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Dave A

Standing Wave Problem

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OK I have questions regarding standing waves. How do you know you have a problem with them and how can you measure for them to determine the effectiveness of  the addition of foam or baffles or both. What general good construction practices help you avoid these?

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21 minutes ago, Dave A said:

OK I have questions regarding standing waves. How do you know you have a problem with them and how can you measure for them to determine the effectiveness of  the addition of foam or baffles or both. What general good construction practices help you avoid these?

One visit to the Klipsch factory I noticed a diagonal baffle in some bass bins.  Roy told me that it was there to break up standing waves inside the cabinet.  Next I asked him how they determined where to put the baffle.  He said they wasted a lot of wood figuring it out.  They'd install one, measure it then bust it apart and install it in another spot.  Measure, repeat until you got the results you desire.  Unfortunately he didn't get into the measurement aspect of it but placement seemed trial and error.

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1 hour ago, Dave A said:

What general good construction practices help you avoid these?

 

Non-parallel surfaces.

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Standing waves can't be avoided.  Every room dimension has a resonant frequency or 2 (first and second, 1x and 2x the wavelength and sometimes more)  The trick is to avoid integer multiples of the other room dimensions.  8' x 8' x 16' would be one of the worst rooms.  Another trick is to build walls with highly different lengths, say 11' x 8' x 19'  Non-parallel walls work well, I did that in my old house (to much derision from the x).  

 

post-2142-13819498863272_thumb.jpg

 

Finally, diffusers and absorbers can help by breaking up the reflections that reinforve each resonance. 

 

You will hear them when walking through the room and finding places where a note is loud and another place where that note is quiet. 

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4 hours ago, glens said:

 

Non-parallel surfaces.

 

Question, in regards to building a rectangular speaker cabinet ... o O (technically, does an 1/8"  over 34" length = "out of parallel" from top to bottom, in relation to the front and back, and side to side for standing wave breakup, even if it looks square ???) 

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9 minutes ago, windashine said:

 

Question, in regards to building a rectangular speaker cabinet ... o O (technically, does an 1/8"  over 34" length = "out of parallel" from top to bottom, in relation to the front and back, and side to side for standing wave breakup, even if it looks square ???) 

 

You have to think in terms of fractions of a wavelength. One-eighth of an inch is negligible when the wavelength is 32' (35 Hz), but it's very significant when the wavelength is 3/4" (18 kHz). Unfortunately we don't get to operate at just one wavelength, so standing waves are almost certain to occur somewhere in the frequency range. We can take steps to reduce them, such as non-parallel surfaces, and dimensions that are not integer ratios, but we can't eliminate them entirely.

 

I recently built a golden ratio enclosure (width:depth:height in ratios of 0.618:1.000:1.618). The box is heavy and strong and well-damped internally, yet an impedance sweep still shows a slight anomaly exactly where a half-wavelength equals the major diagonal dimension. Its amplitude is negligible, but it's still there.

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17 minutes ago, windashine said:

 

Question, in regards to building a rectangular speaker cabinet ... o O (technically, does an 1/8"  over 34" length = "out of parallel" from top to bottom, in relation to the front and back, and side to side for standing wave breakup, even if it looks square ???) 

Nope.

 

An inch for every foot is a general rule thrown around, but even that doesn't work. Sounds travels in all directions so you'll get tangential modes, etc.

 

The best thing to do inside a cabinet is use absorption.

 

PWK would argue that absorption reduces efficiency. Although that is technically true, I firmly believe the 0.1dB trade-off for way lower resonances is worth it.

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Just now, Dave A said:

OK what is this?

 

Mount the woofer in the enclosure and measure its impedance at all frequencies. Peaks in the impedance represent resonances. Resonances (other than that of the woofer itself) come from standing waves.

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I think I might just ask Roy next week and see if he will answer. It's the baffle in the KPT-456 bass bin that started this question in my mind and he had a hand in that I believe.

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1 hour ago, DrWho said:

Nope.

 

An inch for every foot is a general rule thrown around, but even that doesn't work. Sounds travels in all directions so you'll get tangential modes, etc.

 

The best thing to do inside a cabinet is use absorption.

 

PWK would argue that absorption reduces efficiency. Although that is technically true, I firmly believe the 0.1dB trade-off for way lower resonances is worth it.

You going to China or Hope?

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There is no way to completely avoid standing waves. They're either in your speaker cabinets or in the room. Lower frequency standing waves are the most noticeable. I think most speaker makers try to tame those and don't worry as much about high frequency standing waves. I too would like to hear more about this subject from the forum standing wave guru... I wonder who that is?

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5 hours ago, MechEngVic said:

There is no way to completely avoid standing waves. They're either in your speaker cabinets or in the room. Lower frequency standing waves are the most noticeable. I think most speaker makers try to tame those and don't worry as much about high frequency standing waves. I too would like to hear more about this subject from the forum standing wave guru... I wonder who that is?

Most speakers I have been into seem to do nothing internally to avoid these although I guess the size of the cabinet and it's shape can help out. I know there is the point of diminishing returns I just don't know where they are. For instance I was advised to have a smooth faced motorboard with rounded edges on the front face of the speaker and no grilles. I don't buy into the idea these make serious differences when I have had some Klipch pro gear that has recessed motorboards and the ports are square at the end and the exterior corners are square and the grille has expanded metal with grille cloth or perforated metal and they still sound awesome. I have seen way to many torn cones to want to ever do this bare faced speaker stuff.

 

  Roy talks about headroom and perhaps this is part of it. For example the Chorus I uses the same components as the KP-301 except for the crossover which is different and the KP-301 is rated for much more output. The cabinet is also almost exactly the same in cu" but the exterior shape is different. I am interested in why. All these things contribute but which are worth pursuing?

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This is an interesting topic(s), but several different things are being confounded. Standing waves inside the cabinet, standing waves in the room, baffle diffraction.

BTW, it is good to see Dr Who has mysteriously returned. 

Good Luck,

-Tom

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Just now, PrestonTom said:

This is an interesting topic(s), but several different things are being confounded. Standing waves inside the cabinet, standing waves in the room, baffle diffraction.

BTW, it is good to see Dr Who has mysteriously returned. 

Good Luck,

-Tom

You are right Tom two different topics. Just to clarify I am interested in the cabinet interior and not the room.

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Standing waves have very little to do with headroom and overall output. My reference to PWK was about a Dope from Hope article or maybe an AES article where he talks about the mathematics of damping and his obsession with efficiency.

 

The reason few companies dampen the interior of the cabinet is because it saves money. The audible effects are secondary to other larger concerns; same story with baffle edge diffraction.

 

It is however very easy to measure internal standing waves (and baffle effects). It is also easy to hear the difference when they're mitigated. These things are in the last several percent of what matters to the overall sound quality. And it doesn't take much to get enough damping (point being don't over dampen because that's a waste of energy.... again back to the last percent of performance).

 

The audiophile path is about tweaking out every last drop of performance. The speaker business path is about performance per dollar. That dollar might be better spent in other ways from an engineering perspective - especially in the pro cinema world where the effects are much harder to hear and dominated by the big screen in front of the speakers.

 

On the other side of the coin, it's really cheap and easy to stuff a cotton pillow into a cabinet to hear the differences. If you want to be fancy you can use Owens Corning 703 and install it all proper like.

 

I spent years measuring magical speaker cabinet shapes. They all resonate and have standing waves. Even the ones with magical shapes based on magical equations still resonate. The most direct engineering solution is damping. I wish it was a cooler story but that's the way it is.

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9 minutes ago, DrWho said:

I spent years measuring magical speaker cabinet shapes. They all resonate and have standing waves. Even the ones with magical shapes based on magical equations still resonate. The most direct engineering solution is damping. I wish it was a cooler story but that's the way it is.

 

https://stason.org/TULARC/entertainment/audio/general/12-24-What-is-the-best-stuff-to-fill-a-speaker-cabinet-wit.html

 

I used Fiberloft Mountain Mist in my latest project. It only cost $15 for 2 lbs, which was just right for 3.8 cubic feet. It was worth it.

Edited by Edgar
Correction

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16 hours ago, Dave A said:

I am interested in why. All these things contribute but which are worth pursuing?

 

4 hours ago, DrWho said:

The audiophile path is about tweaking out every last drop of performance. The speaker business path is about performance per dollar.

I was gonna say: There are 2 pursuits in speaker making, making a speaker sound good, and making a good speaker sound its best. The first is about profit, the second is a labor of love. I think part of the reason why we're all here (in this forum) is because we believe, and can hear, that for the us, it's the latter.

 

If you have the time and inclination, I think it's worth it to find out which aspects of standing wave control are worth pursuing. With the work you've done so far and the improvements you've made, I you're definitely on to something, and I think the lot of would be very interested in what you find.

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