question about coaxial monitor design

[groover]

In looking at coaxial monitors with horn tweeters, such as the Presonus Scepter S8 and the Equator Q8 it looks to me like the horn would cause interference, for want of a better term, in the low frequencies with the sound waves bouncing off the back of the horn and also the horn blocking a lot of the area of the woofer.

Can someone explain to me the physics of why this isn't a problem?

Thanks!

[jhharvest]

I'm not a speaker designer so I probably shouldn't answer but here's my gut instinct:
a) Low frequencies go through stuff like that flimsy plastic horn in the Presonus monitor, so it's practically not there. The horn is there to guide the high frequencies from the tweeter.
b) Every monitor is a compromise. What you might lose in some of "the sound waves bouncing off the back of the horn", you gain in time accuracy because of the coaxial design.

A pure guess but that's how I'd approach it. If you look at this way: a lot of speakers are rear-ported. That means the woofer is pushing out air at a very funny phase from the back of the speaker as well as the front. Still they work pretty okay in most rooms.

[JWL]

Low frequencies have really long wavelengths that will diffract around smaller objects in its path.

[vxboogie]

There's been a lot of talk and projects about building coax floor monitors on ProSoundWeb over the years and in many cases they have put some kind of material on the back of the hi-freq horn to absorb and limit some of the "noise" and comb filtering that happens when the lows reflect off of the back of the horn.

[vxboogie]

I'm not a speaker designer so I probably shouldn't answer but here's my gut instinct:
a) Low frequencies go through stuff like that flimsy plastic horn in the Presonus monitor, so it's practically not there. The horn is there to guide the high frequencies from the tweeter.
b) Every monitor is a compromise. What you might lose in some of "the sound waves bouncing off the back of the horn", you gain in time accuracy because of the coaxial design.

A pure guess but that's how I'd approach it. If you look at this way: a lot of speakers are rear-ported. That means the woofer is pushing out air at a very funny phase from the back of the speaker as well as the front. Still they work pretty okay in most rooms.

I get what you're saying, but might look at it a little differently:

a) The lows don't go through the horn as much as vibrate the material like a passive radiator to "transmit" the sound through the horn material. I would think that there would also be some distortion to the high frequencies since they are reflecting off of a vibrating surface. That may be why some of them like the old Urei coax monitors had a foam type material for the hi-freq wave guide.

As far as porting is concerned, the port length and distance that the low freq waves travel allows the designers to tune the lows so that a certain frequency is adding together in phase once they combine at the front. As you said, its a trade off since that really only occurs at one frequency due to the amount of time it takes to travel to the front. For other frequencies, there is less addition or cancellation due to the phase relationship.

[digitaldrummer]

and in the case of the Equators, it seems the onboard DSP is used to "tune out" any undesirable frequencies or problems anyway. Equator has even been talking about allowing additional programming of the DSP to make them emulate other speaker's response (like the NS10, Auratones, etc.).

Mike