Interview to the principles of speaker development,
Peter Strassacker asked Joe d'Appolito (5/2004).
Joe, we want to give our DIY-customers the best information, so they have
the chance to build very good kits, which they like 10 or perhaps 20 years.
What is your advice to them?
Yes we have gone well beyond
the "cook book" phase of loudspeaker design. We now have a wide range of
measurement and design tools that allow us to make very accurate speakers.
The other side of this question is that both the general public and the DIY
types demand the better sound, but do not know how to get it. For the DIY
type, professionally designed kits are the only way to get today's level of
As I stated earlier, in modern design we cannot just use some formulas in a circuit design
textbook. We must accurately account for the impedance and frequency
response of each driver and determine the complimentary crossover for that
driver. There are also questions on driver types, multiple driver
integration, controlling diffraction, controlling system impedance, etc.
Yes, the driver, the cabinet and the crossover must fit and the crossover
is just good for one combination of drivers. What are your ideas
on driver types, multiple driver integration, diffraction etc.
Driver types: The most common driver is the dynamic driver.
These include the dome tweeter, dome and cone midranges and cone woofers
driver by a motor consisting of a voice coil and magnetic circuit structure.
This class of driver has been around for over 80 years and during that time
it has matured into an excellent transducer. These drivers are relatively
easy to integrate into multiple driver systems and are therefore most useful
in developing successful DIY projects.
More exotic drivers like ribbon tweeters and ribbon midrange drivers are
appealing to DIY experimentors, but their polar response patterns are quite
different from their cone counterparts, making it more difficult to integrate
them with cone type drivers in multiway systems.
Multiple driver integration: As more drivers are added to a loudspeaker
design getting them all to work together properly becomes increasingly difficult.
Cut and try methods cannot provide the level of performance we demand of today's
loudspeaker. In modern design we cannot just use some formulas in a circuit
design textbook. We must accurately account for the impedance, frequency response
and acoustic position of each driver to determine the complimentary crossover
for that driver. This requires sophisticated measurement equipment and loudspeaker
modeling and optimization software. For the DIY type, professionally designed kits
are the only way to achieve the level of performance demanded today.
Having said that how complex should a multi-way full range system be?
Personally I feel that today's dynamic drivers have reached the point
where you can get all the performance you need with a three-way system.
As one goes beyond a 3-way speaker getting all the drivers to integrate
correctly becomes more problematic. A 4-way, for example, usually forces
us to crossover in the very critical midrange. Getting a seamless transition
in this region is very difficult.
Diffraction: Diffraction is much talked about and little understood.
In home audio application the diffraction we are most concerned with is
that occurring when sound from a driver reaches a baffle edge, where a
secondary emission of sound occurs. There are two issues here:
1) How do we control diffraction? and 2) How important is diffraction to
the sonic signature of a loudspeaker? The first is an engineering question,
the second is a matter of opinion.
1) First a myth. Rounding corners eliminates diffraction. Diffraction,
like everything in acoustics, is wavelength dependent. With vertically oriented
speakers, diffraction is mostly off the vertical baffle edges. If the rounded
edge is to provide a smooth transition for the sound wave off the baffle, the
radius of the edge must be on the order of a wavelength. A one inch (25.4mm)
radius corresponds to a frequency of 13.5kHz. So even this generous edge has
no effect on the critical midrange and presence range. Diffraction is also
angle dependent. Its effect diminishes rapidly as you move off axis. So
diffraction has little effect on the loudspeaker power response. Since we
use the power response to judge a speaker's frequency balance, diffraction,
unless it is extreme, has little effect on perceived frequency response.
2) What about imaging. Here we get subjective. We use first arrival
response to judge source location. This happens in the first 300-400 microseconds.
Diffraction is a response delayed by the time it takes to get from the driver
to the baffle edge. This is also on the order of 300 microseconds. So the
diffracted wave arrives some 300 microseconds after the first arrival if
you are directly on axis. However, the diffracted sound level is several dB
down from the first arrival (Diffraction is well designed speakers typically
adds no more than 1dB of ripple to the on-axis response.). In my experience,
speakers with this level of diffraction still image extremely well.
Thank you Joe.