What's my total impedance?

[dougmichalak]

I am a newbie designing my first speaker cabinet. I have a 3-way crossover that allows 4 Ohm subwoofer, an 8 Ohm mid-range woofer, and an 8 Ohm tweeter. What is the total impedance of the system? I'll gladly provide more info if necessary.

 

[highfigh]

I am a newbie designing my first speaker cabinet. I have a 3-way crossover that allows 4 Ohm subwoofer, an 8 Ohm mid-range woofer, and an 8 Ohm tweeter. What is the total impedance of the system? I'll gladly provide more info if necessary.

You need to show the driver info (brand, part number) and info about the crossover, which makes a huge difference in the impedance.

 

[TLS Guy]

I am a newbie designing my first speaker cabinet. I have a 3-way crossover that allows 4 Ohm subwoofer, an 8 Ohm mid-range woofer, and an 8 Ohm tweeter. What is the total impedance of the system? I'll gladly provide more info if necessary.

I suspect this is a Frankenstein design. If you used off the shelf crossovers, then you already have a hopeless design.

You have to select drivers carefully that actually can be crossed over. Then you have to design and model the cabinet. Then you have to custom design the crossover and model it. The model will tell you the impedance throughout the audio range.

After you have built the speaker then you measure the impedance dynamically with a woofer tester. Dayton audio have a good one available from Parts Express.

 

[dougmichalak]

You need to show the driver info (brand, part number) and info about the crossover, which makes a huge difference in the impedance.

Crossover: Dayton Audio 3-way, low pass 375hz, high pass 3k, 12db/octave, 2nd Order, 3-octave spread between cutoff
Sub: Eminence CA154-15, 4 Ohm
Mid: Eminence Beta 8A 8315U, 8 Ohm
Tweet: JBL Super Tweeter ST200, 8 Ohm

 

[Pogre]

Crossover: Dayton Audio 3-way, low pass 375hz, high pass 3k, 12db/octave, 2nd Order, 3-octave spread between cutoff
Sub: Eminence CA154-15, 4 Ohm
Mid: Eminence Beta 8A 8315U, 8 Ohm
Tweet: JBL Super Tweeter ST200, 8 Ohm

How did you choose your components? Each driver has to be carefully selected based on several different metrics (T/S parameters) to ensure they'll work together cohesively in the cabinet you chose. Then you design the crossover around your components. Not the other way 'round.

Building a good speaker is tricky business unless you have a kit with all of the parts already selected for proven designs. 3 way designs are even trickier and not really recommended for a beginner. Unless like I said you have a kit with all of the parts already.

 

[highfigh]

Crossover: Dayton Audio 3-way, low pass 375hz, high pass 3k, 12db/octave, 2nd Order, 3-octave spread between cutoff
Sub: Eminence CA154-15, 4 Ohm
Mid: Eminence Beta 8A 8315U, 8 Ohm
Tweet: JBL Super Tweeter ST200, 8 Ohm

How will you use this? The 15" is made for bass guitar or PA cabinets and if you go to the Eminence site, they show the frequency response- shows "usable response starting at 45Hz- because most bass guitar rigs don't need to and they certainly don't need to hit anywhere near 20Hz. FYI- most PA systems for a full band are crossed over around 50Hz and bass guitar rigs often (all should) have a high pass around 35Hz. Sensitivity is shown as 96.9dB@1Watt.

The 8" is made for (quoting the description on the web page) "professional audio mid-bass applications or as a woofer in vented enclosures". The sensitivity is shown as 95.1dB@1Watt.

The Selenium tweeter is made to direct the sound in a narrow area- sensitivity is shown as 106dB@1Watt. That means it's made for pro/commercial sound applications.

The sensitivity mismatches alone make this a no-go- using an off the shelf crossover only works if the sensitivity for all drivers is a good match and the difference of almost 10dB means that you'll need to pump close to 10W into the 8" for it to be as loud and the only way to make them similar without having an amplifier for each is by using a level control. That's fine, but it's not using the drivers for their intended applications.

 

[PENG]

I am a newbie designing my first speaker cabinet. I have a 3-way crossover that allows 4 Ohm subwoofer, an 8 Ohm mid-range woofer, and an 8 Ohm tweeter. What is the total impedance of the system? I'll gladly provide more info if necessary.

No one can give you a correct calculated number for your speaker even after completion let alone you are just designing it because or the following (and not limited to):

a) Impedance is frequency dependent so even if you provided the impedance vs frequency graph for each driver, you can only calculate or measure a value at a given frequency.

b) Impedance also depends on your cabinet design and it sounds like you are in the process of designing it.

c) Impedance of the speaker obviously also depends on the crossover.

If you want to come up with a single number, it would be possible if you base it on the so called "nominal" impedance but then you would have to pick a rule from slightly different rules different speaker manufacturers would pick. I would say even then, is better to follow best practice in your design, once you are satisfy with the final design, built it and then measure it on completion.

 

[highfigh]

I am a newbie designing my first speaker cabinet. I have a 3-way crossover that allows 4 Ohm subwoofer, an 8 Ohm mid-range woofer, and an 8 Ohm tweeter. What is the total impedance of the system? I'll gladly provide more info if necessary.

What speakers have you listened to, that you like? It's hard to do everything needed for a speaker design, on paper.

 

[lovinthehd]

IMO you're far from "designing" anything with your current plan. I'd abandon it. Look for proven kits or really dive into the science. Don't do that mickeymouse/frankenstein thing you're proposing.

 

[dougmichalak]

Thanks, highfigh, for trying to answer my questions as politely as you can. There is definitely a lot more to this than I thought in the beginning. My use for it was essentially to create a multipurpose combo amp to be used with multiple instruments combined with a rack mount system containing a preamp and other components. So I wanted it to be more of a PA-type enclosure with less coloration and midrange buzz than a guitar amp, since it's for lots of different instruments. On the Sweetwater website, the Eminence 15" was rated as a great speaker for both bass amps and PA cabinets; that's why I chose it. I'm not looking for the most awesome, sonically perfect cab I can design. If I play anywhere, it's most likely going to come straight out of my preamp into the PA of the venue, so the speaker cabinet at most will be used as a stage monitor, and more likely as a practice amp. I just wanted the whole thing to be all in one system for easy transport and storage, and more than anything I'm just wanting to make sure I don't blow anything up. I seem to have found a basic answer to my original question from another forum completely unrelated to this one, so...thanks, all.

 

[lovinthehd]

I'm glad you've seen that outside your imagination . Speaker design is based as others have said, and just putting together some random passive crossover/driver/box selection is definitely a prescription for failure unless particularly and specatacularly an exception! Go with the guys who have done this....follow a specific kit of deliberately designed box (per t/s parameters and desired response) with specific drivers. Be interesting what specific other forum/answers you're referring to....

There is a lot of good knowledge in diy speaker groups/fora, from the diy subforum here to that at avsforum or diyaudio, etc. I'd encourage the diy aspect if you have the tools/facility for contruction as well as finishing....but dive into the actual design aspects instead of just imagining how you might buy disparate parts.