AUGUST 2004

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PROJECT MAY
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August 10 - TimG

I have crossovers to audition this week for the S9800, MT with response to 13kHz, and the MTM 3 way. I have attached an image of the MTM and you can see the problem with the supertweeter mounting. Yeah, I can hear you laughing, but I don't have a CNC router.

The problem with current MTM is that I am getting comb filtering effects above 6kHz. Without the supertweeter I do not get the comb filtering problems. I'm not sure if the problem is caused by the placement of the supertweeter or if it is some sort of cancellation problem caused by an interaction with the midhorn. I will post an image of the response tomorrow so that everyone can see it.

I would suggest that the industrial desgin team begin work on the cabinet design. I would suggest a modular design with 8 separate pieces. 4 identical woofer cabinets, 2 horn/supertweeter cabinets, and 2 stands. The woofer cabinets should have close to 4 cu ft internal volume and be able to accept twin 4" flared ports up to 18" long with room for the driver as well. I would suggest a baffle of 17" high x 19-20" wide. For the horn subcabinet, the box needs to be 12.5" tall to house both the drivers. With this configuration of cabinets, the system could be used as an MTM, with a 17" stand added at the bottom (which could also house the passive network) or it could be used as a vertical 2.5 or 3 way depending on Don's preferences and needs


 

 


August 11 - TimG

The comb filtering occurs in the predicted response. There is no comb filtering in the raw 045 response. The comb filtering only occurs when the 045 and 435 are used together. I will post some pictures to illustrate.

I will see what happens if I raise the crossover frequency to 13kHz.

Here is the response of the 045Be with and without the crossover at 10kHz.

Here is the response of the 435Be on the Sonoglass horn with and without the 4th order bandpass filter at 800 and 10kHz.


Here is the response of the 2 1500AL woofers in the MTM format. The crossover is 800Hz 4th order low pass. Ignore the response below 200Hz since it captures part of the room response.


Here is the response of the woofers and midrange together. The slopes at the crossover point still need more work to eliminate the dip in response.


 

Here is the woofer and midrange response again and the supertweeter response is overlaid on the image. Notice the comb filtering the occurs well below the crossover frequency. Can someone send me a link to the S5800 crossover so that I can try that out.


 


This image show the individual filtered driver response curves along with the system response. It would be easier to see if I could do the curves in different colors.


 

Here is the response with the crossover changed to 13kHz.



 

 


August 11 - TimG

All right, the comb filtering is caused by the placement of the supertweeter in the front-back or z dimension. I'm not sure how JBL deals with this with their networks for the S5800, but it appears that the midhorn in the S5800 is shallower than the Sonoglass S9800 horn. This presents a problem for the MTM design. The supertweeter may need to be moved backward into the cabinet or delayed electronically to solve the comb filtering problem. I tried adding 180 degrees more phase rotation by using a 6th order slope at 10kHz but that didn't help. Here is the response of the 3 way MTM with the S5800 filter with the 045 moved back 8cm in the z dimension.

 

And here is the MTM crossover based on the S9800 network with the 045 moved back 10cm in the z axis.


 

 

 


August 11 - TimG

One of the assumptions of a Linkwitz Riley 4th order network is that all of the driver acoustic centers are aligned in the z-axis. This assumption was met in the 4430 from what I remember, and the woofer and midrange appear close to being aligned in the S9800 cutaway drawing. Now it is more clear why the 045 is offset in the S9800.

Is this comb filtering problem something people are willing to accept? I don't think the project would have much appeal if it requried digital delay for the supertweeter. Recessing the 045 into the cabinet would likely cause more problems than it would solve unless someone came up with a really creative solution. Also remember that it is best to keep the woofers as close together as possible in an MTM and that the vertical height of the horn/supertweeter assembly is currently 12.5 inches.

I already attempted to add another 180 degrees of phase delay to the 045 by crossing it with a 6th order acoustic network at 10kHz but it didn't solve the comb filtering problem because of the high frequencies involved.

I listened to the MTM with the newest crossovers today, and the high frequencies sound too bright. However, it wouldn't be a problem to add some downward sloping response when I do the system optimization. Based on conversations with others who have built MTM's, they tend to sound bright if they have ruler flat response, so most people add some downward slope or a dip at 3kHz. Then again it may just be my room.

 

 

August 12
- Giskard

According to JBL's Greg Timbers: "The comb filter effect is normal and cannot be eliminated, even with the use of electronic delay. The wavelength at 10k is an inch or less and even if the 045be were delayed to the z plane of the 435Be, even the slightest movement up, down, left or right of the measurement position would change the relative path length between the 435Be and 045Be to where the interference pattern would return.

What I have done is use a 4th order on the 045Be high pass and adjusted the values using substantial smoothing on the frequency response curve. The polarity does seem to matter in getting the smoothest "smoothed" curve.

I originally had the 045Be coming in where the main 435Be cuts off (15k) which cost less parts, but we could hear the 045Be as a separate location and it was distracting. I found that 8k - 10k minimized this effect. In spite of the curve, we have had no negative comments about the 045Be and the choppy response through the crossover range."
 

 


August 28 - Mr. Widget

If Tim will ship the pair of H9800s and drivers along with the bat slayers (045Be) I will whip up a pair of combined horns that will be suitable for listening tests and basic aesthetic study. They will be machined and carved plastic in an MDF frame. (Trust me they will look pretty close to factory, a bit more time and they would look factory.) If I got them by mid week, I can turn them around by the end of the weekend. I will also incorporate a sub enclosure that will be structural so that the 1500AL enclosures can stack on the mid and high horns.

There is no way I can fabricate a pair of wood combination horns in less than several weeks. I am not willing to make more than one pair of them so we will need the industrial design to be locked in before I begin those.

As for the industrial design, if we could get the dimensions of the front layout, i.e. woofer spacing and height of the centerline of the H9800 from the floor that would be very useful.

Once armed with that info I can go back over all of the previously posted drawings and sketches that have been posted by myself and others and put them together in a more coherent form to aide the design process and get the ball rolling again.