the mic around you may find a lobe or null (the two are usually found together)
Looking at the BEYMA CP35 frequency graph shows a broad peak centered around 8KHz, roughly 6 dB in amplitude.
I've ordered components to make a notch filter starting at 6500Hz and stopping at 9500Hz with a 6dB max attenuation. I'll see where that takes me, as I understand it the only way to know for sure is to try...
i hope to flatten the tweeter response enough to be able to remove the variable L-pad I'm currently using.
Agreed, try the notch filter.
I can't tell off hand if/what effect absorption would have on that, it's way smaller than what i have a feel for.
In any case, it wouldn't hurt and on that shape it would be the sides where you would want to put it
I'm currently trying the notch filter.
It does work - it does as predicted, removing the 8KHz bump and allowing me to remove the l-pad (with the bump, the tweeter was really too hot), effectively boosting the relative level of frequencies above 10KHz.
But I have yet to decide if I like the result.
The new extended treble makes the sound more detailed but also more rough and unforgiving.
I still don't know if:
1) the new response shows all the flaws of my equipment
2) the notch filter caps sound rough because they are not broken in yet
2) the notch filter caps (Jantzen cross-caps) aren't good enough
3) putting a notch filter wasn't a good idea.
So I will now wait until the caps settle down and re-evaluate.
I was wondering.
Does that Manger also produce that 90 degree phase shift
at midband with flat frequency response, like direct radiators do?
Same stuff Rogers (and others) used for the LS3/5a.
Dense is not what you want - if it is really dense it will reflect more than absorb.
This stuff is very soft but thick and substantial.
In my unhumble opinion the best you can get.
I couldn't find sheets or rolls of felt suitable for other speakers.
Madisound have some but I'd be stacking their stuff, somehow.
Skeptical Measurer & Audio Scrounger
I have used really dense wool felt on a number of enclosures I've built. I found it to be more effective over a wider range of frequencies at absorbing sound waves than any foam I've tried. My experience in this area was back in the 1980s. Neither sound waves not felt have changed since then, but better foams might now be available.
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