What is the audible result of damping?

Jay Barracato

Jay Barracato

Graduate Student
Jan 11, 2011
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Solomons MD
A room that I was in has a pair of EAW VR51 installed. The coverage and level seem appropriate to the room. They are powered by an appropriate sized amp.

With use, they change tonally, from medium bad to downright crap. Unless there is a flaw in the speakers themselves, the only visible questionable thing about the system is that the original speaker cable installed looks to be 18 gauge with the longer run being about 100 feet. The shorter run which is the side that actually sounds worse has had the speaker cable replaced with a piece of 20 gauge wire of the same type the mic feeds from the stage are wired.

I did not notice any problems in checking with voice only or background music levels but over the course of the first show (bluegrass ) the sound got thinner and thinner until the one side had no low mids to speak of.

Is it a damping problem or is there something buried deeper I haven't found?
 
Re: What is the audible result of damping?

It could be a damping problem, that wire gauge is pathetic for the speaker cord distance.
The VR51 impedance minimum is at 150 Hz, so the lack of damping would make the amp have the least control in the low mids.
That said, lack of damping usually makes a speaker sound "muddy" (sloppy transient response) rather than lacking low mids.

Years ago, with some time on our hands at a corporate show, we compared a 25' and 75' 14 AWG cord on speakers similar to the VR51 being used for center fill.
In a blind test, the consensus between several techs was the longer cord needed 3 dB more gain to sound the same "loudness" by ear.
Until you actually do the comparison, or hear the crap sound that whimpy cable produces, it is hard to believe, as the charts showing loss in % of power do not adequately show the degradation in sound quality.
 
Sloppy transient response would be an adequate description of the sound.

Perhaps there was a loss of total level due to the cable size that was not noticeable until the cable heated up.
 
Re: What is the audible result of damping?

Passive crossovers can shift with heating up... Significant wire resistance will appear in series with the crossover filter components.

Damping factor is generally overstated as an audible problem (with modern solid state amps), but frequency response can be affected by too much wire resistance.

JR
 
Re: What is the audible result of damping?

Jay Barracato said:
Sloppy transient response would be an adequate description of the sound.

Perhaps there was a loss of total level due to the cable size that was not noticeable until the cable heated up.
Click to expand...

With a 100 foot 18 AWG speaker cord, power loss would be in the order of 15%.
Assuming you were compressing the crap out of the music, it would still have at least 10 dB crest factor, with 500 watt peaks the average power level would be less than 50 watts, only 15% being converted to heat in the cable.
I doubt the cable would heat up.

An amp's damping factor is the ratio of the load impedance to the output impedance of the amplifier, usually measured at 1K at 8 ohms.

As a rule of thumb, to avoid “flabby” response, you want a damping factor above 20 at the speakers.

Bennett Prescot wrote an article about damping factor:
http://campuspa.com/downloads/dampingfactor.pdf

Anyway, here is the math that Bennett did not go into boring detail with. I wouldn’t have gone through all this, but didn’t know Bennett wrote an article about it until after I cranked the math.

79 feet of of 12 gauge copper wire (158 feet round trip) is .25 ohms.

An amp with an output impedance of .004 Ohms:
DF=8 /.004=2000
Now, add a .25 Ohm speaker cable and the damping factor at the speaker is:
New source impedance = 0.004 + 0.25 = 0.254 Ohms (at the speaker)
8 / .254 = 31.4 DF
4/.254=15.75 DF (at 4 ohms) Not so good.

With a damping factor of 500:
DF=8/.016=500
Again, add a .25 Ohm speaker cable:
New source impedance = 0.016 + 0.25 = 0.266 Ohms (at the speaker)
8 / .266 = 30.07
4/.266=15.04

The amp with 4 times the damping factor results in barely any difference at the speaker.


79 feet of of 10 gauge copper wire (158 feet round trip) is .158 ohms.

DF=8 /.004=2000
Now, add a .158 Ohm speaker cable and the damping factor at the speaker is:
New source impedance = 0.004 + .158 = .162 Ohms (at the speaker)
8 / .162 =49.4 DF
4/.162= 24.69 DF (at 4 ohms) Now we meet the “not flabby” specification!

With a damping factor of 500:
DF=8/.016=500
Again, add a .158 Ohm speaker cable:
New source impedance = 0.016 + .158 = .174 Ohms (at the speaker)
8 / .174 = 46 DF
4/.174 = 23 DF (at 4 ohms)

Use fat cables and high impedance loads on long runs, if you do the math on 100' of 18 AWG you will see how little damping factor is available from any "modern solid state amps".

Art
 
I am extremely familiar with Bennett's article and I have followed it in my own practice.

I am not concerned with damping as a spec on amps, modern or otherwise. Because in my practice I try to have cables as large as possible and as short as possible I am not familiar with what damping would sound like, and my eyes tell me that 100 feet of 18 gauge is a bad idea and my ears tell me there is a problem. I would like to know if the problem I see is a likely cause of the problem I hear before I recommend rewiring.

Based on Arts math and Bennett's article, the damping factor must be in the single digits. What neither of those sources account for is the change on the sound during the course of the show, unless something was heating enough to change its impedance.
 
Re: What is the audible result of damping?

I already said this but let me repeat with emphasis.

If the wire resistance is significant wrt driver impedance, it is also significant wrt crossover filters.. frequency response errors are generally far more audible than the more subtle effects of damping factor.

Worry about whatever, but IMO this is yet another argument in favor of active crossovers, or better yet powered speakers.

JR
 
Re: What is the audible result of damping?

Jay Barracato said:
I am extremely familiar with Bennett's article and I have followed it in my own practice.

I am not concerned with damping as a spec on amps, modern or otherwise. Because in my practice I try to have cables as large as possible and as short as possible I am not familiar with what damping would sound like, and my eyes tell me that 100 feet of 18 gauge is a bad idea and my ears tell me there is a problem. I would like to know if the problem I see is a likely cause of the problem I hear before I recommend rewiring.

Based on Arts math and Bennett's article, the damping factor must be in the single digits. What neither of those sources account for is the change on the sound during the course of the show, unless something was heating enough to change its impedance.
Click to expand...

Why not experiment to prove the problem is or isn’t the wire. Take some of your heavier gauge wire and temporarily replace the existing wire with it. Play (crank) some tunes before and after and listen to the differences.
 
How much power were the speakers getting? 3kw vs 100 watts, same SPL output - the 100w powered speaker will sound like crap as the peaks are missing. That plus awful damping will make any speaker horrible.
 
Re: What is the audible result of damping?

John Roberts said:
I already said this but let me repeat with emphasis.

If the wire resistance is significant wrt driver impedance, it is also significant wrt crossover filters.. frequency response errors are generally far more audible than the more subtle effects of damping factor.

Worry about whatever, but IMO this is yet another argument in favor of active crossovers, or better yet powered speakers.

JR
Click to expand...
The series resistance of 100 feet of 18 AWG (200 feet total) is 1.277 ohms.
Not significant regarding power loss, and makes no difference whatsoever "wrt crossover filters" used in a passive crossover speaker.
I agree frequency response is important, but series resistance attenuates, simply making a speaker quieter, not changing the frequency response.

The reduction in damping factor 1.27 ohms of speaker wire in series causes an amp with a damping factor of 500 to have only a damping factor of 6.2 at the speaker.
An argument for reasonable sized cable.

A damping factor of 6.2 sounds bad in comparison to one of 20 or more, put 100 feet of 18 gauge on one speaker, and 10 feet of 12 gauge on another, and you will hear a difference with music (assuming your hearing is in the normal range;^) ) though a SPL meter (or RTA) would show perhaps a 1/4 dB difference.

Art
 
Re: What is the audible result of damping?

Art Welter said:
The series resistance of 100 feet of 18 AWG (200 feet total) is 1.277 ohms.
Not significant regarding power loss, and makes no difference whatsoever "wrt crossover filters" used in a passive crossover speaker.
I agree frequency response is important, but series resistance attenuates, simply making a speaker quieter, not changing the frequency response.
Click to expand...
#1 only true if loudspeaker loads were pure simple resistances. Have you ever looked at a loudspeaker impedance plot vs frequency?
#2 wire resistance in series will often add to any other R in a passive LP filter.

I can't predict significance without knowing specific details about the individual speakers.
The reduction in damping factor 1.27 ohms of speaker wire in series causes an amp with a damping factor of 500 to have only a damping factor of 6.2 at the speaker.
An argument for reasonable sized cable.

A damping factor of 6.2 sounds bad in comparison to one of 20 or more, put 100 feet of 18 gauge on one speaker, and 10 feet of 12 gauge on another, and you will hear a difference with music (assuming your hearing is in the normal range;^) ) though a SPL meter (or RTA) would show perhaps a 1/4 dB difference.

Art
Click to expand...

I recall participating in a listening tests where we artificially created single digit DF with some PA cabinets. The effect was subtle and more of a LF response error (to my ears) than some dramatic sonic signature.

In some guitar amps DF is made variable on purpose to increase amp/cabinet interaction ( a desirable feature in old tube guitar amps). I recall one guitar amp where HF DF and LF DF were adjustable independently (while they didn't call it DF :-) ).

======
Getting back on topic this may be a red herring wrt to the OP's speaker issue. Have we considered power compression? If a marginal speaker is over driven, one of the bandpasses may be weakened by overheating, or even going into protection, to save itself. A speaker in protection mode, will not sound very good, just better than complete silence.

JR
 
Re: What is the audible result of damping?

John Roberts said:
#1 only true if loudspeaker loads were pure simple resistances. Have you ever looked at a loudspeaker impedance plot vs frequency?
#2 wire resistance in series will often add to any other R in a passive LP filter.

I can't predict significance without knowing specific details about the individual speakers.
Click to expand...

For a voltage drive amplifier,
The transfer function from Voltage to displacement, given a simplified linear model, has a Q value

Q = sqrt(km)/(b + (BL)^2/Re)

So what I've noticed over the years, BL will typically be the dominate factor in pro-audio systems. For example, High BL speakers will have an over damped frequency response, rolling off at around 6dB/oct way above Fs of the system. Adding series resistance can be quite noticeable if you start sticking large values in there.

Adding excessive amounts of series resistance could, cause the cabinet tuning to go bad on an HP enclosure. Normally when the Q gets big the Frequency response will become under damped, which is what we perceive as that "floppy" sound.

I haven't tried it yet, but if a passive network is built to compensate for the resonance in a HF driver, adding the series resistance could cause the network to not function properly anymore at the crossover region.

Obviously there are a lot more variables involved in the total Voltage/SPL transfer function. But the simplified model does match what you will observe if you grab a variety of magnets and resistors.


PS. This is also why if you start taking passive networks out of boxes and converting them to biamp you should also be checking the porting on the enclosure.
 
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Re: What is the audible result of damping?

Again what does this have to do with a speaker that reportedly changes "tonally" with use?

Wire resistance should not change significantly with use, amplifier DF should not change significantly with use. If anything VC impedance should rise with use, making the impact of wire resistance less not more.

I return to my suspicion of power compression or outright driver protection.

That said I am not familiar with that particular speaker model, and specifics of how it is being operated.

JR
 
Re: What is the audible result of damping?

John Roberts said:
Again what does this have to do with a speaker that reportedly changes "tonally" with use?
Click to expand...

Not sure what it has to do with it. I was just pointing out reasons why cumulative series resistance would change the tonality. Could be a VC which is damaged and when it heats up and expands causes the problem? Damaged cap in the crossover? Who knows.
 
500 watt amp run into 500 watt rated speakers.

I jumped my FOH including a sabine navigator for DSP straight into the amps bypassing the rest of the installed system. I know for a fact the board, the input and the output were clean with no clipping during the show. I checked the amp (upstairs and not readily available) during sound check at about 120% of my show level. It was barely showing level. The show level was about 90 db at 40 feet where FOH was located. The was no significant compression on the signal and the music was bluegrass.

If it was power compression than something is significantly wrong with the boxes themselves which is what I am trying to determine before recommending they invest a couple of hundred dollars in replacing the cables.

That is the same reason a simple swap test is not reasonable. I would have to uninstall either the speakers or the amps to test with cables I have, or go ahead and buy and install new cabling not being sure that is the problem.
 
Re: What is the audible result of damping?

Perhaps it would help us guess, if you described more specifically how the tonality is shifting, more/less? low/mid/high?

Tonality changing over time, does not sound to me like a symptom of wimpy speaker wire, that in my expectation would affect sound pretty much the same, and all the time, not after a while.

JR
 
John Roberts said:
Perhaps it would help us guess, if you described more specifically how the tonality is shifting, more/less? low/mid/high?

Tonality changing over time, does not sound to me like a symptom of wimpy speaker wire, that in my expectation would affect sound pretty much the same, and all the time, not after a while.

JR
Click to expand...

My feeling was that the low mids were getting thinner and more distorted compared to the horn. I did not have the same feeling early in the show. When I heard the distortion I pulled the level back fairly significantly and went up to stick an ear against each of the speakers. That is when I noticed there was also a difference between the SL and SR speakers and very little output from one of the 15 inch cones.

I am not really sure when the tonal change started as during the show I was fighting excessive stage volume as well, but I did not feel the FOH was the same tonally as when I tuned them prior to the show and during soundcheck.

It is not my system and I am not sure how deeply I want to get into making it right. But I think it would be worth my effort to go take some measurements at different levels to see if I can spot any differences in the two bands or in SL vs SR.
 
Re: What is the audible result of damping?

Jay Barracato said:
A room that I was in has a pair of EAW VR51 installed. The coverage and level seem appropriate to the room. They are powered by an appropriate sized amp.

With use, they change tonally, from medium bad to downright crap. Unless there is a flaw in the speakers themselves, the only visible questionable thing about the system is that the original speaker cable installed looks to be 18 gauge with the longer run being about 100 feet. The shorter run which is the side that actually sounds worse has had the speaker cable replaced with a piece of 20 gauge wire of the same type the mic feeds from the stage are wired.

I did not notice any problems in checking with voice only or background music levels but over the course of the first show (bluegrass ) the sound got thinner and thinner until the one side had no low mids to speak of.

Is it a damping problem or is there something buried deeper I haven't found?
Click to expand...

I've recently replaced the crossover in a VR21 so still have one on my desk and it has one of those round yellow self-resetting soft circuit breakers for protection.
from experience if these are exercised they get softer and softer every time.

try running some pink noise at a decent level and listen to the tonal change over time.
when it goes bad, reduce the level to a point where it's not stressing the speaker at all but that you can still make out the tone.
if I'm right you'll hear it slowly come back to normal.

Jason
 
Re: What is the audible result of damping?

+1 PTC (positive temperature coefficient) fuses do shift sensitivity from repeated flogging. That sounds like a likely suspect.

The speaker manufacturer's service department should be aware of the issue.

JR
 
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