Port Placement in DIY Bass Reflex Cabinet

[Clipped and Shorn]

Good, I'll get fini to build me some.

-c7s

[Al Klappenberger]

Hi Guys,

I haven't been following this thread but I did see some references to "group delay". I want to point out a trap that MOST people fall into. "Group delay" and "time delay" are two different things. Group delay is a measure of phase linearity ONLY. 20 mSec of "group dealy" can not be thought of as having anything at all to do with 20 mSec of propogation dealy through the air from speaker to you ear. There is NO way to equate one to the other! Group delay is meaningless! It is a measure of the rate of change of phase with a given change of frequency at ONE frequency only. That is the "slope" of phase change, that is, it is the "Derivitave" of phase. It is NOT time delay!

AL K.

[John Warren]

c

[Al Klappenberger]

John,

Yes, that's what I am saying! You can't hear group delay! The "two-clicks" thing is TIME DELAY! They are NOT related! Group delay can only be heard through headphones and with very specific test signals. Quite a bit of research has been one on it and group delay is NOT very important when reproducing music!

Al K.

[John Warren]

c

[Al Klappenberger]

John,

I too could site a bunch of dueling experts on this subject, but it's too much

trouble! I would rather make the point with a simple exercise. If you do not

believe what I am saying after this experiment, I will simply go quietly away.

This subject is just too controversial to get into a major argument over!

Use the following equipment: An oscilloscope having delayed and intensified

sweeps, A frequency counter and an audio sine wave signal generator.

Set up the oscilloscope to trigger on the input sine wave and set the delayed

sweep to intensify a tiny spot after some known time delay that you simply set

to that whatever you like. I suggest a delay that spans many cycles of the input

waveform. This will represent true "time delay".

Adjust the frequency of the input signal so that the intensified spot is at the

peak of some displayed cycle and read the frequency from the counter.

Move the input signal frequency so that the spot moves to the zero-voltage

crossing of the waveform. This will represent a 90 degree change of phase. read

the counter frequency again and find the difference. That will be the change in

frequency.

Using this formula calculate the "group delay":

group delay = (degrees change in phase) / (change in frequency * 360)

When you do this you will see that the two forms of "delay" are nowhere near

equal! They are two entirely different things!

There is a long list of "experts" who confuse these two. Group delay should be

renamed something like "envelope distortion factor" or something like that.

Calling it "delay" is extremely confusing to spite its universal acceptance. As

the writer of a network analysis program I experimented with this quite a bit.

There is also another form of "delay" to confuse things even more. That's "phase

delay". It has a similar shape to group delay when plotted but does not come to

the same "peak" at the corner of a filter passband as "group" delay.

I have prepared a technical paper on "extreme slope" crossover networks to be

published in AudioXpress magazine where I present an oscilloscope photograph of

the time delay through the highpass channel of a filter having an over 100 dB /

octave slope using a tone burst. The "group delay" was computed and found to be

many times larger than the actual "time" delay through the filter.

Again -- "Group" delay is NOT "time" delay! To blame "group delay" for hearing

two distinct sounds is simply WRONG!

Al K.

[John Warren]

c

[Al Klappenberger]

John,

All I am saying is that many people confuse the tar out of "time delay" and

"group delay". They toss the terms around like they were the same thing, even in

technical papers. The real question is the impact of group delay on your

hearing. I believe it is very minor in comparison to other factors. The B&L

article you mentioned is clearly about "group delay" since they did the tests

with "all-pass" filters. These are definitely "group delay" devices. The tests

were done by listeners who were actually trained what to listen for. I can site

other "experts" who conclude that it does not matter when reproducing music. If

you have a tradeoff between group delay error and other factors you should give

the group delay lowest billing! I have crossover networks in my Belles that have

over 100 dB / octave slopes and have group delay peaks at the crossover

frequency that are huge (about 3.6 mSec at 700 Hz). I have carefully listened to

square waves, tone bursts and pulses adjusted slowly right across and at the

crossover frequency. I could hear no difference in the character of the sound

anywhere in the crossover region. I can see a double burst on either side of the

crossover frequency using a conventional 12 dB / octave crossover that I built

to test the theory. With the extreme slope crossover, there is only one except

right at the crossover frequency. The result is a far smoother dispersion with

the extreme slope network than with the conventional one. This is to spite the

big group delay peak. Two drivers making sound at the same time cause "modeing"

in the dispersion as they interact. You can definitely hear that because it

causes bad frequency response! It is questionable if a dual burst is audible in

itself. I suspect it is not, and that truly is "time delay", not "group delay".

Al K.

[John Warren]

c

 

[Al Klappenberger]

John,

The dealy peak is below the "B&L" limits or I would not have built the network! The fact is that 3.6 mSec IS huge for a crossover network at 700 Hz! Many people would have ruled out such an extreme slope filter just because of the size of that peak compared to the group delay throuout the rest of the passband swearing that the "ringing" would be overwhelming. It isn't! What I am saying is that the ear is nearly deff to "group delay" and phase errors except uner extreme conditions, like through headphones, which is how the writers of the B&L article measured it. At woofer port frequency, I doubt you can hear it at all! Look at his curve and tell me what the group dealy limit would be at 30 Hz! Here's a scan from a letter to the editor that sums up the situation. Phase errors are WAY over rated as a factor when other things are involved.

AL K.

[John Warren]

c

[Al Klappenberger]

John,

Nice program you used there. I got it too. It's WinISD..

The simple answer is NO.. Unless you have an accelerometer under you listening chair connected to an electronic differentiator and something to convert 30 Hz up to 1 KHz so you can hear it or some instrument that will operate at 20 to 30 Hz to linsten to it for you, you will NOT her the difference! At 20 to 30 Hz you feal it more than hear it anyhow! You are putting to much faith in published stuff on "group" delay! Cosider this simple point. If there has been extensive research on a subject, then the results must be in question. You do not need to research the obvious. That's why so much research has been done on the importance of phase distortion, group delay and caffeen in you coffee. It MUST be bad, so we are going to prove it, OR ELSE, SOMEHOW!

On a second reading of you post.. You ARE still confuisng "time" delay with "group" delay! "fast" and "slow" do not apply here. "Group delay" has NOTHING to do with speed! It has only to do with change is the "slope" of phase!

Al K.

[jnorv]

I may be missing something here, but isn't that second plot exactly what a base reflex is suposed to do. At 20 Hz, a half cycle is 25 ms. Just what the program predicts. The port is out of phase with the front of the driver.

Jim N

[Al Klappenberger]

John,

"Group delay" is about the most over-rate bunch of "Bull____" to come along

since God knows what. Look at the attached graphic. It is a composite of the

computed amplitude response and "group" delay of the high frequency channel of

the woofer / squawker crossover I am using in my modified Belles. Look at the

amplitude response. It goes down like the side of a barn and the group delay

peak is almost exactly at the crossover of 700 Hz. Now look at the actual

response take from an oscilloscope screen of a single cycle tone burst at 700 Hz

of that network. The oscilloscope is triggered from the input burst making the

two upper traces locked in time with the input burst. All channels are

"chopped". The group delay is about 2 mSec. If you can find ANYTHING on the

middle trace (the high frequency channel output) that corresponds to 2 mSec in

"time", I will go away!

Impulse response of any waveform, and therefor it's response in "time", is

computed by a method called "Fourier analysis". The math to do it involves the

summation of the instantaneous AMPLITUDE and ABSOLUTE phase of each and every

frequency element in the spectrum. There is NO information required about the

"CHANGE" in the phase of each harmonic element, only the instantaneous number of

degrees. "Group" delay relates only to the rate of "change" in phase of a single

frequency, not to it's delay in time!

Al K.

[Al Klappenberger]

Jim,

Yes.. Who cares about what comes out of a bass reflex speaker below the port frequency anyhow? The woofer is unloading from the box and the sound is shutting down anyhow! The discussion about "group" delay down there is a moot point!

AL K.

[Al Klappenberger]

Oops - double post. I must have hit the button twice!

AL K.

[Clipped and Shorn]

"Yes.. Who cares about what comes out of a bass reflex speaker below the port frequency anyhow? The woofer is unloading from the box and the sound is shutting down anyhow! The discussion about "group" delay down there is a moot point!

AL K."

Close enough for jazz, I say, and certainly even closer for latin jazz and Stravinsky. {What about Jesse Crawford's Organ Favorites you say....well I don't know about that, how many of those organ notes go below 24 hz?}.

OK, I have armed guards posted, I am cautiously moving the plywood out of van and into the shop. The wind is my sails. Might even buy a new saw-blade for the table saw.

"Serenity now."

-c7s

[Al Klappenberger]

Clipped,

Good deal, but watch that group delay. Move the sheets of wood one at a time. If you move them as a "group", they may get there before you move them. Group delay can go negative you know! (Look at the curve of dealy I posted. It goes negative through the "notch" frequency od the filter).

Weird stuff!

Al K.

[John Warren]

c

[Al Klappenberger]

John,

The derivitave of phase versus frequency plot is "group delay". That is the

definition of it. What it tells you is how equally the phase is changing with

frequency over a specific frequency range. If a network has flat (all equal)

group delay over a particular frequency range, and if all the individual

elements of the "group" of elements (sidebands) that make up a particular

complex waveform are passing through that network, the elements will then

reassemble to make the waveform in an undistorted manor. It will usually not

tell you how "fast" it will get through that network. The only thing I know of

that has "group delay" equal to "time delay" is a length of transmission line.

That is a length of RG-59U coaxial cable at radio frequency for example. I have

never seen a direct relationship between "group" delay and "time" delay. In the

filter design world, it seems that the wider the bandwidth over which group

delay is flat, the closer the two will be to each other. I don't know why or

even if it is universally true.

To talk about the group delay at a single frequency is totally useless. There is

no "group" involved! "Envelope distortion" is often described and specified in

two ways. Either flatness of group delay over a given frequency range or in

terms of +-X degrees error from a best fit straight line between two

frequencies. The only time group delay is consider at one frequency is if you

are "phase matching" two separate filters. Even then the matching is usually over

a range of frequencies.

If you like, I will post sample plots of a linear phase bandpass filter and a

Butterworth or Chebyshev filter showing both group delay flatness and phase

error from a best-fit straight line on each. I can also use Fourier analysis to

show what happens with each in the time domain. It's a pain in the butt to do

all the plots and upload them, but I'll do it if you are really interested.

Al K.