time-aligning a super tweeter

[cporada]

This past week, I built a pair of super tweeters for my DIY ACI alpha kit to see what they would do to the sound (the alphas roll off at about 18kHz). I had read an article in the Journal of Neurophysiology that the human brain responds to sounds above 20kHz, and that listeners in a double-blinded study were able to consistently identify music containing information from 22kHz-100kHz (in addition to the audible range) when compared to music only containing the audible range. It appears that the brain processes this information and alters the way the person perceives the audible sounds, and people felt the music sounded more natural when it contained the ultrasonic information, even though they could not correctly identify when only the high frequency information was played by itself. Inspired by this article, I set out to build a pair of these to see what they would do to the sound of my system. I used the Fostex FT17H, since it is very efficient and linear to 50kHz. I built a crossover at 18kHz with a 1uF Kimbercap and a 0.1uF Audiocap Theta, and ran the signal from my binding posts up through an L-pad so I could match the output to my Alphas. The sound is intoxicating. Everything is very natural. It sounds like music now from real instruments right in my house, not like a stereo. The soundstage is considerably wider, deeper and higher, and everything sounds very smooth and nonfatiguing, which I find curious, since I usually think of high frequency sounds as being very fatiguing and harsh. The sound seems to float in the room rather than coming from the speakers.
Having given this background, now my question; does anyone know how to time-align the supertweeter with my main drivers, i.e., where is the acoustic center on a horn-loaded tweeter, is it at the front of the dome? Is there some way to calculate this to get the super tweeters positioned correctly from front to back on top of my ACI alphas?
Thanks
Chris Porada

[adason]

hi cporada

human brain responds to sounds above 20kHz,

thats why people go to live concerts


btw, what signal source do you use?
there is no audio signal above 20kHz from cd

[cporada]

Hi Adason,
I use SACD, vinyl and CD, but the strange thing is the improvement is very apparent with redbook CD as well, which leaves me scratching my head for an explanation. The only thing I can think is that the upsampling on my SACD player adds the high frequency info back to the redbook CD, but this is a guess. I know when Stereophile, Positive Feedback, and other mag's reviewed the Murata and Townshend supertweeters, they also heard an improvement on regular CDs and were equally puzzled with this
Chris

[DaveM]

It makes no sense that the SACD is adding back the high end info. It is simply not there. You can't re-create what does not exist. That is a very good question though as to what you are hearing there.

About live music. I have lately found that I often prefer my home system to the sound of a concert (MTM with PHL 1240's and a Raven 2). The exception is live acoustic music. Most shows available where I live are amplified. We all know that the equipment used in a PA is no where near the quality of most everyone on this boards system. I love going to shows and am so often dissapointed in the sound.

DaveM

P.S. The best show I ever saw was Dave Brubeck when I had front row center seats. I was in front of the PA speakers. The balance was off (cymbals were too hot, Sax was a little down) but I was not hearing the PA. I was hearing the actual instruments. This was without doubt the best sounding show I had ever heard.

[adason]

I know when Stereophile, Positive Feedback, and other mag's reviewed the Murata and Townshend supertweeters, they also heard an improvement on regular CDs and were equally puzzled with this

Chris, those reviewers were paid a lot of money to hear something.....

I suggest you do blind experiment. Instal a switch to your supertweeters and have one of your friends to randomly switch it on and off and see what you hear. If you will be able to guess correctly if they are on or off. If you do it right I bet you will not be able to pick the difference.

[adason]

The best show I ever saw was Dave Brubeck

hi DaveM,
I have a lots of Brubeck's LP's, some are mono, some stereo, but some are recoded live...those are wonderfull

I have lately found that I often prefer my home system to the sound of a concert

DaveM,
I was talking about acoustic music.

the best live concerts I remember were Mozart's and Berlioz's Requiems in a Cathedral

well, this is not a discussion live/recorded music, so let's not go there

lets go back to supertweeters....
Chris, do you think if you would replace the tweeter with the better one which go to 22-25kHz, you would achieve the same effect? Plus save a lots of money?

[cporada]

Hi Adason,
Perhaps this would do the same, but I already had the main speakers (and like the sound), and each Fostex was only $35 from Madisound, so it seemed like a small cost compared to the commercial units that cost upwards of $1000 for a pair. I have attached a picture of my finished products. I think they look pretty cool, especially for the cost.
Chris

[adason]

Chris,
I might have an explanation why the supertweeres help. Your crossover, even 18kHz, is not very steep, so it actualy has a significant output lower the 18 kHz. 18kHz is just the starting point to roll of. Since the Fostex supertweeters have pretty high efficiency, I bet you are actualy hearing increased hights well below 18kHz. Therefore even CD's sound is improved. No magic. Just my explanation.

As long as it works for you. Enjoy the music!

[cporada]

This could be true. I think the way the crossover is designed (sorry I know almost nothing about this, tech support from Madisound helped me pick the cap values) is for the Fostex's to be down 3db at 18kHz, with a slope of 6db/octave. So if anyone could tell me how low the Fostex's are outputting audible sound (like where is 6db down, where is 12db down, etc), I would greatly appreciate it, since I can figure out how to back-calculate that from the slope.
Thanks
Chris

Also,
If anyone could make a suggestion on my original question which was how to find the acoustic center of the Fostex so that I can time-align it with my existing drivers, that would be great, too.

[adason]

Chris,

unfortunately I do not have any correct unswer to your original question. I have noticed once supertweeters bacame fancy, manufacturers started to put them on top of pretty much any loudspeaker. For instance Tannoy on top of their dual concentric speakers. That does not make much sence to me. First you go into trouble of creating a point source radiator, than you put another source far apart from the tweeter? How can that ever align again? Just a thought.