I'm having some troubles with the measured response of the Peerless 811435 neodymium tweeter. The problem may very well be related to the microphone used - I donno - which is a WM60AY combined with Rod Elliot's mic preamp (not line level). The microphone is connected to the microphone input of my Audigy. This is problematic because of the bias voltage for a standard direct connection to the capsule. In the case of my Audigy the mic input seems to be configured quite differently from others. I have measured ground-3.2v-1.8 instead of the standard ground-3v/5v-signal.
Anyway here's the response graphs. I've put a 21uF pp cap in series with the tweeter. This is the gated response:
And the ungated farfield:
What is the problem here? And why is there such a big difference between the gated and the ungated response (especially below 2KHz)? Sorry for the difference in the scales.
Anyway here's the response graphs. I've put a 21uF pp cap in series with the tweeter. This is the gated response:
An externally hosted image should be here but it was not working when we last tested it.
And the ungated farfield:
An externally hosted image should be here but it was not working when we last tested it.
What is the problem here? And why is there such a big difference between the gated and the ungated response (especially below 2KHz)? Sorry for the difference in the scales.
You don't say what you think the problem is, but I suppose you mean the +6dB step above ~4kHz? In that case, congratulations!. You have just discovered the "baffle step" and in this case the baffle is small (I suppose the tweeter was unmounted?). Put it in a baffle and note the difference. Then figure out how to compensate your system for this effect! 
PS Peerless measure all their speakers mounted in a wall, so they don't get any baffle step in their response curves.
HTH
PS Peerless measure all their speakers mounted in a wall, so they don't get any baffle step in their response curves.
HTH
Thank you for your reply, Svante. I know about baffle diffraction step, but I have not seen such an extreme example before.
The tweeter is mounted in a baffle: approx. 26mm from the top, 31mm from the left and 61mm from the right baffle edge. The baffle width is 145mm.
The bass/midrange is mounted 9mm from each side edge. Calculating the f3 point of the BS using f3=4560/Wb where Wb is the width of the baffle, I get approx. 800hz.... but then again - the tweeter is not flush mounted... hmmm...
I using the width of the tweeter frame which is 53mm you get 2185hz.... are you sure that this is due to baffle diffraction step?
The tweeter is mounted in a baffle: approx. 26mm from the top, 31mm from the left and 61mm from the right baffle edge. The baffle width is 145mm.
The bass/midrange is mounted 9mm from each side edge. Calculating the f3 point of the BS using f3=4560/Wb where Wb is the width of the baffle, I get approx. 800hz.... but then again - the tweeter is not flush mounted... hmmm...
I using the width of the tweeter frame which is 53mm you get 2185hz.... are you sure that this is due to baffle diffraction step?
sobazz said:
No, I'm not sure. It just fitted so (well reasonably) well with an unmounted tweeter. Anyway, there is a *big* difference mounting a driver in the centre of a baffle and at the edge. You can try different baffle placements with this program:
http://www.tolvan.com/diffract.exe
I'm starting to feel like a marketer now (I have posted this link a few times the recent days in this forum, stop me guys if I am to persistent).
One way to test if it is the baffle step you see, is to take a piece of cardboard and tape it to the front of the baffle, with the tweeter sticking out of it in the middle. You should then see a change in your curves.
Was any of the measurements taken in the close field? I suppose the woofre (wow, brittish mistyping???
) was disconnected?
sobazz said:
Well, I could be wrong but I always thought it was the distance from the center to edge that determined the frequency where the baffle amplification started to take effect.
That's what I always figured when designing a speaker and it works beautifully.
And square-baffled speakers like the peerless one is a particular problem when not baffled properly.
sobazz said:
I wouldn't think so. It goes out just a few millimetres, right? Then there should only be a minimal reflection there, that certainly could not make a 6dB+ difference at 10 kHz where the wavelength is 3.4 cm.
I ran it through my diffract.exe, and I see a peak at 2-3 kHz and one at 9 kHz. In fact I think see those in your measurement as well, but with a superimposed tilt of maybe +6dB/octave. Hmmm... You mentioned that you have a 21uF cap in series with the speaker, are you sure that it is OK? There seems to be a HP function way up high. Possibly you should check the equipment once more?
Attachments
Svante: Yes, there is 21.8uF in series with the tweeter. It forms a 6db/okt. filter applied at somewhere between 1000-1800hz depending on the impedance, which I have not measured. The filter should not be the problem.
I'm a bit puzzled right now. I have tried the measurement microphone on my rather mediocre Mirage FRX speakers, and the frequency response graph was without surprises.
I'm a bit puzzled right now. I have tried the measurement microphone on my rather mediocre Mirage FRX speakers, and the frequency response graph was without surprises.
AGGEMAM, I don't quite agree. Can you post a screen shot?
Svante: Argh... you're the man. I do know how to read numbers... but - I put a 6.8uF in series with a 15uF and assumed that the total capacitance was the sum of those too... how foolish
Hmm... seems I'll need to do some more measurements...
Svante: Argh... you're the man. I do know how to read numbers... but - I put a 6.8uF in series with a 15uF and assumed that the total capacitance was the sum of those too... how foolish
Hmm... seems I'll need to do some more measurements...
Yes, of course. Here you go - gated at 60cm:
I suppose there's not much you can do about the nasty dips from 5khz to 18khz. But what about the dip at 4khz? It sure is undesireable! Any suggestions?
A closer look:
An externally hosted image should be here but it was not working when we last tested it.
I suppose there's not much you can do about the nasty dips from 5khz to 18khz. But what about the dip at 4khz? It sure is undesireable! Any suggestions?
A closer look:
An externally hosted image should be here but it was not working when we last tested it.
How far away is the mic from the tweeter, sorry if you mentioned this im in a bit of a rush so didnt read everything. I measured a vifa tweeter on a 22cm wide baffle mounted in the centre about 4cm from the top and maybe 30 from the bottom. The response was differnt to yours as the tweet is differnt, but when I measured the response really close up I got a similar result, try measuring at 30cm and see what you get. The vifa did hav a bump between about 12-18k anyway which needs to be compensated for with a notch. Usually however BS is compensated for with an inductor and resistor in parallel with the driver.
The tweeter is mounted off centre on a 145mm wide baffle. The above measurements were done in the farfield at 60cm distance.
60cm IS in the farfield, I believe (blue is 30cm, black 60cm and red 120cm):
60cm IS in the farfield, I believe (blue is 30cm, black 60cm and red 120cm):
An externally hosted image should be here but it was not working when we last tested it.
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