Controlling speaker directivity

[TheWarrior]

Can someone point me to a good source of information for learning to understand and manipulate this through crossover design?

I think this will help broaden my understanding of speaker design, and maybe put a few other things into context.

 

[Swerd]

To keep things simple, let’s think about a 2-way speaker. Let’s assume you’ll use a 6½" mid woofer, and you’ve already decided on a cabinet design that provides decent bass response. For example, look at the Seas CA18RNX coated paper woofer.

How do you choose a crossover frequency that keeps an overall smooth frequency response, and preserves the woofer's high frequency off-axis sound as much as possible? This is critical for a speaker's image creating ability.

Generally speaking, the higher the crossover frequency, the less audible the crossover is, as it pushes the crossover above the frequency range where human hearing is most sensitive. But there is a trade-off. As you increase the crossover frequency, the off-axis response of the woofer deteriorates. This will hurt the speaker’s ability to create good imaging. So a lower crossover frequency is important.

Imagine a 4th order crossover at 2000 Hz. When sound is exactly at 2000 Hz, both the woofer and tweeter are equally loud. Within an octave of the crossover frequency (from 1000 to 4000 Hz), both the woofer and tweeter contribute to the sound enough so that differences in their dispersion are noticeable. If you push the crossover frequency too high, the woofer beams sound instead of dispersing it widely. In this same frequency range, the tweeter will be dispersing it's sound widely. Sound imaging works much better if you avoid this.

This woofer's frequency response is shown below. Note that 3 traces are shown for high frequency responses above 500 Hz. They are measured on-axis (darker black line), and 2 off-axis angles, 30° and 60°.

Note how the off-axis response curves drop off at lower frequencies than the on-axis curve. At 2000 Hz, the on-axis and 30° curves are close together, but the 60° curve has dropped by about 5 dB. So, as a first guess, I'd go no lower than 2000 Hz for a crossover frequency. There are many reasonably priced tweeters available that can be crossed as low as 2000 Hz. To go lower will increase a tweeter's cost by quite a lot.

Just for grins, look at the manufacturer's frequency response curves for a smaller 5¼" and a larger 8" woofer. Comparing their off-axis responses will show interesting differences. You'll see why no good 2-way speaker is made with an 8" woofer and a 1" dome tweeter.

https://www.madisoundspeakerstore.com/approx-5-woofers/seas-prestige-ca15rly-h1216-5.5-coated-paper-cone-woofer/

https://www.madisoundspeakerstore.com/approx-8-woofers/seas-prestige-ca22rnx-h1288-8-coated-paper-cone-woofer/

 

[TheWarrior]

Bam! Not only do I now understand the concept, but also loosely how to manipulate it.

I will be purchasing the Thor kit relatively soon, and I know that will only be the beginning. And I've got KEW inviting me over to hear some Beryllium.

Swerd, I own the Loudspeaker Design Cookbook. To cover topics like this, particularly associated with high end 3-way full range tower designs, can you recommend any further literature? Thanks for your help!

 

[Swerd]

Bam! Not only do I now understand the concept, but also loosely how to manipulate it.

I will be purchasing the Thor kit relatively soon, and I know that will only be the beginning. And I've got KEW inviting me over to hear some Beryllium.

Swerd, I own the Loudspeaker Design Cookbook. To cover topics like this, particularly associated with high end 3-way full range tower designs, can you recommend any further literature? Thanks for your help!

It may be mentioned in that book, but I don't remember any details.

In my above post, I used the manufacturer's published frequency response curves to illustrate my point. But in reality, you should build your own cabinet, mount the drivers, and measure their on- and off-axis performance yourself. Things can be different depending on the dimensions of the front baffle, especially its width.

In a 3-way, the loss of off-axis performance by a mid woofer is much less of a problem. You don't have to try to get low bass from a mid woofer. Choose a mid range based solely on it's mid range performance. That also frees you to select a comfortably high crossover frequency to the tweeter, as long as the mid range doesn't loose off-axis performance.

Choose a woofer (8" or 10") that allows you to cross to a mid range driver at no lower than roughly 400 Hz. You will run into many problems, including very high component cost, if you try to make a passive crossover below that frequency.

 

[Swerd]

I will be purchasing the Thor kit relatively soon, and I know that will only be the beginning.

I've never heard the Thor, but it is widely regarded as very good.

A much less expensive DIY option that you might consider is the ER18 MTM. I've heard it, and was able to compare it to the Salk HT2-TL. I believe the ER18 MTM is the "poor man's HT2-TL". The HT2-TL uses the exact same mid woofers as in the Thor, therefore I would guess they might be similar sounding.

 

[TheWarrior]

Studied the ER18 MTM for a while when I saw it! I was almost tempted, but said no. I want the best. And given that various incarnations of the Thor exist in the form of $4500-$13k speakers, thats about the 'best' that I'll ever be willing to throw money at.

It also helps that I am a carpenter and furniture maker. I've just not built a speaker, but I've been reading and think I can jump in the deep end. Especially a kit with a pre-assembled crossover. I'm fair with soldering, but thats nothing but practice.

Re: Literature; I should have said 'learning to design with D'Appolito arrangements'. I understand that is not a 3-way, but if I learn how to add a 10" woofer to my own MTM arrangement, (oh yes I dream of hearing TLS' similar concept) then I'll really be a happy camper.

 

[KEW]

[QUOTE="TheWarrior, post: 1071439, member: ] And I've got KEW inviting me over to hear some Beryllium.
[/QUOTE]

It's the least I can do. I owe you big for letting me experience your room treatments. I understood the concept and it all made sense, but there's no substitute for actually seeing them properly implemented in a room as well as actually hearing the difference, especially in the case where you moved the one panel while I was listening !
Thanks again!

 

[mattsk8]

I played with this with my Blues MTM ML/TL in my shop. My tweeters are at about 37", and whenever I stood up the speakers would loose a bit of depth. So I put them at a 7 degree slope back, and they're perfect; they sound incredible sitting and standing. That doesn't do much for off axis (but they're actually pretty good side to side), but for sitting verses standing it solved the issue for me. I did it after I saw some Gamut speakers that had a sloped front baffle.

I should play with this on my ER18s as well, because I have the same issue there. Those are set up in my pool table area, so when I'm shooting pool I'm generally off axis with those speakers too.

I should also add that it made the biggest difference at about 7' in front of the speaker, when I'm about 15' away it doesn't really make much difference so obviously it depends on how far you are from your listening position. YMMV

 

[TheWarrior]

[QUOTE="TheWarrior, post: 1071439, member: ]
It's the least I can do. I owe you big for letting me experience your room treatments.

Not at all! And it should he noted I hung all of mine myself. I used to work for a sign company, so its almost effortless for me to hang them straight and level. You said your ceiling is high like mine, so when yours arrive give me a buzz and I can give you a hand!

 

[KEW]

I will take you up on that!
Thanks.
May be May or June by the time I actually pull the trigger.

 

[highfigh]

Bam! Not only do I now understand the concept, but also loosely how to manipulate it.

I will be purchasing the Thor kit relatively soon, and I know that will only be the beginning. And I've got KEW inviting me over to hear some Beryllium.

Swerd, I own the Loudspeaker Design Cookbook. To cover topics like this, particularly associated with high end 3-way full range tower designs, can you recommend any further literature? Thanks for your help!

Does the Cookbook have any way of showing that new information was added after previous editions? I think mine is the Second Edition, so I know there's a lot of new info that mine is lacking.

 

[TheWarrior]

Does the Cookbook have any way of showing that new information was added after previous editions? I think mine is the Second Edition, so I know there's a lot of new info that mine is lacking.

• Chapter 0. How Loudspeakers Work
• Chapter 1. Closed-Box Low-Frequency Systems
• Chapter 2. Vented-Box Low-Frequency Systems
• Chapter 3. Passive-Radiator Low-Frequency Systems
• Chapter 4. Transmission-Line Low-Frequency Systems
• Chapter 5. Cabinet Construction: Shape and Damping
• Chapter 6. Loudspeaker Baffles
• Chapter 7. Passive and Active Crossover Networks
• Chapter 8. Loudspeaker Testing
• Chapter 9. CAD Software for Loudspeaker Design
• Chapter 10. Home Theater Loudspeakers
• Chapter 11. Car Audio Loudspeakers
• Chapter 12. Two System Designs

Thats the chapter list of the seventh edition, which I have. I only did a brief search but I am sure somewhere theres a list of whats been added. If you have any specific questions on what information is found in those chapters, let me know!