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

[ThisBass]

Ohms and Watts aren’t units of sound pressure. Easy to make that assumption.

For the medium "air" and a given number of SPL its possible to compute acoustical Power which is (indirectly) expressed in Watts.
The acoustical impedance of a medium, such as air, is expressed in Ohms.

While acoustical Ohms and electrical Ohms are phyiscally quite different animals, acoustical Watts and electrical Watts are just the same. Both Watts can be measured and expressed in Joule.

In acoustical science the SPL has got the same meaning as the voltage has got in electronics.

 

[Killing Floor]

For the medium "air" and a given number of SPL its possible to compute acoustical Power which is (indirectly) expressed in Watts.
The acoustical impedance of a medium, such as air, is expressed in Ohms.

While acoustical Ohms and electrical Ohms are phyiscally quite different animals, acoustical Watts and electrical Watts are just the same. Both Watts can be measured and expressed in Joule.

In acoustical science the SPL has got the same meaning as the voltage has got in electronics.

Notwithstanding that what you just said is fabricated, here is the empirical definition.
1 Ohm = 1 kg⋅m2⋅s−3⋅A−2.

I’m good with all kinds of anecdotal explanations but I’d have never passed the EIT or PE if I took units and added descriptors I liked. “Acoustic” Ohm, spicy meter, it’s all good.

 

[ThisBass]

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.

Good question bro.
With the bass guitar the most part of energy is located below 400Hz.
While to the contrary a "generic" telephone channels low limit bandwidth respectively lower f3 equals ~400Hz.

While for a bass guitar cabinet it may be of valid interest how much SPL the cab provides at 100Hz oder 200Hz the same consideration is absolutely meaningless for a telephone channel cause the channel is steep high passed at something ~400Hz .
That was done (in the good al days) just to save resources cause generic speech does not neccessarily demand "extended" bandwidth for good speech intelligibility

 

[ThisBass]

Notwithstanding that what you just said is fabricated, here is the empirical definition.
1 Ohm = 1 kg⋅m2⋅s−3⋅A−2.

I’m good with all kinds of anecdotal explanations but I’d have never passed the EIT or PE if I took units and added descriptors I liked. “Acoustic” Ohm, spicy meter, it’s all good.

As I told already above, acoustical Ohms and electrical Ohms are quite different animals. But acoustical Watts and electrical Watts are just the same.
1 Watt = 1 Watt, it doenst matter if acoustical or electrical or mechanical or thermodynamical "steam" Watts ...

Interestingly the ears are sensitive to SPL which is NOT acoustical power. Similar like Voltage which is NOT electrical power

 

[Killing Floor]

As I told already above, acoustical Ohms and electrical Ohms are quite different animals. But acoustical Watts and electrical Watts are just the same.
1 Watt = 1 Watt, it doenst matter if acoustical or electrical or mechanical or thermodynamical "steam" Watts ...

Interestingly the ears are sensitive to SPL which is NOT acoustical power. Similar like Voltage which is NOT electrical power

OK. We’re at an impasse. I know what V means, I’m an engineer. A real one licensed in multiple states with a stamp. And Ohm is not a unit of sound. Let’s agree to disagree. I think I’ll go for a run. It’s 80 umami degrees.

 

[ThisBass]

OK. We’re at an impasse. I know what V means, I’m an engineer. A real one licensed in multiple states with a stamp. And Ohm is not a unit of sound. Let’s agree to disagree. I think I’ll go for a run. It’s 80 umami degrees.

The only thing I tried to tell is there are as well Ohms and Watts present at the acoustical side of considerations.
If we discuss considerations about electrical Watts of an amplifier we "could" also discuss the acoustical Watts "in the house" which are "derived" by the electrical Watts of the amplifier.

SPL isn't electrical Watts, that's true. But electrical Watts are transformed by the loudspeaker into SPL, and the SPL does produce for sure(!) acoustical Watts within the medium "air" which shows a distinct acoustical impedance.
Furthermore, some part of the generated acoustical power (in the nearfield of a loudspeaker) is reactive power which "flows back" into the amplifier.
This means in total that it can't be such as easy just to claim the SPL has got absolutely nothing to do with amplifier Watts.

And Ohm is not a unit of sound.

True, the acoustical impedance of air (in Ohms) is not the unit of sound (SPL).
However for a given number of acoustical power (Watts) and given acoustical impedance (Ohms) the SPL can be computed by formula

SPL = SQRT[Pac*Zac]
where Pac = acoustical power in Watts
Zac = acoustical impedance of air in Ohms

 

[micguy]

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.

If you read my post, you'll see I'm discussing real 4 ohm drivers, which generally have less sensitivity than the 8 ohm ones. As such, they require about the same voltage to develop the same output level. Same output voltage, half the impedance, Ohm's law applies very simply here, and we're at double the current for the 4 ohm driver. Or, looked at the other, way, half the current for the 8 ohm load.

 

[micguy]

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.

It's not a dramatic effect, but it is there - you need decent gear to see it. The presence of 2 nearby driver means the air in front of each driver is constrained by the presence of the other's output - what happens is each driver gets more air mass coupled to it so its resonance moves down a bit.

If you're familiar with end effects of ports, it's similar - think of the 2 cabinets near each other, but in free space, then draw a plane bisecting the structure, so that each driver is on one side. As each driver puts out the same amount, that plane has the same effect as a hard reflecting plane to one of the drivers - it constrains the air motion like the thing is on the floor - just like the end of a tube in a baffle has more end correction (due to mass loading from the physical constraining of the air) than an end in free space, you get more mass coupling from that virtual boundary - the resonance moves down.

In an array, you get even more constraint, and the resonance moves down even more, though it still might be subtle. If you think of an infinite planar array, each driver is not shooting into free space, where its loading is just what's in front of the speaker, it's loaded into a "tube of air" - each driver has its own tube. If the drivers are squares or hexagonal, so the array of speakers is uninterrupted, those tubes can be infinitely long, which will change the loading a LOT. The case of 2 loudspeakers near each other is a step in that direction.

 

[Leigh Semmens]

..................

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

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

My point exactly, the Power of an amp can have little meaning without a suitable reference point. SPL might go a long way to dispelling the myth that more is ALWAYS better.

 

[ThisBass]

If you read my post, you'll see I'm discussing real 4 ohm drivers,

whereas the OP is in the market for

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

...only ONE cabinet!

This means to me the ONE cabinet "might" be loaded with two 8 Ohm drivers which in parallel connection of the drivers provide 4 Ohm load to the amplifier or alternatively a single driver loaded cab where the driver is 4 Ohm.

which generally have less sensitivity than the 8 ohm ones.

yes, a little less efficient with the 4 Ohm driver thing is quite typical for 4 Ohm vers 8 Ohm drivers in the range of the same model.
That can be easily seen (respectively reread) on Eminence websiite or at Faital where you can read the specs of just the same driver model, but different Ohms of the same driver.

As such, they require about the same voltage to develop the same output level.

If you have a look at the reference efficience "eta zero" you may figure that the difference isn't such as big as you try to claim.

My amp is 150 watts at 8ohm and 300 watts at 4 ohms.

Of course there was more of benfit with a dual loaded 4 Ohm cab (two 8 Ohm drivers) versus a single loaded 4 Ohm cab.

 

[Snert]

you need decent gear to see it.

I have excellent gear.

If you're familiar with end effects of ports, it's similar - think of the 2 cabinets near each other, but in free space, then draw a plane bisecting the structure, so that each driver is on one side. As each driver puts out the same amount, that plane has the same effect as a hard reflecting plane to one of the drivers - it constrains the air motion like the thing is on the floor - just like the end of a tube in a baffle has more end correction (due to mass loading from the physical constraining of the air) than an end in free space, you get more mass coupling from that virtual boundary - the resonance moves down.

I understand spatial loading, and simply placing two conventionally sized BG cabinets right next to each other isn't going to make a difference; they're too small compared to the wavelengths involved as this effect was referenced at the bottom of their bandwidth.

 

[ThisBass]

In an array, you get even more constraint, and the resonance moves down even more, though it still might be subtle. If you think of an infinite planar array, each driver is not shooting into free space, where its loading is just what's in front of the speaker, it's loaded into a "tube of air" - each driver has its own tube. If the drivers are squares or hexagonal, so the array of speakers is uninterrupted, those tubes can be infinitely long, which will change the loading a LOT. The case of 2 loudspeakers near each other is a step in that direction.

This infinite tube model is a little bit "delicate" and does not reflect physical principles.
Once a "wave" is encoupled from its source it does not matter anymore how the source looks alike.
For acoustical waves (and also for RF electromagnetic waves) the "wave" is encoupled from its source when it leaves the near field to the source.

If you deal a little bit with RF frequencies you may figure exactly the same "principles" of source coupling and polar patterns such as it is quilty for line arrays in acoustics science.
Electromagnetic waves on its own don't "couple" each other. But its possible to "enlarge" the source by coupling of multiple sources. Just the same thing happens with acoustics.
The basic idea behind is to enlarge the source, rather then coupling the waves. The waves on its own, once these are "independent" travelling thru the medium "air", don't couple to the source anymore.

 

[micguy]

This infinite tube model is a little bit "delicate" and does not reflect physical principles.
Once a "wave" is encoupled from its source it does not matter anymore how the source looks alike.
For acoustical waves (and also for RF electromagnetic waves) the "wave" is encoupled from its source when it leaves the near field to the source.

If you deal a little bit with RF frequencies you may figure exactly the same "principles" of source coupling and polar patterns such as it is quilty for line arrays in acoustics science.
Electromagnetic waves on its own don't "couple" each other. But its possible to "enlarge" the source by coupling of multiple sources. Just the same thing happens with acoustics.
The basic idea behind is to enlarge the source, rather then coupling the waves. The waves on its own, once these are "independent" travelling thru the medium "air", don't couple to the source anymore.

You may have studied RF, but I’ve studied acoustics. I’m not making this stuff up.

 

[HolmeBass]

Good question bro.
With the bass guitar the most part of energy is located below 400Hz.
While to the contrary a "generic" telephone channels low limit bandwidth respectively lower f3 equals ~400Hz.

While for a bass guitar cabinet it may be of valid interest how much SPL the cab provides at 100Hz oder 200Hz the same consideration is absolutely meaningless for a telephone channel cause the channel is steep high passed at something ~400Hz .
That was done (in the good al days) just to save resources cause generic speech does not neccessarily demand "extended" bandwidth for good speech intelligibility

Bro, lol!

Why do you keep talking about ancient telephone technology?

 

[agedhorse]

It's not a dramatic effect, but it is there - you need decent gear to see it. The presence of 2 nearby driver means the air in front of each driver is constrained by the presence of the other's output - what happens is each driver gets more air mass coupled to it so its resonance moves down a bit.

If you're familiar with end effects of ports, it's similar - think of the 2 cabinets near each other, but in free space, then draw a plane bisecting the structure, so that each driver is on one side. As each driver puts out the same amount, that plane has the same effect as a hard reflecting plane to one of the drivers - it constrains the air motion like the thing is on the floor - just like the end of a tube in a baffle has more end correction (due to mass loading from the physical constraining of the air) than an end in free space, you get more mass coupling from that virtual boundary - the resonance moves down.

In an array, you get even more constraint, and the resonance moves down even more, though it still might be subtle. If you think of an infinite planar array, each driver is not shooting into free space, where its loading is just what's in front of the speaker, it's loaded into a "tube of air" - each driver has its own tube. If the drivers are squares or hexagonal, so the array of speakers is uninterrupted, those tubes can be infinitely long, which will change the loading a LOT. The case of 2 loudspeakers near each other is a step in that direction.

Yup, the shift downwards can be attributed to several things happening simultaneously. First there is mutual coupling where each driver affects the output of the adjacent driver (provided they are close together relative to the wavelengths being examined). Second, there is greater diaphragm area, greater motor strength and larger internal volume which contribute to low frequency extension and Third, because of the mutual coupling there is slightly improved coupling to the air which appears like an acoustic transformer with better acoustic impedance matching. All of these combine to shift the -3dB point of the low frequency down slightly. It's not a huge difference, but it's there. For example a single cabinet might have an F3 of 50Hz and a pair of cabinets might have an F3 of 48hz. Where it's most noticeable IMO is in a cabinet like the SVT-810, a single driver is 1/8 the air space of the 810 is going to suffer with a higher F3 (rough guess is maybe 5 or 6 Hz, maybe micguy has a better guess since he is a real acoustical/electrical engineer... I'm just primarily an EE)

My point exactly, the Power of an amp can have little meaning without a suitable reference point. SPL might go a long way to dispelling the myth that more is ALWAYS better.

SPL measurements and marketing is just as corrupt as anything else, in fact I would suggest that it is more corrupt because nobody (well, we do) specifies the space loading that the SPL values are presented as. This alone is a huge issue because it's hard to explain but easy to market.

I have excellent gear.

I understand spatial loading, and simply placing two conventionally sized BG cabinets right next to each other isn't going to make a difference; they're too small compared to the wavelengths involved as this effect was referenced at the bottom of their bandwidth.

The point is that the spacing needs to be small relative to the wavelength.

 

[ThisBass]

You may have studied RF, but I’ve studied acoustics. I’m not making this stuff up.

So what's wrong with similarities in physical laws?

An RF masterdegree of studies does include some additional "studies" in "classical physics" such as thermodynamics and acoustics and others more.

The wave equation
Wave equation - Wikipedia
applies mathematical wise to any kind of a wave. The math applies to RF waves just the same way as it applies to acoustical waves, or to mechnical waves such as a wave in water.

There is "physically" a wide variety of similarities present to "different kinds" of waves.

Both waves, electromagnetic and acoustical wave, may show parts of reactive power, reactive power is (almost always) present especially in the nearfield of a source.

There is also a wide variety of similarities present that in particular applies to polar patterns, coupling of sources, reflecting and mirroring, boundary effects (earth ground boundary effect on RF, side wall boundary effect on acoustics), directional and bidirectional polar patterns, encoupling of a wave from its source, reactive and resistive parts, efficience of a source and its radiation impedance, impedance of medim (reactive and resitive) in free air, and some others more do well apply just the same way to acoustical and RF waves.

What's wrong with all of these similarities which do apply to both kinds of waves just the same way?


Some notes regarding the "infinite tube" model.
In "typical" engineering textbooks its not unusal to refer to rough simplifications just to "demonstrate" in a rather "comprehensible words" rather complex processes.
The "tube model" may in an comprehensive way demonstrate some outcoming of source coupling, and may also demonstrate how the front of the wave looks alike, and also how the wavefront can be "computed", but by no means does the "infinite tube model" explain anything about the "physical" processes WHY the coupling of sources generates a more even wavefront and also an improved directional narrow polar pattern.

If I was asked a question: how does a wave looks like that is generated by a line arry?
ok, I might try to refer to the "tube model" which helps a little bit to demonstrate the generated waveform which is produced by multiple and coupling sources.
I could even tell that in "rough simplified thinking" there where multiple narrow waves "side by side" and these have kinda coupling-effect to each other.

I think this model was just "suited" to "demonstrate" the generated waveform, no more, and nor less
But the tube model is by far not applicable to answer any question about the (origin) development of the wave, neither was it applicable to an acoustical line array, nor to any antenna arrays

 

[micguy]

So what's wrong with similarities in physical laws?

Nothing - my background in Physics deals with many different kinds of waves. RF and Acoustics have many similarities, but when your mindset doesn't allow you to understand why something that we can easily measure in Acoustics works the way it does, you need to try a different approach.

 

[ThisBass]

Nothing - my background in Physics deals with many different kinds of waves. RF and Acoustics have many similarities, but when your mindset doesn't allow you to understand why something that we can easily measure in Acoustics works the way it does, you need to try a different approach.

These effects can be measured with RF waves just the same way. The similarities are just the same.
Where did I tell these effects can not be measured?

The only difference is, you are trying kinda sort of infinite tube model to explain the effects that can be measured, while my approach in this regard "how it all works" is totally different.
Your tube model does not distinguish between the "complex" effects that happen within and between farfield and nearfield, while my approach includes these effects.
In your tube model the wave still couples to the source in the farfield, while my approach regards a wave that travels beyond the nearfield as encoupled from its source. The farfiled is (almost always) characterized by nearly pure resistive fieldimpedance.
If you'd expect nearfield characteristic even for farfield distances, the source would try quite hard to push any "resistive" part of the wave into the medium air.

edit:
don't get me wrong, in any chance but, due to the similarities that apply to both worlds just the same way, if your tube model was really applicable to acoustc waves then it "should" be applicable as well to RF waves, but it don't, by no means!

 

[ThisBass]

Bro, lol!

thanx for your kindness

Why do you keep talking about ancient telephone technology?

sometimes folks do wonder a little bit from where this "ancient" 1kHz thing comes from that is still in use as common standard for amplifiers.

 

[agedhorse]

sometimes folks do wonder a little bit from where this "ancient" 1kHz thing comes from that is still in use as common standard for amplifiers.

Different tones apply to different things being measured. For example you would choose a different measurement bandwidth for a woofer and a tweeter.