Hi, complete newbie here to loudspeaker building. I'm trying to find a dipole tweeter to suit my Coral Beta 10's in open baffles (Slot loaded open baffles). I bought a pair of Aurum Cantus AST 25120 air motion transformers, which should suit the Coral's sensitivity nicely (95-98dB). The frequency response looked quite flat on the spec sheet (see attached pdf). However, I did a quick measurement on them, and found a huge dip above 10kHz (-10 - -20 dB). I measured them with Room EQ Wizard from 3k-22k without crossover, straight from both my 300B amps and a Denon transistor amp. Am I doing something wrong?
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I did, but it made very little difference. At least not 20 dB. The results are similar for both tweeters, so I guess it's not a production fault. Measurement mic is a Minidsp UMIK-1.
What was the distance from mic to tweeter?
Try a near-field measurement (like 5 mm from the tweeter diaphragm).
Try a measurement at 2 meters.
Try measuring a different, known good speaker with flat frequency response.
With those three measurements added, you should be able to tell if it's something related to your mic/measuring system, baffle, your measuring distance and the line source behavior of the tweeter, or something inherent to the tweeter's response.
Try a near-field measurement (like 5 mm from the tweeter diaphragm).
Try a measurement at 2 meters.
Try measuring a different, known good speaker with flat frequency response.
With those three measurements added, you should be able to tell if it's something related to your mic/measuring system, baffle, your measuring distance and the line source behavior of the tweeter, or something inherent to the tweeter's response.
Thanks. That did the trick, see attachment; measurement taken at 15 mm. Seems the AMT is a rather directional tweeter.
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I'm glad it helped.
Typical measurement methods assume point source behavior. You are dealing with a small line source, so path length differences from the mic to different parts of the diaphragm may cause issues depending on the distances involved.
A larger tweeter like this will be more directional vertically than a typical dome. As long as it's aimed at the listening position, it should be OK though. You see similar size/shape tweeters used successfully, so not a big problem in practice if you are careful.
Typical measurement methods assume point source behavior. You are dealing with a small line source, so path length differences from the mic to different parts of the diaphragm may cause issues depending on the distances involved.
A larger tweeter like this will be more directional vertically than a typical dome. As long as it's aimed at the listening position, it should be OK though. You see similar size/shape tweeters used successfully, so not a big problem in practice if you are careful.
I haven't yet tried flush mounting the tweeter - will do so in my next prototype. I also bought a roll of felt, to cover the baffle around the tweeter.
The tweeter does sound nice though. Much more refined than the top end of my Coral Betas, which isn't bad at all actually. I built two sets of crossovers, one at 6.5kHz and one at 8.5kHz. I think I prefer the lower value.
The tweeter does sound nice though. Much more refined than the top end of my Coral Betas, which isn't bad at all actually. I built two sets of crossovers, one at 6.5kHz and one at 8.5kHz. I think I prefer the lower value.