Axiom Home Page Forums General Discussion Technical Questions Lobes

To boil down the meaning of a lobe to the starting point (hmmm sounds a bit like a dinner recipe), it is simply a bump or hole in the amplitude response at some frequency area. Normally it is referred to in terms of room interaction with the frequency response at some particular place in the room. If it existed in the on axis response of the speaker in an anechoic environment then this would be a serious problem. If it exists in only one of the off axis measurements then it is unlikely to affect the performance much in an overall sense. These particular single axis lobes represent an area of research where we spend a lot of time, mainly determining their exact effect in the end overall in-room performance.

Ian,

wow, that was a very informative handful there!

It's hard to believe the owner and engineer of a company would post this kind of info on his/her own website!

I have a question (again) though. I want to know in a little more detail what exactly an anechoic chamber is? I hear it's a room that has special material for the walls, and the walls are angled so it deflects all the sound 'properly'. Is there any easy way to elaborate on this?

Perhaps you could post a new post so people could see the terms anechoic chamber, and they would read it.

Thanks Ian (or Alan or another, if someone else responds) !

Hi Ravi,

The walls, ceiling, and floor of an anechoic chamber are lined with these spike-like wedges of absorbent foam, sometimes 3 to 6 feet long, so that all reflections--and I mean all--are totally absorbed. Thus you have a large chamber with literally no echoes, hence the word "anechoic" to describe it. The chamber is also totally isolated from all outside sounds that might otherwise corrupt any measurements. (When you're in one, it's so silent that you can hear your own heartbeat! It's eerie.)

A metal grid-like catwalk is suspended in the middle of the chamber so a technician can place the speaker to be measured on a remote-controlled pivoting turntable in the chamber's center. A calibrated measurement microphone is suspended by wires in the middle of the chamber about 2 metres from the speaker.

The reason you want to create a "room with no echoes" is to precisely measure the acoustical output of the speaker across the audible frequency range (20 Hz - 20 kHz), uninfluenced by any room surfaces because, if you think about it, there is no such thing as a "typical" room. EVERY room is different; even moving a speaker six inches in any given room changes the mix of direct and reflected sounds radiated by the speaker, so if you tried to measure the speaker in the room your results would not only reflect the speaker's output but also the room's influence or "output". The results would only be valid for that speaker in that position in that particular room, which is of no use to the designer!

So, the technician leaves the chamber, closes the door, and starts the measurement process. As frequency sweeps are fed to the speaker, it's tilted and rotated in all axes and the results from the measurement mike are recorded and graphed, so you have an untainted record of the speaker's output from every angle--on-axis, off-axis, above, below, etc.

You can also achieve fairly accurate quasi-anechoic measurements by mounting a speaker outside on top of a tall pole and rotating and measuring it the same way. You have to be certain there are no environmental sounds (wind, traffic, thunder, etc.) to corrupt the measurement. Ian Colquhoun uses this technique at the Axiom plant and compares the results for accuracy to the actual anechoic measurements he has done at the National Research Council's anechoic chamber in Ottawa, Ontario.

Anechoic chambers are large (if you wish to get accurate measurements of low frequencies because of the wavelengths involved) and very expensive, which is why few speaker companies can ever afford to build their own. The NRC's chamber is available (for a fee) to any speaker manufacturer and it has benefited Canadian speaker designers (and some US ones) because there are no similarly available facilities in the USA.

Regards,

Alan,

great information! you might want to save that one for the next person who asks!!

i'd like to listen in one of those chambers one day... must be a creative experience!

Thanks Ian,

OK I got some if it...

So is this frequency interaction caused by the exact same frequencies being produced (and thus dispersed) by two different drivers and arriving at the same location at the same time? And a hole would occur if the signals somehow were 180 degrees out of phase and a bump occurs if the signals are in phase (and thus reinforced)?

Or am I totally lost here.....

Randyman

Randyman, you are certainly on the right track. For an in-room environment, and especially with low frequencies, you will end up with canceling and doubling happening to various degrees around the room. This effect is not really related to the interaction between the different driver components though. In the initial speaker design the interaction between the drive components will show up as a doubling or a canceling exactly as you have pointed out, in an anechoic environment. This needs to be dealt with during the design of the speaker initially.

OK Ian,
If I can now go a bit further - So I am beginning to get an understanding of the effect (in my thick skull) but if the effect is not related to different driver components, then it is (my assumption) that it IS related to driver placement/arrangement. If it is not then I don't fully grasp your comments about your "discovery"(my word-not yours) of the higher quality sound from the configuration for the VP150 center channel speaker (i.e. WTW in the VP100 vs the TWWWT in the VP150)[or for those that prefer, use M instead of W]

It was references to some other message board posts and their comments about lobing in the VP150's driver arrangement that started all this. Now I am just trying to understand the science (or is it mystery) around the lobing effect.

Appreciate any other info you could deposit here. Thanks!!

Randyman

A lobe is the result you see in the down axis measurement of the standard tweeter above woofer array. There are, though, many other things besides the driver placement that can create lobes. In a WTW array this particular lobe would now be located to the right and left axis measurement as opposed to the down axis measurement. In the TWWWT array this lobing axis is gone. Sorry for the confusion but the term lobe is quite broad as it is a term describing an effect on the amplitude response measurement and can have many different reasons behind it.

Alan,

Thanks so much for the information. I think I pretty well understand the terminology/effect now.

It's pretty cool that the owner of the company would spend time on their message board and answer/explain technical questions like you do.

Thanks for the great speakers too!

Randyman