Mounting Horns/waveguide on wooden baffle

[ttan98]

Most people know that mounting Dome tweeter the flange needs to be flush(same level) with the baffle.

Does this applies to horns or waveguide as well? Please explain.

Thanks..

[Corax]

Hi ttan98,

who said so that it must be mounted that way?

In most cases are just cosmetical reasons involved why the flanges are flushed with the baffle.

However there's a so called baffle-step diffraction (google for it and you'll find as much info as you want). If this baffle-step is taken into account at the time the crossover was designed there's no big need to worry about that - basically. However any sharp corners close to the point where the sound exits the path of the wave should be avoided (if possible) to suppress any reflections from such corners even if there's no 'wall'. Believe me or not but that's physics and it happens but I'm not going to explain that any further right now.

Nevertheless another effect is getting more and more important when dealing with waveguides and horns. The geometrical point where the sound is produced (the voice coil) and where it leaves the waveguide/horn is in some cases far away from each other. This needs a time-aligned output in a multi-way loudspeaker for a smooth over-all frequency response curve. The wave just travels with the speed of sound which would be approx. 340m/s therefore giving the constant on which you can calculate what the necessary time-delay would be if all 'ways' of the whole speaker are flush mounted. In this case the voice coils of the chassis are not directly mounted vertically under each other since for instance tweeter, midrange and bass horns usually are of different length - most likely the lower the frequency the longer the horn.
Furthermore crossovers are known to 'delay' the sound, because of their capacitors and inductors forming a complex load for the driving amplifier where the voltage is not in phase with the current. Depending on the order of the crossover the phase can be shifted quite far and therefore the time from the input of the crossover to the output to the chassis.


After all it's just a question of design (ease), costs, compromises and other (more or less understandable) factors why in most cases the flanges are flush mounted. But it is in deed not absolutely necessary! Of course you will find some audio enthusiasts which have a completely different opinion about this and I hope someone else will jump on and will give us his opinions, proof or whatsoever.

[EspenE]

Originally posted by Corax

who said so that it must be mounted that way?

In most cases are just cosmetical reasons involved why the flanges are flushed with the baffle.

Definetely not. Flush mounting of the tweeter is absolutely essential, or +/- 4-5dB ripples will occur. See http://www.zaphaudio.com/mtg-surface.html for a very good explanation.

Same goes for waveguides.

[augerpro]

The more directive the horn the less the baffle (and sharp edges) are illuminated. So diffraction *should* be less pronounced, although it would vary on case by case basis so actual measurements would probably tell the real story.

[EspenE]

Yes. Measurements will tell the real story.

Anyway - in my experience: even though waveguide tends to restrict the tweeters radiation angle, I have always seen the diffraction effects when the waveguide is not flush mounted. A 6" waveguide usually exhibits a significant dip at around 5-6 kHz. A larger waveguide will have less significant effects, but they are there.

A smooth termination to the front baffle is important.

[Corax]

Thanks 'augerpro' for your comments.
That's what I had in mind when I was trying to explain the (un)necessity of the flush mounting with the baffle.

As an example: If you take a look at many line arrays, which are made of stackable boxes covering just the mids and heigths or even full range, with a waveguide in the center for the hights and right besides to the left and right are the mid horns positioned there's often no additional baffle space for the waveguide(s). I'm sorry that I had a specific design in mind (which might be more or less common) and didn't post a picture to make that more clearly.
Another example: If you look at many (professional) PA speakers you find very often the tweeter horns mounted right in the middle of the mid(/bass) horn mouth. However they're 'flush' mounted with all the other chassis to the baffle but not directly connected to the baffle.

Of course there're some advantages and disadvantages, as I mentioned, i.e. ease of construction, available space (and therefore compactness, less weight, etc. which is often more desirable).
You always go for compromises and you've to find yourself the best way for your specific purpose.

[Paul W]

Though I was initially surprised, diffraction artifacts are clearly visible in waveguide measurements (11" & 12" OD WGs). Baffle diffraction appears to be reduced with a waveguide (relative to a dome on a flat baffle) but it still exists. So I'll continue to flush mount, round cabinet edges, etc.
Paul