Nube questions tweeter selection and cabinet shapes

[late boomer]

Hi,
First post!
Q1: Some documents on tweeters have a recommended crossover frequency posted. Example 3k.
Can I crossover below this recommended frequency as long as I'm clear of fs?
The tweeter I'm considering is the TB 28-1177SB.
Crossover no lower than 2.5k.
Q2:Cabinet shapes referencing Loud Speaker Design Cookbook 7th.
Cabinets with beveled baffles in chapter 5.
Is the effect in question relating to interior shape and sound cancelations inside the box? ( box, cylinder, sphere, pyramid etc)
i.e. Can I install wedges and get the same result while maintaining a square exterior?

My first project is 7 X two way systems for 7.1 HT. Sealed.
The woofers should perform well up to 4K but I am aware of off axis roll off at the upper frequencies.

Thanks for your assistance.

 

[nickf]

1 besides keeping well clear of fs the manufacturers recommended crossover frequency may also take displacement limited output into account. Some tweeters simply don't have the linear excursion to produce much volume at lower frequencies. I personally try to stick to within the manufacturers recommendations.

2 Although it isn't that clear from the first few paragraphs of that chapter I am certain Vance D is referring to the outer shape of the speaker. Standing waves inside the box can be addressed with damping materials. "The edge" software can be used to indicate the effect of a baffle shape. My own experience is that rectangular is ok as long as the tweeter is a different distance from every edge of the baffle. and even if that mistake is made the speaker can be very enjoyable.

Enjoy your speaker building!
Nick

 

[late boomer]

Thanks Nick,

Here, https://www.madisound.com/store/product_info.php?cPath=45_229_324&products_id=8799, they post F Response 775-25K Hz, Fs at 775 Hz and post recomended XO at 3Khz.
Some vendors post the lowest F of the response range as the lowest possible XO frequency.
I want to do this right.
If not the lowest F of the range and not Fs, then is there a rule of thumb or more scientific means of calculating the XO frequency?
A "recommended xo frequency" is not always posted.

I'll keep my tweeter mount at unequal distances from cabinet edges.

Thanks again,

Hugh

 

[kbgl]

Thanks Nick,

Here, https://www.madisound.com/store/product_info.php?cPath=45_229_324&products_id=8799, they post F Response 775-25K Hz, Fs at 775 Hz and post recomended XO at 3Khz.
Some vendors post the lowest F of the response range as the lowest possible XO frequency.
I want to do this right.
If not the lowest F of the range and not Fs, then is there a rule of thumb or more scientific means of calculating the XO frequency?
A "recommended xo frequency" is not always posted.

I'll keep my tweeter mount at unequal distances from cabinet edges.

Thanks again,

Hugh

The higher the x-over frequency, the louder you can play it. If you never intend to play a speaker loud, then you can sometimes get away with a lower x-over point, or slope. It can be risky though. I had my thumb on the wrong button of my remote one time. I tried to turn the music down, but it kept going up instead. Stuff happens! Rounding the outside edges of the box will smooth the tweeters response. I like large roundovers, but I have yet to find a good way to make them.

 

[phivates]

do it right

Us late boomers have run out of time to do it right. We have to just do it, Phil Knight notwithstanding. This forum shows plainly there's no single right/best/One way to do it.

 

[late boomer]

concepts confirmed

Good,

This confirms my understanding that many rules of design can be deviated.
Or, just do it!

My prototype box happens to "just" fit the mid woofer on the narow face, typical of the "new" box , driver orientation.
There is little baffle to the immediate left or right of the driver its self.
Is this a bad thing?
ie: There is no room to do a roll over for the sake of the mid woofer its self.
I could roll over the top of the box for sake of the tweeter.

If there was a right way, all speakers would follow the same formula.
Judging by the wild variety of speaker designs, almost anything goes.

 

[late boomer]

BTW, what radius makes a "large" round over?
Exceeding the largest router bits I presume.

 

[Norris Wilson]

Hi Boomer,

There are several options here if you do not mind using MDF.

Look at the quarter rounds under millwork.

I have not used any of these corners rounds. But, would be interested trying them in the future.

Curved Plywood, Dieboards, Drumshells Aitwood Home

 

[Borus]

late-boomer
The rule of thumb would be Fs + 2 octaves if xover is 6Db/O w/o Ferroflud
that's 3100Hz for your tweet but you have Ferroflud so you can fuge the rule.
Two octaves is still a good idea but at 12Db you could get down to 2500Hz with out too much trouble. If the woofers are good to 4KHz why even worry about 2.5K. If for a 7.1 you will want constrained dispersion. No?

 

[late boomer]

Excellent input

Thanks Borus,

I like the numbers.
As for the 7.1 constrained dispersion...
I'll have to think it over.

I had bought a set of retail speakers prior to Christmas.
WTW config.
They had a nasty dead zone which has me cautious about dispersion although I think the real problem was phasing / interference and a poor tweeter match to boot. (low spl)
They were returned.

I'm open to hear the case for constrained dispersion.

 

[Borus]

constrained dispersion

late-boomer
your FR & FL speakers with large dispersion will have first reflection sound reaching the viewer at a time not suited to match with your SR & SL speakers not a problem with 5.1 but with 7.1 it can be. See pic

 

[late boomer]

Makes sense

Thanks Borus!

 

[kbgl]

BTW, what radius makes a "large" round over?
Exceeding the largest router bits I presume.

I think a 1.5" radius will do. 🙂

 

[head_unit]

BTW, what radius makes a "large" round over?
Exceeding the largest router bits I presume.

Something like 4" but I forget the source. I believe it was an AES paper from JBL. Any radius helps somewhat, better than a sharp edge. But you could also mount the tweeter sticking out a bit but surrounded by absorptive felt (don't know what current materials are good for that, sorry).

It's correct that rectangular faced cabinets are fine if you stagger the tweeter from the edges.

Tweeter power handling has three limits:

Thermal: only so much heat will dissipate before the tweeter melts. The higher the crossover, the less musical energy comes through.
Interestingly, I made a number of simulations for an AES paper and found that steeper slopes do NOT reduce the thermal power to the tweeter (except if you're crossing at like 10k or something). That sounds odd, but the reason is that higher order crossovers cut more sound below the cutoff frequency, but actually let MORE sound through above the cutoff frequency since the shoulder is steeper-especially in the linear plot that corresponds to power transmission, not the decibel plot that corresponds to SPL.

Mechanical: the tweeter can only move so much before it breaks. Higher crossover = less motion. Higher slope = less motion.

Clipping: turning up an amp loud enough to clip can kill a tweeter both thermally and mechanically. Picture a 100W amp making 99W of bass alternating with 1W of treble. Now clip it a chunk: it's putting out 200W of clipped bass and 2W of treble. Keep turning it up, and in theory with enough gain you could be putting out 200W of clipped treble. Your tweeter will have vaporized long before that.

The moral of these stories? The power handling limit is really the user. Make the crossover low and low slope if you turn the music down when it sounds bad. Otherwise, better be conservative.