Port Tubes

Randy Beavers · Post by **Randy Beavers** » 13 Jul 2004 4:04 am

I have one of the light weight speaker cabnets Duane Marrs builds. It is a closed back, and it has 4 2" holes in the speaker baffle board to let the sound from the rear of the speaker get out. It had a nice bass response the way it was but...

I was on the Parts Express web site and was reading about fine tuning an enclosures response with the use of port tubes. I called their customer service for details. After giving him the details of the enclosure he said I had a severe roll off in bass responce at about 200 hertz. He told me for the size enclosure I have, I should put four 1-7/8" port tubes in, that are 4-5/8" long to get the ultimate response. What they sold was 1-7/8" X 4". He put that into his program and said with those the bass roll off would occur at 62 hertz rather than at 58hertz with the 4-5/8" long tubes. I don't think I could hear that much difference, especially with steel.

For the test. Rather that drive them in all the way I just pushed them in about half way so I could take them back out for comparrison. What I can tell you is the difference was dramatic. Maybe someone here can explain why that sound going through those tubes increases the bass responce. I know Bose has made a fortune with this technology. The tubes cost about $1.50 each. Probably the best $6.00 I've spent in awhile.

Paul Graupp · Post by **Paul Graupp** » 13 Jul 2004 5:15 am

Randy: I may be wrong but my feeling is that it relieves the forced cancellation of the waves within the cabinet. I tried to get around that by using many speakers instead of just one, or two. I used 8x5" Bose type speakers in each closed back cabinet for a more complex wave action within the cabinet to reduce that cancellation of equal and opposite waves. That got a good bass responce from them and open backs showed a similar curve.

Regards, Paul

David Doggett · Post by **David Doggett** » 13 Jul 2004 6:37 am

Randy, this is very interesting to me because I also just got a couple of Marrs closed back cabinets for 15" speakers. I have been using two larger closed back reflex slot Thiele type cabinets with JBLs. I think they are bigger and heavier than I need for pedal steel, so I'm hoping the smaller Marrs cabinets are adequate.

The first thing I noticed is that the Marrs backs are very flimsy 1/4" plywood. This can't be good. Even bookshelve home stereo speaker cabinets are typically made of very rigid 3/4" material. I immediately made some thicker backs. I just painted them flat black and don't intend to cover them with tolex. I haven't tried them yet, but plan to do an AB comparison with the original thin backs.

I too worry that those four tubeless front ports allow too much cancellation of the speaker output spilling over between he front and back of the speakers. But reflex tubes not only prevent such cancellation, they also can provide a resonance that is tuned to just below the frequency where the low end roll off occurs. By reinforcing the frequencies around that point they extend the roll off to a lower frequency.

It is not clear to me how much of a problem this is for pedal steel. It is a major factor for bass guitar or a speaker intended for full range sound such as a PA or home stereo. But even the lowest strings of a pedal steel may not have fundamental frequencies at or below the roll off frequency of the cabinet. I will check some frequency charts to see what the lowest frequencies are for a C6 or uni. Do you know what the roll off frequency is for the Marrs cabinets and a typical 15" speaker?

Jim Smith · Post by **Jim Smith** » 13 Jul 2004 7:03 am

Randy, please keep us informed. My Stereo Steel speaker cabinets also have the four holes in front, maybe they're the same design as the Marrs cabinets.

I have no complaints about the bass response, but every once in a while, I'd like to show the bass player up.

Larry Moore · Post by **Larry Moore** » 13 Jul 2004 7:13 am

Randy,
What is the size of the Mars Cabs.?
I am thinking about making a pair the same size of the NV400. Would this same size port work,if I closed the back of them?
Larry

Jim Smith · Post by **Jim Smith** » 13 Jul 2004 7:37 am

There are pictures and dimensions on Duane's site here: http://www.duanemarrs.com/Products.0.html

C Dixon · Post by **C Dixon** » 13 Jul 2004 8:20 am

"porting" a speaker cabinet is probably one of the most scientifically challenging things I know of. Literally volumes have been written about it.

Of course the "answer" to the engineer's dilemma of course is the computer which can easily make millions of computations in less than a second. A feat that would require 100's of engineers 100's of years to do.

The science of sound and sound perception, particularly when it comes to speakers and speaker enclosures, is sooooo very complex. And many things one does not even suspect can make a decided difference. This of course I believe is what happened to Randy Beavers in what he and that phone call accomplished.

By the way folks I want to take the opportunity to bring those of you who may not know it, some history concerning this incredibly talented steel guitar player name a "Randy Beavers".

Many years ago at the old Chase Park Plaza hotel at the ISGC, I rented a room next door to Randy. I did not know him from the ole proverbial "Adam's housecat". In fact I still have never met him personally. Which I am sad about.

Anyway, I kid you not; this kid (and he was at that time) played ALL night long. Yes, all night long. I was reminded of what Buddy Charleton once said about Buddy Emmons and how he would practice endlessly hours on end.

Well Randy was no different. And all I could think of listening to these repetitive sparkling and crystal clear stacatto notes emanating from his speaker, "that person is going to be great one day."

My thoughts have been fulfilled. Randy was of course a protege' of the late and great Zane Beck. In fact, the first time I recall Randy appearing on the stage was with Zane in Stlouis. Again, when I realized it was he who had dazzled me with "sound spectacular" all night, I said it again.

"That kid is going to rattle their timbers one day".

And has he ever. Randy cut his own path. Yes he was a child protege' of Zane. But Jesus rest Zane's soul, I am sure he is proud as can be at how far Randy has progressed in the realm of what we all love sooo dearly.

Every note is a clear as a bell. And with soooooo much taste. Let me describe it. I will repeat an 'oft used phrase on this forum. "Some just shoot 22 rifle shots. In other words they ain't hittin nuthun', they are jes shootin".

Well this does NOT describe Randy Beavers. When Randy plays the fans LISTEN! And the notes do indeed say something. Musically too, like few others.

Many in the steel guitar world are very proud of you Randy. There is little likelyhood I will ever have the pleasure of meeting you, and that is sad. Because had I, you would be very high on my cherished lists of my all time greatest aquaintances in the world of steel guitar.

May Jesus richly bless you for what you have done. He has inately gifted you with something all the money in this world could not buy. I trust you will never fail to thank him for what he has given you. And may Jesus richly bless all the rest of you always,

carl

David Doggett · Post by **David Doggett** » 13 Jul 2004 8:26 am

Okay, this is easier to figure out than I thought. The A above middle C is 440 hz (duh). The A hit with the boowah pedal on C6 is two octaves below that, or 110 hz. So we don't need to worry about reinforcing roll off below that. The guy who was calculating the length of the port tubes for you was looking at the the natural roll off frequency for a Marrs sized box and the natural resonance frequency of your 15" speakers. Maybe you could go back and ask him for that frequency. If it is below 110 hz, reinforcing it with reflex ports wont help much.

Okay, I see in your post the tubes you got will reinforce roll off around 62 to 58 hz, depending on length. This will not necessarily help the lowest notes a pedal steel guitar can make, which as I calculated above are around 110 hz. On the other hand, roll off is not at a precise point. On frequency charts you can see that it is a rounded shoulder. More important, the frequencies might not be perfectly flat just before roll off. If not, then it is possible that reinforcing below the lowest note of your instrument will help flatten out the frequencies around the your lowest notes and help the sound there. It would still be nice to know the resonant frequency of the Marrs-JBL combination.

The fundamental frequency of the lowest string on a bass guitar is 82.4 hz. A bigger box with a lower natural resonance and reflex ports that reinforce frequencies down to 83.4 hz make a big difference for bass guitar, or for a PA speaker that is used to mike bass guitars or bass drums. [This message was edited by David Doggett on 13 July 2004 at 09:37 AM.]

Brad Sarno · Post by **Brad Sarno** » 13 Jul 2004 8:38 am

David, I believe that you may be off by one octave. Isn't the low E on a bass 41.7Hz? And isn't the low A of a boowah pedal at 55Hz? I thought the low E on a regualar 6 string guitar was at 83.4Hz.

The port tube tunes the resonant frequency of the cabinet by lenghthening the path thru which the waves must pass. I've seen people mess with telescoping port tubes where one tube fits tightly inside another. Then you slide the inner tube back and forth until the bass sounds right.

Brad Sarno

William Peters · Post by **William Peters** » 13 Jul 2004 9:07 am

There is a pretty neat free program to calculate speaker enclosures call WinISD. It has an pretty good library of speaker types, and plots gain, phase, etc.

You can download it from this site...
http://www.linearteam.dk/

------------------
Bill

http://www.wgpeters.com
Cougar SD-10, PV-260, Tubefex, PV TNT-115, Gibson SG, Squier P-Bass, Berhinger V-amp Pro

David Doggett · Post by **David Doggett** » 13 Jul 2004 9:46 am

Yeah, Brad, I think you are right. I think a regular guitar goes from E above middle C to two octaves below that, which would be 82.4 hz. So the A of the boowah pedal would be 55 hz, and the low string on a bass guitar would be 41.2. So the numbers the Parts Express guy was giving Randy, in the low 60s and high 50s, would be right around the lowest strings on a pedal steel, and boosting those with reflex tubes should make a noticable difference. Randy's experience seems to verify that.

I just placed an order for some tubes that are 2 1/8" ID and 5" long uncut. I'm hoping the exact size of the tubes is not so critical and maybe anything in the right vicinity will help flatten out the lows.

C Dixon · Post by **C Dixon** » 13 Jul 2004 9:47 am

With sincere respect for Dave, While a fundamental frequency may endeed be above the cutoff frequency of a speaker and/or its tuned port cabinet, there ARE sub harmonics that come into the equation.

This same phenomenon occurs in the other direction. IE, the highest frequencies of a steel guitar are not very high in the realm of 20 to 20KHZ, but the overtones (harmonics) of a steel guitar in both directions (sub and positive) extend much further than the fundamental frequencies.

carl

Brad Sarno · Post by **Brad Sarno** » 13 Jul 2004 10:34 am

Yea Carl, I agree. To me the steel guitar is the ultimate overtone generator! That's why tuning is such a hot issue. Not only are we concerned with our fundamental harmonies, but also with the complex array of overtone harmonies. When the the overtones harmonize, that's when the steel can make us cry, or at least be hypnotized. Like you said Carl, that happens both above and below the fundamental notes.

Brad Sarno

David Doggett · Post by **David Doggett** » 13 Jul 2004 12:34 pm

Well I was just thinking of first things first. If the fundamental frequencies are not flat, there is no hope for the harmonics. Obviously higher harmonics are important for the sound texture or tone, but I never thought much about harmonics below the fundamental. Unless they are tuned to reinforce the low at the roll off, they might be undesirable in speaker cabinets. I guess in a sense the beats we listen to while tuning are subharmonics, but do you really get hearable subharmonics from a single string the way you get higher harmonics?

C Dixon · Post by **C Dixon** » 13 Jul 2004 1:04 pm

Good question Dave, but consider this. When we tune our A (6th string B pedal down) to 440, then try to tune our A pedal C#, most PSG player's ears WANT to hear 550, not 554.37 (440 ET reference). Now follow along.

WHY do we want to hear 550?

If you think about it for a moment, that number is arrived at (and it is flat by almost 5 HZ) by dividing 440 once, then again obtaining the 2nd sub harmonic, added back to 440 equals 550*.

IE, 440 divided by 2 equals 220 divided by 2 equals 110, added back to 440 equals 550.

So even as high as 440HZ, a speaker would HAVE to reproduce a minimum of 110HZ cutoff for those combined strings to be reproduced faithfully by a speaker and/or its enclosure.

It is this, as Brad so poignantly points out, that drives many a steel player crazy trying to tune our confounded tinkertoy. While we may NOT be able to pick out the "subs", nonetheless our brains certainly do, as the above clearly shows.

carl

*Note: this same phenomenon occurs for our E note referencing the A at 440. IE, divide 440 once getting 220 now add it back to 440 equals 660. Steel players ears insist on wanting to tune our E's respect to our A's slightly sharp of the standard (ET) E note=659.26.

If a regular guitarist did this, an E chord would sound absolutely beautiful, IF, the G# note was also tuned beatless-almost 5HZ flat as shown above; but a C chord at the same time would sound absolutely horrible.

The above shows why.

Randy Beavers · Post by **Randy Beavers** » 13 Jul 2004 3:20 pm

Carl, a sincere thank you! Someday we will meet. This time I promise to not keep you up all night. These days I go to bed around 9:00pm. However, I may get you up very early! I'm usually sitting at my guitar around 6:00am.

The customer service rep I talked to at Parts Express said from the dimensions I gave him, along with the speaker combination, D-130F, the roll off was starting to occur around 200 hertz. That's pretty far up into our register. It's not very hard to hear the difference when doing the comparison.[This message was edited by Randy Beavers on 13 July 2004 at 04:27 PM.]

C Dixon · Post by **C Dixon** » 13 Jul 2004 4:59 pm

Randy,

You may keep me up ANY time you want to. It did not bother me then and it will not now. You play what is in my head. I love it.

bless you friend,

carl

John Daugherty · Post by **John Daugherty** » 16 Jul 2004 11:45 am

What a complicated subject. I might add that before you even TRY to calculate what is happening inside the speaker enclosure, you must eliminate the reflections. Insure that you have a lot of sound absorbing material in that cabinet and build the cabinet from material that will not vibrate and produce audio frequencies.
If I remember correctly, The dimensions for an anechoic chamber are 1 x 2 1/3 x 2 2/3.
I think most of the present knowledge about speaker cabinets came from making changes to the cabinet, then obtaining a response curve. ....... JD

John Daugherty · Post by **John Daugherty** » 23 Jul 2004 8:55 am

Randy, did you notice if the Marrs speaker cabinet has any sound absorbing material inside? ........ JD

David Doggett · Post by **David Doggett** » 23 Jul 2004 9:56 am

John, the Marrs cabinets have some sort of sound absorbing material on the interior of the sides, top and bottom. It is some sort of synthetic board that has a consistency somewhere between styrofoam and foam rubber, and it is covered with some type of foil. This seems to be designed to absorb lows, but to reflect highs.

The 2 1/8" ID tubes came and they are too big for the 2 1/4" holes in the cabinet. I ordered the next smaller size, and also one adjustable telescoping tube for each cabinet. Seems like the best way to tune these cabinets is by trial and error with these adjustable tubes. I'll report back the results when I try this. [This message was edited by David Doggett on 23 July 2004 at 11:02 AM.]

John Daugherty · Post by **John Daugherty** » 23 Jul 2004 10:46 am

David, The higher the frequency the easier it is to absorb (damp). I doubt that the low frequencies you are interested in are affected by this material. ....... JD
Thanks for the info.

Steven Welborn · Post by **Steven Welborn** » 23 Jul 2004 9:03 pm

Randy, I had the good fortune to make the 1rst Mesa,Arizona convention. I was inspired by your obvious dedication and mastery. The sound of your guitar and rig was the favorite to my ears. Were you using the Marrs cabs with the D130F's at that show if you dont mind my asking? Hope you come back out west again. Thanks.[This message was edited by Steven Welborn on 24 July 2004 at 05:34 AM.]

Randy Beavers · Post by **Randy Beavers** » 24 Jul 2004 6:49 am

The material Duane uses to dampen with, I believe is the insulation that is used on the side of a house under the vinyl siding. It looks like expandable foam with a foil backing.

Steven, many thanks. I had a great time in Mesa, and Judy and I looking forward to returning in January. On that show I played through Jeff Newman's Peavey 1000. All in all, still my choice of amp for most every situation. I am going to have to try the 1000 through the Marrs enclosure. I really like the idea of splitting up the combo by putting the speaker in a separate cabinet. Who wants to carry around a 70 to 80 pound amp anymore?