Results of waveguide "measurements" not so good ...

[paulspencer]

I've been keen on the idea of waveguides for a while, but this weekend did some experiments. I turned up 5 waveguides on the lathe, put them on my rough open baffle prototypes and ran auto EQ on ultracurve to get some crude quasi "measurements".

Generally I found little useable gain, which may in some cases be due to the passive crossover 2nd order @ 3.5k

Measurement setup isn't ideal - measuring in listening position 2.8m away or so in 4x5m room using pink noise with a diy electret mic.

I measured with and without waveguides to get a comparison. No gating or nearfield measurement. My theory was that by only changing one thing (if the waveguide is there), it should give some indication of what it is doing. Valid assumption?

I don't have anything yet that I'd be happy to use based on results or listening tests. With eq to get it flat, it doesn't sound noticeably different with or without the waveguides.

Waveguide 5, the last one I did was based on the one used by John Zaph:
http://www.zaphaudio.com/hornconversion.html
Based on this I'd expect to see some reasonable gain of 6dB around the low end of the tweeter, but no, not even with the same profile (although slightly smaller but almost identical).

Anyone with suggestions on how to get better results with a waveguide, both in design of the waveguide and testing?

Here is the profiles

[paulspencer]

So what would I like to achieve with this?

Ideally I'd like to:
* match the polar response of the mids on open baffle
* reduce distortion
* increase power handling
* crossover probably around 2.4k later when I go active with Ultradrive
* achieve boost in the 2 - 8k range and keep a smooth response

Here are the measurements. Gain offset 10db so dotted line is the zero gain point. Gain = acoustic boost of the WG

[noah katz]

Paul,

From what I've read previously, my guess is that all the action is at least an octave below the XO freq.

[MarkMcK]

Paul,

This also relates to the project that extends the upper frequency output of the P17.

When you front load a transducer (action of a horn or wave guide) you should not ignore the energy storage in the resonance of the air mass defined by the front loading structure.

Further, dome tweeters produce most of their output below 10 kHz through an energy storage (resonance) mechanism. This can be seen in either a delay response or waterfall response plot.

This is not a good thing. When you then compound this problem by adding an additional energy storage mechanism from the front loading structure this is not a good thing.

Again, this phenom can be easily seen in transient testing or in delay or waterfall frequency response tests.

Further, while the calculations are not simple, good predictive results for frequency of difference are achieved by calculating air mass resonance and interference multiples as defined by the front loading structure.

The free program from Audua (Speakerworkshop) will allow for acceptable quasi anechoic, far field testing with adequate resolution.

I have used what others have called wave guides to achieve time alignment on a flat baffle. I use an extended range low frequency driver. I target for a 10 kHz crossover. I design the front loading structure to keep air mass resonances well below 10 kHz. And I limit sub 10 kHz tweeter output to minimize excitment of either the tweeter intrinsic low frequency storage mechanism or the front loading storage mechanisms.

Doing this produces a very different sound. A sound that is better scientifically because it more closely reproduces the signal supplied to the transducer. A sound that is better artistically because it is clearer, more open, with more precise imaging, and less harsh.

Best,

Mark

[noah katz]

"Further, dome tweeters produce most of their output below 10 kHz through an energy storage (resonance) mechanism. This can be seen in either a delay response or waterfall response plot.

This is not a good thing. When you then compound this problem by adding an additional energy storage mechanism from the front loading structure this is not a good thing."

??!! So you're saying a horn is not an acoustic impedance transformer but a resonant device?!

First I've heard of energy storage in the air in front of a tweeter. AFAIK it's only at Fs and in the diaphragm's breakup resonances.

[Few]

I don't have access to my "audio lab" as I write this so I'm afraid I can't provide much detail, but I did some experimenting with a quasi-waveguide this weekend. I started with a block of extruded polystyrene insulation and carved/cut/sanded a waveguide that was primarily intended to reduce some diffraction effects around a tweeter in a d'Appolito speaker I'm re-building. As I said, I don't have dimensions or precise measurements with me, but I saw quite a pronounced increase (about 5 dB) in tweeter output in the 1 - 2 KHz region when I mounted the waveguide in front of a D2900-9300 tweeter. The mouth of the waveguide didn't terminate tangent to a plane as the MCM one does. Rather, the curve of the waveguide continued to wrap back around and connect smoothly with the beveled edges of the cabinet. The mouth itself had a diameter of roughly 7" and the dome tweeter was mounted about 1-1/2 to 2" deep, meaning 1-1/2 to 2" behind the plane that would normally define the mouth of the waveguide.

Some preliminary work did show evidence of some "hash" in a CSD plot, but I find the appearance of such plots to be very dependent on the parameters used to form them---and often in ways that aren't at first obvious. For example, delayed reflections can look very similar to resonances. For that reason I'm not yet sure I have a handle on the origin or nature of what I saw. More work ahead...

Few

[john van ommen]

So basically what you are saying is that the waveguide is rolling off the highs and boosting the low end? This is what you would expect I believe.

In the graphs, I noticed a rolloff in the upper frequencies. As I understand it, this is due to the fact that the tweeters energy is now spread across a wider coverage angle.

For example, a dome tweeter on a flat baffle may have only 20 or 30 degrees of coverage at 10,000khz. But restricted to a waveguide, it could have as much as 90 degrees of coverage. And due to the same amount of energy being spread across a wider coverage angle, the overall response on axis is LOWER.

Conversely, that's what is boosting the frequencies lower in the range.

Is that conservation of energy?

JOHN