Countersunk (recessed) tweeters

0 Members and 1 Guest are viewing this topic. Read 460 times.

panash

  • Jr. Member
  • Posts: 9
Countersunk (recessed) tweeters
« on: 7 May 2022, 04:38 pm »
I'm building three 9.1 litre acoustic suspension bookshelf speakers based on Peerless SDS-P830656/Peerless XT25BG60.

To build three seems odd but the TV is next to the fireplace and I'll switch between fireplace and TV using a DPDT relay switched from a USB socket on the TV but that's not the point of this listing.

I'd like to mount the tweeter on the rear of the front baffle to protect the patented twiddly bit from damage and use a round-out router bit which will form a very short horn, so short in fact that's it's probably negligable.

However reading around on the web leads me to the conclusion that tweeter output is boosted at lower frequencies but at the upper limit the response returns to that of an unloaded tweeter.

The physics says that horn loading is an impedance matching structure which presents quite a high impedance match to the tweeter which is closer to the 'output impedance' or radiation resistance of the tweeter so more acoustic energy is transferred.

However considering the spectral energy and angular dispersion it strikes me that we get a fall in response with frequency because the HF energy is more evenly spread out giving a better off-axis performance. The downside is that you have to compensate for this by boosting the drive at high frequencies which won't be a problem as the XT25 is way more sensitive than the P830656 at crossover at its upper limit.

I won't be designing any equalisation network until I've measured the units in the cabinets.

Can anyone with experience of this give me a sanity check on all this?  Why is is considered beneficial to have good off-axis response.  Does this benefit on axis listening?

Thnx PN

richidoo

Re: Countersunk (recessed) tweeters
« Reply #1 on: 7 May 2022, 07:28 pm »
You are correct that a horn is a mechanical transformer that increases impedance at the surface of the diaphragm. This increases efficiency, lower freq extension, and dispersion control. A shallow waveguide like you are planning can also help to improve physical time alignment on the Z axis, if you incorporate this aspect in your design. The waveguiding effect of a horn can reduce beaming into higher frequencies which increases the size of the sweetspot where stereo imaging can be perceived. However the shape of the horn affects tone due to internal reflections causing amplitude and phase distortions. Very few horn/waveguide designs are free of these tonal aberrations ime. See Earl Geddes for mo info on this. A DIY waveguide cut with roundover bit will have some of these effects but you won’t know if it is tolerable until you try it.

The total FR of the speaker and room response together is called “power response” and is more like what we really hear because we listen inside a room and we hear the reflections. Speakers with flattish power responses are said to sound better than speakers with flat on axis FR but otherwse poor power response. This is because the off axis radiation lights up the reflective surfaces which then color the direct sound. When direct and reflected sound are both relatively flat they add together to a relatively flat perceived FR by the listener. All listening positions tend to benefit from good power response. Thisnis why Harmon group speaker brands all tend to use waveguides or horns, because the waveguide controls how the HF is dispersed into the room. Their chief scientist Floyd Toole and team determined over many years of listening experiments that flat power response was preferred over speakers with only flat on-axis FR. This has become somewhat of a moral obligation in some speaker design circles. IME it can sound better and it can sound worse. In a well damped room its less important but few rooms are well damped. It isnt the only important parameter but worth considering. I personally prefer the sound of unloaded tweeters to horn/waveguides. ymmv

panash

  • Jr. Member
  • Posts: 9
Re: Countersunk (recessed) tweeters
« Reply #2 on: 8 May 2022, 09:28 am »
Yeah, thanks for that.

My lounge is quite lively as it has laminate flooring so the off axis energy will excite (I like that) unwanted reflections.

I'm recessing the tweeter for practical rather than acoustic reasons and the profile can only be the profile of the round-out router bit. As you say I'll have to see if I like the sound or not.

Cheers

PN