LMS pros, cons, and alternatives for measuring speakers

[head_unit]

Inherited some LMS and LEAP setups, which I used some time ago.

LEAP I really liked as a simulator...LMS, less so. My application was measuring 2-way car speakers in a not-large space, and simulate crossovers. With MLSSA, I could get good matches between simulation and results. With LMS, not. Basically as I recall, the LMS does not truly measure phase but calculates it via Hilbert transform. That's fine if you have linear drivers and a good measurement...but if you measurement chops off below a few hundred hertz, the calculation freaks out. Also, if there are non-minimum-phase nonlinearities due to acoustic cancellation and such, then I think Hilbert is not valid (?).

Anyway, I would welcome any hints and comments as to:
- How to make good measurements with the LMS
- Alternative systems which make an actual phase measurement, thereby bypassing those problems, and their pros and cons. (I already know MLSSA well-works great, expensive. But what others still exist or have come out newly?)
- Any simpler/cheaper packages than LEAP which number crunch actual magnitude and phase data to simulate crossovers?


P.S. If some think this should be in the "software" forum, apologies, but when I looked there it was basically SPICE...plus I figured the users I want are here in this forum...

[Michael Chua]

I use LMS. Mine is the early DOS version (386 days). Works just fine even after all these years. Very happy with the performance. Extremely accurate.

LMS is not a simulator. It does frequency and impedance sweeps. A useful function is the Gated Sweep. With this feature, it is like measuring in an anechoic chamber.

By the way, LMS comes with a Full size AT card. You'll need an ISA slot (vintage motherboard). I think Linear-X has an external case for the newer LMS versions.

[head_unit]

What do you do with the LMS? (i.e. what are you measuring/designing? What is your application?)

What hints do you have for using this?

I want to import data into LEAP to use it for crossover work, but don't have a big measurement space.

[Michael Chua]

LMS basically measures a driver's response in real time. You can measure the frequency response of, say a tweeter with a crossover network in place. Likewise for the woofer. When the two of them are combined as in a 2-way speaker, you will be able to tell if the crossover network is correct.

For more information, click here for a project that uses LMS exclusively in designing a speaker. It is quite detailed, starting with Thiele/Small measurements.

Click here for newer speaker designs.

[head_unit]

Sorry, I was not clear. I'm familiar with LMS, just haven't used it in a long time.

I'm wondering, if I don't have a big measurement space available, if I can really use it for crossover work-or if the phase-deriving transform will freak out and yield garbage...

[Michael Chua]

All you need is 1 meter without any obstructions all round. With a Gated measurement, you should be able to measure down to about 700-800Hz. Below that, you'll need space. Either ground plane or pit.

I believe you can export LMS to LEAP and use the crossover design software there for simulations. I don't use LEAP, nor do crossover simulations, so not much help to you there.

The Minimum Phase feature in LMS allows you do certain things, like merging the port output of a Bass Reflex with a Near Field Response of the woofer. You can also generate the Group Delay of a Low Pass Filter.

Minimum phase plots can look very messy. It is fine if it is simply a phase plot of the impedance sweep.

[Erik F]

I know this thread is old, but, I stopped using LMS about 10 years ago. I find it isn't very flexible and behind the times. LMS doesn't really offer very much for the money and like has been stated relies on frequency data to produce phase data...also you cannot find true acoustic center of drivers with LMS. I use Praxis and before that CLIO. Praxis can do everything that LMS can do and more. There are other sound card based programs out there that are more advanced than LMS also if you don't mind doing a little work. Praxis isn't cheap either (around $1000), but still less than LMS or MLSSA and you don't need an ISA slot. I have put together a nice very portable set up behind Praxis that all fit into a 1x1x2' case. Everything I need is in that case besides a mic boom and laptop. If you are on a budget look into SoundEasy, but buy John K's guide for $11.

[head_unit]

Yeah, I like LEAP a lot but LMS not as much. It's nice to actually measure the amplitude instead of calculating, but I hate the assumption that phase is linear. And it's not always convenient to try and do all that splicing and so on.

Isn't there some cheap sound card based thing that will do a quick and dirty basic measurement by now? I thought there used to be something for like $190 but everything seems very expensive...

[Erik F]

I have thought about giving ARTA a shot it is cheap and has a good reputation.

[1audiohack]

LinearX is working to catch up with the immanent? release of the LX500.

As has previously been stated LMS is painful to use, it looks like the new platform will be a huge improvement.

The old ISA card has real trade in value, like $500 USD trade in value although part of the savings is in not buying the mic again as it is the same M31 used with LMS, but the LX500 is still expensive.