Designing Locomotive Acoustics for On-Board Sound Systems

ModelRailroadForums.com is a free Model Railroad Discussion Forum and photo gallery. We cover all scales and sizes of model railroads. Whether you're a master model railroader or just getting started, you'll find something of interest here.


R

railandsail

Guest
Sound from Front of Tender?

As I re-read this PDF document several times I became aware of a possible speaker installation that was not covered in this document....excerpt:

"Separate back wave from front wave: It is important for sound quality that the inside cab area be sealed as much as possible to prevent the back wave from escaping through grills, vents, fans or other openings in the diesel or electric locomotive cab or through the openings in the chassis where the motors connect to the trucks. If there is any leakage of the back-wave to the outside, it will mix with the speaker front-wave and cause destructive inference in some base tones and perhaps constructive interference for some of the higher frequency tones. The respective path lengths for the front-wave and the escaping back-wave and the position of the listener will determine which frequency components are degraded or changed. Usually, since the acoustic chamber and path lengths are short, any back wave escaping from the locomotive causes degradation of the sound. If the back wave is allowed to escape close to the front-wave, the degradation is more severe. As the scale of the model decreases, this becomes more of a problem since the distances between front and back waves become smaller.

Vent the front-wave sound under the locomotive: Propagating sound upward into the open air seems to produce lower quality sound unless you are directly over the speaker. The sound has no opportunity to reflect against different parts of the layout such as buildings, mountains, etc. that add both volume and presence. Our experience is that the best design choice is to propagate the sound under the locomotive. The next best choice is out the sides of the locomotive through vents and grills. If sound is vented under the locomotive, always be aware of the affect of trucks and other obstacles and other factors that can either improve or degrade sound quality. Do not vent sound straight down too close to the track where it can be reflected back and decrease volume and sound quality. Venting the sound directly under the fuel tank usually does not produce good sound in smaller gauges (HO and N) since there is little space between the bottom of the fuel tank where the sound is vented, and the top of the track. Sound usually reflects back from the track resulting in poor volume and presence. Venting through the gear tower and chassis over the open truck areas seems to produce the best sound."


What I find interesting here is no mention of venting the 'front-wave' from the front of the tender, verses out the bottom, or out the coal load?? Why does this interest me? I have at least 4 locos with vandy tenders (C&O) that I wish to install sound into. He covers vandy tenders to some considerable degree, but does not mention this possibility.

I happen to have a friend who has a relatively cheap little IHC steamer that has its sound vented out the front bulkhead of the tender right into the cab area of the loco....the sound is GREAT. On the other hand I have a 2-8-8-2 with the sound vented out the coal load that I consider substandard. So I am giving real serious consideration to experimenting with one of my Spectrum C&O Heavy Mountains with their vandy tenders and venting the sound forward out of the tender.

Has anyone else seen such an installation? Does anyone have a photo and/or illustration of the 'factory sound' installation in these Spectrum models??
 

hminky

Member
Sounds like "overthink" to me. Since the locomotive is moving the sounds don't reflect back that much. If an article doesn't have pictures of installations then it is just armchair theory. Ya gotta have pictures otherwise it is just "sound":eek:.



I got pictures
Harold
 
R

railandsail

Guest
Sounds like "overthink" to me. Since the locomotive is moving the sounds don't reflect back that much. If an article doesn't have pictures of installations then it is just armchair theory. Ya gotta have pictures otherwise it is just "sound":eek:.
Are you saying that the article I referenced is just 'armchair theory' as it contained just illustrations rather than pictures??

Yes, it may be a little 'overthink', but I once sold a boat to the two guys who own Polk Audio. I don't think they would term speaker technologies as overthink. The greatest amps in the world are useless unless they 'speak properly'. ;)
 

hminky

Member
Yes unless it contains pictures of actual installations where the writers have compared actual application it is just arm chair theory.

Like the illustration of having multiple smaller speakers will not improve the sound. The sound will be the same as a single speaker of that size, you need dissimilar speakers. I am told that is wrong, but I know it is true because I have an actual application. Speakers in locomotives isn't rocket science.

Harold
 
R

railandsail

Guest
Speaker Orientations in some Diesels ??

...from an emailed question I wrote recently

I was just listening to your 'sound samples', and had a question about the EMD 567 Sound of the F7 locos. (BTW, great presentation :cool: )
http://www.ulrichmodels.com/Sound_Samples/SoundSamples.htm

I assume the speaker is installed similar to the factory MRC installation on the rear of the metal chassis. But I believe I have been exposed to two different speaker orientations:
1) the front wave of the speaker faces downward and the rear face of the speaker is cover by a round plastic back-wave cavity enclosure.
2) the front wave of the speaker faces into the round plastic back-wave cavity enclosure while its rear face faces downward in the loco.

I would think the second orientation would be less effective than the first ?? Doesn't the already small front face of these small speakers produce a much more positive sound than the back face that is greatly cover by the metal frame of the speaker itself ?? Why would one install these small speakers 'backwards' ??

I've run into a similar situation for the P2K PA/PB locos where the speaker has been installed in a relatively shallow box up in top of the shell. First off in one case its installed with the front wave of the speaker projecting down into the shell. In the second case the speaker's front face projects into the small cavity box.....Strange??
And in both cases I would question the size of the 'back-wave' cavity enclosure box. It seems so shallow as to not really allow for full speaker excursions. Shouldn't these back-wave cavities be 'allowably large' so as to not impend full speaker excursions??
 
R

railandsail

Guest
Speaker into P2K E8

....I've run into a similar situation for the P2K PA/PB locos where the speaker has been installed in a relatively shallow box up in top of the shell. First off in one case its installed with the front wave of the speaker projecting down into the shell. In the second case the speaker's front face projects into the small cavity box.....Strange??
And in both cases I would question the size of the 'back-wave' cavity enclosure box. It seems so shallow as to not really allow for full speaker excursions. Shouldn't these back-wave cavities be 'allowably large' so as to not impend full speaker excursions??
....from another forum...
With the Revolution I installed the QSI HB106R extended bass speaker and the QSI baffle for it in a P2k E8, pointed down at the opening behind the front truck. Although I was pleased with the frequency response over all, I thought it was a little too "plastic" sounding. I covered the back (top) and 1/2 of the sides of the baffle with museum putty. It helped the over all tone quite a bit. If there is a reduction in bass response, it's hard for me to tell, cause of the improvement over that plastic sound. Someday I want to try a baffle made out of a tube of cardboard, fiberglass or G10. I can't "seal" up my shell, cause I need those top vents and rear door open for cooling the RC board that runs the motors and the Revolution.
 
R

railandsail

Guest
Speaker Orientation, front-wave vs back-wave & shell vs fuel tank mounting

...from an emailed question I wrote recently

I was just listening to your 'sound samples', and had a question about the EMD 567 Sound of the F7 locos. (BTW, great presentation :cool: )
http://www.ulrichmodels.com/Sound_Samples/SoundSamples.htm

I assume the speaker is installed similar to the factory MRC installation on the rear of the metal chassis. But I believe I have been exposed to two different speaker orientations:
1) the front wave of the speaker faces downward and the rear face of the speaker is cover by a round plastic back-wave cavity enclosure.
2) the front wave of the speaker faces into the round plastic back-wave cavity enclosure while its rear face faces downward in the loco.

I would think the second orientation would be less effective than the first ?? Doesn't the already small front face of these small speakers produce a much more positive sound than the back face that is greatly cover by the metal frame of the speaker itself ?? Why would one install these small speakers 'backwards' ??

....And in both cases I would question the size of the 'back-wave' cavity enclosure box. It seems so shallow as to not really allow for full speaker excursions. Shouldn't these back-wave cavities be 'allowably large' so as to not impend full speaker excursions??
....interesting reply from Steve at Ulrich Models

There are rules in the speaker game but there also is a large abstract area where rules do not seem to apply. I have been playing around with speaker combinations for two years and I still find trial and error the best method. This is what I do know about the MRC speaker and baffle. It is an outstanding combination and out performs almost any other round 28 mm speaker and baffle I have used, except for the QSI large base speaker. It has more base and more volume than the other baffle combination I have tried. I think there are two reasons. (1) the speaker is sealed into the baffle with a sticky gasket. (2) I think the rounded corners also help. To tell you the truth, it works so well, I have not flipped the speaker to try it. Also, it comes as a sealed unit. The seal is really important as any leakage can destroy the base response. The exception is a well designed port. It also nice that the wire terminals are exposed in the face-down orientation.

In the case of the Loksound decoder that uses a 100 ohm speaker, I removed the MRC speaker and installed a 28 mm Loksound round speaker with a new gasket into the baffle. The combination out performed the LokSound speaker in its factory baffle significantly. I then tried flipping the Loksound speaker in its factory baffle to be like the MRC, but there was no improvement or degradation. So far I have not seen much difference based on the speaker orientation in the baffle. I wish I could take credit for finding this speaker but it was suggested by one of my customers.

The other thing I have found is that placement of speakers inside shell is important. I have some oval enhanced base speakers that I put into SD70Ms and such. In the Athearn, I thought they would sound best with the speaker surface pointed out the radiator fan openings. However, I later flipped the speaker over and pointed it down at the trucks. There was a significant improvement. I believe it is due to the shell acoustics and the sound resonance from the shell itself. When the speaker was open to the air, I did not get any of those effects.

I am afraid that I cannot explain the physics behind the results. I only know what seems to work. Too bad for MRC. Nice speaker and lousy decoder. I hope they keep making the enclosures.

As a rule, the decoders will sound even better when you have the locomotive in front of you and on the track than in my presentation. I did everything I could think of to obtain real sound. I use a professional mike and a preamp but there is nothing like being there.

http://www.ulrichmodels.com/Sound_Samples/SoundSamples.htm
On this page, note this reference, "An Enhanced Base Speaker, 40 mm x 18.5 mm, was mounted in the shell for these samples. This speaker and enclosure produces better sound than the tank mounted speaker and eliminates the problem of metal debris being picked up by the speaker. Click here to see a typical install. "
 

kenw

5th Generation Texian
Sounds like "overthink" to me. Since the locomotive is moving the sounds don't reflect back that much.
actualy the sounds waves travel a wee bit faster than the locomotive, so yes, they do reflect back.

IMHO, yes, it is a bit of overkill; however, I have seen and heard the improvement gained from a bit of engineering of small speakers in notebook computers, so I know it is real. For the most part a sealed box with an open front is going to give you the biggest bang of the buck with these extremely low power (low movement of diaphram) speakers.

Porting the rear (venting, whatever) calls for some pretty high powered science and the wrong size, shape, length of the opening will make it less effective. iow, it's easier to get wrong than right.
 
Interesting reading.

Yes unless it contains pictures of actual installations where the writers have compared actual application it is just arm chair theory.

Like the illustration of having multiple smaller speakers will not improve the sound. The sound will be the same as a single speaker of that size, you need dissimilar speakers. I am told that is wrong, but I know it is true because I have an actual application. Speakers in locomotives isn't rocket science.

Harold
The example of using multiple smaller speakers was not to get improved sound over a single speaker(or actually two in their example), it was to get the same speaker area in a narrower space. The example was a somewhat rounded tender and if you mounted the speakers half way up in the tender you could fit 32 mm speakers, but if you mounted them on the bottom of the chassis, you could only fit 18 mm speakers.
 

beiland

Well-Known Member
...
What I find interesting here is no mention of venting the 'front-wave' from the front of the tender, verses out the bottom, or out the coal load?? Why does this interest me? I have at least 4 locos with vandy tenders (C&O) that I wish to install sound into. He covers vandy tenders to some considerable degree, but does not mention this possibility.

I happen to have a friend who has a relatively cheap little IHC steamer that has its sound vented out the front bulkhead of the tender right into the cab area of the loco....the sound is GREAT. On the other hand I have a 2-8-8-2 with the sound vented out the coal load that I consider substandard. So I am giving real serious consideration to experimenting with one of my Spectrum C&O Heavy Mountains with their vandy tenders and venting the sound forward out of the tender.

Has anyone else seen such an installation?
Here is another 'front of the tender' venting reference onboard an Athearn Genesis steam loco:
http://www.modelrailroadforums.com/forum/showpost.php?p=161942&postcount=6

..and a Youtube reference here:
http://www.modelrailroadforums.com/forum/showpost.php?p=125220&postcount=1
 

beiland

Well-Known Member
Single Oval Speaker vs Dual Round Speakers, Hi-Bass

I recently ran across this comment over on 'wiringfordcc.com'

"SUGGESTION 11-20: Finally! A Good Speaker for HO I Like!

The speaker is the Intervox 511x16VN. It is an 8 ohm, 1.2 watt speaker. It will fit in most HO tenders other than maybe some small switchers.

It is an oval shaped speaker with rectangular frame. The oval gives you all the speaker surface area possible that will fit in an HO tender. This maximizes the volume and bass.

It can handle the full power output by Soundtraxx modules, as well as many other sound products out there, without audible distortion that otherwise ruins your locomotive's sounds.

Here's the best part. A speaker's surround, the part of the speaker that attaches the cone to the speaker cage, is critical to a speaker's ability to move air. After all, moving air is what making loud sounds and good bass is all about. Most small speakers had a surround that clearly don't allow much travel. (Other things limited their travel as well.) This speaker is capable of significant travel. You want this! Frequently, speaker frames don't include enough depth for all the travel the speaker may cover. This is because the extra depth is usually provided by the mounting enclosure in some way.

I was quite happy with the performance of this speaker when used singly. I saw no real need to hook additional speakers together in any of the serial or parallel combinations "


This was an older submission (2002) so I wonder how applicable it might be, and if there are newer oval speakers that might offer comparible, OR better results??

I'm beginning to appreciate the possibility of not having to put multiple speakers in serial or parallel combinations if possible, and still get good sound. The serial or parallel combinations seem to make some decoders work too hard, or create more heat, when most are designed to look at 8 ohms
 
Last edited by a moderator:

beiland

Well-Known Member
Speaker Enclosure Considerations

Here are a few pictures of how I added the QSI High bass speaker.
Thanks very much for those pics.

I would offer a copy of observations from my reading and research so far. I quote from Litchfield Station, "As a general rule, the larger free air volumn in the enclosure, the more efficient the acoustic conversion. Thus keeping the speaker magnet outside the enclosure will help the efficiency". (in other words relying on the back side of the speaker for the sound rather than the front side)
This would be particularly true where one was utilizing a h-bass speaker with its large deep magnet structure into a limited space enclosure speaker 'box'. The back side of this speaker eats up a lot of that free space you were needing to get a proper size enclosure.

If at all possible I would prefer to get my sound from the front face of the speaker unimpeded by the physical structure of the speaker magnet/cone support. So where possible I believe it is wiser to try and use the shell structure of the tender/loco as the 'enclosure' for the speaker, and face the front face of the speaker out of the train structure.

Custom styrene box, gasket and QSI speaker. Sort of a mini sub-woofer box
From that discussion above it might suggest that your hi-bass installation in that box might have been less than optimal.?

I keep imagining that confined air inside the 'enclosure' for our speakers. If it is too small a volume I would imagine it might try to restrict the excursion of the speaker cone to some degree as this speaker movement tries to compress and decompress that air in that enclosure in a rapid sequence. I wonder in cases where the enclosure is smaller than we would like, if a small pin hole in the enclosure would be a good thing to allow for this 'pressure relief' without allowing the back side sound to cancel out the front side sound?
 

RexHea

RAIL BENDER
Cheap baffle suggestion without rocket science: Use a standard plastic medicine bottle cut down to fit space. A 1" round speaker will fit the standard bottle, but I am not sure about speaker size for the larger bottles (you can always make a styrene adapter plate). It will have plenty of air space in rear of speaker providing very good sound...for the size...and it is an easy installation ;).
 

beiland

Well-Known Member
The photo film canisters work well also as an 'enclosure'.

But, usually the problem is more of 'how to fit that bulky cylinder into a space in the loco'.
 

beiland

Well-Known Member
Bass Reflex Speakers, Ported Speaker

....
Porting the rear (venting, whatever) calls for some pretty high powered science and the wrong size, shape, length of the opening will make it less effective. iow, it's easier to get wrong than right.
I'm recalling some of this speaker technology I once read about back in college when I was seeking to get the best hi-fi set up. I believe it was termed 'bass reflex' and involved having another 'ported tube' from the interior of the speaker enclosure that vented to the front of the speaker, ie:
http://en.wikipedia.org/wiki/Bass_reflex

Actually I think one would find that many of todays home stereo speakers still utilize this ported concept.

So here is one model railroad sight that is offering such speakers:
http://www.railmasterhobbies.com/Speakers.htm
...quoting their site...
Bass Reflex Speakers
There speakers are self contained (sealed) and ported. The port allows just enough air to reach the back of the speaker to give a better bass response than standard speakers


Has anyone here had occassion to experiment with these speakers??
 




Affiliate Disclosure: We may receive a commision from some of the links and ads shown on this website (Learn More Here)


ModelRailroadForums.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com

RailroadBookstore.com - An online railroad bookstore featuring a curated selection of new and used railroad books. Railroad pictorials, railroad history, steam locomotives, passenger trains, modern railroading. Hundreds of titles available, most at discount prices! We also have a video and children's book section.

ModelRailroadBookstore.com - An online model railroad bookstore featuring a curated selection of new and used books. Layout design, track plans, scenery and structure building, wiring, DCC, Tinplate, Toy Trains, Price Guides and more.

Top