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Dirty-South Blues Harp forum: wail on! >
Do bigger speakers = more feedback?
Do bigger speakers = more feedback?
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Rick Davis
1229 posts
Jan 27, 2013
2:52 PM
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I've seen that claim here and elsewhere a few times: Bigger speakers are more prone to feedback than smaller speakers. That does not really comport with my experience, but I am frequently wrong so I thought I'd ask.
Smaller speakers are generally less efficient than larger speakers (8-inch vs 12-inch, for example), so the bigger more efficient speaker would reach the feedback threshold sooner with less power. That may be what is happening.
I am curious to hear your opinions on this.
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-Rick Davis
The Blues Harp Amps Blog
The Mile High Blues Society
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Willspear
273 posts
Jan 27, 2013
4:45 PM
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Disagree
I don't think it's that black and white. Certain speakers in certain amps feedback extremely bad. But my ga40 with its 12 is less prone than several amps with 8s I have played.
I gig with a 15 in an ampeg that feeds back less than alot of amps
The mini 15 meteor is not feedback prone at all.
I am not sure of the science or anything but my experience is more that a given amp feeds back more readily which leads me to believe its all in its gain structure and how susceptible a mic is to creating a loop at a frequency and the amp/speakers representation of that frequency.
Last Edited by on Jan 27, 2013 4:46 PM
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528hemi
358 posts
Jan 27, 2013
4:59 PM
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My understanding is feedback is caused by certain frequencies which has to do with t he amp design. Some amps can filter these frequencies which will be more feedback friendly. Also the Speaker characteristics can also cause more or less feedback depending on its frequency response. That is why normally when you turn the treble up you usually have more feedback. I would think if you have a good Equalizer, you could probably cut some of the frequencies causing Feedback.
Then again I graduated with a BSEE many moons ago. :)
More experienced Amp tech/designers can better answer.
BIgger speakers might project more sound so might also contribute to feedback?
528hemi
Last Edited by on Jan 27, 2013 5:00 PM
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5F6H
1529 posts
Jan 28, 2013
4:26 AM
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@ Rick "I've seen that claim here and elsewhere a few times: Bigger speakers are more prone to feedback than smaller speakers."
In my experience, comparing speakers with *the same motor & impedance*, just larger cones & baskets - "Yes". As cone diameter increases so does high end response & efficiency. Resonant frequency, however, may drop. Ability to get out of the speaker's range and inhibit feedback, by positioning, also reduces.
However, as 528Hemi says, a speaker works as a frequency filter, a speaker that is very active in feedback sensitive range, or with a very high efficiency, or resonant frequency, will feedback more, irrespective of cone diameter.
So there will be some 12"s that feedback less than some 10"s, for instance, but I wouldn't say that was typical.
It's not always an easy thing to test, given impedances & parallel vs series wiring (comparisons are rarely based on a level playing field)...you might find that 2x or 3x 10" feedback less than one 12"....but then 1x10" may feedback more than 2x or 3x10" too.
The amp itself is a factor, very high idle currents feedback before lower currents, so an old cathode biased PA style head might work great with 2x10" & struggle with feedback with 4x10" (using the same speakers). A moderate/cool biased 4x10" might work great, but simply switching to 2x10" might require more idle current to get the volume up, whereas a sympathetic 12" or 15" will work better than 2x10" at the moderate/cool current (but usually not as well as 3 or 4x10").
Basically, once you have your speakers, mess around (whilst matching impedance) to see what gives best volume (at the speaker, not on the vol knob). Tone is king, "enough" volume may be enough.
We've done quite a bit of testing with the Brown tolex 2x6L6 Fenders (Bandmaster, Pro, Concert) all have the same circuit (6G#-A), operating voltages...we found 4x10" cool bias the loudest, 3x10" cool to moderate next loudest, 1x15" cool least loud (but certainly still useable with a restrained band/mic'd). 2x10" regular/hot idle, loud but difficult to hear on stage (making a cab with 2x10" arranged one on top of the other can help here, rather than side by side).
My pal's Blues Deluxe was a good example too, started off as 1x12" 8ohms (8ohm tap), cool current (~20mA), tried a couple of 12" speakers, went to 2x10" at 4ohms (4ohms tap) moderate current (25-30mA), less feedack, sweetest tone & a little louder, then to 3x10" at 2.7ohms on the 4ohm tap, cool current (15-20mA) - loudest & easiest to hear on stage. He has another with an 8ohm 4x10" array that's louder still (but most folks would just plump for the DeVille as it comes with 4x10").
It's as much about balance, actual volume before feeedback, what you can hear from the backline...etc. Pretty much anything can be made to work satisfactorily with enough testing & development.
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Last Edited by on Jan 28, 2013 6:31 AM
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Rick Davis
1230 posts
Jan 28, 2013
7:40 AM
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@Mark: "Tone is King." Amen.
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-Rick Davis
The Blues Harp Amps Blog
The Mile High Blues Society
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Greg Heumann
1964 posts
Jan 28, 2013
8:35 AM
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I agree with what Mark said above.
Here's a way to think about it: For a given amp setup, sound pressure level at the mic is THE SINGLE DETERMINING FACTOR. No question different speaker setups will produce different SPL's for the same amp, but I'm willing to bet if you level the playing field - i.e., speakers of same efficiency and frequency response - that you'd find that, at the SAME SPL, speaker size has very, very little to do with it. Speaker resonant or most efficient frequencies will make a small difference - but usually not to the degree you need when feedback is an issue in the first place.
I hold that the single most important factor in the USEFUL management of feedback is an amp's gain. This has MUCH more to do with the nature of the feedback than the absolute volume at which it occurs in an anechoic chamber. It is also important to understand that what sounds loud in your practice room may well simply not be loud enough to be heard at a typical jam - which means you will keep turning it up until you can hear it, or, if the amp is too small, it feeds back. You can't bring a knife to a gunfight.
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/Greg
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Rick Davis
1231 posts
Jan 28, 2013
9:14 AM
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Greg, I believe you are right. it makes sense...
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-Rick Davis
The Blues Harp Amps Blog
The Mile High Blues Society
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dougharps
326 posts
Jan 28, 2013
9:28 AM
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Not to disagree in any way with Mark, whose knowledge I greatly value, but I need to ask a question about one issue in his post above: using different impedance taps.
When you start using different impedance taps don't you also change the amp's sound and behavior, aside from the speaker(s) tone?
I first considered this when a pro guitar player friend of mine commented that he liked the sound of his amp better when using the 4ohm output to matching impedance speakers. He said it sounded warmer.
Isn't there more current when using lower impedance outputs, which could effect the overall sound of the amp with possible power tube distortion?
I don't want to get into this too much in a speaker thread, but I think it may make a difference. I have an old Stromberg-Carlson PA head that has less feedback with more volume when I use two speakers in parallel using the 4ohm tap as opposed to one speaker with the 8 ohm tap, or two in series with the 16ohm tap.
So my thought is that volume before feedback and tone is impacted by the inherent frequency profile of the speaker and the specific behavior of the amp as relates to the impedance of the tap.
If this is incorrect, please educate me!
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Doug S.
Last Edited by on Jan 28, 2013 9:29 AM
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MrVerylongusername
2468 posts
Jan 28, 2013
10:38 AM
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You can't ignore the angle of dispersion - larger speakers will have broader dispersion of the midrangey feedback causing frequencies making it harder to stand outside of the beam of sound.
the higher frequencies that a driver produces will always be the most directional ones - that's why in hifi and PAs you get stacked, progressively smaller drivers to give an optimal dispersion at a range of frequencies.
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5F6H
1530 posts
Jan 28, 2013
10:53 AM
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@ Doug "When you start using different impedance taps don't you also change the amp's sound and behavior, aside from the speaker(s) tone?" Indeed you do, that's why I mentioned the "level playing field" any one change has a knock on somewhere else...so there is always some interpretation to fit in with speaker W ratings, practicality, impedances, etc.
"Isn't there more current when using lower impedance outputs, which could effect the overall sound of the amp with possible power tube distortion?" Yes indeed, but only when you start mismatching impedances. If OT & speakers are a reasonable match voltages, current & clean W RMS stay constant. But even within matched loads: lower impedences tend towards, livelier, airier, perhaps brighter sounds. Higher impedences towards darker, thicker tone. What works best for you will depend on your starting point & what you need to achieve, of course with a multi tap OT simply using different amounts of the secondary winding is thought by some to influence tone, I'm not rich enough to buy 3 or 4 OTs every time I want to test something, so I'm happy to take credible testimony there ;-)
Unless you are seriously mismatching things (I don't recommend that you do without manufacturer's consent) nominal clean W RMS won't vary significantly (say keeping a pair of 6L6s between 2.5K-7.5K). Normally mismatching taps means a jump of double, or half speaker load in ohms, which may/may not affect clean power some & will certainly change the voltage & current ratio. Running 8ohms load on a 4ohm tap (safe) will raise the voltage & drop OT primary current by a factor of 0.7. 16ohm load on a 4ohm tap (check with amp manufacturer) will double OT primary voltage & halve current at a given W power out. Conversely, mismatching down by half (2ohms load on a 4ohm tap - I STRONGLY RECOMMEND YOU DO NOT MISMATCH DOWN without consultation) will reduce voltage & raise current by a factor of 0.7. 4ohms load on a 16ohm tap will double current & halve voltage. Current tends to kill quicker than voltage for a given W. But be aware that W RMS clean ratings are of little relevance to harp, they are diagnostic states, all that really matters to us is average power at the speaker (dirty or clean) and the amp's (inc speakers) ability to get that accross at useable volume with a tone that suits us, individually. We largely live in non-linear territory, "through the looking glass" if you like?
"I don't want to get into this too much in a speaker thread, but I think it may make a difference. I have an old Stromberg-Carlson PA head that has less feedback with more volume when I use two speakers in parallel using the 4ohm tap as opposed to one speaker with the 8 ohm tap, or two in series with the 16ohm tap." I have found similar using 4x10" in parallel (2ohms load), compared to the same speakers in series-parallel (8ohm load). I typically like to use multiple speakers in parallel, others have found different - again there's that starting point vs target. These are matched loads (with your PA head) so things that may be having an effect are the parallel speakers sharing the load over one single speaker, having to do less work each, with more current at speaker/OT secondary @ less voltage. The series pair uses less speaker current at a given power out. Sure a W is a W and which way you divvy up the amps & volts should make no difference, but we are starting with measured power, not theoretical, and in my experience speakers in a harp amp like current over volts. Perhaps due to the need to keep a sustained note, rather than a peakier guitar signal? Guitar playing amp builders often say the opposite: volts over current.
Everything affects everything else: mic, preamp, power amp, speaker array. You might find high gain at one point works with low gain elsewhere, or medium all through the chain. Inevitably, you need a certain amount of gain/efficiency and high end somwhere to stop everthing turning to mud & to keep dynamics...that will also vary from player to player, as we make different sounds when we play
Last Edited by on Jan 29, 2013 2:14 AM
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dougharps
327 posts
Jan 28, 2013
3:01 PM
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@5F6H
It is reassuring that you also have noticed that parallel speaker arrangements seem to work better with harp.
I really liked this:
"Everything effects everything else...mic, preamp, power amp, speaker array..." and "we make different sounds when we play."
That has certainly been my experience!
Thanks for a comprehensive thoughtful answer to my questions and observations.
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Doug S.
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