However, if you have read through some of my posts on the subject and downloaded my wall bounce calculator, then you know there's a problem. Freestanding speakers have -6dB shelf filter in their high frequency response. This compensates for an effect know as "baffle step" or "diffraction loss". At wavelengths longer than the width of the speaker cabinet , half of the acoustic energy wraps around the front baffle and radiates towards the back. The high frequencies with shorter wavelengths are forced to radiate in the forward hemisphere only. The -6dB shelf filter gives a flat anechoic on-axis response, but the total acoustic power emitted into the room is +6 dB heavy in the bass. Now, when you flush mount a freestanding speaker, all the frequencies are forced into the forward hemisphere. So, the on-axis response exhibits a +6 dB step in the bass in accordance with the power response imbalance.
To illustrate the effect plug these values into the wall bounce calculator:
D = 0 cm (flush mount)
L = 200 cm(doesn’t matter)
W = your speaker width
F3 = 1 Hz (to better illustrate effect)
See the big bass shelf? So, what is the solution?
Answer: a do-it-yourself -6dB low shelf filter.
I've designed a high performance filter that will level out the power response step of a freestanding speaker, yielding both a flat power response and a flat on-axis response when flush mounted. If you have some experience building electronic circuits, this project should be relatively easy for you.
The circuit is simply a stereo -6dB low shelf filter with buffered inputs and outputs. The filter response depends on the width of your speaker cabinet, so I've included a table of component values for a range of widths. If your speaker doesn't match up exactly with one of these dimensions, don't worry. Just round to the nearest dimension. The response error will be a fraction of a decibel. If you really want to be more exact, then choose a resistor value that falls in between the higher and lower values.
I've given two choices of op amps, but there might be others that are pin compatible. The Burr-Brown OPA604 is an excellent and affordable amplifier. It will give very high quality, transparent performance in this circuit. For a no compromise circuit choose the Analog Devices AD797. This a little monster of an op amp, combining extremely high input impedance, extremely high output current, extremely low noise, and extremely low distortion. It's just extremely good in every respect. At almost $10 US each it also happens to be a rather expensive little bugger. The price, however, isn't too prohibitive in this project because we're only using 4 of them. A buffer built from an AD797 is just about as good as it gets. You can possibly find a discreet circuit that will perform a hair better, but you'll need to search long and hard.
All resistors should be 1% metal film types.
I've included an optional delayed relay circuit that prevents output thumps during power up. The relay grounds the outputs (these op amps can take this indefinitely) for a few seconds after the power is turned on, giving the power supply and filter circuit enough time to stabilize. This option doesn't affect the sound because there is nothing in the circuit path when the relays open. Keep in mind I haven't actually tested this relay circuit. It should certainly work, but I'm not 100% sure about the delay time. It could be too long. You might need to play with the 15k Ohm resistor and 220 uF capacitor values to get the time constant right. I can suggest some other values.
If you don't want to build the relay circuit, then just make sure you turn this filter unit on before you turn on the power amplifer.
You can download the schematic of the unbalanced circuit here: Baffle Step Decompensation Filter - Single Ended.
I will update this thread with a balanced version.
I haven't included a design for a dual 15 Volt power supply, but you can readily find them on the web. Here are some designs:
http://headwize2.powerpill.org/projects ... on_prj.htm
http://headwize2.powerpill.org/projects ... w1_prj.htm
http://headwize2.powerpill.org/projects ... on_prj.htm
And here are some places where you can purchase an appropriate power supply.
http://www.marchandelec.com/ps10.htm
http://www.arrow.com (Power-One HAD15-0.4-A or HAD15-0.8-A)
http://www.bmm-electronics.com/Product. ... duct_ID=31
Enjoy!

Thomas