Ampeg Isovent - Jack Plate Circuit Board.

[agedhorse]

Modern high performance cabinets is a combination of science, engineering, advancement in materials and experience knowing what works well in practice.

There's less and less wild ass guessing, and more measurement and analysis.

 

[Redbrangus]

There were several variants of the Isovent over a brief period of time, and with only about 357 or so made, it can be seen there was a struggle with the design it's short run, and it is clear the Isovent didn't become a marketing hit for Ampeg, or they would have continued running it for a whole lot longer.

I'm interested in knowing where you uncovered this bit of data...is there a Web site or forum for aficionados of this odd box that you could share with us?

 

[Jim C]

Modern high performance cabinets is a combination of science, engineering, advancement in materials and experience knowing what works well in practice.
There's less and less wild ass guessing, and more measurement and analysis.

Here's a delicate question.
Before computer modeling (and computers for that matter) how well could a skilled designer predict that outcome of a cabinet without building numerous prototypes?
As with most things, some of these early designs worked much better than others.
Was their some black magic (or dumb luck) involved with design and manufacturing (say pre 1995), or could we still create a great design given the speaker technology and manufacturing of the day?

For that matter, was the main design decisions based purely on a SPL meter in an anechoic chamber (or outdoors), by listening, or a combination of both?
A similar but different question might be, did cabinets start sounding much better because of driver design or box tuning.
Yes I know, not a black and white answer but curious to the historical development.

 

[mexicanyella]

I would like to know more about that too.

Here's a delicate question.
Before computer modeling (and computers for that matter) how well could a skilled designer predict that outcome of a cabinet without building numerous prototypes?
As with most things, some of these early designs worked much better than others.
Was their some black magic (or dumb luck) involved with design and manufacturing (say pre 1995), or could we still create a great design given the speaker technology and manufacturing of the day?

For that matter, was the main design decisions based purely on a SPL meter in an anechoic chamber (or outdoors), by listening, or a combination of both?
A similar but different question might be, did cabinets start sounding much better because of driver design or box tuning.
Yes I know, not a black and white answer but curious to the historical development.

 

[agedhorse]

Well, the expectations of performance, low frequency extension, power handling, sensitivity, reliability, size and weight were very different back then, so close (combined with some good luck and choices) was good enough.

As things evolved, the math models became more accurate and we did more math.

Computer modeling just makes the math easier and more efficient, and the results prettier.

 

[Jeff Scott]

Before computer modeling (and computers for that matter) how well could a skilled designer predict that outcome of a cabinet without building numerous prototypes?

 

[Jim C]

Well, the expectations of performance, low frequency extension, power handling, sensitivity, reliability, size and weight were very different back then, so close (combined with some good luck and choices) was good enough.
As things evolved, the math models became more accurate and we did more math.
Computer modeling just makes the math easier and more efficient, and the results prettier.

Was there a test plan used by manufacturers back then to study and document the results or more like listening tests? (assuming bass guitar cabinets).

 

[fdeck]

Here's a delicate question.
Before computer modeling (and computers for that matter) how well could a skilled designer predict that outcome of a cabinet without building numerous prototypes?

I can't speak for the pro's, but for us DIY'ers, we're living in a golden age. Perhaps beyond anyting else, there's the information that's available on the Internet. Also, decent modeling software is available for free and runs on any computer, including your phone.

When I built my first speaker, all I had was one book from the public library, and the formula I needed to use was wrong. (The units of measure didn't work out, that's a clue). Also, I had no access to information about how a bass speaker might differ from a hi-fi speaker, etc. And no way of testing anything.

Today, I can read about speaker theory online, I can find a spec sheet for any driver on the market (if it's a legitimate supplier), and even some of the measurements are starting to be democratized thanks to PCs and their audio hardware, as well as auxiliary gear becoming affordable such as high quality multimeters. We still have a long way to go before DIY testing can produce results that are believable for widespread consumption, but they can be useful for troubleshooting and verifying design assumptions.

It's my guess that new components such as neo drivers are more DIY friendly, because for many of us (including myself), we can easily buy more performance than we really need, and not worry about squeezing every last dB out of a design.

Glue has gotten better.

On the negative side, there's also a blizzard of false information and folklore to wade through. That's not unique to bass speakers. I don't know how you can reliably separate truth from crap on the Internet without being a physicist and mathematician, because I've never had to do that.

Short of actually designing cabs and cutting plywood, the same golden age applies to being a sidewalk engineer and just enjoying the conversation and new design ideas coming out.

 

[Jim C]

Do you ever use pink noise and a RTA to look at the response curves or is this not an accurate way to measure a MI cabinet?

 

[fdeck]

Do you ever use pink noise and a RTA to look at the response curves or is this not an accurate way to measure a MI cabinet?

Aha, you walked right into my trap.

I use my toneClusterWaveform.

bassistTech/toneClusterWaveform

It's a mixture of perfect sine waves, but each frequency is chosen to have precisely an integer number of waves within a data block (4096 samples, a convenient power of 2) at the sampling rate of my audio hardware (44.1 kHz is good enough). Whew. The benefit is that a single sweep of a FFT analyzer with same block length and sampling rate will produce a stable RTA-like readout, but with no signal averaging or other processing needed.

With wired measurements (such as, er, HPF boxes), I get results that are good within a tiny fraction of a dB. With speakers, of course I'm affected by room noise, but it still seems to work better than pink noise based methods. This is the season when I make my measurements in between when my home furnace is running.

Now my method will miss very narrow response spikes, but those are absent from most basic measurements such as simple filters and the low frequency response of speakers. I'm not averse to letting theory guide what I think is necessary to measure. You always have to be aware of how a particular measurement can overlook things that you're assuming either don't matter or are impossible.

An area where I use a noise waveform is measuring the signal-to-noise of a gadget. That's where I'm actually looking for artifacts that I don't expect.

 

[seamonkey]

This is a great value software for measuring cabs and rooms.
REW - Room EQ Wizard Room Acoustics Software

It's good for objectively comparing one cab over another.

And an example of one way to use REW
Invalid Link Removed

 

[farace]

This discussion got me reminiscing. My first cabinet in 1980 was a 2x15" from plans found in a Radio Shack book on building speaker enclosures. The drivers were also from Radio Shack (God only knows who actually built them). With the Music Man 65-watt tube head I bought, it sounded pretty good. At least compared to running my Ric through my Panasonic stereo (blowing a chip in the process).

 

[Wasnex]

See attachment for the schematics of the SVR:
www.riverband.demon.nl/SVR212 Schematics.PDF

The drivers a wired the same - to - , + to +, so bipole?

Anyway when I look at the values of the components of the crossover on that schematics, the crossover frequencies would be 300Hz (LPF rear speaker) and 6000Hz (HPF tweeter) while the manual states 150Hz and 4000Hz. Both speakers and the tweeter are 8 Ohm. Or am I wrong?

I believe you have to use the actual driver impedance at the crossover frequency for the calculations rather than the nominal impedance.

 

[DJ Bebop]

This discussion got me reminiscing. My first cabinet in 1980 was a 2x15" from plans found in a Radio Shack book on building speaker enclosures. The drivers were also from Radio Shack (God only knows who actually built them). With the Music Man 65-watt tube head I bought, it sounded pretty good. At least compared to running my Ric through my Panasonic stereo (blowing a chip in the process).

I had this one:

I built two 1x15 cabs like you with Radio Shack 15s and ran them with this, a Marantz 8B

And a Heathkit SP2 preamp:

 

[Wasnex]

I'm pretty sure no one here needed any pics or explanations of how a folded horn works, for instance.

 

[farace]

I had this one:
View attachment 3683007

I built two 1x15 cabs like you with Radio Shack 15s and ran them with this, a Marantz 8B
View attachment 3683008

And a Heathkit SP2 preamp:
View attachment 3683009

Yup, that's the book. It might even still be around here somewhere, packed away in a box I haven't been through in decades.

 

[BassmanPaul]

@DJ Bebop I've got that book too. LOL

 

[delta7fred]

I don't have a copy of that book but the picture looks familiar, I must have seen it in "Tandy", that was the UK name for Radio Shack IIRC

 

[Nicholas Orick]

When you 'test' the speakers outside the cabinet, are they still connected to the other speakers via the crossover you are not testing? And if so, are the other speakers still in the cabinet producing sound?

Still connected at rhe Crossover/Terminal Block.

.

 

[agedhorse]

Certainly there is a lot of measurements made, but from what I have seen on the internet speaker sites, there's no shortage of bad data, improperly acquired data and completely illogical conclusions made.

It's like developing perpetual motion machines, the less you understand, the easier it is to become a "believer"