Speaker Impedance Question

[audioguru]

Did they have ultrasonics when you were young?

The old geezers who designed them thought nobody could hear them so they left them turned on all the time. They were very loud to me.

Did they even have burglars?

The concert halls and art galleries had ultrasonic burglar alarms so they must have also had burglars.

 

[ericgibbs]

Do you mean a Leslie rotating speaker? No, I never played an electric organ.
I also never played a guitar.

I played the piano when I was very young and played the trombone in my high school band. If I continued with studying music then I would have developed "perfect pitch". I was very close to having perfect pitch and could identify some frequencies by their sound.
When I was young I could hear all the ultrasonic burglar alarms in town.

hi agu,
The 'wobbulator' was a poor mans audio sweep generator.
The speaker cone was modified to carry a piece of metal foil, close to that moving foil was a fixed piece of foil, the two pieces forming a capacitor.
You could actually buy ready made units.

The speaker was fed with a low frequency 'saw tooth' waveform, which of course changed the foil capacitance value.
Very useful for sweeping a tuned RF amplifier.

IIRC we had to use a freq mixer/changer to cover audio, about 100Hz thru 20KHz.

By synchronising the scope with the sawtooth it was possible to sweep thru the audio band of an amp and display the response curve.

 

[audioguru]

I don't test the frequency response of speakers with a sweep genertor. If the sweep is too quick then resonances are not activated. Echoes ruin the measurements.

For years I used pink noise outdoors with the speaker pointing up towards the sky and the microphone propped above it on a stick. Then there are no reflections.

Lately I have used FFT for measuring frequency response of speakers.

 

[Speakerguy]

Yeah sweeps are no good. Though Praxis has this thing called a 'chirp' which sounds very much like a sweep, but supposedly there's some real fancy math going on with it (and I assume something in the signal that don't recognize as being in there along with the sweep) but it gives better results than I've gotten in the past with standard pink noise or MLS techniques. Anyway....

The standard way these days for measuring frequency response is by using a maximum length sequence technique. The fancy computer program that uses it (Laud, MLSSA, Praxis, LEAP, etc) gets you an impulse response, and you visually inspect the impulse response for any ringing that may occur as a result of room boundaries (using the speed of sound, the distance of the boundary to the floor and the mic, and some math you get the expected arrival time of the first reflection). Anyway, you look for that first reflection glitch in your impulse response, window the time you are going to FFT to just before the start of the impulse to just before the start of the first reflection, and FFT that. Only problem is that this limits you to a few hundred Hz on the low end. Good thing is most speakers are really really linear <300Hz or so so you can just close-mic the speaker to measure frequency response and let the direct sound from the cone swamp out the room reflections (you want the mic 1/2" from the cone in such a test).

Anyway, that's the way I do things when designing a speaker. I haven't had much time for the hobby since I graduated and started working a year ago, but I'll dig up an old design or two to show off.

 

[audioguru]

Hi Speaker Guy,
Did you hear about "The Sweet Sixteen" speaker system?
It used 16 very cheap and small speakers but they were all different. The idea was that their frequency resonses would all average into a very flat response, and all the cone area would add into a huge and powerful speaker.

It sounded horrible. All the resonances would smear the sound, no bass and no sizzling highs. It was extremely directional and off-axis made comb filter effects.

 

[Speakerguy]

Well you guys get to see the first full-range speaker I ever designed & built on my own This must have been circa 2001 or so.

This is one of the first pair, in unfinished quartersawn white oak:
**broken link removed**

This is the frequency response measured in Laud w/MLS:
**broken link removed**

It's too flat. I had a full 6dB baffle step compensation in there, so it sounded a little too bassy in anything but a large room pulled away from the wall.

I refined the crossover a bit, this is a picture of the revised crossover:
**broken link removed**

It also got a new cabinet at the same time as the crossover redesign. Those drivers/xo now reside in the fancy pre-made cabinets from PartsExpress.com (which look REALLY nice). The 0.5cf size was just right for this particular Peerless woofer. I still need to work on it a little, I shelved the tweeter level just a smidge too much, I need to raise it maybe not quite 1dB. I don't have any of the new measurements up on the web but those should give you guys an idea of what I do, or at least USED to do, for a hobby back when I had the time and access to friends with cabinet saws....

 

[Speakerguy]

Audioguru,

Never heard of it, but it sounds like a heckuva mess I have difficulty believing that anything more than a 3-way is every really necessary (well, 3-way with a separate active sub I can deal with I guess).

Did they even use crossovers in the sweet sixteen? I'll google it and see if I can find anything.

 

[Nigel Goodwin]

Hi Speaker Guy,
Did you hear about "The Sweet Sixteen" speaker system?
It used 16 very cheap and small speakers but they were all different. The idea was that their frequency resonses would all average into a very flat response, and all the cone area would add into a huge and powerful speaker.

It sounded horrible. All the resonances would smear the sound, no bass and no sizzling highs. It was extremely directional and off-axis made comb filter effects.

As kids we needed a speaker for doing discos, but couldn't afford to buy one, so we collected lots of speakers out of old TV's (from a local tip), mounted them, and carefully connected them in series/parallel for the correct impedance.

We then connected them to our 50W amplifier, turned the volume up, and they all died in a cloud of smoke I've still never really understood why?.

 

[Bob Scott]

Hi Speaker Guy,
Did you hear about "The Sweet Sixteen" speaker system?
It used 16 very cheap and small speakers but they were all different. The idea was that their frequency resonses would all average into a very flat response, and all the cone area would add into a huge and powerful speaker.

I recall that late `1967ish magazine "sweet 16" construction article. I was about that age too. I built the speakers using the suggested 16 5" speakers per box using cheap 5" speakers listed for sale in the same magazine. The idea was that mounting the speakers close to each other (and wired in phase combination of series and parallel for 8 ohms) would make the cones accoustically couple, making for a good bass response.

It sounded horrible. All the resonances would smear the sound, no bass and no sizzling highs. It was extremely directional and off-axis made comb filter effects.

The result was worse than that for my set. When the speakers arrived, they were such low quality that I had to bend some of their frames just to stop the voice coils from rubbing. The speakers didn't even have enough high end to cause any comb filter phase interference. They sounded as bad as an AM car radio. I could not make cabinets large enough for any bass whatsoever. The front panel assemblies had impressive bass, but only with no cabinets, and suspended on a horizontal plane about 2 feet above the floor. (speakers firing downward)

In the language of the 60's, it was a "bad trip, man".

Bob

 

[audioguru]

Hi Bob,
I was raised in "Hongcouver". I left for Toronto when all the hippies in Toronto went to Vancouver, and before all the Chinese took it over.
I don't miss the liquid sunshine since there were 364 days per year with rain.

Sorry, but I think Vancouver will disappear when the big earthquake hits. It is long overdue.

 

[Bob Scott]

Hi Bob,
I was raised in "Hongcouver". I left for Toronto when all the hippies in Toronto went to Vancouver, and before all the Chinese took it over.

Hippies all got educations and jobs, and are upper management capitalist conservatives now.
I like my neighbours from Hong Kong. We exchange small gifts at Xmas.

I don't miss the liquid sunshine since there were 364 days per year with rain.

True, it makes no sense to build a sun-tracking solar cell array here. Like Seattle, the weather stays heavily overcast for months on end. Stores can't seem to sell astronomical telescopes.

Sorry, but I think Vancouver will disappear when the big earthquake hits. It is long overdue.

I have earthquake insurance. The fault line is the other side of Vancouver Island, and we don't have any of your mosquitoes or other biting insects at all here.

Cheers, Bob

 

[audioguru]

There are a few hippies still in Toronto. They are homeless and jobless bums who have huge beer-bellies, have some long white hair and ride very noisy motorcycles that have no mufflers.

The tsunami from the big earthquake will be huge and will wash over Vancouver Island. The island also might be gone. The waves and everything in Vancouver will splash over Mount Seymour and over Grouse Mountain.

We used to have many mosquitoes and bats in Toronto. Not anymore. We got rid of them. But the Canada Geese are out of control! Millions of them everywhere.

 

[Gaston]

when i was a young kid with no money. i had a cheap sears stereo witht the two speakers that had four inch drivers in them with a six inch circle painted on the wood behind the cloth to make them look bigger. the magnets on the back of the drivers were about 3/4 o an inch in diameter. my uncle gave me two home made speakers that must have been modled after the sweet sixteen. it had three inch speakers in it though. i must say it was a big improvement over the sears speakers. although i didn't even know what bass was back then. but there was a lot of ozzy and ac/dc poured out of those

 

[audioguru]

Many cheap console stereos had a photo of a good tweeter pasted on instead of a real tweeter.

Remember vinyl records? I always had the second-best magnetic cartridge made by Shure, because my friend bought the latest ones and gave me his old (1 year old) one.

 

[Gaston]

do i remeber vinyl? i still have my records from when i was a kid.. 45 and 33....but no 78 . you up early on a sunday audio? what time zone are you in? i'm in american central. i wouldn't be up but i have to catch a plane.

 

[audioguru]

I have some vinyl records and my record player is still connected. I haven't played it for about 20 years.
I am in The Eastern North American time zone.
I am retired so I get up whenever I want and do whatever I want.

 

[mneary]

I just replaced my record player with a new one.

It has an internal A/D and a USB interface (also has mag and line level outputs).