Calibrated Microphone

Anyone familiar with these, and how they are used to measure freq response of speakers ? I am about to get into this.

Once I have the mic, I figure I got a Wavetek 111 which is good enough for now. I thought about square waves and wil give that a try but do not have g reat hopes for anything just yet, but I am after something else.

Speakers. I am about to put some together and I am looking for something mo re than the average. In fact later, if my pardner can figure out a process for it, we might be in a brand new ESL tweeter business. Either direct coul led or using a differewnt kind of amp. Long story that isn't long enough ye t anyway. Fuck all that for now.

Regular dynamic tweeters and whatever, I wil have to buy them off the shelf though I do intend to do alot of product testing. Well this is part of it.

When I take a calibrated mic and put a speaker to it, when I have the scope on the actual generator outpur, the second trace (from the moc) should mov e. I should be able to calibrate something by measuring that change in phas e and correlating it with the distance.

Part of what I am doing other than screwing arond and learning something, i s getting drivers for speakers I want to build. I am going to order a bunch of them, all differnt, from MCM. If I can do like Bose and compensate, we will be fine.

Oh. by the way, I intend to use (eventually) a separate amp for each speake r. THAT will allow aloty more leeway int he crossover, and control ovewr ph ase.

That is actusally later though, I want to get the mic setup sooner than tha t. I need minimal phase shift and pretty flat response. If not, I need it p lotted. Anyone know where is the best place ot get these things ? Iknkw the re are a bunch of them and each and every one of them is the best there's e ver been, if you believe their websites.

So, does anyone deal with this stuff ?

Reply to
jurb6006
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Anyone familiar with these, and how they are used to measure freq response of speakers ? I am about to get into this.

Once I have the mic, I figure I got a Wavetek 111 which is good enough for now. I thought about square waves and wil give that a try but do not have great hopes for anything just yet, but I am after something else.

** Think about making one of these:

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A "tone burst gate" turns sine waves into burst of a few cycles with pauses in between. Very handy for testing loud speakers and any display device where transients need to be read accurately - ie VU meters etc.

With speakers, if the signal from the mic look reasonably close to the output of the gate across the audio band - you have a *very good* speaker.

The dead time allows you to see ringing & delayed resonances plus the amplitude of the burst follows the frequency response. Can be used for indoor and outdoor testing - but keep the mic close to the driver when indoors to avoid room sounds contaminating the result.

... Phil

Reply to
Phil Allison

Definitely the way to do it if you want to avoid harming the speaker.

A square wave is too easy and ambiguous because of its harmonic structure. A pure sinewave gated after a variable number of cycles is a much better test but you can't apply a continuous tone for long.

You can damage a loudspeaker pretty rapidly by playing continuous sine tone if you hit a mechanical resonance you will do serious damage to it.

Pink noise and a spectrum analyser is another approach.

You have to pay a fair amount of attention to the room acoustics or have your own anechoic chamber if you don't want biassed results.

The microphone might be calibrated but nothing else is.

--
Regards, 
Martin Brown
Reply to
Martin Brown

Something I might try.

At the moment I am more intereste in phasing and all thart and optimizing a crossover. Seems something simpler should do that, at least in the begionn ing stages. This seems like something that'll reveal ringing, and if I got the drivers damped well enough woth the crossover I gues I'll have to dumpp them.

The plane is to probably buy a bunch of regular commercially available unit s and do some testing.

Another thing I would like to do is stick a nice clean Wavetek sinewave to them and see how linear they are, as much as is possible using a mic in air .

Tone burst is one thing, have you ever heard of the FFT method ? One tick p ut the speaker and a computer reads out the entire response. This is from t he 1980s actually.

I do not intent to try to do that. I just want a reliable way to measure ac tual sound. I WILL take the thing outside in the middle of the night if I h ave to.

This tone burst thing, if I can learn the math, could tell me alot about wh atever room I am in. (well the speakers) Hwever, I think it can be done wit h a PC and a little buit of costom software a hacker could write for a nick el bag or whatever. I might be able to get somneone on Craigslist or someth ing. that would solve all the problems at once.

I saw the other thread and I wanted to ask why the guy wanted to do this, b ut I see it.; He is looking for echoes. OK, maybe so am I.

But my idea really for that is to just do it totally digitally. Somehow, yo u can just specify a waveform and the soundcard will supply it. Start and s top on a dime. That's what MIDI has been doing for the last thirty years or so. It should not be hard technology.

I ownder why the OP in that thread didn't just do that. Well, maybe there a re reasons, sometimes they don't tell us everything.

I duno, it's late here and I am really ready to hit it. Tomorrow I will loo k at all this again. Thanx.

That is boiling down to an echo detector. Hmmmmmm...

Reply to
jurb6006

Better to use a psuedo random waveform or pink noise. You don't get a lot of signal to noise off a delta function click.

DAQARTA (sp?) might be able to do most of what you want out of the box.

It will easily do the realtime audio spectrum analyser. I have used it before for lecture on the physics of music making to show how the waveform and harmonic structure affects the perceived sound. And how easy it is to trick the ear into hearing things that are not there.

Even if it doesn't do tone bursts I expect if you ask him nicely and pay the license fee it could appear in the next version. My copy is out of date since I don't often use it apart from for lectures.

--
Regards, 
Martin Brown
Reply to
Martin Brown

I did a bunch of audiofoolery experiments in the '70's. What I found was the perceived sound was affected by: The room. Speaker placement. Furniture. Wall/floor/ceiling treatments. Listening position.

If the wife doesn't want the speaker exactly 2.4454' from the wall, guess where it ain't gonna go!

Basically, you can trade amplitude for phase...which gets transformed right back to amplitude when it reflects/interferes. Crossovers are bad at that.

A half-decent microphone is plenty good to see all the horrible artifacts that you can't easily fix.

Only thing I did that I thought was better was motional feedback and negative output resistance and some filtering to take out the primary cab resonance in my Datsun Pickup.

In the house I bought some Klipsch Horns and put them the only place in the room they could go. If I put a watt into them, they were so loud that I couldn't hear all the imperfections. ;-) I put away the level meters and the pink noise generators and listened to Pink Floyd instead.

I did try some ribbon speakers. Really liked the sound, but they had to be so far from the wall that they took up more space than the corner horns. And my 100W amp wouldn't drive them effectively.

Bottom line is that, unless you're willing to sit inside an anechoic chamber, most anything you do is a waste of time.

Reply to
mike

Sounds to me as if you have a long, long road ahead of you. People (some of them very smart) have been working on speakers for the last 100 years (approx). Digital compensation has been tried (and is used quite a bit) and it isn't that simple. If you are just at the point of asking about how to measure frequency response I suggest that before you buy any gear then start reading stuff. There are loads of ways of measuring speakers, partly depending on what you want to test. You will find that you need to automate your measurements to some degree

- other wise it will just take too long. If you are trying to measure spatial characteristics against frequency then you will need to sweep the frequency, move the microphone and record the results. It's easy for each test to require >1000 measurement points (100 frequencies, 10 positions). You'll need somewhere to test, outside free field is OK apart from noise, rain, neighbors etc. You can buy a cheap microphone from Behringer (ECM800 microphone) but pros would use B&K or GRAS (about 10x the price).

Michael Kellett

Reply to
MK

Before you get a mic, consider just getting a test CD/DVD with some tone/sweep waveforms on it, and use your ears. There's lots of inexpensive mics that will do relative-loudness work just fine; do you really care about the dB_spl absolute loudness, or is 'these are less efficient' enough?

Speakers have crossovers and resonances, because they are lightweight, in terms of kilograms per watt of power. Microphones, kilograms per watt, are MUCH heavier, lack crossovers, and even the cheapest have smooth response over frequency.

Reply to
whit3rd

You could start by reading thread in rec.audio.pro. Measurement microphones.

I've been through some of the ordeals of measurement. First read up. Sine wave measurement, NO. Most of my luck starting from scratch, get graphs from driver manufacturer. You also have to deal with averages, on axis, off axis, and how it's going to sound in a real room. Forgot distance from driver.

Greg

Reply to
gregz

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