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Old 24th September 2001, 03:47 AM   #1
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I'd like to build some two-way nearfield monitors using some speakers and crossovers which I already have laying around.

The speakers are for automobiles however I'd like to utilize them in some low cost cabinets. For each cabinet I'd like to use:

1 MB Quart QM130TX (mid-bass)
1 MB Quart QM19HC (tweeters)

1 Passive MB Quart crossover, 3500Hz, 12dB/octave

This crossover came packaged with these 2 speakers when I bought them years ago.

I'm looking for some direction here - how to build some cabinets based on the speakers and crossover which I have.
Nothing too elaborate, just something that I can use as monitors to edit some recordings. Any help will be appreciated.

Regards,
Alex Douglass
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Old 24th September 2001, 04:19 AM   #2
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Alex,
In order to build a proper cabinet, you'll need the Thiele-Small parameters for the mid-bass drivers. Once you have that information, it will be a simple matter of plugging the numbers into a program (any number of which are available--people tend to develop an emotional attachment to them and defend them against all comers) which will, in turn, give you the volume for the cabinet. Your next step will be the woodworking.
But first...the T-S numbers.

Grey
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Old 24th September 2001, 06:36 AM   #3
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Alex:

The tweeters and crossover came as unit, not much use monkeying around there.

The bass/mid will require the enclosure. I took the liberty of looking up the Thiele-Small specs at
http://www.speakerspecs.com

Here they are:

Resonance Frequency [Fs]
80.90 Hz

Mechanical Q Factor [Qms]
3.74

Volume Equivalent of Air at Cas [Vas]
4.76 liters

Volume Equivalent of Air at Cas [Vas]
0.16 ft-3

X Linear [xmax]
7.00

Electrical Q Factor [Qes]
0.69

DC Resistance [Re]
2.90

Nominal Impedance [Z]
4

Total Q Factor [Qts]
0.58


Sensitivity [SPL 1W/1m]

86.0 dB

I have my own ideas for an enclosure for this speaker. But I will let Grey go first.
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Old 24th September 2001, 12:09 PM   #4
Super is offline Super  United States
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I'm hoping for some pyramids and cylinders this time around (pyramids in particular. I think we can assume that the crossover doesn't compensate for driver alignment.)

Also, with an efficiency of only 86db, looks like we're going to need a fairly powerful amp here. But I agree, lets listen to Grey first

[Edited by Super on 09-24-2001 at 07:13 AM]
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Old 25th September 2001, 02:03 AM   #5
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And the three young men sat watching the flames of the dying fire, discussing the weighty matters of the day. Of boxes and ports and amplifiers strong they spoke, and methods of prying into the secrets of nature.
"Do you think he'll come?" one asked.
"I don't know," said another, shaking his head.
"Huh, I don't know if I even want the old buzzard to show up," muttered the third. "All he'll do is tell us we're up past our bedtimes."
And in truth, it was said by some that the one of whom they spoke was a fool, yet by others claimed to be wise. He was reputed to be everywhere--and nowhere.
In fact, he was...
...behind them.
Some small sound made one of them turn, and there, framed in the doorway against the dark of night, stood a cloaked figure, regarding them silently.
The young man gasped, causing his companions to turn also.
A gnarled, calloused finger raised, pointing at Alex. "I have consulted with the powers of the unseen world. The numbers have been returned on the wings of a white raven. You are seeking a ported enclosure of .51 cf volume, with a resonant frequency of 55.26 Hz." His eyes narrowed, flashing. "But I fear that you will not be entirely happy with your project."
"But, but...why?" Alex asked.
"It is not the destiny of a car driver to make a good monitor. For that, you need better drivers."
"Better?" the kelticwizard asked.
"He means it'll take eye of newt and toe of frog to make this come off properly," said Bryan. "And I went and left mine at home."
"I think I've got some in my pouch," the kelticwizard said, rummaging in the leather bag suspended from his belt.
"But one thing more will this take," whispered the shadowy figure in the doorway.
The three looked up in alarm, fearing that they might yet come up short in their efforts. "What's that?"
"Beer!" cried the old man, throwing back his hood, laughing. As he came towards the fire, he pulled off the latex prosthesis that had made his finger appear misshapen. "Everybody knows you can't build speakers without beer."
*****But seriously, folks*****
Actually, red wine or a good single malt Scotch will do just as well, if not better.
(Bryan, close your ears, lest I get arrested for contributing to the deliquency of minors...)
Jeez, fellas, you made it sound like I was supposed to arrive tap-dancing or something.
Honestly, Alex, I'm not sure that car speakers are the ticket to nirvana, here. But, if they're what you've got on hand and money is tight, then go ahead and build them. If you've got a little cash on hand, it might be better to look into other options. Just for giggles, I tossed those numbers into the program I use, and it said ported, which honestly surprised me, as I tend to think of car drivers as being in enclosed chambers. I mean, really, you just don't see ports in the rear decks of cars every day of the week.
That said, I couldn't seem to manipulate the box to give a truly flat response. It comes down to about 100Hz or so, shelves down a dB, goes to about 50Hz, and drops like a stone. If you think that a low end in that region will get you where you need to go, then get out your woodworking tools. It's up to you.

Grey

[Edited by GRollins on 09-24-2001 at 09:05 PM]
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Old 25th September 2001, 02:48 AM   #6
Super is offline Super  United States
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First, I'd like to think Grey for making my evening worth while. Nothing can quite compare to a combination of DIY audio and a good chuckle now and again. But anyways...

As Grey (sort of ) said, these drivers definitely aren't ideal, but if you have them on hand and don't need audiophile quality, then why let them go to waste?! So, lets start with the materials.

MDF: Material of choice for the DIYer. Heavy, fairly easy to work with, and better characteristics than hard wood in term of resonance. Depending where you're from, the cost for the MDF (medium density fiberboard) can be between 12-40 bucks, and can vary with your cabinet design. The thicker the better. 3/4 inch is widely available here, so I often double it up for 1 1/2 inch width. However, this stuff is MESSY. Its made from compressed sawdust, so its a good idea to work in a well ventilated area. And I STRONGLY suggest wearing a painters mask. It helps a lot. Also, its not a bad idea to have carbide bits, and a few of them for each tool. MDF is extremely hard, and wears bits down like none other.

Tools: Basically a must have: A router (though a dremel or Roto-zip with router attachment can be substituted), a table saw (unless you dont like straight cuts), plenty of clamps, and a drill or drill press. Again, carbide bits and replacement bits are strongly suggested to have on hand.

Putting it all together: There is some debate in this area. For attaching the driver, t-nuts and threaded screws seem to be the way to go. For laminating boards together, any strong wood glue or Liquid Nails will do, although I like to use "Rue glue". I get this from a nearby woodworking company. It dries slightly rubbery, giving it excellent damping, but once it dries, you're more likely to rip the face off of the MDF than you are to break the bond. If you are going to use screws to hold it together, I suggest drywall screws. They have a bigger thread, so they grip the MDF better. All vented cabinets must be sealed very well in all areas except the port. You can use your choice of materials for this, although I and many others use silicon caulk. Its a good idea to let this dry first, because the fumes can damage the driver. Also, you need to line the inside of the cabinet. A heavy felt can be used, but I prefer to use acoustic foam (the egg-crate stuff). It helps that I get this extremely dense foam for free from "Hi-Tech Packaging"; a company who my father, a UPS man delivers to.
Also, if you want to keep the cabinet from resonating, you're going to have to brace it. Be sure to compensate for all bracing when calculating the final volume.

Port tubes can be made of PVC, or there are several online sources to find them, for a neater look, and not much money. Now, we get to the shape...

If you want to keep things simple, you can make a box. Is this the best enclosure? In my opinion, no. Whenever you have a right angle, you have standing waves, and this we want to avoid. I can point you to a few pyramidal designs online, similar to the Wilson WATT appearance. For a small monitor speaker, I'd point you in the pyramid direction. If you have no woodworking skill or know how whatsoever, then come back and let us know, and we'll continue this further...
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Old 25th September 2001, 05:58 AM   #7
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Smile Thanks for the feeback!

You guys are outstanding! Thanks for all of the feedback. The epic yarn by GRollins still has me laughing.

Some of posts have me thinking a little more about this project. I expected to lose some bass but didn't quite think that I'd have as much dB loss from 50 - 100Hz. I guess I'll just have to sacrifice some bass. Also, I saw an estimated 86dBSPL W/m. Would this be the overall sensitivity for the speakers in the enclosure, or just the speakers by themselves? I don't plan to listen to the speakers very loudly. They will only be about 3 feet away from my head, played at modest levels.

I guess that I'd be losing some more sensitivity if I go with an outright sealed box. Any chance I can get something reasonable (better than regular Yamaha PC speakers) with a sealed box?

My setup will be PC soundcard -> audio receiver -> monitors.
Currently, I am using Yamaha YST-MS25 (R, L, and subwoofer). The subwoofer is OK but the two small L and R speakers are nothing to brag about. My goal is to construct these monitors to replace the R and L Yamaha speakers, and later on the sub. as well.

Thanks again,
Alex Douglass
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Old 25th September 2001, 12:12 PM   #8
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Sensitivity, I believe, is almost always measured by itself, primarily because different boxes change the sensitivity (horns for example). The industry standard is to present an 8-ohm speaker with a 1 watt (2.83 volts) input signal and measure its output in an anechoic chamber with a microphone at a one-meter distance.

Being rather inefficient as they are, I wouldn't suggest placing them in a sealed box. It will take more amp power to reach the same output levels, and you'll lose even greater bass response. A sealed application doesn't seem to be the appropriate choice in this application, especially if they are only going to be played at moderate volumes.

Trust me on this one. Its not very hard to outdo almost ANY mass produced set of PC speakers on the market. Even with these drivers, I think you'll see a world of improvement.
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Old 25th September 2001, 04:37 PM   #9
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Sorry...I guess I got rather carried away above...
*Sigh*
Anyway, Alex, the T-S specs carry within them strong hints as to whether one should use a ported or sealed enclosure. The numbers for your drivers (assuming that the data from that site that kelticwizard found are correct) point towards a ported enclosure. That's not to say that you can't use a sealed enclosure, but be prepared to sacrifice a bit of performance in the process.
If you're going to use them nearfield and at 'moderate' volumes (moderate for some folks around here means that the trees in the back yard are swaying, but those out front are merely trembling) you need not worry about efficiency or sensitivity. A modest amp will drive them well enough.
Whether car speakers or PC speakers are better...oi! I think I'll go have another drink.

Grey
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Old 25th September 2001, 07:13 PM   #10
tvi is offline tvi  Australia
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Your other option might be a <a href="http://www.infomaniak.ch/~bonavolt/tqwt.htm">Tapered Quarter Wave Tube/Voigt Pipe</a>

Theres two excel spead sheets <a href="http://melhuish.org/audio/software.htm">here</a> for calculating the enclosue.

As to its suitability for near field?

Regards
James
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