Ported RS100-4 surrounds

[bvbellomo]

Still a newbie, looking to build some 3-ways to replace my front and center, and wanted to build surrounds first (to learn, since they are easier, cheaper and less visible). My plan is to use RS100-4s for the mid-range of my 3-way, and originally was to have the same drivers and crossovers for the surrounds.

After considering budget and placement, I am thinking of using the RS100-4s as full range in a ported enclosure, with both the port and driver facing upwards. I have a good sub, so if I can hit the 72Hz F3 listed on parts-express for this driver, I don't need a low range driver. With the top-firing design, the extra detail from a neo-tweeter will probably get lost (not much detail here on most movie tracks anyway), which adds $40 to my cost (per speaker) mostly due to crossover parts. I am also considering dipole, bipole and transmission line configurations, if I can get them to go low enough without taking up to much space or costing too much money. Another issue is these will be placed relatively close to the listener, possible as close as 3 feet, which is why I like the top-firing idea.

I don't understand ported designs as well as the other designs. There is plenty on the Internet about theory, and lots of calculators to give dimensions, but putting it to practice is different. I know getting precise enclosure volume is important, and know to add roughly 20% for stuffing with acoustic absorption and take off volume used by the driver and port. However that gives me a very rough estimate, and I need a precise volume.

I also know people 'tune' these to specific frequencies by measuring impedance and changing the port length. And I think I understand how to do it. What I don't understand is why this is important. If my actual port is tuned higher than the frequency I want, I lose low end - a bad thing. But what if my port is tuned too low? Is that really a bad thing? I can imagine it wouldn't be good, otherwise everyone would tune a 3" driver to 20Hz, but I don't why it is bad or how precise it needs to be.

 

[chris661]

Download WinISD Pro and play around with it - you'll soon see why nobody tunes a 3" driver to 20Hz.

I'd seriously consider the likelihood of dust and other crap getting into the drivers. IIRC, they use phase plugs instead of dustcaps, so there's a very real risk (when pointing upwards) of stuff getting in and ruining the driver.
A fabric cover or a different driver would be needed.

Chris

 

[IG81]

Though I'm not really interested in home cinema (stereo movies are plenty good for me) and I may lack a bit of knowledge in the area, I think that I would not bother with vented enclosures for surrounds and find a driver that can have the desired bandwidth in a sealed or aperiodic enclosure. f3 between 150Hz and 200Hz might be plenty good with a mild Qtc, 0.65-0.75. Picking smaller drivers for better dispersion is a good idea and I would forget the up-firing thing. If possible, high-passing them would allow for greater power handling as well.

IG

 

[bvbellomo]

I think that I would not bother with vented enclosures for surrounds and find a driver that can have the desired bandwidth in a sealed or aperiodic enclosure

Why?

f3 between 150Hz and 200Hz might be plenty good with a mild Qtc, 0.65-0.75

Are you suggesting I cross everything below 200Hz to my sub? That would be very localizable and sound bad.

I would forget the up-firing thing

Why?

Too close and speakers pointed directly at you are not optimal. Too close to a wall, and rear ports are not optimal. This leaves up firing, down firing and sideways. Sideways (with a dipole or bipole) could work well. Downfiring could also work. These are for a living room, not a sawmill. I would think they wouldn't see enough dust in 10 years to actually damage the driver. As chris661 said, a cloth cover is an option.

 

[IG81]

Why?

Simpler implementation, no real need for bass on surround speakers AFAIK.

Are you suggesting I cross everything below 200Hz to my sub? That would be very localizable and sound bad.

No. Your mains will handle bass down to the sub, the surround don't need to reach the sub.

Why?

Too close and speakers pointed directly at you are not optimal. Too close to a wall, and rear ports are not optimal. This leaves up firing, down firing and sideways. Sideways (with a dipole or bipole) could work well. Downfiring could also work. These are for a living room, not a sawmill. I would think they wouldn't see enough dust in 10 years to actually damage the driver. As chris661 said, a cloth cover is an option.

Going sealed would resolve the port problem and unless you can find a driver with a large rise in HF response, it may sound dull 90° off-axis.

IG

 

[bvbellomo]

Simpler implementation, no real need for bass on surround speakers AFAIK.

No. Your mains will handle bass down to the sub, the surround don't need to reach the sub.

Going sealed would resolve the port problem and unless you can find a driver with a large rise in HF response, it may sound dull 90° off-axis.

IG

IMO Surrounds for movies should be full range or cross to the sub. These are mainly for effects, and there are a lot of effects below 200Hz.

Part of my problem, and the reason I started this thread, is I don't understand how to build a ported enclosure. Not using a ported enclosure is a poor solution to that problem.

Off axis response is a major drawback to anything other than a front firing design. Front firing with a top, bottom or side port is worth considering.

 

[IG81]

IMO Surrounds for movies should be full range or cross to the sub. These are mainly for effects, and there are a lot of effects below 200Hz.

Part of my problem, and the reason I started this thread, is I don't understand how to build a ported enclosure. Not using a ported enclosure is a poor solution to that problem.

Off axis response is a major drawback to anything other than a front firing design. Front firing with a top, bottom or side port is worth considering.

I'm fairly confident that you may find that some 4"-5" drivers could almost reach 100Hz (f3) in sealed enclosures and have useful reach beyond that.

As I said, I might not have all I need to know to advise on HT setups, but aside from differences in sealed or reflex enclosures for surrounds, have you downloaded and played with WinISD? It's a good start to figure out what kind of responses will be produced by different enclosure and vent geometries for a driver with a given set of parameters. Quickly and simply, a given volume of air and a vent with X length and Y diameter will produce a resonant frequency (Helmholtz). Making that vent longer will lower that frequency, and making the diameter larger will raise that frequency. Things to check for are the transfer function (loosely the bass response), cone excursion at required power levels and vent airspeed (you want to stay below 17m/s). A simulation software such as WinISD and time to play in there really is a good way to learn. Try known designs as input and you can have a reference of sorts for your own designs.

IG

 

[bvbellomo]

I'm fairly confident that you may find that some 4"-5" drivers could almost reach 100Hz (f3) in sealed enclosures and have useful reach beyond that.

As I said, I might not have all I need to know to advise on HT setups, but aside from differences in sealed or reflex enclosures for surrounds, have you downloaded and played with WinISD? It's a good start to figure out what kind of responses will be produced by different enclosure and vent geometries for a driver with a given set of parameters. Quickly and simply, a given volume of air and a vent with X length and Y diameter will produce a resonant frequency (Helmholtz). Making that vent longer will lower that frequency, and making the diameter larger will raise that frequency. Things to check for are the transfer function (loosely the bass response), cone excursion at required power levels and vent airspeed (you want to stay below 17m/s). A simulation software such as WinISD and time to play in there really is a good way to learn. Try known designs as input and you can have a reference of sorts for your own designs.

IG

I have played around with WinISD, and the effects of small changes to enclosure volume and port width and length are much smaller than what I've read on the Internet would lead me to believe. This is good news if it is accurate, but I take it with a grain of salt, since it has no problem modeling a 1 liter box tuned to 1 Hz.

 

[IG81]

The WinISD suggested alignement for a driver is usually a fair starting point that can be further customized to your needs. Looking at the SPL, excursion and vent airspeed graphs is really a good idea, along with progressively using a stronger signal level, as it will set limits within which the design can be modified to the needs. I wish I had learned of this sooner myself. Of course the program will take any input a user feeds is, such as that 1L/1Hz box. Hehe, a 1" vent would need to be ~60000" long...

IG

 

[bvbellomo]

The WinISD suggested alignement for a driver is usually a fair starting point that can be further customized to your needs. Looking at the SPL, excursion and vent airspeed graphs is really a good idea, along with progressively using a stronger signal level, as it will set limits within which the design can be modified to the needs. I wish I had learned of this sooner myself. Of course the program will take any input a user feeds is, such as that 1L/1Hz box. Hehe, a 1" vent would need to be ~60000" long...

IG

What I am really interested in using this for is to see how the design tolerates error. Suppose I decide the optimal enclosure is 2 liters with a 1" wide port that is 3.5" long and uses the manufacture's specified T/S parameters. Suppose I actually build a 1.95 liter enclosure with a driver that deviates from the manufacture's spec. I can use this program to predict how much tolerance I have (which is much more than I'd think it would be without using the program). I can possibly even use it to model what I really built, and see how various modifications fix problems. If this is accurate, it is definitely the way to design ported speakers.

If this is accurate, doing it by guesswork, adjustment, measurement and/or hearing is also a lot easier than what I've read on the Internet leads me to believe. If a 2 liter enclosure is perfect, a 1.95 liter enclosure is probably not a total disaster.