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Old 18th March 2009, 09:54 PM   #3041
jm_kzo is offline jm_kzo  France
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Quote:
Originally posted by Sabbelbacke
Doing "free field" measurements gets harder the longer the wavelength ist. One good "trick" to get good results is the ground plane measuerement.
Putting the speaker on a hard surface like a parking space and putting the mik on the floor creates a situation where the floor acts as an acoustical mirror. If there is enough space between mic, box and surrounding objects (cars, buildings), one can easily do good measurements, especially for bass. I have no good online resource on the matter, but the book by Jospeh D'appolitto covers it in detail and itīs the method used by most sub-measurements.
Doing measurements in the horn mouth can have funny effects on the results.. I didnīt try it on a TH but know the effect on frontloaded horns quite good (and because I know measurements like this can be misleading, I never bothered again, always went straight to groundplane).
Hi Saddelbacke,
would you use "gated" measurement for this (like in SW) because my undertsanding is that that would indeed eliminate reflexions but they are no good for bass ?

I assume you would do this outdoor ?

That would be great if you could suggest dimensions for the set up ? How far to put the mike, how high above the ground, etc.

Many thanks in advance.
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Old 18th March 2009, 10:44 PM   #3042
MartinQ is offline MartinQ  Canada
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jm_kzo, the first thing that comes to mind is:

- looks like it's made from particle board which is very flexible, is there any bracing?

- how thick is the material?
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Old 18th March 2009, 10:52 PM   #3043
jm_kzo is offline jm_kzo  France
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Quote:
Originally posted by MartinQ
jm_kzo, the first thing that comes to mind is:

- looks like it's made from particle board which is very flexible, is there any bracing?

- how thick is the material?
Hi Martin,
no bracing (again, I only experienced quite modest SPL... I think), this is just for experiment, if I build a final one, it will be braced.

This is 18mm particle board.
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Old 18th March 2009, 11:03 PM   #3044
jm_kzo is offline jm_kzo  France
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Quote:
Originally posted by littlemike
@ jm_kzo

Can you measure impedance? That may help identify leaks.
Hi littlemike,

here are the impedance measurements...
Click the image to open in full size.


Quote:
Originally posted by littlemike

Some more questions:
How did you seal the top of the cabinet?
Is your microphone/preamp/measuring system accurate below 60 Hz?
Are you using any sort of response weighting in your mic preamp?
Are you measuring indoors?
Some answers (as much as I can)...
The top of the cabinet was screwed and I used foam joint (you know, for doors and windows) to improve sealing...

I do not really know if my measuring equipment is accurate. It is a Panasonic WM-61A mic and a "Eric Wallin" v2 preamp, but it never got calibrated. Can it be that off ?
I can borrow a digital SPL-Meter at the locakl Home Depot, would it be better ?

No response weighting either, how do you produce this ? Calibration ?

Yes, I am measuring indoors.

Quote:
Originally posted by littlemike

Perhaps you could try measuring with FreqRespPlot or a another free swept-sine measurement software as a check of Speaker Workshop?

FreqRespPlot (in French)

There is an English version of the helpfile and software available too.
I think I will be Ok with the French version of Freqresplot .
I have measured with REW and SyncRTA as well, measure more or less match.... unfotnately. .


Thanks for your help.
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Old 18th March 2009, 11:20 PM   #3045
Mark Kravchenko --- www.kravchenko-audio.com
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Default Low end Missing In Action

jm_kzo

Your measurement position is one part that is causing your problem. The horn mouth is designed to fired into a corner. The corner of the room then acts as an extension and thus an enlargement of the horn mouth. This will give better support for the low end of the frequency response.

For measuring check this out.

http://www.xlrtechs.com/dbkeele.com/papers.htm

You want #6.

It will tell you what to do. All I could do is quote from it so I might as well give you the source. As for the rest of the papers dig in!

Mark
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Old 18th March 2009, 11:58 PM   #3046
fb is offline fb  Australia
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Default Re: fb_TD10H

Quote:
Originally posted by GM
You may need one or more resonators to damp the peaks depending on the amount of stuffing used.

GM
Is that the peaks in the passband or above? I'll be using computer eq anyway, not sure if there'll be problems or not.

Quote:
Originally posted by tb46
Hi,

The experts advise a compression ration below 3:1. In other words S2 should not be smaller than 115cm^2 for this driver. If you watch the comments bar at the bottom of the Hornresp input screen, it will tell you the compression ratio you are using.

Regards,

Doom doom doom..... Hehe. What effect will a ratio of 4.35:1 have? S2 of 115 makes freq response not happy again.

In a related note, designing tapped horns does my head in - can anyone share (or make for me ) a TH with 10" driver that does 120db 35-90hz? All designs I've seen with 12" or less exceed xmax in this range.



Thanks all for your help
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Old 19th March 2009, 12:11 AM   #3047
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Keeles Paper focuses much on the nearfield measurements and primes for techniques how to convert a nearfield measurement into an farfield equivalent (combining Port Level and direkt radiation from the cone or taking the baffle step into account). I didnīt read it completely but recognise most of the grafix and topics and since its referenced a lot in later works, the topics are familiar.
As far as Iīve seen, ground plane measurements are not covered in this text.

A gated measurement would be no good with a subwoofer, since the time-window would be tooo long because of the very long wavelengths. Since groundplane measurements are +6dB than farfield halfspace, a 2m distance is a good starting point for "practical" measurements, since the level derived is comparable to normal measurements (2m is a doubling of distance to 1m which equals a 6dB loss, so youīre even). Also the distance to other objects is relevant, my rule of thumb is that objects need to be at least 3 times the distance than mic to speaker, some even say 5 times is necessary. Since I sometimes do those measurements in a Club-Discothek, the 2m/6m rule of thumb is a good enough compromise for me.
I also use sine-sweeps most of the times, since itīs signal to noise ratio is better for bass-measurements in suboptimal enviromnents. Look up farinas work on this, afaik he did the latest research on the matter.

I googled a little and found this, which might come in handy as a starting point:

http://www.mh-audio.nl/Groundplane.asp

Hope i could help.

Oh, I forgot, edit:
Measuring a cone and measuring a horn is not directly comparable. In front of a cone youīre always in the nearfield and have a pressure chamber-like situation. In a horn you have alternating perassure maxima and minima, so it is very likely to pick a different response at the hornmouth than if youīd measure outside the speaker. At least thatīs i discovered in most of the cases when dealing with basshorns. A reflex(ported) enclosure always is easy to measure with keeles techniques, basshorns or TLs most of the time failed, but worked good with groundplane.
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Old 19th March 2009, 12:34 AM   #3048
JLH is offline JLH  United States
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Default Re: My tapped horn experiment with a Beyma 10G40

Quote:
Originally posted by jm_kzo
Hi tapped horn gurus,

I have been lurking at this thread for while and posted a few times, but last week-end, the time did come to make some saw dust and play some sine sweep...

Just to put things into context, I am looking for a clean sub to cover from 80/100Hz down to as low as possible but realistically lets say 30Hz to go with this project that I have described here.

The only speaker that I have lying around is a Beyma 10G40 that will go later in my center front speaker and I decided to give it a go as it was simulating quite well.

The simulation with the T&S parameters from the specs are as follows.
Click the image to open in full size.

It is even flatter with an added 2.2mH inductor.

Following advise here, I "measured" the T&S parameters of one of my 10G40 and it models as follows in the same tapped horn (quite similar).
I am putting measured in quotes because:
- I used SpeakerWorkshop measure in free air method
- I did not do the added mass or closed box bit, so do not have Vas
- my speaker stand was not necessarily as rigid as it should be.

Click the image to open in full size.

Again, it is a bit better with an added inductor.

I then started the build...

Here is how it looks before closing the box.
Click the image to open in full size.

Where the driver will go...
Click the image to open in full size.

The driver in place ready to be sealed...
Click the image to open in full size.

The thing in place for measurement (mic in the mouth)...
Click the image to open in full size.

Up to this point, I was a happy man...

With the pieces cut to dimension (as we can get from our Home Depot equivalents here in France) it took me 4 hours prompt to fully assemble the thing.

[tbc]
You need to make the hole opening into the throat the full size of the cone. The S2 area only concerns the horn, make the woofer hole as big as the cone. Your picture labeled "Where the driver will go..." shows me yor problem. I made the same mistake on my very first tapped horn. Do not feel bad, it looks like you have a big enough access panel to cut the woofer opening larger.

Rgs, JLH
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Old 19th March 2009, 12:52 AM   #3049
Mark Kravchenko --- www.kravchenko-audio.com
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Default I messed up with to many open windows to copy from!

Thanks Sabbelbacke your correction was well presented!

I posted the source in error .

Your comments are completly correct for the nearfield response from a horn to.

I have made a small PDF directly from D'Appolito's Testing Loudspeakers. It's a bit big to post on the forum. So here it is!

My apologies!

http://www.filefactory.com/file/af6e...asurements_PDF

I'm still looking for the paper on low frequency horn measurement. Never have the right stuff at hand when you need it!

Mark
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Old 19th March 2009, 01:00 AM   #3050
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Default Re: I messed up with to many open windows to copy from!

Hy Mark,

Quote:
I have made a small PDF directly from D'Appolito's Testing Loudspeakers. It's a bit big to post on the forum. So here it is!
Thanx. thatīs very helpful as a basic starting point. Looks familiar I was wondering if one could lookup the book in google-books? Its restricted in germany, how about in the US oder canada?

Quote:
I'm still looking for the paper on low frequency horn measurement. Never have the right stuff at hand when you need it!
Oh yes, I know the situation I hope after I switch to MAC on my desktop the "search" will help me out on situations like this. Similar mechanisms on windows didnīt appeal to me, fed up too much ressources.
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