Speaker stuffing problem

[Phil Olson]

I have designed a nice two way and tested it and it worded fine so I made a bunch more to convert my 7.1 theater.

The problem I am having is that a couple of the speakers won't come down enough in Q no matter how much stuffing I put in the box. The speakers are all sequenced from the same run, (Seas 15LY) so there shouldn't be any difference there. The boxes look the same.

Is there some common problem as to why s box will be too high in Q, (eg. .77 vs .707)? Can drivers vary that much? If there is a leak in the box, what does that look like?

Thanks,
Phil

[gedlee]

Yes, drivers can vary that much. Box stuffing only works to a point and then more stuffing actually takes the Q higher as the box now appears smaller from the "too much" material in it. The variation that you are seeing is almost certainly from the drivers and is not that atypical.

Smaller boxes that raise the resonance more will tend to swamp out the compliance variations (the dominate one) and this will tend to stabalize the effect that you are seeing.

If you can run impedance curves on them, then you should as this would tell you precisely what is happening.

[Phil Olson]

Thanks! I was hoping you wouldn't say that though! The amazing thing is that all the drivers are from the same run, 55, 56 ,57, 61, 63, 64, 65, 66, 67, 68, 69, 70. I wouldn't think they would vary that much, particularly from Seas.

The sad thing is that it is an MTM and the first one I built had both drivers that could be tuned correctly. Now I have six more boxes and don't know how many drivers are too far out. One is OK and three seem to be out. I have eight more to check.

I guess I'll have to mod the boxes, what a bummer! They are already finished but the backs can be carved out without damaging what shows.

Is there an easier way? Is there anything that can be done to the speaker itself without damaging sound quality?

[gedlee]

First, make sure they are well broken in before concluding anything. Rub them at high excursion, out of the box, for several hours at least. After that they won't change much, but they will change during that exercise.

Other than that all you can do is try to trace the problem, but even knowning that, what can you do? You can't rebuild them, or remagnitize them.

[Joachim Gerhard]

Is your temperature very high at the moment ? That could account for the differeces you observe. In winter it could be even worse. Stuff the boxes only that much that the wrincles disappear in the impedance curve. More stuffing only takes back the energy.
What kind of stuffing material are you using ? If you are a perfectionist you could tune all boxes to the same Fr by adjusting the acoustic volume eg. making the internal volume slightly different until all measure the same. That can be done with styrofoam put in bags. SEAS has excellent quality control, so when you really found a flaw, call up the manager Olav Arntzen and he will try to help. Unfortunately SEAS is closed for summer hollyday at the moment until the 8th of august.

[Phil Olson]

Temperature is low 70's, quite comfortable. Here are the Qt's of the raw drivers just sitting on a carpet pointed up:

.5518, .5620, .5631, .5708, .5943, .5965, .6118, .6239, .6285, .6490, .6677

I ran them all at 1W pink noise over night before measuring. Is that not enough? Should the breakin power be increased? Are these number variations typical?

I would hate to modify the boxes but that looks like my only option at this point. I would like to tune them to the same .707 so the slopes are the same. Only four of the 14 I've bought will tune this low. Perhaps I'm too much a perfectionist?

[Joachim Gerhard]

Also look at Vas. The Q factor variation i see is big but normal. Put in a small box you are fast running out of steam to end up with a butterworth alignement. The boxes seem to be already build. One option is to use a big cap and then you have a 3rd order system with flat response.

[sreten]

Hi,

You do not need to mod the boxes. 0.77 is different to 0.7 but not a disaster
by any means, its only a ~ 1dB or so maximum difference. Sort your drivers
and then pair them , e.g. do not use your two lowest and highest together.

The differences in Qts will average for each box, in fact for two similar drivers
in a box, lowering the Qts of one will lower the Qts of the other somewhat
and vice versa for higher Qts.

For smaller speakers a lot of people prefer Qts = 0.8 over 0.7. If its a 7.1
system as you say your quibbling over near nothing if bass management
is applied to your 7 speakers.

Also the raw drivers variations of Qts are almost certainly mainly due to
variations of Vas and hence Fs, boxes will reduce these differences but
looking at your measured range a 0.7 target is unrealistic, the box size
for 0.67 would be massively oversized.

Whats your driver Vas to box volume ratio ? for sealed Vb should dominate.
What are the nominal figures for Qts, Vas and Fs for the driver ?
What is the box volume you are stuffing ?

/Sreten.

[gedlee]

Also, measure the same speaker Q several times to find out what your measurement variabiity is. This could be ALL of the variability that you see.

[Phil Olson]

The driver is Seas W15LY001 Specs: Vas 12 L, Qts .45, FS 49 HZ
When I run these numbers through WinISD I get a Vb of 8.2L for 707

I built the box as 10L which I thought was plenty. I just did a mod that added 2L more and am still running out of room.

If I go the other way and plug the highest measued Qt into WinISD I get the results that .707 is indeed impossible, (100L!!!).

The question is if the spec is .45 why am I getting as high as .67? I would have thought that the variation would center around .45 and go lower and higher.

My measurement setup is a Dell Studio 17 running Windows 7 professional 64 bit. This feeds a M-Audio Firewire 410 into a Bryston 4BST amp.

The software is Soundeasy v16. I have a 10.0 ohm resistor as the measurement standard and all volume controls are max except the Soundeasy slider control which is at 10%, (perhaps a watt or so at most).

Anyone played with these drivers? I wonder if I'm just not measuring them correctly.

Right now the drivers are individually chambered. If I build one box the same footprint and series the low and high drivers. I get a free air Qt meaurement of .60. Plugging that into WinISD I can get down to .707 with 35L which is doable, (33.8L box plus stuffing).

Right now I guess I'll run them all at .75 and be done with it. Since I'm retired, if I get bored I can always go back do the new boxes later.

I just shot off an email to seas so see what they say.