HD15 Measurements

Greetings!

Since I could'nt find any measurements of HD15 when my friend and I thought about building our first ones and since there are people asking for those over and over again, I decided to take some measurements before we sell our HD15 and post them on here.

We did measurements of 1, 2 and 4 HD15 loaded with Oberton 15B450 drivers in a parallel configuration. They were done in a field with just some bushes to the side (2 Pi) on the ground with approx. 3,5m distance from the stack. The Mic was the Beyerdynamic MM1 which is not calibrated for actual soundpressure, so just ignore the absolute dB-levels.

This is the result and some pics of the setup/measurement-conditions:

bob4 wrote: Hi Julian, great post! Thank you for sharing this. This is highly useful! It is lacking a bit of circumstantial information though, which would add greatly to its value. Amping, power levels......  Could you also please add information about the processing? I assume you have had a high pass filter in place? If so, please add cutoff frequency and filter order to your previous post. This is crucial.

Hey Bob,
since I wanted to provide accurate frequency response plots, there is no processing involved. No EQ, no HPF or LPF, just the fullrange signal to one ampchannel of a T.Amp TSA 4-1300. I did not alter the gainstructure when connecting more Speakers in a parallel configuration (so its a load of 8, 4 and 2 Ohms)
I would assume the cabs got something in the range of 10W but i just made sure its loud enough for a good measurement. Like i said, the mic is not calibrated for absolute levels, so the 130dB have no meaning. I can just provide the relative difference in soundpressure when using 1, 2 and 4 cabs and the change in the frequency response :)

But if it helps: We used the Yamaha Steinberg UR12 audio interface connected to a DCX2496 (without any processing enabled) which itself was connected to the TSA 4-1300 with just one channel used. There where no changes made when connecting more cabs :)

A+

odc04r wrote: Ah yes, not so sure about ARTA. Tend to use REW for this type of thing. Quick check of ARTA's manual online says it will do THD, doesn't seem too complicated. Investigating THD as a function of power would be interesting, maybe gives a clue as to why people think HDs can honk high up. Yes fair point re. that resonance that is suppressed with increasing mouth area. It is an interesting one albeit a bit of a moot point as no-one would ever play the cabs that high. It could be related to the L/A relationship of the bandpass port. I wonder also if the presence of that resonance is another reason that people find HDs honky up in that region near 200Hz.

Sooo...we tried to measure the THD but had limited success because of an less then optimal location. I will update you with the THD when we have time to drive to the location of the first measurements :)

djkeet wrote: Out of interest JulianDA what horns have you replaced your HD's with and what was the xover freq used when they were part of your set up. Also based on your findings in general do you think you could make an educated guess on how HD's will perform based on physical attributes and drivers used. Its interesting to note that the bigger mouth has little effect on bass extension. I always seem to view HD's as a take on a 'Turbo' Turbosound so any variation in design, drivers defo have to be considered so is this design flexi enough for other drivers or is it just the 15B450

We replaced them with my own design. You can see the development of it in this thread:
https://forum.speakerplans.com/15-horn-to-go-over-x1s_topic103148.html

The HD15 were used as kicks above X1 Subs and the XOver was set to LR24 at 71Hz and LR24 at 151Hz (71Hz provided a nice coupling with the X1s and above 151Hz they just dont sound right...i.e. the honkyness) But i cant tell you the delay and phase settings from the top of my head, sorry.

Regarding your other question, there are a lot of forum threads about suitable drivers, but i cant comment on that, since we just tested and used the 15B450 and were quite happy with them. 

I dont know what you mean with the physical attributes, maybe you can elaborate that a bit more?

PS: The thing about the bassextension and the hornmouth isnt really that supprising. Any horn has its characteristic "base-resonance" which is dependant on its lenght. The lowest resonance is the quarter-wave-resonance of this lenght, so the resonant frequency has a wavelenght of 4 times the lenght of the horn. The hornmouth has to have an adequate size to ensure a good impedance-match to the outside world at this frequency. So if you put horns together that are to small on their own, you can get a better loading (more efficiency) down to that frequency. But below that you dont get much more gain, since the horns are still to short (to be exact, you will see the resonance frequency shifting a little bit lower, since the horns gain some "virtual" lenght because of the boundarys)

If you translate this to the HD15 with its roughly 60cm hornlenght you get a quarter-wave-resonance at ~140Hz (the wavelenght is 240cm, so 4x60cm). Below that, the bandpass chamber dominates the output. You can see this effect in my measurements, since the ripples above ~140Hz get smaller with more cabs, while the form of the graph below that frequency stays nearly the same.

I dont know what you mean with the physical attributes, maybe you can elaborate that a bit more?
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What I was alluding to was variations on the same theme/other ppls designs. But if your noticing honking @ certain freq prob the design as gone as far as it can go I will look @ your thread with your own design cheers.