Why do some commercial speaker use 7" drivers for midrange?

Bogwan

Member
2006-07-24 4:52 pm
I was sufficiently intrigued by Usher’s 8955A woofer (8 inch diameter) that I bought a pair from Parts Express during their sale in December 2007. In Usher’s own designs, they cross this woofer over at 276 Hz or 282 Hz to their 7 inch diameter mid-bass driver, Model #8948A. Other manufacturers such as Wilson with the Watt/Puppy also use 8 inch woofers with a 7 inch mid-bass driver. I don’t understand the logic behind crossing an 8 inch woofer over to a 7 inch woofer. Are these manufacturers letting the 7 inch driver roll off naturally, thereby actually creating 2.5 way systems? I can’t argue the success of speakers like the Watt/Puppy, but wouldn’t greater midrange resolution and transparency be achieved using a proper midrange driver? My plan is to combine the 8955A with a true midrange driver using a crossover point around 500 Hz, but I could be convinced otherwise. Does anyone foresee a problem using the 8955A at this high of a LP crossover point? Thanks in advance for your comments.
 

Hezz

Member
2002-12-22 6:52 am
Utah
Bogwan said:
I was sufficiently intrigued by Usher’s 8955A woofer (8 inch diameter) that I bought a pair from Parts Express during their sale in December 2007. In Usher’s own designs, they cross this woofer over at 276 Hz or 282 Hz to their 7 inch diameter mid-bass driver, Model #8948A. Other manufacturers such as Wilson with the Watt/Puppy also use 8 inch woofers with a 7 inch mid-bass driver. I don’t understand the logic behind crossing an 8 inch woofer over to a 7 inch woofer. Are these manufacturers letting the 7 inch driver roll off naturally, thereby actually creating 2.5 way systems? I can’t argue the success of speakers like the Watt/Puppy, but wouldn’t greater midrange resolution and transparency be achieved using a proper midrange driver? My plan is to combine the 8955A with a true midrange driver using a crossover point around 500 Hz, but I could be convinced otherwise. Does anyone foresee a problem using the 8955A at this high of a LP crossover point? Thanks in advance for your comments.


What is a proper midrange driver? The idea of a proper midrange drivers has changed dramatically in the past 30 years. So what is the midrange?

It is generally the range from 100Hz to about 3000Hz. A properly designed six or seven inch driver performs well in this range with some qualifications.

A smaller driver of 4-5 inches may do better in the upper end of this range in terms of dispersion but often is no better in terms of breakup resonances. In fact, most dedicated midrange drivers break up in about the same region as 6-7 inch mid/bass drivers so there is little or nothing to be gained.

Since 90% of the most important fundamental frequencies are between about 100-1500Hz the larger drivers do very well as long as the break-up modes around 3000Hz are attenuated.

While the ear is most sensitive around the 3000Hz region this region is really the boundary region between most of the fundamentals and higher order harmonics. So if you have a 2 or 3 way it is often the best compromise in size to use a larger midrange driver and place the crossover in the 2500-3000Hz region. A shallow null can be easily created in this region where the ear is very sensitive if necessary to get good sound.

Also most speakers with high end asperations will be high passing the midrange so that it doesn't get much bass energy. The low notes can modulate the higher ones with more doppler distortion so even crossing to a 8 inch woofer from a 7 inch midrange can improve the midrange quality of the 7 inch driver. Plus the larger midrange driver can more successfully reproduce what bass information it does get without straining the driver.

In the old days a smaller, cheap midrange was used and crossed to the woofer in a three way in around the 500-700 Hz range. Probably the worst possible range to do a crossover and requires very good crossover design. But even then, having the woofer reproduce much of the female vocal range usually doesn't work very well. Ideally the midrange driver should approach the theoretical ideal of the perfect wide range driver by reproducing all or most of this all important range. And if you don't get this region right the speaker can never be anything but mediocre.

In addition, a larger midrange driver can more successfully integrate with the woofer at the lower crossover frequency because it is not so different in size and is likely to be a better timbre match for the woofer.

Also, a lot of so called midrange drivers do not perform or sound good in the lower midrange region and why most manufacturers don't use them except in four or five way systems.
 
I'm not going to answer your question but here are a couple comparitive measurements of an 8" midbass and 7" midbass used in the Wilson Duette, Usher 718-BE in the NRC chamber. These are good measurements. Both of these speakers are expensive, with the Wilson playing in Bill Gates Territory.

The distortion measurements are the most interesting comparison. The Wilson uses a 8" SB Acoustics driver.

http://www.soundstagenetwork.com/measurements/speakers/wilson_duette/

Here is the Usher 718-Be which uses the 8948A. Take note of the distortion measurements.

http://www.soundstagenetwork.com/measurements/speakers/usheraudio_be718/


As far as midrange, it depends on whats important to you. Neither of these drivers have midrange T/S parameters. They are midwoofers meaning that their suspension, motor and cone are built with dual purposes in mind. If you where going to design a dedicated midrange you would choose other properties.
 
I can't offer any numbers, but the tri-amped system I've been using for years consists of an 8" poly woofer (unknown surplus pedegree) in a sealed box, a 7" Dynaudio mid-bass (17W75, I think) in an open baffle, and a 1" dome tweeter (Morel MDT28). The results have been far more satisfying with less work, than anything I tried using smaller mids. I had particularly poor results with 2" convex dome midranges (no known application AFAIK), and only so-so results with inexpensive sealed back mids. It would seem like the same thing can be accomplished with just the tweeter and the the 8" woofer alone, but it just ain't so. You need that clarity without crossover issues right in the middle where it counts.