Normal waveguide result?

[augerpro]

I'm doing some testing with the HiVi K1 in the MCM waveguide. The interface was very smooth and I didn't expect any weird nulls. And for the most part I didn't except for the WG loaded response was LOWER than stock from 7k-20k Hz. Under 7k was quite a bit higher as expected. Check it out:






Sorry they are both in red. WG loaded starts out higher and ends lower than stock. But that's not what's really bothering me. I went to Zaph's site to compare results. I think HiVi changed the K1. They seem to be using parts from the X1R. Zaph's FR for the X1R starts to dip at 10k Hz, mine starts a little sooner, but they both have that sharp valley at 16k Hz. Mine also starts to rolloff sooner, again closer to the X1R FR than the K1 from Zaph.And my CSD is closer to Zaph's X1R than his K1:







What do you guys think of the WG result and switcharoo HiVi pulled? I'll get some better pics up tomarrow so the FR is easier to read.

[augerpro]

I was just looking at Zaph's waveguide article. The higher frequencies drop off on his a bit too. Give and take I guess.

[JonMarsh]

Can you spell, reflections and cancellations? A dome to a WG mouth that large is going to have problems, I'm afraid. I've seen the same thing in my own testing with the MCM waveguide, particularly with hard dome tweeters, where it seems to be aggravated (more reflection from the hard dome?)

Off to Montreal this morning- a long day sitting in aircraft ahead of me.

~Jon

[noah katz]

Not sure if this is what you're asking about, but a CD (constant directivity) WG rolls off high freq because instead of beaming more w/increasing freq it spreads them out, attenuating them on-axis.

[dlr]

I see it from the other perspective

Not sure if this is what you're asking about, but a CD (constant directivity) WG rolls off high freq because instead of beaming more w/increasing freq it spreads them out, attenuating them on-axis.

In playing around with the KEF drivers, my perspective is that a well-designed guide doesn't attenuate, it reinforces as frequency drops due to it re-directing the energy forward at the lower end rather than allowing it to spread.

If you were to take a (good) waveguide tweeter, measure the on-axis response and note the sensitivity at, say, 10K, then measure that same tweeter without the waveguide, the sensitivity will change little at the high end, thus my take on there being no attenuation. The tweeters own directivity at that end is not altered much by the waveguide, other than some intereference effects related to wavelength and the geometry of the waveguide close to the tweeter.

But it is all frame of reference.



This tweeter from a more recent KEF isn't likely to maintain 110db @0.5 as shown when removed from the woofer/waveguide. The slope will certainly level out, but it will probably be somewhere around 100db, maybe even less, without the waveguide.

[dlneubec]

Brandon,

If you have not seen it, take a look at this write up on wave guides. I think it might answer some of your questions.

http://www.aeronet.com.au/waveguide.htm

[augerpro]

Dave> that Kef is interesting. Now if we can just find a waveguide that increases directivity on the low end but leaves the top end untouched. I wonder though if it's possible for waveguide to keep offaxis response even with out lowering the on axis SPL at higher frequencies. I guess in my mind the total power radiated is the same, the WG just changes the angle of radiation. On the lowend it pushes the offaxis power onaxis, resulting in an SPL bump. Conversely if the WG brings up the normally drooping high frequencies offaxis it must get that power from somewhere. My assumption is it "moves" this power from the onaxis direction to the offaxis.

Dan> I'll check that site out, looks interesting. BTW how is the omni coming?


I'm still not happy with apparent change HiVi made to the K1 though. I have a Vifa DX25 on the way for testing though.

[augerpro]

Here is the SPL and Z of the first K1 I measured:









Here are the measurements of the other K1 I have:





The first one is out of spec I think. Although it still looks like maybe an X1R dome or suspension maybe?Even if the second is in spec, it still won't work with my PA dipole. Hope that DX25 lives up to it's low Fs and high efficiency billing...

[dlneubec]

Dan> I'll check that site out, looks interesting. BTW how is the omni coming?

Hi Brandon,

I assume you are talking about my second omni project, the one with the waveguide's and concave cone diffusers.

My first omni project (the Mentor) has been complete for some time, except for completing the veneer and finish on one speaker. They went to the Indiana DIY with me. There was no official voting/review but I had a lot of folks come up to me and say they were the best there! That made me feel good, since it was my first complete design.

I also took the prototype of the waveguide omni and played it in mono. It also recieved a good amount of praise, though the crossover was not very good at that time. I've since redone the crossover a couple times and have one that is pretty good, with crossover points at 690 and 3900, IIRC. Unfortunately, the waveguide omni is just not as good as the original Mentor omni. I think I may need to change the mid dome out and use a high efficiency pro mid, 6.5"-8" in size, with no waveguide. Also, I could stand to get better at 3way design, which might help as well. I have decided to put it aside for the time being to work on a more direct update of the my first omni project. I will probably start a thread on it soon. Here is a pic of what the waveguide omni looks like prior to the temporary shelving of the project.

I hope to start making dust on the new project this weekend.

[dlr]

Good points, though I differ somewhat in terminology

Dave> that Kef is interesting. Now if we can just find a waveguide that increases directivity on the low end but leaves the top end untouched. I wonder though if it's possible for waveguide to keep offaxis response even with out lowering the on axis SPL at higher frequencies.

IMO this isn't likely to happen. The smaller the tweeter diameter, the better I would expect it to be, but since the tweeter is not a true point source that would allow for a tiny waveguide entry opening, there's always going to be some amount of time-delayed interference effects.

I guess in my mind the total power radiated is the same, the WG just changes the angle of radiation.

This is a key point and one that is overlooked or mis-understood. References to on-axis or off-axis power response are mis-nomers IMO. The response on any given individual axis is the SPL response on that axis, not power response on that axis. Power response is the response of the driver/system integrated over a sphere that encloses said driver/system. What this means is that except for whatever small change occurs in the acoustic impedance for diaphragm coupling with the air, the actual total power (power response) will change little. What does change is the direction in which the power is radiated.

If the power response changes, then the driver efficiency (not sensitivity) would be the indicator of changes in power response. Power out = power in - losses. Conservation of energy.

On the lowend it pushes the offaxis power onaxis, resulting in an SPL bump.

I would phrase it as being part of the polar radiation pattern.

Conversely if the WG brings up the normally drooping high frequencies offaxis it must get that power from somewhere. My assumption is it "moves" this power from the onaxis direction to the offaxis.

It does change the direction of radiation, so you could say that it alters the radiation pattern, thus the power output direction. Still, power response is generally used to indicate the total in-room response, meaning the combined output indicated by the (SPL) polar response pattern.

This is more easily understood by looking at the power response of systems with crossover in place. Differing driver spacing and different crossovers both result in different power responses in-room. The only reason that we generally care about power response is the affect in-room. Anechoically, power response means little.

[augerpro]

I see. Thanks Dave for the insight!