Geddes et al on Measuring Loudspeakers

[nullspace]

Hi All --

Earl's thread on waveguides has recently veered into touching on measuring, and I figured it's worthy of a thread of its own. My hope is that not only with Dr. Geddes give us his thoughts, but also for Lynn Olson, Duke, SY, ScottG, GM and others who know what they're doing to drop off a pearl of wisdom or two on the topic.

As for me, I'm laboring through my first complicated speaker project. I've built some widerange-on-an-OBs before, and now I'm working on a 2x12" on OB + horn/compression driver. I made a preliminary post here, but that was a while ago and it's changed since then. In addition to the Jensen A12s, I picked up a couple of the Tone Tubby 12s. And, as well as the Altec 802s I now have one each of the BMS 4550, B&C DE250, Beyma CP380M, and Radian 475PB in the mix.

I have a copy of D'Appolito's "Testing Loudspeakers" and I'm working my way through that. And, I've picked up a Behringer ECM8000, a Rolls MP13 preamp, a licensed copy of ARTA, and built a LM3886 amp (because I didn't have a single SS amp in the house) and a bunch of jigs.

I've made some measurements, but how to know if they're worth anything? Also, while ARTA I'm sure is a boon to those who know what they're doing, its flexibility is a bit daunting for those without the background. What kind of signal generator to use? What are the relative advantages/disadvantages of periodic noise, swept-sine, MLS? Sequence length? What sample rate? How to figure out the delay in order to get usable phase data? Anyways, you get the point.

In addition to a theoretical disucssion on the merits of different measuring techniques, I would think it would also be helpful if those that can speak on some of the practical hurdles in measuring chime in. For instance, I live in suburban Philadelphia. The only time of day quiet enough for me to contemplate measuring drivers outside would result in the police being summoned. I'm stuck doing measurements in my 13x18 living room -- any helpful suggestions?

Hopefully this is enough of a jumping point for a discussion that will prove a boon to both experienced and n00bs alike.

Regards,
John

[AJ]

Hi John

You beat me to it.

I'm in the same boat - I've found a speaker system that I like enough to want to get that last 5% from, but I'm going to need a much better understanding of meaurement tools to do it. I'm confident that I'm getting good FR data, but beyond that, I'm unsure. This is partly due to my misconceptions, lack of knowledge, basic stupidity, and other personal traits that make me the lovable guy that I am.

On Earls' suggestion, I went back and took a fresh look at how I've been using the tools I have, and for the life of me I can't repeat the sound card issues that I was sure existed. There is still a warm-up period needed for the preamp, but from a cold start, I was able to get stable readings within 30 seconds everytime. I think I got things in my head from my initial learning curve that stuck, in spite of them not being totally correct, and probably stem from changing too many things at once and assigning the result to the wrong one. Anyway, I'm ready to move on. The card is good.

" What are the relative advantages/disadvantages of periodic noise, swept-sine, MLS? Sequence length? What sample rate? How to figure out the delay in order to get usable phase data? Anyways, you get the point."

Perfect. I would like to see some discussion on these things as well. My grasp of them is limited to the pictures - I can understand the graphs and the waterfalls, but I need to know more about the settings and parameters used to create the output (preferebly without needing a Phd to get a practical understanding of them).

[Wayne Parham]

Keith Larson wrote a pretty good post on the subject that is probably worth linking here:

• Measurement Signal Types

[gedlee]

There is much more made of the signal IMO than is necessary. Anything that gets us to an impulse response is fine.

Once you have the impulse response then interpretation becomse the issue. If you can't get an impulse response that is believable, repeatable and clean, then thats the first goal.

In theory any and all signals MUST yield the same impulse response. It used to be that time and processing were major limitations and some signals excel at speed, etc. But today, those things are not such an issue, and for us they are not so important. On an assembly line where you have to take a measurement ever few seconds time can be a factor. But none of us is in that much of a hurry.

Don't worry about the signal type, use whatever gets you an impulse response.

Below is a fairly typical impulse response - note reflection starting after the initial response. Note that the waveforem MUST be above zero as much as it is below zero. The woofer is raised by 10 compared to the waveguide. You need to be looking at nice clean impulses like this before you can begin to do anything. You need to be able to find the direct sound and the reflections. If you can't tell where one starts and the other ends then you have too small of a space.

I get back later when I have more time.

[nullspace]

Originally posted by AJ

...I can understand the graphs and the waterfalls...

You're a step ahead of me, AJ. CSDs still pretty much confound me.

Originally posted by gedlee
Below is a fairly typical impulse response - note reflection starting after the initial response... You need to be looking at nice clean impulses like this before you can begin to do anything. You need to be able to find the direct sound and the reflections. If you can't tell where one starts and the other ends then you have too small of a space.

Thanks Earl. I'll take another look at my results over the weekend. I don't remember seeing such a strong first reflection -- I did have a 2-foot deep pile of pillows and such to dampen the floor reflection, at LO's suggestion.

Regards,
John

[SY]

What I would do also, besides playing around with gating, is to see the effects of various windowing schemes. Don't worry about "good" data yet, just get a feel for what spectra look like when you do crazy things like start the FT somewhere after the impulse start or include vs exclude the first refection. If you're creative, you can even put jumps and discontinuities in the impulse response. You just want to get to the point where if you see something funny, you'll know what caused it.

You've got an incredibly valuable resource in the d'A book.

[gedlee]

The key to taking good measurements in smallish rooms is to set up the system as far from reflections as possible. here is what I do.

First I clear the room, which isn't so tough.

Then I put the speaker on a stand that gets it at about the rooms mid-point. I have found that its just not possible to get rid of a reflection, so I don't bother with damping etc. I put the line from the mic to speaker diagonally across the room. This gives the greatest distance between the source and the first reflection. The floor or ceiling tend to be the limiting factors.

It really is important to get a decent time from first arrival to first reflection as this limits the LF capability. Making the speaker to microphones distance shorter helps this but at the sake of getting too close to the source. I would never get closer than about three feet and prefer more like ten. I use a stand that rotates to get polars.

I go into the nearfield for the low frequency and then mate the two measurements together. This yields a very reliable and useful set of polar responses from which to develop a crossover or test the final design. I can't over emphasize how important it is to do more than just the axial response. No single response could ever tell you how a loudspeakers sounds.

[gedlee]

Originally posted by nullspace

Thanks Earl. I'll take another look at my results over the weekend. I don't remember seeing such a strong first reflection -- I did have a 2-foot deep pile of pillows and such to dampen the floor reflection, at LO's suggestion.

Regards,
John

Thats another reason why I just leave the reflections, it makes them obvious. There is nothing worse than finding that the crummy data you have was because you didn't get rid of all the refections, but didn't realize it. Even a small reflection - barely visible in the impulse response, can have major effects. Thats because the impulse is linear and FR's are log. This is why some people square and take the log of the impulse (Energy-time) because it highlights the reflections and things that make differences in the FR in dB.

[nullspace]

Great stuff Earl; thanks very much.

A 'lazy susan' is on my project list. Maybe next weekend...

Regards,
John

[gedlee]

Originally posted by nullspace
Great stuff Earl; thanks very much.

A 'lazy susan' is on my project list. Maybe next weekend...

Regards,
John

Listen - its trivially easy. Just drill a hole in a board and place it onto the top of the stand with a pin in the middle. I use a 1/2 dowel - brass in my case. Now grease the layer between the two boards and mark off the angular increments to a nail put in the side. I have used this "polar test stand" fo several years and it works so well that I don't see any reason to change. Although in Thailand we had a remote actuated one which was fun, but certainly not ncessary to do the job. Hey, its all about getting the job done, not spending money.

My test stand is a few 2X4s and a MDF top - net cost about $10.

And oh yes definately buy the $49 Beringer mic and I also use their preamp. I've had $1000+ mic and preamp and they got exactly the same response. Absolute mic accuracy is not nearly as important as doing good comparative measurements.