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Optimum ear/speaker vertical height

A.S.

Administrator
Staff member
During the design of (any) loudspeaker system, the designer must fix his design reference axis above the floor, perpendicular to the front baffle, an imaginary line along which he will position his measurement microphone. The crossover will usually be optimised so that the sound wave contribution from the individual drive units that arrive at the measurement microphone in a defined and integrated loudness and phase way.

The only place in the listening room where the speaker designer's precise intentions of how the speaker system should behave is replicated from the design lab is when the listener's ears are aligned with the main tweeter - at least in a Harbeth speaker. Obviously, Harbeth speakers sit atop speaker stands, and those stands are available in different heights. Add to that the variation in human height, what is domestically acceptable, and there are many reasons why the sound enjoyed at home falls short of the perfection achieved in the design laboratory where the listener's ears and the reference axis can be aligned with precision.

It's pretty obvious that under normal domestic arrangements, the stand height will tend to be shortened, especially if there are young children around, but if you are serious about sonic perfection, then you will have to place you ear at the same height as the tweeter, or technically 'on the reference design axis'.

Here are some measurements of the centre of the tweeter above the outside bottom surface of the speaker cabinet. Don't be concerned about a mm here or there: placing the ear within, say, 10cms of the tweeter will give virtually lab-standard results.

So armed with these measurements, and awareness of how far above the floor your ears are when in your favourite chair, and the height option of commercial off-the-peg stands you can see how close you are to the vertical sweet spot. Don't be surprised if these therortically perfect stands are rather taller than you'd see in a hi-fi store. We are talking of lab standard precison, and the domestic compromises needed to bring the R&D lab sound to the ordinary home.

Of course, when designing, I'm very alive to the fact that for domestic reasons many people would like to hide the speakers on the floor and in the corners (ouch!), but they wouldn't expect those conditions to give the ultimate sound. So I have desensitised the vertical reference axis in criticality as much as I can during design, to give a good sound over as wide a range of ear/speaker heights as technolgy allows.

Above all - don't become overyl concerned about this! Every pair of speakers you have owned or can buy has (or should have!) a declared vertical design axis. If you never thought about this before, there is no need to be overly concerned now!

TO FOLLOW
 

MikeM

Active member
A.S. said:
Here are some measurements of the centre of the tweeter above the outside bottom surface of the speaker cabinet. Don't be concerned about a mm here or there: placing the ear within, say, 10cms of the tweeter will give virtually lab-standard results.
Alan, just to be clear on this one for all of us. Does it matter whether the tolerance of up to 10cms or so puts the tweeter above or below ear level, as in a 70cm stand is perfect for the P3ESR in my case but a 60cm stand would also make no discernible difference even though the tweeter is below ear level?
 

A.S.

Administrator
Staff member
There is no link as I have not had the time to finish the post.

About +/-10cms. In a strictly lab sense, consider the path length to your ear at +10cms from the woofer and from the tweeter as individual sound sources. Clearly, there will be a slightly longer path from the woofer to your ear at +10cms than that at -10cms. Conversely for the tweeter. Now 10cms might not sound like much, but consider the frequency band where the tweeter and woofer are both contributing (i.e. in the crossover region).

The wavelength of sound at, say 3400Hz, around the crossover frequency for a typical speaker, is itself about 10cms. Not a lot is it. So, you can imagine that a competent speaker designer will have factored that into his crossover and/or physical mounting of the woofer and tweeter to make sure that at one point in space and only at one point in space vertically, the woofer and tweeter sound beams are in step with each other. In other words along the vertical reference axis, a distance off the floor.

When additional time-of-flight is introduced, be it to the ear above the reference axis (or TOF is reduced, below ref axis), the path lengths from the drive units to the ear are in one case are shortened (the source is nearer the ear) and in the other, extended (the source has further to travel to the ear). {You don't ever read this in hifi reviews do you, but it should be discussed}.

If - if - the woofer and tweeter sonic loudness contributions are perfectly matched in loudness and phase, then the net result of listening a little above axis would be approximately the same as listening a little below ref. axis , and that is in a general sense true. However, as the diameter of the bass/ midrage driver is so much wider than that of the tweeter, its sound power off-axis diminishes much faster than the practically omnidirectional tweeter (at the bottom of its acoustic pass-band) so the effect of listening further and further above the reference axis has much more impact on the resulting integration of sound from the woofer and tweeter than listening a bit below, closer to the centre radiating area of the woofer (hence, less off-axis, more on-axis). It's the width of the bass/ midrage unit relative to the frequencies it is reproducing that's the enemy here, not the tweeter. And you can't fight physics, except in a marketing department.

Again, depending upon the awareness of the designer, his understanding of the norms of domestic acceptability concerning stand height, how much time he spends alone in his man cave away from the real world and so on, he may just shrug his shoulders and say 'what does the customer expect when he doesn't use nice, tall stands...'. My view - as a wholly real-world family man - is that optimally tall stands* are rarely domestically acceptable. In which case, during design, I place maximum attention on achieving the best possible drive-unit integration on the reference axis and a little above it, settling upon the physical necessity that the integration much below reference axis (where not one user in 1,000 lies, on the floor) is sub-optimal.

The designer has some ability to steer up (or down) the vertical reference axis both by position of the drive units, perhaps staggering them front to back and in the crossover provide that he has sufficient components in the crossover to manipulate the inter-driver sound contribution. If he is of the minimalist persuasion of using just a handful of components he has no means to steer the point of best integration between the drive units. That's one reason I completely reject the minimalist crossover design approach. It ties both of the designers hands behind his back.

We're planning to make several videos in 2018 to explain this sort of issue from a designer's perspective.

Hope this helps.

* For the Bristol show we had especially tall stands made to try to strike a best-possible sound between seated and standing listeners in our demo rooms at the hotel. ANd yes, it did raise one or two comments such as "Wow! Those are tall stands .... are they really necessary...?". The answer was/is, of course, no, but if you have total freedom over stand height, have no small children or animals (or even a domestic partner to take into account!) then yes, to get the studio-sound experience, they are absolutely ideal.
 

MikeM

Active member
Alan, many thanks for the detailed explanation which was very helpful for my knowledge bank and understanding.
 

A.S.

Administrator
Staff member
OK, here are the heights of the main tweeter up from the outside of the bottom of a cabinet, as it would sit on a table top.
Model Distance to tweeter
P3ESR 230mm
C7ES-3 440mm
SHL5+ 475mm
M30.1 / M30.2 320mm
M40.1 / M40.2 660mm















These measurements have been made with adequate precision for the purpose of this exercise.
 
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