8 ohm/4 ohm = rarely use amp's full potential

[micguy]

Start with one 8 ohm cabinet. If it’s plenty loud, you’re done. If not, buy a second 8 ohm cabinet. The result will be louder and more robust than a single 4 ohm cabinet.

Worrying about “ not getting all the watts” is silly. Focus on whether your rig works for the gigs you do.

 

[bolophonic]

Pretty much all amps are twice as much watts at 4 ohms, but in reality most bass players are only going to use a single cab. Seems like a waste to me. I'm in the market for a cabinet. My amp is 150 watts at 8ohm and 300 watts at 4 ohms. But I only want one cabinet

1. get a 4 ohm cab.

2. stop worrying about everyone else’s rig.

 

[2saddleslab]

This is what I learned from a recent experience. I have an Ampeg 112 [portabass] w/ a 4ohm Black Widow run by an Ampeg pf800. It has girth, volume, commanding lows, punchy mids, and never farts out. Decided to try an 8ohm Black Widow. I noticed a decrease in volume, but more importantly, the lack of commanding lows was very noticeable.

Since Peavey makes speaker basket kits it was easy to return the speaker back to 4ohms on my own. It was a useful experiment proving [in my case] that 4 ohm when using a single cab is the way to go.

 

[mazdah]

The EQ is made for using the amps full potential.

The volume difference between single 8ohm and single 4ohm cabinet is very small.

The volume boost while using tho 8ohm cabinets has nothing to do with watts that amp is pushing.

 

[Tim Skaggs]

Sensitivity, sensitivity, sensitivity. That is the speaker characteristic that most people seem to overlook. It also seems to be the parameter where 8 ohm and 4 ohm speakers are most often unequal. If you go to a site like US speaker and look at multiple speakers of the same size, you start noticing many 8 ohm speakers have sensitivity ratings around 100 db or more and a lot of 4 ohm speakers have a sensitivity rating around 98db or less. This is definitely not a hard rule, but it seems to be a trend.

Someone with more knowledge of speaker construction than I have might know if making a lower impedance voice coil sacrifices sensitivity and why it does. I don’t know for sure, I just know I see a lot more 8 ohm speakers with SPL over 100 db and a lot more 4 ohm speakers with an SPL under 99 db.

So, if you have an 8 ohm cabinet with a sensitivity rating of 102 db and a 4 ohm cabinet with a sensitivity of 96 db, the doubling of amp wattage does nothing to increase the sound pressure level coming out of the 4 ohm cabinet as compared to the 8 ohm cab. In the example numbers I chose, the 4 ohm cab is probably going to be less SPL than the 8 ohm cab even though the 4 ohm cab is receiving twice the power.

If a person is speaker cab shopping, it’s best to filter search results to whatever impedance you want so as you look at cabinets of equal impedance, you can ignore the impedance and concentrate on comparing other parameters, sensitivity being one of the more important parameters on which to focus.

 

[agedhorse]

The reason there aren’t a lot of 4 ohm drivers is that it’s hard to trade-off the lower impedance without losing sensitivity. Combine this with the fact that adding another 8 ohm driver also gets you another 3 dB of low frequency coupling gain and you can easily see why the industry has more or less standardized on 8 ohm drivers.

I have designed a few combos over the years with 4 ohm drivers, that’s what the marketing folks wanted, but in reality it wasn’t the best approach.

 

[Jefenator]

As others have pointed out, 4 ohm cabs are a thing. If you plan on doing a lot of moderately loud stuff, it can be a good way to go.
I had a good run with my 4 ohm 2x12. Then my back started giving me trouble and the rock shows became less frequent, so I'm back to using a smaller 8 ohm cab most of the time, with the occasional 4 ohm stack.
More speaker area & lower impedance is one way to mitigate limited amp wattage, but sensitivity also matters. With a 103 dB rating, my 8 ohm Bag End S-15D is pretty good for filling a stage with a 50 watt tube head. I definitely notice a difference from other 8 ohm cabs rated 95 dB or lower.

 

[Phlipper]

Thanks for the math. I remember reading somewhere years ago about the curve of SPL (Db) related to wattage. But I've always noticed a definite increase in volume when adding an additional cab and had attributed it to the additional wattage as well as the additional speaker area. But in most cases I had no idea what the sensitivity of the speaker was unless I had replaced it. Looks like the wattage has only a little to do with it in many cases. Interesting.

 

[mrjim123]

I always use two 8 ohm cabs. I wouldn't want a 4 ohm cab, even though my Mesa head will do 2 ohms, because my backup amps WON'T do 2 ohms.

OP - you only want one cab now but you might change your mind in the future, and if you get a 4 ohm cab now you're stuck. I'd still recommend you get an 8 ohm cab for future growth.

 

[Aloe]

Unfortunately, 4ohm drivers are often 1-2db less efficient than their 8-ohm counter parts.

is it the same for typical 8 and 16 ohm drivers with latter generally being more efficient? I’m thinking of a 212, manufacturer has it in 4 or 8 ohms.

 

[BassFalcon]

Your car is probably capable of hitting 120 mph. Maybe more if it’s a fancy car. Is driving it slower a waste?

 

[InhumanResource]

The reason there aren’t a lot of 4 ohm drivers is that it’s hard to trade-off the lower impedance without losing sensitivity. Combine this with the fact that adding another 8 ohm driver also gets you another 3 dB of low frequency coupling gain and you can easily see why the industry has more or less standardized on 8 ohm drivers.

I have designed a few combos over the years with 4 ohm drivers, that’s what the marketing folks wanted, but in reality it wasn’t the best approach.

I don’t understand, you are suggesting marketing is not in touch with reality?

 

[FunkHead]

I have a single 1x15 and a single 2x10 cab. Both are 4 ohm and both will hang with a loud drummer by themselves.
I also have a 4 Ohm Barefaced 2x12 BT3 that will keep up with an 8X10.

For folks who want to carry 2 cabs, I suggest 2 very light 8 Ohm 2x10's stacked vertically. But why bother when a 2X12 can cover any gig you may ever get.

 

[Analogeezer]

All of my cabinets are 4 ohm cabinets, pretty much by choice.

For using one of my three regular heads (two conventional, on Class D) which only pull down to a 4 ohm load; I use a single cabinet because those amps are for smaller venues and I only need one cabinet.

If I go with two or more cabinets I am using a rackmount rig with a QSC Power amp, which pulls two ohm loads.

But even then for the "big rig" scenarios I run two four ohm cabs (a single 18" and a 2 x 10") on each side of the amp so only a four ohm load.

I could run four 4 ohm cabinets with that setup but never felt the need to use that much gear; even outside or in a large venue, 1,000 watts through two cabinets does the job and barely breaks a sweat.

Back in the day a lot of just regular heads (Acoustic and Peavey for example) would pull a two ohm load, but that was because amps were not as powerful and the cabinets by design were efficient. Most 2 x 15" cabs of that era were 4 ohms, so you could run two of them with the heads of that era to get all the power out of the amp.

In the case of my Acoustic 220 head, that increased the power from 125 watts at 4 ohms to a rip roaring 160 watts! LOL

Analogeezer

Analogeezer

 

[lug]

Your car is probably capable of hitting 120 mph. Maybe more if it’s a fancy car. Is driving it slower a waste?

Depends on where you are. If you are in post-apocalyptic Australia with a guy named Max, then yes.

 

[bherman]

Another vote for 2-8 ohm cabs. I did have a 4 ohm 210 and it was great, but two cabs always sounds fuller to me.

 

[seamonkey]

Pretty much all amps are twice as much watts at 4 ohms, but in reality most bass players are only going to use a single cab. Seems like a waste to me. I'm in the market for a cabinet. My amp is 150 watts at 8ohm and 300 watts at 4 ohms. But I only want one cabinet

Powered PA cabinets do their max power to the built in speakers.
Many combos do this also. (If they don't have external speaker jacks)
Maybe switch to combos so you don't feel cheated about pushing maximum power

The difference in sound output between a single speaker type at 8 or 4 ohms is hardly noticeable.

 

[Bassamatic]

Because of the way that electronic power and acoustic power work, going to a 4 Ohm speaker to double the amp power will sound only a tiny tiny bit louder. You might not even be able to tell the difference.

However, doubling the speaker cone AREA by adding a second 8 Ohm cab for 4 Ohms total, doubling the amp power will double your perceived volume.

Keep your 8 Ohm cab so you can add a second one later if you want. The reason for large amps is to be able to run multiple speakers.

 

[conttador]

Pretty much all amps are twice as much watts at 4 ohms, but in reality most bass players are only going to use a single cab. Seems like a waste to me. I'm in the market for a cabinet. My amp is 150 watts at 8ohm and 300 watts at 4 ohms. But I only want one cabinet

Couldn’t you just buy a 4 ohm cab and get all the power from your amp?

 

[HolmeBass]

The following is a copy of: Help me figure out the impedance of a driver I made one slight edit to correct a typo.

Maxing out the amps power may be less beneficial than you think.

Here is the deal. The minimum decibel change we hear clearly is 3dB. In order to get the 3dB, the amp has to double power when the impedance drops from 8ohms to 4ohms, and the sensitivity of the 4-ohm cab must be the same as the 8-ohm cab. Unfortunately, 4ohm drivers are often 1-2db less efficient than their 8-ohm counter parts.

Results: If the amp doubles power, but the 4-ohm cab has a 2dB lower sensitivity rating than the 8-ohm cab, you only gain 1dB which is not enough to hear.

If you have an amp that is 4ohms minimum, the best choice is often a pair of 8-ohm cabs. If your amp doubles power when you add the second cab, you get +6dB over the max SPL one of the cabs will make by itself. If the amp holds the power at the same level when you add the second cab, you get +3dB...this is how tube amps operate. Many solid-state amps will increase the power when a second cab is added, but not double the power. So you get +4dB to +5dB.

Let's assume an amp makes 225W at 8ohms and 380W at 4ohms. This is the actual specs for a GK series I RB700

Our 4-ohm cab has a sensitivity rating of 95dB 1W/1m, and our 8-ohm cabs have a sensitivity rating of 97dB 1w/1m

With one 4 ohm cab the amp makes 380W. The decibel change from 1-380 is ~25.8dB. Let's round this up to 26dB. For the calculated max volume add the decibel change to the sensitivity rating. 95+26.=121dB max

With one 8 ohm cab the amp makes 225W. The decibel change ~23.5. Let's round this up to 24dB. Max SPL is 97+24=121dB. Exactly the same as the 4 ohm cab. The amp made less power, but the 8-ohm speaker is 2dB more efficient.

With two 8 ohm cabs the load is 4 ohms, so the amp makes 380W. The 380W is shared equally by the cabs, so each cab gets 190W. The decibel change is ~22.8, which we will round up to 23dB. The max SPL of one cab 97+23=120dB. If you tight pack the cabs, you get +6dB for frequencies where the driver spacing is within about 1/4 wavelength. I.E. you get more lows. Combined output of both cabs: 120+6=126dB.

Summary of Max SPL
One 4ohm cab = 121dB
One 8ohm cab = 121dB
Two 8ohm cabs = 126db

So in this example you get the same SPL with either a 4-ohm or 8-ohm cab. With two 8-ohm cabs you get +5dB.

My friend I usually find your posts very informative, but I have to disagree with your characterization of 3 dB as the “minimum we hear clearly”. That is 1 dB, which is precisely why decibels sound pressure were defined the way they were.

dB: What is a decibel?