Accuton C25-6-012 in a waveguide

[davepellegrene]

I saw the distortion effect at Zaph's web site. Dave: Next time you run a HI test, check the distortion box with flat baffle ad WG so we can see what the distortion curves look like in HI.

I can still check the box on the measurement to show distortion. Although the measurements are only taken at 90 dbs. I was also told before to show third order instead. I'll post what i have tonight. Measurement are at home.

Davep

[jonasz]

From Zaph's blog a XT19 on flat baffle:


The same tweeter in a wg and a 4.7uF in series:

[DanHine]

Remember the flat baffle measurement is on a 2'x4'x3" thick piece of rigid fiberglass.
Dave

Pardon my ignorance, but why would you compare measurements from two completely different baffles? Why would it not be better to compare a measurement of the driver on a baffle with a WG and then on the same size baffle without one?

[davepellegrene]

Pardon my ignorance, but why would you compare measurements from two completely different baffles? Why would it not be better to compare a measurement of the driver on a baffle with a WG and then on the same size baffle without one?

You bring up a good point. First of all I would have to recess the tweeter in the baffle then have another baffle to recess the guide into. Every tweeter I work with I would have to have two sets. What I have found using the rigid fiberglass as a baffle seems to eliminate any difraction which I think is important designing the guide. I would think the baffle difraction should be taken care of with the crossover since the baffle size and round over will be different in each application. Measuring the tweeter set flush into the 2'x4' panel then setting the wave guide on top leaves about an 1 1/2" to 2" space from the baffle edge of the guide to the fiberglass. It doesn't seem to make any difference with the measurements as long as I put at least a couple of inches of the ridged fiberglass around the edge of the guide. I have sheets of 1" rigid fiberglass and I have laid them around the guide to fill in the whole 2'x4' area. This makes no difference than just a strip around the edge of the guide. The part I still am not clear on and have asked without an answer is does it matter if I measure the tweeter on a flat baffle then set the guide on top and measure with out moving the mic or the tweeter. This moves the baffle an 1 1/2" to 2" closer to the mic if you measure from the face of the guide. Depending on the depth of the guide. The way I'm doing it I would be measuring from the same acoustical center instead of the baffle face. Not sure it matters. To answer your question what I explained is the only real difference. This really only matters when comparing flat baffle to guide to see if it's worth using the guide or not. Or if I want to see how much boost I am getting from the guide. As far as developing the guide as long as I make no changes as I'm trying different guides I am getting the same results and that's what really matters to me. Comparing the flat baffle to the guide isn't that important.
Hope that makes sense.
Dave

[rpb]

Pardon my ignorance, but why would you compare measurements from two completely different baffles? Why would it not be better to compare a measurement of the driver on a baffle with a WG and then on the same size baffle without one?

When making waveguides, and testing them, it's sometimes easier to get a lot more done by deviating from the ideal comparison, to one that's reasonably close. Perhaps someone looking for "proof" might prefer a more exact comparison. Here's some measurements I made a while back. The waveguides were not anything special, and the tweeters were pretty good even without a waveguide. The waveguides did reduce distortion, which can be difficult to compare to other distortion measurements. These tweeters did not measure this low on a flat baffle. I can't find the picture, but one comparison I made showed a 10 dB decrease in distortion over a pretty wide range. If you're making a speaker that will be pushed hard, a waveguide helps a lot.

Here's one that I made a wacky filter to take the response as low as I could make it go. This was made in response to a friendly challenge by Ryanbouma. The filters used on these were not as simple as a single cap, but I like to make comparisons after the response is flattened. If it takes 6 or 7 x-over parts, that's what I use.




Here's one with an RS28. This is the lowest distortion I've been able to measure with my equipment.

[mzisserson]

What levels are we talking about here? 85dB 1/m there are a lot of non WG tweeters that yield low distortion results that can be comparable. Can you change the relative scale to Db?

[rpb]

Positives

1. Lower cross over (thus larger woofer is possible)
2. Simpler cross over
3. Lower distortion
4. Improved directivity
5. Recessed acoustic center

Negatives

1. Larger CTC spacing
2. Physically complex to construct
3. Directivity (if trying to achieve omni)




That third graph I posted, is an ND28F-6 on a very small oval waveguide with a single 3.3uF cap. Granted it could probably use a shunt inductor to back it up to 1700hz xo for power handling. But that was a $12 tweeter back when I did this. Not many tweeters under $50 can pull off such a low cross over.

The first graph is an SB29 in an 8" Dayton. I'm using it crossed at 1000hz with one 4.7uF cap. Without any issue. That's compression driver territory.

Waveguides aren't for everyone. And that's fine. But the benefits are there on this particular accuton waveguide, imo.

You forgot to mention reduced tweeter diffraction from the baffle edges.

[rpb]

What levels are we talking about here? 85dB 1/m there are a lot of non WG tweeters that yield low distortion results that can be comparable. Can you change the relative scale to Db?

Are you asking about my measurements? I think these were about 90 to 95 dB at 5 or 6 feet. At one time I was testing at 100 dB, but later reduced the SPL I test at. Signal to noise ratio makes getting low distortion numbers easier as the SPL is increased. At some point, the distortion starts to rise. That's usually where I stop increasing the level. Sometimes, I reach the imput limits of my soundcard, and have to stop there, or move the mic further away. I don't test quite so loud now.

[mzisserson]

Are you asking about my measurements? I think these were about 90 to 95 dB at 5 or 6 feet. At one time I was testing at 100 dB, but later reduced the SPL I test at. Signal to noise ratio makes getting low distortion numbers easier as the SPL is increased. At some point, the distortion starts to rise. That's usually where I stop increasing the level. Sometimes, I reach the imput limits of my soundcard, and have to stop there, or move the mic further away. I don't test quite so loud now.

That is why it is important to specify. 90dB at 6' is far more impressive than 90dB at 1', though you sound unsure of the actual details and dB readings. Trying to get a sense of scale, that is all.

[rpb]

That is why it is important to specify. 90dB at 6' is far more impressive than 90dB at 1', though you sound unsure of the actual details and dB readings. Trying to get a sense of scale, that is all.

It's possible that it wasn't that loud. But judging from the -5 dB level. I believe it might have been. I never really standardized an spl level for testing. For a while I was really cranking the spl. What I usually do now is adjust till I get the lowest level of 3rd order that I can. Usually measured from 2 to 4 feet. I run sweeps starting at a modest spl, and increase about 4 or 5 dB each sweep. I find the sweep level that produces the lowest distortion. If I sweep 1 dB louder, or 2 dB quieter, the distortion looks slightly worse. Some of the tweeters I've tested don't get much below -55 dB tested this way, before some portion of the response shows an increase in distortion. I think the results are impressive, but they really shouldn't be compared with any other measurements. When I get a chance, I'll run a sweep on something, and see what the spl is.

[mzisserson]

It's possible that it wasn't that loud. But judging from the -5 dB level. I believe it might have been. I never really standardized an spl level for testing. For a while I was really cranking the spl. What I usually do now is adjust till I get the lowest level of 3rd order that I can. Usually measured from 2 to 4 feet. I run sweeps starting at a modest spl, and increase about 4 or 5 dB each sweep. I find the sweep level that produces the lowest distortion. If I sweep 1 dB louder, or 2 dB quieter, the distortion looks slightly worse. Some of the tweeters I've tested don't get much below -55 dB tested this way, before some portion of the response shows an increase in distortion. I think the results are impressive, but they really shouldn't be compared with any other measurements. When I get a chance, I'll run a sweep on something, and see what the spl is.

Food for thought anyway. Will the horn start amplify issues at in-room volumes of 90+dB, Real world kind of distortion like a listener woudl get in their room.

[rpb]

Food for thought anyway. Will the horn start amplify issues at in-room volumes of 90+dB, Real world kind of distortion like a listener woudl get in their room.

Well, you got my curiosity up. Here's my 30 plus year old Phillips tweeter in one of TN Allens waveguides. Baffle is approximately 9 x 16 (Guessing). I used my Radio Shack analog spl meter to measure the spl, so the values are approximate. I measured the mic distances, and distance to the spl meter within a couple inches on the 6 foot distance. The louder traces actually look lower, but that's because I had to turn down the mic preamp to prevent clipping the soundcard. Filter consists of one cap, and two resistors.

[TN Allen]

Well, you got my curiosity up. Here's my 30 plus year old Phillips tweeter in one of TN Allens waveguides. Baffle is approximately 9 x 16 (Guessing). I used my Radio Shack analog spl meter to measure the spl, so the values are approximate. I measured the mic distances, and distance to the spl meter within a couple inches on the 6 foot distance. The louder traces actually look lower, but that's because I had to turn down the mic preamp to prevent clipping the soundcard. Filter consists of one cap, and two resistors.

Hello rpb,

Could you tell me which of the guides you used for this, just curious? tna

[mzisserson]

It looks good, and more representive of what I expected to see! I believe the loading really helps not reduce, but keep in check the distortion that is already there naturally. This seems to yeild better percieved performance due to flat-out less non-linear distortion as the volume goes up. Thanks for the work!

[rpb]

Hello rpb,

Could you tell me which of the guides you used for this, just curious? tna

It's one of the 6" round ones.

[TN Allen]

It's one of the 6" round ones.

Thank you, tna

[rpb]

It looks good, and more representive of what I expected to see! I believe the loading really helps not reduce, but keep in check the distortion that is already there naturally. This seems to yeild better percieved performance due to flat-out less non-linear distortion as the volume goes up. Thanks for the work!

A waveguide reduces the distortion at high spl simply because the tweeter is playing at a lower spl to achieve the same spl on axis. For example. If a specific tweeter has lower distortion at 87 db at 1 meter than 95 dB at 1 meter, then a waveguide that boosts the output eight dB will reduce the distortion since the tweeter will be playing at what would be 87 dB on a flat baffle, but measuring 95 dB on axis due to the waveguide. So if you were to compare 95 dB with the waveguide to 95 dB without the waveguide, you would be comparing the tweeters distortion percentage at 87 dB with its distortion percentage at 95 dB. (Assuming 8 dB of boost from the waveguide.)

At low SPL the advantage goes away. You might be comparing 75 dB with 83 dB then, and the differences would be marginal because the tweeter is probably very low distortion at 83 dB.

On the other hand, if you are really pushing things hard, the advantages are better. The distortion comparison might be 98 dB vs 106 dB. I suspect that most tweeter have some distortion at 98 dB, but they would have a lot more at 106 dB, so the waveguide prevents that additional distortion from being generated.

So, to hopefully end my thread hijack, (Sorry about that Dave!), A sweep for the purpose of x-over work, or off axis response, is not stressing the tweeter enough to make a judgement on distortion. Instead, the room noise, or equipment signal to noise ratio will make the distortion appear higher when it's really not. What is important is how much boost is provided by the waveguide. Mine have been around 8dB or so. Where there's boost, there's potential for lower distortion at high spl.

[davepellegrene]

After talking with Xavier we came to the conclusion that it would be easier for the wave guide to be able to support the weight of the tweeter in his application so I doubled up the baffle edge of the guide and also added some ribs to strengthen the base plate.



I haven't cut down the outside edge yet but you can see how I braced the guide up.


This is the new plexiglass I tracked down. It has a satin black finish on one side and a high gloss on the other.


I reworked the phase plug a little this weekend and it helped the off axis quite a bit and also boosted the 10 to 20 kHz range a few dbs. I'll post graph's later.

Dave

[xavier]

<cue the mad not-really-a-scientist wringing his hands and drooling with anticipation>

[davepellegrene]

Finally finished these up last night and took measurements on my new set up. Flat baffle and guide baffle are identical so the results will match.

Flat baffle 0 20 30 40 50 60


Finished guide 0 20 30 40 50 60


Finished guide vert. 0 20 30 40 50 60


Some pics
Ribs added before mounting base plate to attach the tweeter to.


base plate mounted


Dave