Full power into one speaker?

[Chienmortbb]

4 hours ago, agedhorse said:

Basic electrical engineering absolutely links voltage to power. Since (solid state) amps are generally considered an ideal voltage source, it’s also an ideal power source. 
 

While a speaker has a reactive component, when the capacitive and inductive regions are averaged out with the DCR of the speaker, the net result is an average or nominal impedance.
 

The amp must provide voltage x current to drive this load which is power. In the inductive regions the current lags voltage and in the capacitive regions the current leads voltage. 

 

Regardless of phase angle, the amp must be able to deliver both voltage and current, this is something an experienced power amplifier designer has to deal with.

 

A 500 watt, 4 ohm single driver compared to a 300 watt 8 ohm driver is likely to have a lower sensitivity, quite possibility 3dB lower, which would make the 8 ohm speaker (slightly) louder at 300 watts than the 4 ohm speaker at 500 watts.

I could not have put it better myself.

[DGBass]

Perhaps I could pose an example to the more knowledgeable forum users which does still bear a relation to the op's original question about essentailly squeezing more out of his setup by having a 4 ohm load. If I have an amplifier that the manufacturer says will deliver 500w into a 4 ohm load by design, and have two identical speaker cabs each loaded with single 15 inch drivers of a suitable power rating from the same manufacturer, same driver model, with same power rating and manufacturers sensitivity rating, and one is a 4 ohm version and one is an 8 ohm version. For a given identical output setting on the amplifier when used with either speaker cabinet using an identical sound source, should there be any expected difference in loudness between the 4 ohm setup or the 8 ohm setup?

[Bill Fitzmaurice]

The 4 ohm will be 3dB louder, as the cone of the 4 ohm driver moves further with the same applied voltage. However, where maximum output is concerned they're the same, as both have the same maximum excursion. It just takes more voltage into the 8 ohm to get there, which takes only a slight increase of the volume knob, say from 5 to 6. Not from 5 to 10.

[DGBass]

Thanks for that, if I can add one more scenario:  If I add an identical 8 ohm cab to the original 8 ohm cab, maintain the same given volume knob setting as in the first comparision and same sound source settings, what measure of loudness increase could I expect compared to the single 8 ohm cab and the single 4 ohm cab? And... will the amplifier in theory be supplying the same voltage and current to the paired 8 ohm cabs in parallel as it would have been with the single 4 ohm driver at the same amplifier volume setting? Hope that makes sense.

[Bill Fitzmaurice]

The 2x8 ohm will be 6dB louder than  1x8 ohm. That's from the combination of 3dB additional sensitivity from the halving of impedance and 3dB additional sensitivity from the doubling of cone area. It will also have 6dB more maximum output, as the Vd (displacement) is doubled. That makes it 3dB louder at the same volume setting as the 1x4 ohm, and 6dB higher maximum output than the 1x4 ohm.

There's a saying in automobile circles that there's no replacement for displacement. It applies to speakers as well, because loudspeaker cones are pistons.

Edited January 5, 2023 by Bill Fitzmaurice

[Phil Starr]

3 hours ago, DGBass said:

If I had ....two identical speaker cabs each loaded with single 15 inch drivers of a suitable power rating from the same manufacturer, same driver model, with same power rating and manufacturers sensitivity rating, and one is a 4 ohm version and one is an 8 ohm version

This is an additional problem. The voice coil would be changed to achieve the change in target impedance; effectively the motor driving the cone has then been changed. In practice manufacturers often use a standard voice coil for any particular variation on a design, and since they sell almost exclusively 8ohm units the speaker is often optimised for that. To make a 4ohm coil you need either shorter or thicker wire in the coil and the tendency is to either shorten the voice coil or use fewer layers. This in turn changes the maximum excursion of the speaker and the efficiency as well as a number of other parameters. You can end up with a reduced maximum output from a 4 ohm speaker if excursion rather than amplifier power is the limiting factor. Each case will be different as each manufacturer will make different decisions about how to make a 4ohm version.

 

You may well have heard a small change in the sound between the 4 and 8 ohm versions of the speaker for this reason. If the 4ohm coil was just a shorter version of the 8 then it may have raised the efficiency enough to hear on an A/B test. Loud sounds give the illusion of more bass. Subjective listening tests are never really definitive either. I do a lot of listening and still manage to make a fool of myself with the acoustic version of placebo effect

[DGBass]

Thanks both, all makes sense and if you are interested in the speaker i've used over the past few years to make 4 ohm and 8 ohm comparisons, its a Faital Pro 15PR400 in the same cab with the same set of amps of varying power ratings. There are a few differences in specifications on the speaker vendor website between the 4 and 8 ohm versions but at the time I chose this driver it seemed a good fit and in practice it sounds good and has worked extremely well for me. I do hear and feel a difference using the 4 ohm version. It may actually be small in technical terms but it is noticeable and keeps me in one cab single speaker 4 ohm territory. My amps all work well at 4 ohms and I'm happy that a 4 ohm setup gives me an edge over an 8 ohm setup.

 

[Phil Starr]

On 05/01/2023 at 17:16, DGBass said:

Thanks both, all makes sense and if you are interested in the speaker i've used over the past few years to make 4 ohm and 8 ohm comparisons, its a Faital Pro 15PR400 in the same cab with the same set of amps of varying power ratings. There are a few differences in specifications on the speaker vendor website between the 4 and 8 ohm versions but at the time I chose this driver it seemed a good fit and in practice it sounds good and has worked extremely well for me. I do hear and feel a difference using the 4 ohm version. It may actually be small in technical terms but it is noticeable and keeps me in one cab single speaker 4 ohm territory. My amps all work well at 4 ohms and I'm happy that a 4 ohm setup gives me an edge over an 8 ohm setup.

 

Out of curiosity I checked and Faital have used the same length of voice coil in both variants you used and the specs are very close, they have gone to a lot of effort to keep the impedance as the only variable. They also give the same sensitivity at 1W/1m.  Unfortunately they don't state their method so this could be with a 2.83V input which is the usual method which gives 1W into 8 ohms but 2W into 4ohms which would give you an extra 3db. You don't say how you did your listening tests; if you have two cabs for example so the test was done quickly or of you simple removed the driver but kept the amp settings the same. The latter would of course mean you were testing the 4ohm speaker with twice the power.

 

There's an old trick used a lot by retailers that when demonstrating sound systems you set up so that the one you want to sell is the loudest, people universally like the louder sounds and we are very sensitive to changes in volume which we perceive as a change in tone. Listening tests are notoriously difficult to do well. Moving a speaker or you head position even a few cm can create a perceived change in sound because of room acoustics. Faitals own data suggests that you probably wouldn't hear any difference between these two at the same power, even at full power so long as the amp was capable.

 

It's a good choice of driver though, I'm increasingly looking at Faital first when designing cabs and this gives me even more respect for what they are doing.

[Chienmortbb]

2 hours ago, Phil Starr said:

They also give the same sensitivity at 1W/1m.  Unfortunately they don't state their method so this could be with a 2.83V input which is the usual method which gives 1W into 8 ohms but 2W into 4ohms which would give you an extra 3db.

This seems to have come from the early days of solid state, particularly car audio manufacturers, to make their very inefficient drivers seem louder. This comes from Ohm's law (V = I x R) and the Electrical Power Law (P = V x I)*, This means that to obtain a power of 1 watt into 8 ohms you need 2.828 Volts, but the same power into 4 ohms only requires 2 V.

 

So by feeding a 4 ohms speaker with 2.828 volts, you magically find an extra 3dB. This trick only became possible once solid state became a thing and transformers disappeared.

 

*P = power in watts, V = Volts, I = Current, measured in Amperes or Amps for short. N.B. Current is often incorrectly called Amperage.

[DGBass]

12 hours ago, Phil Starr said:

You don't say how you did your listening tests; if you have two cabs for example so the test was done quickly or of you simple removed the driver but kept the amp settings the same. The latter would of course mean you were testing the 4ohm speaker with twice the power.

Just the same one cab i've been using for the last nine years (a circa 1980 Musicman RH115-75) which has had the driver swapped from 4 to 8 ohms depending on which amp I choose to use with it.  I couldn't say that testing was scientific in any way. It was more down to percieved change in loudness and feel from being very familiar with the setup and playing regular venues where I know how it should sound. It may even be down to how much my trousers were flapping 😉 at 4 ohms (a lot) and 8 ohms (not so much) respectively with the same amp volume. I tend to leave a particular amp i'm using always at the same settings whether rehearshal or gig to a sweet spot where everyone in the band is happy with levels and its loud enough for a gig. I think thats when I first noticed the 4 ohm cab seemed to squeeze that little bit more from a given amplifier at a given volume.

BFM mentioned that turning up the master a bit would also likely level the difference between a 4 and 8 ohm cab. That does seem to work in practice and might be the difference between the master at 10 o'clock or 2 o'clock for me. I suspect the op was originally wondering if there was a way to get as much benefit from his amps rating without the need for an extra cab or perhaps his master was already way up (and maybe dimed ?).  An extra 3db from a 4 ohm speaker might just be enough.

It's a valid question when so many people buy large powerful amps with ratings mostly always quoted at 4 ohms but the amp ends up always being used with an 8 ohm cab and maybe turned up much more to achieve an end result. It's like considering buying an 800 watt amp to go with your 400 watt 8 ohm cab these days and realising after a few gigs you still need a second cab to get the full benefit of any available headroom. All great for new amp and cab sales at the end of the day.

One other benefit i've had with a 4 ohm cab is when using smaller amps like for example a TE AH200 GP12, or a TE AH250 GP11 i've used in the past. All worked really well for smaller venues and I could make good use of the extra 3db at 4 ohms without the need to bring a much larger amp.

The 8 ohm speaker is currently back in my cab as I have a newer bigger amp (600 watts) that would probably over stress a 4 ohm 15PR-400 after a few shows. If the new amp works out and my old MM cab doesn't shake apart, I may be using the 8 ohm version in future or until I can afford a much higher capacity 4 ohm driver and for the op's benefit that won't be cheap. It was all much simpler when I used Marshall Superbass amps, impedance matching gave me a selector on the back panel and I always got 100 watts whether it was 4, 8 or 16 ohms and it always seemed loud enough.