There is no standard rule that can apply towards all speakers and listening rooms. As you move off the direct line from the center of a tweeter the high frequency response gradually falls off. This applies to a vertical offset which is affected by the speaker's height as well as horizontal offset by setting the desired toe in angle. So you can fine tune the speaker's high frequency balance by experimenting with height and toe in. Depending upon the speaker's design and crossover type there will also be an optimal height where the drivers blend is more seamless, meaning that you can't easily locate the sound coming from the individual drivers as the sound field becomes more coherent. So you either need to follow the specific speaker manufacturer's recommendations or experiment to find the best position in a given room.
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What does 'height' mean...really?
Isn't 'height' just relative to make sure that the tweeters are 'aligned' (pointed) to your ears?
Suppose your tweeters are 10 inches higher from an absolute perspective, but angled down to be in alignment to fire directly into your ears...isn't 'alignment' of more importance to 'height'?
There is an old rule of thumb that is said to apply only to conventional dome or cone tweeter/midrange pairings and that is to put the tweeters just a little above ear height. However, the reason for that advice has little or nothing to do with the highs. It's really to smooth out a response wrinkle in the upper mids. But, even among the conventional speaker designs this might apply to, there can be exceptions. The reasoning that Erikt and Lowrider are referring to can sometimes be taken advantage of to place the soundstage as high (or low) off the floor as you require.
I have an old pair of German Magnat speakers that are 3-way cones using titanium domes. The 'slightly above' rule works good for them, but I had to go through some experimenting to eventually confirm it for myself. The odd part about these particular speakers is that both tweeters are actually offset to the left of the vertical center-line of the 2 woofers and midrange, making the pair NOT a mirror-image one. I was never really quite sure exactly why that was done, unless it had to do with trying to reduce the cabinet diffraction peak that happens when the distance from either side and the top of the cabinet edge from the tweeter are all the same distance (which works to combine the individual diffraction peaks at the same frequency - spread out the individual edge distances and you spread out the overall diffraction peak). You'd think, since both tweeters are offset to the left, you'd be able to hear this somehow in the soundstage, but I haven't been able to yet, under any circumstance, in the 24 years I've owned them. Not sure why that is.
for better results contact the manufacturer of the speaker and find out where the measurments were taken from. some measure on tweeter axis, some measure on woofer axis and some like my product measure where the woofer touches the tweeter making it the musical center. toe angle is something very different and depends on the radiation characteristics of the speaker, uniformity of response and personal taste.
Thanks guys. Unfortunately the stands that came with my speakers are not adjustable for height or rake. I was messing around with toe in last night which seemed to help get the coherence right. I will test out different height chairs today to see if getting my ears up to tweeter height helps.
And Lowrider I was actually planning to add some absorption on my ceiling in the near future.
Assuming the speakers are the Sonus Fabers listed in your system profile, you do not have to sit at the tweeter axis. Sitting slightly lower than the tweeter will not substantially change the treble balance. Stereophile reviewed the loudspeaker and performed measurements regarding dispersion characteristics. The review did note that loudspeakers where particularly sensitive to setup.
Thanks Onhwy61. That Stereophile article was a good read. Given that I am only 3" below the tweeter height at a horizontal distance of 86" away from the tweeter means that I am only off axis by less than 2 degrees. According to the the Stereophile measurements I should not hear any difference. Thank you again.