Technology Overview
Units from all manufacturers fall into two catgeories - those where the sound of an engine is digitally synthesists from electronic waveforms and those that replay recordings of the sound of real engines. The former usually have the advantage of lower cost and the latter have that of increased realism. If you are a serious marine modeller to the extent that you research the number of rivets on the hatches then you need to dig into your pocket for a recorded sound variant!

Synthesized Sound
There is a wide variety of sound quality in the available units - ranging from those sounding like a buzzy bee or a woodpecker on ecstacy to those that capture the beat and the spirit of an engine in much the way a cartoon captures a person's likeness yet lacks the accuracy of a photograph. I'd like to think my 'Combo' and 'Programmable' units are in the latter category, but you must judge for yourself. I believe the sound algorithm employed in my units to be unique and would advise that it is best suited to slow revving, 'slogging' engines typically found in fishing boats and tugs.

Recorded Sound
I know of no units that give truly authentic sound* but within the limitations of the technology employed they are leagues ahead of the synthesised sound units. The technology employed is to repeatedly play a short loop of sound and vary the playback rate according to the throttle demand to give the effect of the engine running through its rev range. Whilst this has the result of speeding up or slowing down the recording, it also has the side effect of changing the pitch of the sound at the same time, resulting in a Pinky & Perky effect (you may need to Google P&P unless you are of a certain age!). These links illustrate the problem using a passage of speech.

* for example the changes in volume and timbre when the engine is accelerating or being put under load are not catered for - but please enlighten me if you know different!

With certain engine types running over a limited rev range the results don't deviate far enough from real life to be unconvincing, but other engine types don't lend themselves well to this treatment. In the latter case some units attempt a compromise where two distinct sound samples are used - one for tickover, switching upbruptly to a quite different second sample for the remaining rev range.

Algorithms to adjust the tempo of a sound without altering its pitch do exist (that's how the third sound sample above was produced) - but they make a desktop computer sweat with the effort and as such these algorithms are unsuitable for real-time processing by the humble microcontrollers typically used in these engine sound units

There is also some skill involved in finding a good point in the sound sample at which to loop it. A poor 'join' manifests itself as a rhythmic 'hiccup' as the speed rises.

The FE100 and FE101 units I am currently developing are no better and no worse in these respects than the others out there in the market, but I hope to win out on price, size, ease of use and features.

Speaker Considerations
Inevitably the size and construction of a model may preclude the recommended speaker being used and if in addition the speaker installation is less than ideal, then both the volume and the quality of the sound will be compromised. If in any doubt as to a sound unit's capabilities, try connecting it to a music-centre loudspeaker and hear the result. Your model sounding any less rewarding than this represents the magnitude of your personal battle to defy the laws of acoustics!

Speakers are ideally mounted on a baffle – this is usually a piece of wood whose width is about twice the diameter of the cone with a hole in it about the size of the cone (determined by the speaker mounting arrangement). The purpose of the baffle is to prevent the anti-phase sound waves from the rear of the cone 'leaking' round to cancel out the in-phase sound waves from the front. Hi-Fi speaker cabinets are generally sealed to achieve this, though a carefully placed and sized hole in the cabinet can cause a phase delay to the sound waves from the rear of the speaker bringing them back into phase with those from the front thus re-inforcing the sound (especially the bass). The hull of a boat makes an excellent substitute if the speaker can be mounted beneath the cabin using the entire deck as a baffle and the sound can escape through open portholes, windows, doors or ventilation grilles in the superstructure.

Follow the links below to YouTube videos I've posted to hear examples of two of my sound units using the competing technologies. Expect to be disappointed if you listen on a laptop, tablet or smartphone, you need the fidelity of a desktop speaker system to fully appreciate them.