The DIY Near-Field Monitor Project, Round Two

[burdij]

It appears, in this case, that time alignment consists of moving the tweeter back in the cabinet by bolting the tweeter baffle onto the inside of the front baffle. Doesn't time alignment mean that all of the voice coils appear in the same plane? And I am curious how you would tune a sealed cabinet for best "impulse response". Perhaps he is using special "C37" lacquer on the wooden cores in his crossover networks, thus enhancing their performance by $2K - $3K.

For a passive crossover, if you use the Leap example on page 31 of the Madisound catalog (2.5KHz xover, 12db/octave) and select all the high end components (Hoviland, Solen, Goertz, etc.) this design would cost about $80 each.

 

[Moby]

It appears, in this case, that time alignment consists of moving the tweeter back in the cabinet by bolting the tweeter baffle onto the inside of the front baffle

Something like that. But, if you like to read about here;s the link.

http://www.silcom.com/~aludwig/Time_alignment.html

But, as I mention before, moving tweeter few cm back will make non flat front baffle what can be really serious problem in the MTM design. My experience tells me that DSP crossover is more flexible, you can change slopes , cross points in the second, and most important you can do usec delays and compensate driver misalignment. Also, additional eq for each band can be nice fine polish tool. :wink: At last, in the studio environment we are looking for accurate speaker, nor audiofool color Are we :?:
To fine tune some accurate passive crossover you must be real expert, and if you are, you will need some serious equipment to check the result. Just my 2 cent based on personal experience...
About tunning the sealed box Qtc is the parameter you are looking for.
Certain values of Qtc [The total Q of a driver and sealed enclosure] have specific response characteristics, which can be characterized as follows:

Qtc= 0.5 Critically dampened; Transient perfect.
Qtc= 0.707 Butterworth response-max amplitude response with minimum cutoff.
Qtc >0.707 Chebychev response-equal ripple response; max power and handling efficiency.
Qtc=0.5 is usually regarded as excessively taut and over dampened, however some standards still consider this value to be optimal.

Check this...http://www.geocities.com/kreskovs/Box-Q.html

Personally I will make calculations for both and choose to make bigger volume box, then fill the inside to fit into smaller volume calculation and finally choose by ear
:wink:

 

[burdij]

Moby - thanks for the info and links.

Do you know what equipment would be useful for evaluating these designs? I have been collecting audio test equipment recently and do have a calibrated microphone (B&K 4133) and have also added two distortion analyzers. I am located near Madison WI and noticed that Madisound has an anechoic chamber. I am contacting them to see if this facility can be rented.

 

[Bauman]

Ok, I dont know much about speakers other than use then so I thought I would ask here

I like the idea of DIY monitors, have been checking the Thor and some other MTM types, very nice.

Looks that right now I will not have time to spend on the building, at least not to do it with proper care. I have this Spirit Absloute 2 monitors that I have used in the past, really dont like the thing but... the idea was to change the drivers and to use a external crossover to have more control and use it biamped.

I saw some decent Seas and Vifa drivers from $50 to $80 the other day...
My question is, the box itself worth the try? If I dump the original drivers and choose better ones can I make this box have a good sound, or the size and everything else doesnt help much?

Keep in mind that my go here is to make it quick, not spend much and have a better than average sound, something a little better than Behringer Thruth maybe?? :roll:

some links
http://www.fm-transmitter.com/audio/speakers/spirit-absolute-2-passive-speakers.htm
http://sound-on-sound2.infopop.net/2/OpenTopic?a=tpc&s=215094572&f=884099644&m=2803097467
http://www.soundonsound.com/sos/1995_articles/mar95/spiritabsolute2.html


thanks!
Fabio

 

[Guest]

With line arrays I have a nearfield anywhere...

 

[Moby]

[quote author="burdij"]Moby - thanks for the info and links.

Do you know what equipment would be useful for evaluating these designs? I have been collecting audio test equipment recently and do have a calibrated microphone (B&K 4133) and have also added two distortion analyzers. I am located near Madison WI and noticed that Madisound has an anechoic chamber. I am contacting them to see if this facility can be rented.[/quote]
If you know what you are lookin' for that's quite enough. Anechoic chamber is what we all miss :sad:

I have this Spirit Absloute 2 monitors that I have used in the past, really dont like the thing but... the idea was to change the drivers and to use a external crossover to have more control and use it biamped.

I tried absolutes few times and I was impressed with lacking details :twisted:
BTW, drivers are better that most of low priced monitors, but something with design must be wrong....Simply not sure about:roll: Right now I don't have info about crossing points and box volume but "hunting" the "better" drivers can be nightmare. If you are not so experienced with speaker building why you don't try something like this? I was really impressed with this puppies :wink:
http://www.madisound.com/vifastudio.html

 

[mhelin]

[quote author="Moby"]
If you know what you are lookin' for that's quite enough. Anechoic chamber is what we all miss :sad:
[/quote]

You could do the measurements in open air (poor man's anechoic chamber).

Regarding the cohice of the drivers (and the boxes etc.) always pick the ones with the least distortion, the flattest frequency response, the shortest and the most even Cumulative Spectrum Decay (CSD). Otherwise you'll have a design that will color the sound. Choose the crossover frequencies so that the off-axis respones of the crossed drivers matches (perfectly). Choose the depth of the filter with the response (frequency and phase) of the acoustic system in mind.

 

[Moby]

You could do the measurements in open air (poor man's anechoic chamber).

I was digging the hole in the ground to simulate the inf. baffle. :wink:

 

[Moby]

Ok, I almost forgot to post this pic's  ;D
True 5.5 sealed nearfields crossed passive (crossover designed by Tony Gee). Yes,enclosure design is inspired by Magico  :

 

[Gus]

NICE!

Were the panels CNC cut or were they cut with a pattern and tracing router bit(a lot of work)?

Mult ply Baltic birch?

Looks like all good parts

That cab design looks like it is very dead and strong.  It would be cool to see an accelerometer waterfall graph of impulses to the cab.

 

[Kev]

yes nice work there
lots of effort and sheet used here
did you use a press during the glueing ?

what is the difference between the green and the red trace ?
is it left and right ?

looking down through the open unit
it occured to me that there might be some advantage if there were more than one position to the cross bracing
then alternated the pieces
but would make it harder to install the stuffing

again
nice project

 

[Moby]

Thanks guys. Yes, a lot of work but I enjoyed  Thanks to my friend I used CNC. All sheets were glued and pressed, but aligning was a real pain. I used the fake frontplate to align all the layers.
Yes, green is left and red is right speaker 

Mult ply Baltic birch?

Yes  ;D

That cab design looks like it is very dead and strong.  It would be cool to see an accelerometer waterfall graph of impulses to the cab.

Hmm, I'm not sure how to measure that  :-\