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8 ohm/4 ohm = rarely use amp's full potential
- Thread starter Tommy TGrip
- Start date
- Feb 12, 2006
- 60,986
- 5
- 220,768
- Disclosures
- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
Additional mass of air load.
For the most part of considerations the "loaded" air mass to the loudspeaker can be neglicted cause the loaded air mass is quite small versus total moving mass of the loudspeaker cone itself.
If I remember correctly even for the large volume in a horn loaded system the loaded air mass can be considered as small versus the moving mass of the loudspeaker.
Furthermore, the moving mass of modern high powerd loudspeakers can be easily three times as big as the moving mass of a (lets say) EVM 15L.
The larger the moving mass of the loudspeaker itself, the lower the impact of the loaded air mass.
Coupling depends on frequency and distance of the sources to each other. Radiation impedance depends on frequency and cone surface area.
The lower the frequency then the better multiple coupling sources can benefit from coupling cause it helps to improve the radiation impedance.
I tried to run a "quick and dirty" modelling with antenna arrays and yes, its possible to improve radiation impdance at the lowish side of the spectrum a little more then in the middle of the bandwidth.
Unfortunatelly I'm missing about one or two octaves at the "low end" side to prove the effect of lowered f3 that can be measured with "generic" bass cabinets.
May be there are some additional "smallish" effects neccessary to hit the point.
For the most part of considerations the "loaded" air mass to the loudspeaker can be neglicted cause the loaded air mass is quite small versus total moving mass of the loudspeaker cone itself.
If I remember correctly even for the large volume in a horn loaded system the loaded air mass can be considered as small versus the moving mass of the loudspeaker.
Furthermore, the moving mass of modern high powerd loudspeakers can be easily three times as big as the moving mass of a (lets say) EVM 15L.
The larger the moving mass of the loudspeaker itself, the lower the impact of the loaded air mass.
Coupling depends on frequency and distance of the sources to each other. Radiation impedance depends on frequency and cone surface area.
The lower the frequency then the better multiple coupling sources can benefit from coupling cause it helps to improve the radiation impedance.
I tried to run a "quick and dirty" modelling with antenna arrays and yes, its possible to improve radiation impdance at the lowish side of the spectrum a little more then in the middle of the bandwidth.
Unfortunatelly I'm missing about one or two octaves at the "low end" side to prove the effect of lowered f3 that can be measured with "generic" bass cabinets.
May be there are some additional "smallish" effects neccessary to hit the point.
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Just some food for thought - or amusement:
I've recently come across a discussion in a German forum where some people defended the opinion that you should not run an amp at its minimum impedance rating because you will shorten its life span doing so since the power section will be able to produce 100% of its potential. According to them, it's better to run a min 4 ohm head with an 8 ohm cab, because limiting the power amp by impedance will make it live longer.
I've recently come across a discussion in a German forum where some people defended the opinion that you should not run an amp at its minimum impedance rating because you will shorten its life span doing so since the power section will be able to produce 100% of its potential. According to them, it's better to run a min 4 ohm head with an 8 ohm cab, because limiting the power amp by impedance will make it live longer.
- Feb 12, 2006
- 60,986
- 5
- 220,768
- Disclosures
- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
If an amp is designed to run at 4 ohms, there’s no reason not to do so.
(This assumes the designer was at least adequate)
Never underestimate the Dunning Kruger effect when discussing with hobbyists.
Some of the arguments are nearly making sense ;-)
Its even sometimes funny to read how so called "experts" do frequently claim they really can audible wise "detect" a rather "shorty" full power burst capability of (lets say) 300msec of a class-D amplifier.
And I'm asking then myself, damn, do these folks really deal with sinusoidal waveforms in "real world pratice" rather than with real world audio program material that (almost always) shows some crest, even on its highest dynamic/transient peaks?
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I agree with what others have said about wattage not meaning very much. Your drivers have a lot to do with perceived volume. I have been through many amps and cabs over the years. I had a 20watt combo amp that would rattle the walls, and also a 200 watt amp that was only moderately loud when it was maxed out.
Currently I run a 450 watt head into an 8 ohm 115 (so it's putting out maybe 225 watts) and it's so stupid loud it's ridiculous. It's not just about the watts, it's about the entire combination of the amp, cabinet design, and drivers. Just looking at a spec sheet doesn't always tell you anything about how it will sound, it just tells you what the engineering limits are.
You also have to be aware that a high wattage killer rig won't sound as good when the volume is turned down because you aren't giving the drivers enough energy to "express" themselves so it will sound very mousy. In those situations, lower wattage actually makes a fuller sound.
Currently I run a 450 watt head into an 8 ohm 115 (so it's putting out maybe 225 watts) and it's so stupid loud it's ridiculous. It's not just about the watts, it's about the entire combination of the amp, cabinet design, and drivers. Just looking at a spec sheet doesn't always tell you anything about how it will sound, it just tells you what the engineering limits are.
You also have to be aware that a high wattage killer rig won't sound as good when the volume is turned down because you aren't giving the drivers enough energy to "express" themselves so it will sound very mousy. In those situations, lower wattage actually makes a fuller sound.
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Till about the end of the 80s I had nothing but my 70s SVT rig.
Sometimes it happened that I had to play at bar gigs or wedding partys or something else where "little loudness" was a requirement to the whole band.
Even at small loudness levels the svt amplifier + 810 fridge sounded damn good.
My 212 cab is loaded with 3" VC drivers and handles ~900 Watt (AES)
Even at rather smallish loudness levels which take a couple of Watts at most the cab sounds pretty good to me.
Another consideration on somewhat down shifted f3 that is measurable.
With microphones its well known that the microphone shifts up very noticeable its low frequency response in the very nearfield to a loudspeaker device.
The standard distance for SPL measurements equals 1 meter to the source. For very low frequencies, such as bass frequencies, 1 meter of distance to the source is still within the nearfield at some part, or at least within transission area between nearfield and farfield.
The consideration, what might happen if the coupling of "narrow" multiple speakers expands the nearfield arround the coupling sources at some content in "on axis" direction versus only one single loudspeaker device.
If the nearfield would expand a little bit for two sources, and even expands a little more for four coupling sources, was it imaginable that the microphone at 1 meter distance to the sources then would measure a somewhat "improved" low response cause it was a little closer within the nearfield?
Just a thought, of course.
With microphones its well known that the microphone shifts up very noticeable its low frequency response in the very nearfield to a loudspeaker device.
The standard distance for SPL measurements equals 1 meter to the source. For very low frequencies, such as bass frequencies, 1 meter of distance to the source is still within the nearfield at some part, or at least within transission area between nearfield and farfield.
The consideration, what might happen if the coupling of "narrow" multiple speakers expands the nearfield arround the coupling sources at some content in "on axis" direction versus only one single loudspeaker device.
If the nearfield would expand a little bit for two sources, and even expands a little more for four coupling sources, was it imaginable that the microphone at 1 meter distance to the sources then would measure a somewhat "improved" low response cause it was a little closer within the nearfield?
Just a thought, of course.
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- Feb 12, 2006
- 60,986
- 5
- 220,768
- Disclosures
- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
Are you really sure you mean 10m ground plane?
Ground plane - Wikipedia
damn antennas, you know!
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even at 2m distance it was imaginable that low response would benefit at the lowish side of the spectrum a little bit from a somewhat expanded nearfiled of the coupling sources.
Of course for 10m of distance to the source the effect was anyway obsolete, if present at all.
As I told above, its nothing but a thought!
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Yes.
Terms interchange between fields. I know what a ground plane means at RF and the term is also used in PCB design at AF and even DC to designate a layer that covers (usually) the entire PCB or a large area of it where all ground referenced items are connected.
How at 2m at the LF end of a BG cab are multiple adjacent cabs not going to couple? You are barely off axis even to several in a horizontal row.
10m is because some of the gear I test has some very high SPL capability and I'm a little protective of my expensive mics.
Of course nearfield and 1m measurements are also done.
You really have got those sufficiant equipment available to measure 10m ground plane?
I'm not familiar with all of technical terms in English language. I tried to google for "ground plane" to figure the exact technical (acoustical) meaning of this term
Interestingly I didn't get at once a hit that applied to acoustics, the first hit I received applied to antennas, which made me a little bit chuckling in this moment,
and I thoght by myself:
"damn antennas"
Where did I tell they won't?
If the row was longish enough in length, the polar pattern that shows uniform SPL numbers may then not show pure circular shape anymore (also not pure spherical shape)
And you are really doing these measurements for such a big gear at 10m ground plane?
Is there any difference "measurable" for the shape/slope of the low response curves that varies with different distance of the microphone to the gear?
If I understand correctly you do measurements for different distances:
nearfield
1 Meter
2 Meter
10 Meter
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Thanks a lot for the link.
I think I do comprehent now what the meaning is. It was already clear to me when I looked to the "antenna stuff" that I received from google search.
It was just amusing to me to receive "antenna stuff" as first hit while searching for something acoustical, and yes it was me myself who already refered to antennas quite a lot here in this thread!
Do not use the serial loudspeakers connection for bass (only parallel), it is killing the loudspeaker LF damping factor. However - it sounds very good for lead guitar. Not so good for rhythm guitar, ac. guitar, keyboards, Hi-Fi, etc. "clean" sounding sources.
