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Old 17th February 2010, 10:34 PM   #1
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Default Tweeter depth, trigonometry, waveguide

I am building a simple little 3-way. Accuton C25N tweeter, Accuton C173N midrange, dual SEAS W26FX001 woofers.

My speakers will be 8 feet apart with my listening position 15 feet from the speakers (wife mandated couch positions).

The tweeters will be 52 inches from the floor.
The midrange will be 45.5 inches from the floor.
My ears will be 40 inches from the floor when seated on my couch.

The tweeters cone center point is around 1/4" deep from the faceplate.
The midrange's around 1" deep from the faceplate.

My ears will be almost a foot lower then the tweeters, and 5.5 inches lower than the midranges.

Trigonometry calculation question;
If my ears are 12 inches lower than the tweeters, and 5.5 inches lower than the midrange, 180 inches from these drivers, with the difference between the drivers diaphram depths being 3/4 of an inch, do I need to recess the tweeters further into the baffle?. This will be a rather small amount, or should I not worry about it?.

If I do not need to recess the tweeters, than my waveguide question is partially unneeded.

BUT; should I recess the midrange, with a slight bevel on the face, and mount the tweeter onto the back of the 1" face (baffle) and apply a waveguide?.
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Old 18th February 2010, 12:14 PM   #2
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Cousin Billy, can you pls reformulate your question. "You are building a 3-way, and..." what's next?

The Accuton C173N midrange, looks nice as a main speaker driver. Small glitches at around 1KHz, a high frequency roll off and high Fs.

Last edited by Inductor; 18th February 2010 at 12:29 PM.
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Old 18th February 2010, 01:22 PM   #3
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by Cousin Billy View Post
I am building a simple little 3-way. Accuton C25N tweeter, Accuton C173N midrange, dual SEAS W26FX001 woofers.

My speakers will be 8 feet apart with my listening position 15 feet from the speakers (wife mandated couch positions).

The tweeters will be 52 inches from the floor.
The midrange will be 45.5 inches from the floor.
My ears will be 40 inches from the floor when seated on my couch.

The tweeters cone center point is around 1/4" deep from the faceplate.
The midrange's around 1" deep from the faceplate.

My ears will be almost a foot lower then the tweeters, and 5.5 inches lower than the midranges.

Trigonometry calculation question;
If my ears are 12 inches lower than the tweeters, and 5.5 inches lower than the midrange, 180 inches from these drivers, with the difference between the drivers diaphram depths being 3/4 of an inch, do I need to recess the tweeters further into the baffle?. This will be a rather small amount, or should I not worry about it?.

If I do not need to recess the tweeters, than my waveguide question is partially unneeded.

BUT; should I recess the midrange, with a slight bevel on the face, and mount the tweeter onto the back of the 1" face (baffle) and apply a waveguide?.
Nothing is simple about a 3-way build! You need to back up some (figuratively).

There are many problems with your approach. First, the center to center distance between drivers is wrong, but for reasons somewhat different than what you are trying to achieve.

Generally, you will need to squeeze the tweeter and mid as close as you can get them together. There is a formula for determining the minimum spread based on the crossover frequency, but I just don't have it handy.

In addition to the vertical center to center alignment, you need to consider the ZDP (Zero Delay Plane) in the horizontal axis. There have been discussions about ZDP versus the acoustic center of a driver (they are different) and Vance Dickason states that ZDP is the only thing that really matters. The ZDP and vertical center to center distance are essentially the issue that you are concerned about here.

I would suggest reading sections 7.10 onward in Dikason's The Loudspeaker Design Cookbook to get a better understanding of the subject. I think that the book in general will also reveal some holes in your understanding of what you are trying to build and help you avoid some pitfalls before you commit to carving up wood or even purchasing drivers and parts.

Lastly, there are some very sharp people here on the subject, so please keep us informed on your progress.
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Old 18th February 2010, 01:29 PM   #4
Loren42 is offline Loren42  United States
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I also see the mid and tweeter nominal impedance is 6 Ohms and the woofer is 8 (actually measures 6). If you use two woofers in parallel your impedance will not match the mid and tweeter. The woofer may also be a little light in the SPL department with a combined SPL of 90 dB/m, so you probably will need to pad down the mid and tweeter a little.

Are you going to bi-amp? That would solve the problem.
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Old 18th February 2010, 11:29 PM   #5
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Thank you Loren, your right.

Yes, the tweeter and midrange will be moved as close as possible. I now went back to my original calculations. The two have a combined radius of 110mm, so they need to be crossed below 3300Hz. Our original LEAP design has them crossing at 2700Hz.

The amplifiers used will be the Atma-Sphere MA-1 OTL tube amps. They will not drive dual woofers in parallel, so we will be driving them in series. This was input into LEAP. The amps also have an output impedance of 2.3, and this was used in the crossover calculations. Because of this output impedance, box volumes where chosen to give us a Q of 0.577.

I cannot find much information on zero delay plane. I do not have the cookbook. Can you give me a quick rundown?.
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Old 19th February 2010, 01:15 AM   #6
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by Cousin Billy View Post
Thank you Loren, your right.

Yes, the tweeter and midrange will be moved as close as possible. I now went back to my original calculations. The two have a combined radius of 110mm, so they need to be crossed below 3300Hz. Our original LEAP design has them crossing at 2700Hz.

The amplifiers used will be the Atma-Sphere MA-1 OTL tube amps. They will not drive dual woofers in parallel, so we will be driving them in series. This was input into LEAP. The amps also have an output impedance of 2.3, and this was used in the crossover calculations. Because of this output impedance, box volumes where chosen to give us a Q of 0.577.

I cannot find much information on zero delay plane. I do not have the cookbook. Can you give me a quick rundown?.
Ah, you ask a very hard question. I'll try to hit the highlights, but I am sure that there will be sizable gaps of knowledge here.

ZDP can be considered to be the physical center of the voice coil. You can actually measure that position at the crossover frequency, but the author states that the mechanical center of the voice coil is good enough.

As you know, there should be phase coherence at the crossover point between two drivers in an ideal world. When that coherence is not present, the vertical lobe can be tilted up or down causing a non-linear pattern.

That tilt can be expressed with this equation:

Beta = ArcTan(d1/d2) in degrees

D1 is the vertical displacement between the two centers of the drivers.

D2 is the horizontal ZDP displacement between the drivers.

You can also calculate the delay between two drivers at a point directly between the two drivers at some equidistant point with:

t = d2 / C

Where C = velocity of sound. Make sure your units are all the same (i.e., inches, metric).

Basically, the larger the value of D1, the greater the possibility of lobing.

This subject is one where there is a lot to understand and getting the Cookbook will go a long way towards understanding it better.

Lastly, while coherence is an important issue, you are now getting into the finer details of a design. If you are working at this level it presumes that you have a lot more of the fundamentals of the design worked out that have a bigger impact on the quality of sound.
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Old 19th February 2010, 01:26 AM   #7
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by Cousin Billy View Post
The amplifiers used will be the Atma-Sphere MA-1 OTL tube amps.
Forgot to mention... Nice amps!
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Old 19th February 2010, 09:57 AM   #8
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Quote:
Originally Posted by Cousin Billy View Post
...Trigonometry calculation question;
If my ears are 12 inches lower than the tweeters, and 5.5 inches lower than the midrange, 180 inches from these drivers, with the difference between the drivers diaphram depths being 3/4 of an inch, do I need to recess the tweeters further into the baffle?. This will be a rather small amount, or should I not worry about it?.
Quote:
Originally Posted by Loren42 View Post
...
D1 is the vertical displacement between the two centers of the drivers.

D2 is the horizontal ZDP displacement between the drivers.
Cousin Billy,
The design process should be, taking in attention "delays and angles" at the xover for, in this case, mid and tweeter, so that, you can not decide the recess if you don't know if you need it. So, back and forth, "tell me about your xover" and you will know if it's needed. The xover can introduce time delays or angles with phase. If you don't do the design process with phase (not frequency output as everybody does) in mind (on paper/simulation) you will never get it. (This was the theme of an argument I had before @Loren42 post).

Last edited by Inductor; 19th February 2010 at 10:11 AM.
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Old 19th February 2010, 12:24 PM   #9
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Just for the fun of it, "phase more important than output", by the late great John Dunlavy,
John Dunlavy Interview by Stereophile
and also to complement with, the exceptional tech article, by Pi Speakers Wayne Parham.
Baffle spacing, phase angles and time alignment, revisited

Last edited by Inductor; 19th February 2010 at 12:34 PM.
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Old 19th February 2010, 12:48 PM   #10
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First Loren

Nail has been hit on the head, thank you. The calculation can now be made. I will design this parameter with two people in mind, myself at 6'3" and my wife at 5'5". The lobing will be accepted by me, I can lower my head, but it is my wife I want to please.

I have looked at a number of commercial speakers that have some form of time/ZDP correction. Most tilt the speakers back ~6 degrees, my ProAc Response 2 has the tweeter offset, and some (YG Acoustics) have the tweeter recessed quite a ways back. There are too many others to mention.

Inductor

Group delay has a peak at 10Hz of 6.9ms, dropping quickly from there. A slight bump at 160Hz, futher dropping to .35ms at 150Hz, and stays at or near 0 the rest of the way up. The midrange is to be wired out of phase. This is from the simulation.

I will start testing various tweeter depths on my test baffle. Nothing is glued together so this will be easy.
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