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

[BOOG]

75.44 per cent accurate. Close enough for rock n roll

Meh…
Prove it.

 

[Johnny Fingers]

I have always preferred 2 cabinets for bass guitar. Two of the same, or that sound the same, give much more coverage. Two amps is cool also, with a little adjustment.
All my cabinets now are 8 ohms. I don’t switch them around, but I can if I want to. An extension cabinet helps fill the room. Only with an extremely small stage/room or group would I opt for just one.
SVT is better with 16/10s than 8/10s. 370 is better with 2/18s than 1/18. Two cabinets for my 320B and even my B15N sound better than one.
I am spoiled, but I know what I like. And I don’t have to crank up the volume until my ears ring.

 

[agedhorse]

I got 9/10 at 1db, but I had to put in both earbuds and listen to the tone a few times to detect the change. That’s an interesting site to compare changes in sound levels. I definitely couldn’t detect changes of any less than 1db, and doubt I could even detect the 1db change in a noisy environment.

Try the test at 80-100hz and see if you can detect 1dB, it's a lot harder and especially with a speaker. Ear buds are the most accurate, headphones less accurate and speaker the least accurate regarding resolution of low frequency level.

 

[themickster]

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.

Would two drivers not give more low end than one?

 

[Tommy TGrip]

Thanks everyone who responded. What an excellent source of knowledge this thread is. I am still not sure what kind of cabinet I will get but am much more informed than I was 24 hours ago. I can at least rule out a 4 ohm cabinet because yes, one day I might want a second. I will look closely at sensitivity ratings. I'll probably look for a 12 inch cab, maybe a 210. The amp is a markbass nano 150/300 watts. Thanks again

 

[Wasnex]

Would two drivers not give more low end than one?

It should.

 

[Snert]

Would two drivers not give more low end than one?

Two identical drivers and enclosures, it will go louder, but not deeper.

 

[pepj]

One of my amps does 1200w into 2.
I also use it as my main practice amp.

Surprisingly it works.

 

[Leigh Semmens]

One item that has only been touched on in all of this discussion it the reported Power Output of same Amps. PPO is quite different to True Power Output (AKA RMS) If an amp is driven at it's peak it is a)going to sound cr*p and b)not last very long. The RMS (root mean square = approx .7071PPO) is the average that an amp can produce without distortion over an extended period and will produce longevity in the equipment. Some mfgs cheat and quote PPO and thus, young guns think that their amp is "better" than a bloke whose amp is quoted as having only 80% the power of the other amp, albeit that 80% is true RMS power. More power does not necessarily mean more sound, it should mean better sound though.

 

[J-Bassomatic]

One of my amps does 1200w into 2.
I also use it as my main practice amp.

Surprisingly it works.

Yes, it's either loud enough for your given situation or it isn't.

 

[HolmeBass]

if I remember correctly then it was Mr. Bell himself (the inventor of the dB scale) who referenced 3dB in difference of SPL as threshold of perception.

edit:
some folks may need 4dB of difference just to rcognize a noticable difference in loudness level, while other folks just 2dB or even less.
The "average" is referenced to 3dB, no more and nor less

Did you check the site I linked? It’s from a university course and clearly shows the JND is 1 dB.

Here it is again- go here and hear for yourself: dB: What is a decibel?

 

[ThisBass]

Safe is a relative thing. One of the key threats to electronic gear is heat. If you have an 8 ohm cabinet that produces comparable output to the 4 ohm one, but with half the current draw, that creates less heat -

The factor for reduced current equals 0.71 but not factor 0.5
Its about 30% reduced current on the 8 Ohm load vers 4 Ohm load, respectively about 40% more of current on the 4 Ohm load vers 8 Ohm load.

both in the speaker

It depends.
The OP didn't ask for a distinct cab format nor did he ask for a distinct driver size. So the cab in question could be anything, it could be a smallish 2x8", or a 110 cab, or even a 4 Ohm fridge.


If the OP was comparing a dual loaded 4 Ohm cab versus a single loaded 8 Ohm cab then he might figure that the "heat" thru each speaker VC in the 4 Ohm cab will be considerably smaller vers the "heat" thru the single speaker in the 8 Ohm cab.
Its not valid to claim "heat" thru the speaker with a 8 Ohm cab would be always reduced. It can also happen the other way round, this means less "heat" thru the speakers in the 4 Ohm cab.

and in the amplifier.

where the current isn't halfed at 8 Ohm load. The current is reduced by ~30%
Even for ~30% reduced current there will be less produced "heat" in the amplifier but, 30% less of "heat" is not the same as 50% less of "heat".

For reduced current prediction at different loads you can do simply the math with formula:
Irms = SQRT[P/R]

400 Watt RMS cab - 4 Ohm nominal
Irms = 10 A

400 Watt RMS cab - 8 Ohm nominal
Irms = 7.01 A

Will the amplifier have a decent lifetime running flat out into a 4 ohm load? Maybe.

Persoally I'd rather expect a "decent lifetime" for a sophisticated designed amplifier rather than expect "Maybe" speculations.
Personally I trust in sophisticated engineering. While you may not.

Will it last longer into an 8 ohm load? Absolutely.

I don't know if the amplifier really would last considerably longer into 8 Ohm load, where current was then reduced by ~30% into 8 Ohm
The only thing I really do know, the amplifier would likely last longest if the amplifier was not powered on at all.

So, let's say your car has a 200 horsepower engine, and it's rated to pull a 5000 pound trailer. No matter what, it will break down faster pulling a trailer all the time than if you use the motor to pull just the car. Rougher service is rougher service.

Sometimes car anologies do well apply, sometimes they do not at all.
I could try to give an answer if you would tell which kind of car engine you are talking about.
The principles of gas oil engines are basically way different versus petrol engines.

Diesel cycle - Wikipedia
Otto cycle - Wikipedia

Here you are with another car analogy:
Two 200 horsepower engines. One engine is a gas oil engine while the other one is a petrol engine.
For just the same speed of the car on the highway the diesel engine may run with 2.500 rpm whereas the petrol engine may drive about 5.000 rpm for the same speed of the car
Same horse power both engines, which one would last longer?
May be the diesel engine will last longer, cause it does only half the rpm for the same speed of the car.
Does it really apply?
Components such as crankshaft bearings may last longer due to reduced rpm. At the other hand side the combustion pressure inside the diesel engine is way bigger versus the petrol engine and it might happen that the cylinder head gasket of the diesel engine does not last as long as the one inside of the Otto engine.
As usual, it all depends!

Your amp is rated for 4 ohms MINIMUM load - that's means it's "OK" to do that. It doesn't mean it's ideal.

I think most part of your arguments might better apply to crappy China made "firecrackers" that are (possibly) designed close at the edge of total thermal capacity at full output.

But personally I have got "alil" more trust in engineering craftmanship that was done in US and/or EU countries by engineers who do their job with quite a good sense of responsibility.

Furthermore, as long as the design of the total thermal capacity of the amplifier is quite unknown then its rarely valid to make any assumptions in this regard.
If the amplifier runs "at full output" at about 30% of its total thermal capacity then some change in the Ohm load has got way less influence on its expectable "life time" versus an amplifier that runs already at about 90% of its total thermal capacity.

And as long as I don't know anything about the "usage" how hard the amplifier was taken powerwise by the user itself its anyway hard to make assumptions.
If in doubt, I'd try to ask the user first before I make reccomendations.

 

[Jefenator]

Sensitivity is nice to have in a cab when available wattage is limited. But there are other important considerations. (Like dimensions, weight, sound, availability...)
OTOH more wattage has never been easier to acquire & work with (at least with class D solid state). So if you find 150 @ 8 isn't quite cutting it, rather than adding speakers to maximize your wattage potential, you might want to think vice versa. Might be cheaper - and easier on your back - over the long haul to upgrade to a 500 or 800 watt head that will drive a good 350 watt 8 ohm cab closer to its full potential.

 

[themickster]

Two identical drivers and enclosures, it will go louder, but not deeper.

My two matching 1x12s are deeper than one by itself. The same goes for my other matching set.

 

[agedhorse]

Two identical drivers and enclosures, it will go louder, but not deeper.

This is generally not true, the low frequency knee generally shifts downward when identical low frequencies cabinets are doubled up.

One item that has only been touched on in all of this discussion it the reported Power Output of same Amps. PPO is quite different to True Power Output (AKA RMS) If an amp is driven at it's peak it is a)going to sound cr*p and b)not last very long. The RMS (root mean square = approx .7071PPO) is the average that an amp can produce without distortion over an extended period and will produce longevity in the equipment. Some mfgs cheat and quote PPO and thus, young guns think that their amp is "better" than a bloke whose amp is quoted as having only 80% the power of the other amp, albeit that 80% is true RMS power. More power does not necessarily mean more sound, it should mean better sound though.

Need to be careful about the (various) definitions of “peak”.

In the engineering sense, based on a sine wave signal, peak VOLTAGE is 1.414x the RMS voltage and peak POWER is 2x the RMS power.

In the marketing sense, there are multiple definitions, many making little to no sense in the world of science.

Did you check the site I linked? It’s from a university course and clearly shows the JND is 1 dB.

Here it is again- go here and hear for yourself: dB: What is a decibel?

Not at low frequencies, the resolution decreases due to the effects described by Fletcher-Munson. This non-linearity is fairly significant, and is also influenced by boundary conditions in the environment.

 

[Snert]

This is generally not true, the low frequency knee generally shifts downward when identical low frequencies cabinets are doubled up.

I've never found this to be true in any of the measurements I've done, so please feel free to back your point up with data and references.

 

[Snert]

Did you check the site I linked? It’s from a university course and clearly shows the JND is 1 dB.

It's not very clear in the link, but I think they're using sine waves for the testing. JND is more like 3dB with actual program material at best is my understanding.

My two matching 1x12s are deeper than one by itself. The same goes for my other matching set.

Measurements to support please.

 

[ThisBass]

Did you check the site I linked? It’s from a university course and clearly shows the JND is 1 dB.

Here it is again- go here and hear for yourself: dB: What is a decibel?

Back in the old days (before 1923) in the telephone industry the common damping measure unit has been "m.s.c."

1 "m.s.c" equals the damping that is caused at the frequency 800Hz by a „19 gauge“ telephone cable at a length that equals "1 English Mile".

And interestingly this unit of damping at 800Hz frequency was in "the oldish days" already refered as threshold of acoustic perception.

With very good accuracy 1 "m.s.c" equals 1dB
But it must be noticed that this "threshold of acoustic perception" was taken at a "rather highish" frequency that equals 800Hz.

Its not very surprising WHY they used 800Hz back in the days.
The bandwidth of telephone channels was quite narrow and it was a good idea to reference the damping measure to a frequency somewhere in the "essential" middle of the bandwidth.

It can also be (easily) seen where the later and worldwide common 1kHz standard (which is still in use) originally comes from.

 

[LiquidMidnight]

I have a Divided by 13 TBL200 that has a 16-ohm tap. I believe Glockenklang made a 16-ohm cab at one point and there may be some vintage cabs with that impedance, but I've NEVER run into a 16-ohm situation in a real world playing situation. I'd much rather have a 2-ohm tap.

 

[HolmeBass]

Back in the old days (before 1923) in the telephone industry the common damping measure unit has been "m.s.c."

1 "m.s.c" equals the damping that is caused at the frequency 800Hz by a „19 gauge“ telephone cable at a length that equals "1 English Mile".

And interestingly this unit of damping at 800Hz frequency was in "the oldish days" already refered as threshold of acoustic perception.

With very good accuracy 1 "m.s.c" equals 1dB
But it must be noticed that this "threshold of acoustic perception" was taken at a "rather highish" frequency that equals 800Hz.

Its not very surprising WHY they used 800Hz back in the days.
The bandwidth of telephone channels was quite narrow and it was a good idea to reference the damping measure to a frequency somewhere in the "essential" middle of the bandwidth.

It can also be (easily) seen where the later and worldwide common 1kHz standard (which is still in use) originally comes from.

And what bandwidth contains, say 90% of the variance in a bass guitar signal?

I don’t know exactly, but my guess would be 200 to 800 Hz.