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

[GWGuitar]

Does a second cab make it seem louder because of increased coverage, even if there isn’t much difference in dB output?

 

[Wasnex]

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.

@agedhorse is more qualified to answer the question; maybe he will comment.


Regardless of impedance, various models have different sensitivity ratings. So, I would suggest using the actual sensitivity ratings rather than any sort of generalized rule. Also makes sure you are using the 1W/1m sensitivity ratings, rather than the 2.83V/1m rating

Explanation:
In some instances, you may see a sensitivity rating expressed in terms of 2.83V/1m rather than 1W/1m. 2.83V at 4 ohms results in 2W. 2.83V at 8 ohms results in 1W.

No conversion is required for the 2.83V rating at 8ohms rating since 2.83V at 8 ohms = 1W.

We know that a 2:1 or 1:2 power change results in a 3dB decibel change. So to convert the 2.83V at 4ohms sensitivity rating to the 1W/1m rating, simply subtract 3dB.

Example, lets a assume the following sensitivity ratings:
99dB 2.83V/1m at 4ohms
97dB 2.83V/1m at 8ohms

This makes is appear that the 4 ohm driver is 2 dB more efficient than the 8 ohm driver. But lets convert to the 1W/1m rating so it's more equitable comparison.

- A sensitivity rating of 99dB 2.83V/1m at 4ohms is equivalent to 96dB 1W/1m. (Here we subtracted 3dB)
- A sensitivity rating of 97dB 2.83V/1m at 8ohms is equivalent to 97dB 1W/1m. (No conversion was required)

Another factor to keep in mind is the process for measuring sensitivity is not standardized. So, if you are comparing specs from two different companies, the comparison may not be very accurate. Use these as ballpark estimates only.

Even comparing specs from different product lines within the same company can sometimes be misleading, because different teams designed and tested the products. Comparing specs is most useful when you are looking at the range of options within a single product line

 

[coy garcia]

Big Boom Cabinets are worth a looksie. I have a 4ohm 112 paired with a 50w tube head that keeps up with my loud rock band. They have bigger 4ohn offerings.

 

[Snert]

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

But the Max SPL = sensitivity + power rating (dBW) isn't really valid as it doesn't account for when the driver hits Xmax.

Basically agree with the rest of the (partially) quoted post as lower impedance drivers have lower efficiency (not sensitivity) and power amps don't double power into half the impedance so the net is approx zero and never going to be heard in use.

 

[Wasnex]

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?

The 3dB rule is a standard assumption in audio work.

The key word is "clearly"

Under controlled circumstances perhaps you can hear a 1dB change, or even less. Probably not in the middle of a loud band stand.

No idea how accurate this is, but here's a test for you:
Blind Testing a 1 dB Level Difference (audiocheck.net)

How to take the test:
Select the dB difference for the test at the top, left of the page (Click a link next to the heading "Change the target here". Click the buttons under "Files being tested" so you can hear the examples.

Scroll down to the "Test" heading and click the question mark on the left.

 

[Wasnex]

Does a second cab make it seem louder because of increased coverage, even if there isn’t much difference in dB output?

Speakers convert mechanical energy to acoustic energy. The transfer is more efficient with a larger radiating surface.

The dispersion pattern will be narrowed along the axis that runs between the drivers. Also as the frequency goes up, secondary lobes will start to form along the axis. The angle of the lobes are dependent on frequency and spacing between the drivers.

This is the results for a three box horizontal array of 12"+horn pa cabs.

Ref: Array Simulations - Pro Sound Testing

 

[Wasnex]

But the Max SPL = sensitivity + power rating (dBW) isn't really valid as it doesn't account for when the driver hits Xmax.

Mechanical power handling is definitely a very important factor that most companies don't mention. If necessary you run an appropriate HPF and/or limit the max power and SPL to the mechanical power handling of the design.

Assume the mechanical power handling is not exceeded in my examples.

Summary of Max SPL
One 4ohm cab = 121dB (380W)
One 8ohm cab = 121dB (225W)
Two 8ohm cabs = 126dB (380W total and 190W per cab)

 

[Snert]

Assume the mechanical power handling is not exceeded in my examples.

But it's an assumption that's very seldom made and the common SPL max is simply not true, in I'd say the majority of cases with all sorts of pro speakers. Just looking at the BG world, how many have a HPF at tune on a ported cab to ensure the driver isn't moving unto unloaded below tune in use? Almost no one. How many have a HPF at all? Again, almost no one.

With that covered, with modern drivers power handling is so good that I don't recall the last well thought out low frequency enclosure that was actually power and not excursion limited in application.

The arrays sims and measurements you posted are interesting for BG cabinets at about 1k and below, but few BG cabs have a horn properly integrated like a PA enclosure would. Most are cobbled together afterthoughts and don't have a very even DI.

 

[-Asdfgh-]

Pretty much all amps are twice as much watts at 4 ohms,

For older lead sleds, it's more like 50% more, at least for everything I've owned, not twice as much. I am not sure what the deal is with the new-fangled class D stuff I have not yet moved to.

 

[-Asdfgh-]

I have a Barefaced Super Twin (4 ohms). Whether I use a 100 watt Traynor Custom Special, a 150 watt GK MB150, a 280 watt GK 400RB, or a Glockenklang Heart Rock (750 watts @ 4 ohms), it's loud enough to be heard in a band with three guitarists who use 100 watt Marshall heads and 4x12 cabinets.
View attachment 4660215
View attachment 4660216

Three guitarists? Was there a special deal on at the guitarists shop?

 

[-Asdfgh-]

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 have learned something today. One of my cabs is 4 ohms, but then I got it because it was a good deal on a light cab rather than anything else. It seems to be loud enough. At some point I will find two 8 ohm cabs I like that are also light and then never use one of them if I can possibly do that. It's nice to have the option if you need it even if you never use it.

 

[BOOG]

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

How accurate do you suspect your conclusion is that “most bass players are only going to use a single cab”?

 

[Wasnex]

But it's an assumption that's very seldom made and the common SPL max is simply not true, in I'd say the majority of cases with all sorts of pro speakers. Just looking at the BG world, how many have a HPF at tune on a ported cab to ensure the driver isn't moving unto unloaded below tune in use? Almost no one. How many have a HPF at all? Again, almost no one.

With that covered, with modern drivers power handling is so good that I don't recall the last well thought out low frequency enclosure that was actually power and not excursion limited in application.

The arrays sims and measurements you posted are interesting for BG cabinets at about 1k and below, but few BG cabs have a horn properly integrated like a PA enclosure would. Most are cobbled together afterthoughts and don't have a very even DI.

The max SPL should be limited to the lower of the amp's max power out or the cab's max power handling. For bass you need to differentiate between the cab's RMS power rating (thermal power handling) and mechanical power handling which is often significantly lower than the RMS rating below 100hz.

In my example the amp's max power is 380W at 4 ohms and 225W at 8 ohms. This is a pretty modest power rating these days and I have specified that our imaginary cabs are not mechanically limited, so the Max SPL is certainly valid in the example.

But your point is well taken that in real-world applications you need to pay attention to the cab's mechanical power handling limits. Unfortunately most companies do not provide this type of data. If you like heavy lows, pushing a cab to it's RMS rating is likely to push the driver beyond it's excursion limits.

Mesa publishes useful mechanical power handling data for their Subway line. FYI the Mesa Subway 210 is mechanically limited to 425W at 8 ohms at 81.3hz, so it should be fairly safe with a 225W amp and a suitable HPF. The Subway 410 is mechanically limited to 925W at 4 ohms at 79.90hz, so it should be fairly safe with 380W amp and a suitable HPF.


Here are some dispersion examples with subwoofers. As I mentioned the angle of the secondary lobes is dependent on driver spacing and wavelength. So if you change the spacing, you can get the same dispersion/lobing patterns at different frequencies.

For reference: Wavelength at 100hz = 3.43m, and Wavelength at 50hz = 6.86m.

The first two examples show the dispersion when driver spacing is about 30% of the wavelength.

The second two examples show dispersion when driver spacing is about 60% of the wavelength.

The last two examples show the dispersion when driver spacing is about 230% of the wavelength.

 

[Snert]

But your point is well taken that in real-world applications you need to pay attention to the cab's mechanical power handling limits. Unfortunately most companies do not provide this type of data. If you like heavy lows, pushing a cab to it's RMS rating is likely to push the driver beyond it's excursion limits.

Mesa publishes useful mechanical power handling data for the their Subway line. FYI the Mesa Subway 210 is mechanically limited to 425W at 8 ohms at 81.3hz, so it should be fairly safe with a 225W amp and a suitable HPF. The Subway 410 is mechanically limited to 925W at 4 ohms at 79.90hz, so it should be fairly safe with 380W amp and a suitable HPF.

Which is excellent info to have, and good on Mesa for supplying it, but max safe SPL is nowhere near as simple as Efficiency + power.

Here are some dispersion examples with subwoofers. As I mentioned the angle of the secondary lobes is dependent on driver spacing and wavelength.

I know this, and they look more like sims than measurements as they're too smooth. However, my point was that PA cabs tend to have much better controlled DI vs F than bass cabs do, so whilst illustrative I don't think are representative of most bass cabs as few actually care about dispersion in their designs.
Measurements once the new Earthworks mic arrives and I have some time.

 

[seamonkey]

At low levels say 40 to 41 db the average person can hear a difference
At higher levels say 90 to 91 - you'd have to be a highly train listener to hear a difference.

 

[HolmeBass]

The 3dB rule is a standard assumption in audio work.

The key word is "clearly"

Under controlled circumstances perhaps you can hear a 1dB change, or even less. Probably not in the middle of a loud band stand.

No idea how accurate this is, but here's a test for you:
Blind Testing a 1 dB Level Difference (audiocheck.net)

How to take the test:
Select the dB difference for the test at the top, left of the page (Click a link next to the heading "Change the target here". Click the buttons under "Files being tested" so you can hear the examples.

Scroll down to the "Test" heading and click the question mark on the left.

Well I aced that test @ 1dB difference in a moving car with the radio playing.

My other experience is in mixing a recording, changing the volume of different tracks by as little as 0.5 dB could have a noticeable affect on the mix.

All that said, in general I agree that getting a 4 ohm cabinet instead of an 8 ohm one is almost never worth it. If you’re running your amp up against its protection circuit with an 8 ohm cabinet, switching to a 4 ohm version of the same cabinet is not going to solve your problem.

 

[Snert]

At low levels say 40 to 41 db the average person can hear a difference
At higher levels say 90 to 91 - you'd have to be a highly train listener to hear a difference.

From all the research I've read on it, including data from people who do this for a living and a number of AES papers, that might be the case with pure tones, but not with broadband material. Broadband 3dB is about right and not difficult to test.

 

[Rip Van Dan]

There are tons of single cabs available that are loaded with two 10"s or two 12"s
Most amount of these available cabs provide a nominal load that equals 4 Ohm.

Sounds like you're saying that most cabs that have 2 x10 or 2x12 are 4Ω cabs and that's just not so. I have a 210 cab that is 8ohm myself and there are a LOT of 210 and 212 cabs that are 8 ohm. They typically use 16Ω speakers wired in parallel to get an 8Ω cabinet. Or perhaps I am misunderstanding what you are trying to say?

 

[Wasnex]

Sounds like you're saying that most cabs that have 2 x10 or 2x12 are 4Ω cabs and that's just not so. I have a 210 cab that is 8ohm myself and there are a LOT of 210 and 212 cabs that are 8 ohm. They typically use 16Ω speakers wired in parallel to get an 8Ω cabinet. Or perhaps I am misunderstanding what you are trying to say?

AFAIK 8 ohms for 210 and 212 cabs has become more and more common.

 

[agedhorse]

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.

Sometimes, but generally 8 and 16 ohm drivers are closer than 4 and 8 ohm drivers IME.