Usable frequency response for TAD 4001

[Rocky]

I just aquired a pair of TAD 4001 drivers, and have recently been involved in some threads discussing a possible setup. Yesterday, i stumbled across a page that really caught my attention.. Sierra Brooks manufactures a line of mahogany tractrix horns, very good looking indeed, and very expensive ofcourse, but what I found especially interresting was the DIY-projects section of the site. They show a number of setups using the TAD 4001 driver, but each and every one of them utilizes one of their largest horns, a 140hz tractrix horn as shown below (the big one, mated with two Onken tweeters):

So this makes me think.. as I'm using affordable diaphragms from Radian (aluminium) much more rigid than the fragile and expensive beryllium ones, I am not too afraid of blowing them although I would rather not... Radian's datasheet on the phragms indicates a 600Hz XO with 18dB/oct slope, but says nothing about power handling at this XO config...

My question is if these drivers are usable below 600Hz, and down to what point, assuming a maximum power handling of 1 watt only, and active 24dB/oct XO slopes? These japanese guys don't use 140Hz tractrix horns just for the fun of it, do they?

[roddyama]

Well, where I don't have the direct experience going that low with the 4001's, I did mke a couple phone calls.

The word from Radian is "absolutely not to be run below 500Hz."

My other conversation was with Paul Butterfield Audio (blues musician's cousin) who services TAD drivers. He has heard of the 4001's being used down to ~250Hz, but no direct experience that low.

[Rocky]

Thanks Rodd. Interresting indeed.

I suspect that I will not endanger the Radian diaphragms when using them in configs where the original diaphragms work successfully, as they are supposed to take alot more beating than the originals... What might be an issue is rise of distortion near Fs.. I dunno if Radian's diaphragms exhibits more resonance distortion than the original TAD diaphragms, and therefore might be unsuited for a low XO?

I'll contact Sierra Brooks and gather available info on their TAD 4001 setups... like XO points, slopes, power handling, etc. I'll return with more info when they respond.

It is possible they do a low order XO at a somewhat high frequency, but gets the most out of the attenuated response down to 100-200Hz by hornloading it I guess.. It would make some sense..

[roddyama]

If you work from the Radian Spec Sheet for the 4001/2 diaphragm, it is capable of 128db @ 60W input @ 1kHz. If we lower the frequency 2 octaves to 250Hz, we would need 4X the excursion for the same SPL (actually acoustic power). Now at 250Hz, if we lower the excursion back to XMaz (assuming it was excursion limited in the first place), we will lose 12db of acoustic power leaving us with 116db SPL. But this is not entirely accurate because SPL is not directly proportional to acoustic power. The difference is in the sound field the acoustic power is radiating into. If we assume a controlled dispersion at 1kHz equivlent to 1/16 space and a relatively uncontrolled dispersion at 250Hz of 1/2 space, we will lose about 9db more to the wider dispersion at the lower frequency. This leaves us with 105db at 250Hz @ 60W input. All this assumes that the driver has the same loading throughout the BW and into the stop band, and that it can take 60W at 250Hz.

Two things are apparent (to me), one: you're not going to get the SPLs with a flea powered SET, and two: you probably will never get to 140Hz at that SPL.

[Rocky]

Rodd, your posts are most useful, you are really a great help for me on this.. Thanks.

250Hz would be a very nice XO point, as it'll cover almost the entire voice area. Although I am building 425Hz tractrix horns already, I will build a single bigger horn when I get the calculations done, and try to use the TADs in 2 stages of a 4-way.. lower and upper mids.. Mono setup.

I can run the upper TAD on El-Spuddo (1W) to start with, but I understand I will need more power if it should be possible to run the second TAD as deep as 250Hz. My setup will be for modest listening levels, so based on your calculations I figure I'll need a power handling of 15 watts at 250Hz to reach 99dB, I need to find out if radian provides such a power handling then....

[Rocky]

They claim to use 1st order passive crossover at 300Hz, on a 140Hz horn, with great results... Any comments on this?

Tad specified to me that it will play at 300hz if it's used for HOME use only. Not concert PA stuff. See its specified for 600hz because it was intended to be played with mega watt amps for pa purpose so they wanted to be safe in putting it at 600hz. But with SET then playing them below 300hz is harmless.

[roddyama]

Originally posted by Rocky
250Hz would be a very nice XO point, as it'll cover almost the entire voice area. Although I am building 425Hz tractrix horns already, I will build a single bigger horn when I get the calculations done, and try to use the TADs in 2 stages of a 4-way.. lower and upper mids.. Mono setup.

I personally do not like this idea. I think going mono is a huge compromise. Remember, "...by the Fanatics, for the Fanatics."

Originally posted by Rocky
I can run the upper TAD on El-Spuddo (1W) to start with, but I understand I will need more power if it should be possible to run the second TAD as deep as 250Hz. My setup will be for modest listening levels, so based on your calculations I figure I'll need a power handling of 15 watts at 250Hz to reach 99dB, I need to find out if radian provides such a power handling then....

I have made a lot of assumptions in my previous post so be aware. It would be a very interesting experiment to see how SPL you could get from the 4001's @ 250Hz.

Originally posted by Rocky
They claim to use 1st order passive crossover at 300Hz, on a 140Hz horn, with great results... Any comments on this?

You could run a passive first order and an active @ 140Hz just in case. Then you can vary the active parameters as needed.

[Rocky]

Rodd,
What would you say about a setup similar to the Avantgarde Uno speaker, with a TAD tractrix of about 200Hz? (stereo this time ) Even though I am seeking to use compression drivers as low as possible, I still fancy the good ribbons or AMTs over horns in the high frequencies, so Raven R1 or R2 might be suitable as tweeter unit..

I know the avantgarde uno midhorn plays down to 220Hz using a horn loaded 4 inch dome, wouldn't I have a good reference in the Uno to find a suitable low-frequency extension for such a project?

What do you think?

[roddyama]

Originally posted by Rocky
Rodd,
What would you say about a setup similar to the Avantgarde Uno speaker, with a TAD tractrix of about 200Hz? (stereo this time ) Even though I am seeking to use compression drivers as low as possible, I still fancy the good ribbons or AMTs over horns in the high frequencies, so Raven R1 or R2 might be suitable as tweeter unit..

I know the avantgarde uno midhorn plays down to 220Hz using a horn loaded 4 inch dome, wouldn't I have a good reference in the Uno to find a suitable low-frequency extension for such a project?

What do you think?

The idea has possibilities. I heard the Ono at the '03 CES and they were nice, but not as nice as they make out on their web page. It was being played at moderate levels and the sound was smooth and accurate (from what I remember). What I was expecting and didn't hear was the impact and dynamics that brings the music to life.

That said, I noticed right away that the Radian diaphragms are more dynamic then the TAD's, although not quite as smooth and not quite as musical. It could very well be that the TAD integral surround/diaphragm actually limits the dynamics of the stock 4001 driver.

Paul Butterfield (mentioned above) has my old TAD diaphragms and is having some special secret surrounds made to use with the beryllium diaphragms. I will be very interested to hear the outcome of this experiment.

So the point I'm trying to make is that the radians should do better than the TAD's at the lower frequencies. The surrounds are more flexible and the voice coils can handle more power.

Don't forget about the BW each driver will need to cover. Your scenario is using the TAD's to the lower, and possibly the upper limit of their BW. As BW increases so does IM distortion. Couple the wide BW with an already stressed driver from operating at its extreme, and you'll began to sacrifice the sound quality.

[Rocky]

Originally posted by roddyama
Don't forget about the BW each driver will need to cover. Your scenario is using the TAD's to the lower, and possibly the upper limit of their BW. As BW increases so does IM distortion. Couple the wide BW with an already stressed driver from operating at its extreme, and you'll began to sacrifice the sound quality.

Sounds like a good advice. If mating 200Hz horns with Raven R2s, the crossover to the raven can go quite low.. however, I would ofcourse like to have the TAD horns play the entire human voice band up to at least 2000Hz.. This is one decade, or some 3.32 octaves from the horn cutoff frequency.. With active filters the Ravens could be crossed at 2kHz with no problems.. thoughts?