Large Scale Central

An experiment in porting for better sound

Electronics Power and Sound
#1

The thing that’s missing in large scale sound is bass frequencies–small speaker, small enclosure equals weak bass. It’s the law of physics. To improve bass response in small enclosures like bookshelf speakers or table radios, speaker designers generally go with a ported cabinet–a cabinet with a small hole in it. In the hole is typically a cardboard tube, say 1 inch in dia. x 2 in length. The port is usually “tuned” to emphasize certain frequencies, by adjusting the length and the diameter. There are standard parameters for doing this. But the basic principle is if you want small and loud with good bass response, you go with a ported speaker. I know this not as a speaker designer, but as a semi-pro bass player So large scale sound–you have small enclosures and small speakers. Why not port the enclosure? So I did–I recently installed a QSI soundcard in an Aristo Pacific. I put the speaker in the loco up by the smokebox, under the boiler shell, and then put a port in by drilling through the backhead and the back wall of the boiler shell. (I know, but I had already drilled a hole through the backhead for an aristo code set switch, so I just made it bigger.) Then I put a piece of scrap brass tube about 1/2 by 2 though the hole and hot glued it in place

"img> Here’s a sound comparison. For the first clip, I plugged up the hole with tape, so this is the sound with no port http://chnm.gmu.edu/courses/magic/westover/plugged.mp3 For the second clip, I opened the port http://chnm.gmu.edu/courses/magic/westover/ported.mp3 These are downsized mp3s, so some of the difference has been lost, but to my ears the ported cab is much much better. It’s hard to tell on small speakers and mp3, but it seems apparent to me. Try listening to it with a decent pair of headphones. If anyone is interested, I can post the full size .aiff file

#2

Ported speaker cabinets require less power than a sealed cabinet to produce the same number of decibels.
As a speaker cone moves back and forth pressure builds up behind the speaker cone requiring more effort to move the cone.
The port in a Bass Reflex enclosure allows this pressure buildup an avenue of escape. Tuning is an effort to keep this pressure from introducing unwelcome frequencies.
Acoustic Suspension cabinets, which have no ports, are much more accurate but require more power.
I’m sure your loco port is indeed giving you an increased volume. What is important is that it sounds better to you.
Ralph

#3
Ralph Berg said:
Ported speaker cabinets require less power than a sealed cabinet to produce the same number of decibels. As a speaker cone moves back and forth pressure builds up behind the speaker cone requiring more effort to move the cone. The port in a Bass Reflex enclosure allows this pressure buildup an avenue of escape. Tuning is an effort to keep this pressure from introducing unwelcome frequencies. Acoustic Suspension cabinets, which have no ports, are much more accurate but require more power. I'm sure your loco port is indeed giving you an increased volume. What is important is that it sounds better to you. Ralph
Ralph--there's not much volume increase to my ears, and volume increase was not what I was looking for anyway. It was low end response. I think it sounds much better, but it's always possible I'm imagining it!
#4
mike omalley said:
Ralph Berg said:
Ported speaker cabinets require less power than a sealed cabinet to produce the same number of decibels. As a speaker cone moves back and forth pressure builds up behind the speaker cone requiring more effort to move the cone. The port in a Bass Reflex enclosure allows this pressure buildup an avenue of escape. Tuning is an effort to keep this pressure from introducing unwelcome frequencies. Acoustic Suspension cabinets, which have no ports, are much more accurate but require more power. I'm sure your loco port is indeed giving you an increased volume. What is important is that it sounds better to you. Ralph
Ralph--there's not much volume increase to my ears, and volume increase was not what I was looking for anyway. It was low end response. I think it sounds much better, but it's always possible I'm imagining it!
Like I said..if it sounds better to you that is what matters. If you port a full range speaker.....it is most likely that the volume of all frequencies have increased. You may have "lucked out" and placed your port in some way that is benefiting the bass response more so than the treble response. Ralph
#5

It’s not entirely luck–I did do some calculations of the interior volume and the port size

Also in my experience, porting is less about volume than it is about low frequencies. Greater speaker efficiency is a gain from porting, but not the onlyh one

http://thespeakerguys.blogspot.com/2008/03/how-port-on-speaker-works.html

#6
mike omalley said:
It's not entirely luck--I did do some calculations of the interior volume and the port size

Also in my experience, porting is less about volume than it is about low frequencies. Greater speaker efficiency is a gain from porting, but not the onlyh one

http://thespeakerguys.blogspot.com/2008/03/how-port-on-speaker-works.html


Cool,
You did your homework.
I’m glad it worked out as planned for you.
Ralph

#7

Mike,

Oh, yeahhhhh, I hear it tooooo… :lol:

Seriously, I do hear a difference, there is more base in the ported sample, even on Com Pewter’s speakers.

My speakers for my mallet and my mikados are in the Vanderbilt tenders. What would happen if I took a 2 inch diameter tube, say of fiber board, and placed that between the speaker and the speaker grill at the bottom of the tender. Would that produce a similar effect?

#8

Steve:

I woulg guess yes–the tender seems like an ideal candidate for porting to me and it’s much easier to conceal the port. I’ve been meaning to try it for a while now and the QSI install gave me an opportunity

#9

Here’s a picture of my inspiration for this, the Kloss Model 1 table radio:

(http://www.ccrane.com/images/medium/tivoli-audio-henry-kloss-model-one-radio-beige.jpg)

I have one in my kitchen. It’s about the size of an aristo long tender. It gets remarkably rich sound–there’s no real bass, but there’s a lot of low midrange and it sounds deep. Low frequencies, like say a upright bass in a jazz combo–come out as a low hooting, like blowing across the mouth of a bottle. But it has a very effective sound overall, and when I opened it up I found just a simple 2 inch speaker. When I hear the phoenix card in my aristo long tender, it sounds good but it’s missing low frequencies, and it ought to be possible to improve it

#10

Somehow you ended up with two threads on this topic. I posted this on the other one before I noticed everyone was posting responses on this one.

Jon Radder said:
Nice work. Takes up a lot less room than my solution…

(http://www.cvsry.com/images/SoundCar-3-640.jpg)

#11

Dear Jon,

I was wondering where my speakers went !! LOL

Very imaginitive. (One could even say “Krafty”).

Dear All,

Back in the early 1980’s I passed an Acoustics prof in the hallway at school and asked him what the best way to make a speaker was (which could have been a 2 semester long lecture). His 5 second answer: take apart a DLK speaker and copy what they did.

As far as I can recall, a tuned port speaker has to be sealed (exept for the port hole) and the box has to be stiff.

Big driver + big box (tuned to driver) + tuned port (tuned to driver and box) = good bass response. The bigger the driver and box, the deeper (and louder) the bass response.

I made a 15" pair that had insane bass response. I gave up trying to get the mids and highs equalized out (lack of money and skill), so they ended up as bass cabinets on a separate (stereo) amp, loafing along at about one-tenth the power of the (smaller DLK’s used as) mid-high speakers and main (stereo) amp. The combination sounded fantastic to my ears.

As I recall, “Breakdown” by Tom Petty was often heard and sung along with (screamed?) at very high volumes. A drop or two of Bheer may have been spilled during those times.

What sounds good (is least tiring or irritating) to the human ear? (Psycho-Acoustics)

I’ve heard of a formula something like “lowest frequency x highest frequency = 400,000 with a smooth response in between”.

20Hz to 20KHz fits that formula. For a telephone earpiece that is limited to 3000 Hz on the high end, the low end must be around 133 Hz to not tire your ear.

I hate horn midrange drivers. They are efficient, but have an irritating spike in frequency response that bugs my ears.

Here is something new and interesting:

http://soundtraxx.com/documents/surroundtraxx%20sell%20sheet.pdf

Hope this helps.

Sincerely,

Joe Satnik

#12
Joe Satnik said:
Dear Jon,

I was wondering where my speakers went !! LOL

Very imaginitive. (One could even say “Krafty”).


When I first posted this on MLS the response I got was “Cheesy” so it has been known ever since as the Cheesy Sound Car. It has been moidified a few times since that photo and now works on either battery or track power.

BTW - I used to sell, rent and install pro-audio systems. Did PA work for several folks you have heard of, but got tired of being broke all the time so I got a regular job. Sound is still a passion, but I don’t do design work any more. Your theories are simplifications, but accurate. Each driver has unique characteristics that dictate the size of the box and the frequency to tune the port to. As you have learned, you can guess at it and get good results, but if you want the absolute most from any given driver, the enclosure needs to be tuned to the driver. Electro Voice used to publish specs for all of their commercial drivers and design tips on building tuned boxes for them. None of their stuff will fit in anything smaller than ride-on scale.

I don’t know how Radio Shack did it, but the sealed Minimus 7 and Minimus 3.5 speakers still sound great and compare favorably with more recent stuff for low power applications. I just installed a set of Mini 7’s I found in the basement into my camper for DVD / TV sound. They are 100% better than the stereo TV.

#13

Jon,
I also do PA work. So far,nobody you would have heard of.
I guess I better keep my day job.
Ralph

#14
Ralph Berg said:
Jon, I also do PA work. So far,nobody you would have heard of. I guess I better keep my day job. Ralph
Most of my clients were regional. The only national acts I ever got involved with were Rick James, Michael Walden and Blue Oyster Cult. I didn't do the PA for these bigger acts, just supplied some equipment when they were in town. I built some pretty impressive systems for the local bar bands though :D

Definitely keep the day job. When I look at my Social Security statement, I earned the lowest of my lifetime during those years. I made more doing landscape work part-time and summers while in school :o

#15
Jon Radder said:
Definitely keep the day job. When I look at my Social Security statement, I earned the lowest of my lifetime during those years. I made more doing landscape work part-time and summers while in school :o
I should have talked to you before I spent that big pile of money on that even bigger pile of equipment! Ralph
#16

Does anybody know where I can get small speakers AND the specs? There are lots of sites selling speakers but they never make the specs available

I’m interested in trying some more experiments with this partly because I’m a bass player, and I’ve been amazed at how good some of the smaller, lighter rigs are now. I came up playing big bass cabs–2 15s, 8 10s, and now I get away with small light stuff. My criteria now is “how does it sound at 2 am, when I’m loading it in my car.”

#17

Small speaker specs can be found here…
http://www.soundlabsgroup.com.au/c/80-Visaton-33/3.3+Inch+-+8cm.html
good little speakers & Soundlabs ships fairly quick!

#18

Thanks–although I’d prefer not to ship from Austrailia if I can help it

#19

Interesting discussion on improving sound here. Take a listen to the difference that Ray has managed to produce.

#20

(http://www.trainweb.org/rgs/tndr04.jpg)

(http://www.trainweb.org/rgs/tndr05.jpg)

All I did was use the biggest speaker I could fit in the tender. No enclosure or port, but I can’t turn the volume all the way up because sound overwhelms the engine. The speakers I got from Radio Shack, but Wal-Mart and K-mart both sell the same kind in the car audio dept. However car speakers are usually used in pairs and are 4 ohm so a 4ohm resistor may be needed to keep the sound from cutting off. At least, that’s what I’ve found. The RS speakers were 8 ohm. PS ignore the bowl of spaghetti beside the speaker…:wink: