mooc waveflex part 7 - waveflex caraudio · mooc dsp a8 + rew v5 – part 7 ... and open the first...

MOOC DSP A8 + REW V5 – Part 7

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Part 7

12) SPEAKERSRESPONSESIMULATION/PART2:EQMENUThis is the more pleasant and instructive step from this tutorial, it will show in detail the behaviour of EQ and

filters when applied to a real response curve, thanks to the simulation tool in REW.

With this tool, one can simulate all the filters available in DSP A8, either the filters (high‐pass, low‐pass, band‐

pass), the equalization (PEQ, High Shelving, Low Shelving, Notch,LT), and also all‐pass filters and many more.

u) SETTINGSINEQMENU:

Still in the EQ menu, hide the Waterfall panel in the lower part of screen. This waterfall tab behaves in the

same way here as in the main tab of REW, but here it displays a forecast of Waterfall with filters engaged.

Then display the settings column on the right. And open the first panel Equaliser and start by choosing the

Generic DSP type, as in the screenshot hereunder.

Open the second panel Target and fill‐in the settings in the green box if not already done (cf. previous

chapter). If you are using your own target curves, then you must strictly use the values from this screenshot,

so that only the House Curve that you previously loaded is displayed on the screen.

Indeed, both curves, the House Curve (the one you manually loaded) and the Target Default (the one you drew

in REW) will add to each other to create the target curve Target in EQ menu.

If you have different values for the fields: LF Rise Start/End/slope and HF Fall start/slope (which define the

global response shape of the system, see chapter 2) don't forget to replace these with the proper values.

To start, we need to define the level of the target curve with the Target Level in the yellow box.

It’s important to understand that this setting will directly affect all the corrections to apply.



For example, if the measurement curve is too far from the target curve, you will need more corrections and

stronger ones, than if both curves are very close.

As we’ve previously seen, some dip areas of the response curves will not be able to be fixed, it’s useless to set

the target curve too high.

However, slight boosts can be applied, but only for a low value of Q (wide frequency area, to act on a large

listening zone).

To ease this task, smooth‐out the curve with 1/1 or 1/2 octave and adjust the target level so that this matches

with the dips to fix.

Write down this level value in the notes attached to the measurement (EG: house curve level=71 dB), see the

yellow boxes.

Now, open the EQ filters panel (red line). This panel will allow us to apply filters to the response curve.

The four first lines will be used for filtering and for using asymmetrical filters, they will have a Manual Control

(see red box).

The next five lines will be 5 PEQs from the DSP A8 available for each channel and will have Auto Control and PK

type.

Why Auto control ? because only these filters will be able to be modified by automatic simulation EQ.

We will see this in depth shortly.

To define the Match range, i.e. scope of simulator action, use the green lines from the screenshot hereunder.



These 2 green lines define 2 frequencies where the level is attenuated of at least 30 dB.

You can choose your own value instead of 30 dB depending on your needs, but generally 30 dB is enough.

A lower value will cause more overlap in frequency between 2 speakers without correction, and this is not

always desirable.

Settings overall max boost and individual max boost define the maximum allowed amount of the global

correction to apply (many cumulated PEQs), and the maximum boost of single PEQ filter.

Setting Flatness target is a precision threshold of the simulator. Corrections smaller than this value will not be

processed by the automatic simulator.

NB: all settings in each panel are specific for each measurement. This is the same for filters from the EQ Filters

panel.

v) AVAILABLEFILTERSINREW:

Here are all filters types that REW can simulate and that are compatible with DSP A8:

o PK: Parametric equaliser PEQ (defined with Frequency, gain, quality factor Q)

o Modal: parametric equaliser where Q is adjusted depending on needed Decay

o LP: 2nd order Butterworth low‐pass filter (Q=0.707)

o HP: 2nd order Butterworth high‐pass filter (Q=0.707)

o LPQ: 2nd order low‐pass filter, with adjustable Q

o HPQ: 2nd order high‐pass filter, with adjustable Q

o LS: Low Shelving



o HS: High Shelving

o HS 6 dB and LS 6 dB: High and Low Shelving with a slope of 6 dB/Oct.

o HS 12 dB and LS 12 dB: same with slope of 12 dB/Oct.

o NO: Notch filter (rejection filter, a very narrow PEQ with a high level to suppress a band of

frequency)

o AP: 2nd order All‐Pass filter with adjustable Q (add 360 degree of phase in lower frequencies,

and constant delay for all frequencies)

w) SPEAKERFILTERING

This is a usual practice to tune a system without simulator:

o first fix the speaker response with EQ

o then apply the x‐over

o hence the filters have the expected behaviour because the response curve is flat.

But in our case, we can use a great simulation tool which allows us to simulate all the filters types at the same

time. They do not prevent you from using it for filtering and equalisation to fix issues in the response curve.

The DSP A8 is capable of defining any shape of filters, which we will simulate so that it exactly matches our

needs.

Hereunder, there is a simulation for a high‐pass (purple curve) and a low‐pass (orange curve) with different

settings. One is a high‐pass with low Q smaller than 0.7 (over damped, wide band) and another one with a high

Q greater than 0.7 (under damped, narrow band). The blue curve is the result of the action from both filters.

It’s easy to understand that by adjusting the shape of the filters, we will be able to smooth‐out a large part of

the response curve; as if we were using PEQ filters but instead we are doing filtering and equalisation at the

same time. The only difference between low‐pass and high‐pass with a PEQ filter is in the shape of resulting

action.

The LP, HP, LS and HS filters have asymmetrical shapes (the behaviour is different before and after the center

frequency), and the PEQ, notch and modal filters have symmetrical shapes. So this not a single filter, but a

combination of filters that will interact between themselves to create a "single filter" to apply to the response

curve.

To get a proper result that matches our simulations, our DSP needs to be able to cope with all filters type we

will use; otherwise, simulations will provide wrong results.



Let's take back the response curve from last example and let's apply many filters to match the closest we can

the target curve before starting the automatic correction.

3 asymmetrical filters have been chosen (see green box):

One high‐pass HPQ filter with low Q to damp the peak at 200 Hz

One filter is enough here because the speaker response already has a natural roll‐off of 12 dB/Oct. and a

high Q factor. Adding a 2nd filter would create an acoustical slope of 36 dB/Oct. which would be too

steep for the chosen target curve. In this situation, even if a natural roll‐off of the speaker acts as a filter,

it will be necessary to apply an electrical filter to protect the speaker against damage and distortion.

So one PEQ filter will smooth‐out the peak at 200 Hz

2 low‐pass LPQ filters with low Q (< 0.7)

They act slightly above the expected cutting frequency, but with a low Q, this will damp between 1.2

and 4 KHz. The response curve being nearly smooth above 1.5 KHz a slope of 24 dB/Oct. (2 filters) is

adequate here. We can notice this in 3.2 to 8 kHz area with the blue line.

Let's see the result in screenshot hereunder:



We can notice, in the yellow circle, the area where the combined action of the 2 filters HPQ and LPQ decreases

the level of 2.5 dB between 500Hz and 2 KHz. It will be necessary to add a low‐Q PEQ to increase the level to

retrieve the initial level.

The red circles show the areas where PEQ actions will be needed in order to match the target curve.

Probably 3 cut PEQs and 1 boost PEQ will be necessary to get the expected result.

The blue circle, match range setting value has been decreased to 4000 Hz, because above this frequency the

response curve is fine.

x) AUTOMATICEQUALISATION

Now let's use the automatic function available in REW to equalize the response curve. To achieve this, a

simulator will probably perform better than ourselves to find the optimum PEQs.

Before starting with the automatic equalisation and after the latest deductions, we need to modify the

settings of the simulator. They are in the red box in the screenshot hereunder.

Setting match range has been decreased (from 6 KHz to 4 KHz) because LPQ filter has proper action.

Settings max boost has been increased of 1 dB so that the automatic simulator behaves properly to

boost the area between 500Hz and 2 KHz.

Setting flatness target has been decreased to be set to 3 dB so that the simulator detects the

anomaly at 2.8 KHz, which is less than the previous value of 5 dB



Fill‐in these new setting values, then click on the match response to target button in order to start the

automatic process. Results will be displayed in the EQ filters panel.

Our last conclusions (3 cuts and 1 boost) are well seen in the simulation results (see EQ filters: 1, 2, 3 and 5). A

boost appeared to try to fix the dip at 320Hz, although this area is not minimum phase. This area will probably

be disabled because this filter may not have the expected acoustic result. It is necessary to add a note in this

measurement to check by listening if this filter is needed or not.

For each filter that REW will add, and mainly the boosts, it will be necessary to check if the covered frequency

area may be fixed or not, depending on the listening tests.

Central boost (EQ 2) is at maximum level that we allowed, Q=1.71 is less than half the gain (+4dB) then

this area is wide enough to avoid tonics or unexpected distortion.

EQs 1 and 3 can't be avoided, they are necessary to fix these big peaks in the measurement curves. Q

factors are low enough to act on a wide area. These big fixes, more than ‐6dB, even if they are

necessary to get acceptable simulation results, can be too aggressive in a practical situation.

EQ 5 has a high Q factor value and a high frequency value, and may not be able to properly fix the

issue. According to what has been seen in the distortion chapter, this boost should be improved in

order to limit the distortion. But a high Q value will represent a small listening area. The higher the

frequency is; the lower the Q factor should be to have an impact in a large listening area. In order to

confirm this filters usefulness, you need to check if the issue is present for all the position

measurements.



For each cut filter with high value (more than ‐6dB), the listening tests will allow to check that subjective

dynamic is not compromised.

Add this step in the measurement notes.

To get an overall view and to check the result, we need to use a resolution close to ear resolution, then choose

a smoothing of 1/3 Oct. Changing smoothing during the EQ operations will allow to fix problems with a high Q

factor and big errors we could not see with our too strong smoothing value. On the other side, a 1/3 Oct.

smoothing will mask issues that are not audible.

Let's see if we use 1/3 Oct. smoothing.

Not bad?

You can improve the results to closely match the measurement and the target curve by restarting the process

with new filter settings, or doing it manually for experienced users.

It’s also possible to click on Optimisation gains, Qs & freqs to easily relaunch one optimisation process with an

already computed equalisation.

When you've got a proper result, save the simulation in a new folder named Simulation (see red box). Choose

a file name "EQ MD" if this is Medium and Right (Gauche/Droit=Left/Right side)



Generally, here are a few advices for equalization operation:

In most cases, Q values for PEQ should be less than 5.

Limit boost to 3 dB (or more depending on the type of issue to fix and/or dependencies with many

filters) with the settings overall max boost and individual max boost; listening tests with high volume

level should be done to check the power handling (new distortion measurement can be done).

The higher the frequency is, the lower the Q factor should be.

Limit cuts: if values are greater than ‐6dB, listening tests are needed.

Redo these differents steps for each speaker.

I strongly advise you to write down used LP and HP filters in each measurement note. E.G. HP380 Q0.5/LP3300

Q0.64/LP3150 Q0.57

Indeed, Optimisation gains, Qs & freqs operation can sometimes disable the filters, even if there is manual

control in EQ filters tab.

y) INSTRUCTIONSFORTWEETER,WOOFERANDSUBWOOFERSPEAKER

To simulate tweeters, don't hesitate to use a strong high shelf filter from as early as 8 KHz, because a car

interior deeply absorbs these frequencies.

The slope to use for the high‐pass is often 24dB/Oct. because tweeter response is generally flat in their use

band.

For woofers, one of the 2 is generally weaker in low frequency, it will be necessary to limit the action of PEQs

for the boost between 50 and 100 Hz.

In a 3 ways system, woofer high‐pass and low‐pass (E.G. 70 and 300Hz) are often very close to each other and

can interact and decrease general level. Then you must be careful about the "coincidence" of chosen filters

(For those who know, see the JMLC spreadsheet, coincidence curve shows this phenomena)

Usually, we uses slopes of 12 dB/Oct. for high‐pass (natural acoustical roll‐off for a speaker is often 12 dB/Oct.,

but this depends on the speaker mounting) and 24 dB/Oct. for low‐pass (response curve is flat or rising).

For subwoofers, due to the 1rst order modal ringing and the length of the car, a dip in response curve is always

present between 55 and 95 Hz.

The response curve for this type of speaker is then, in 99% of the cases, a peak between 30 and 55Hz followed

by a dip (modal ringing) and then a high fall (nearly 12 dB/Oct. between 65 and 150Hz) and often there is a rise

or cut between 100 and 200Hz.

The natural low‐pass slope is then usually strong and one single 12dB/Oct. filter with a high frequency high

enough (higher than expected target) often allows to get an acoustical slope of 24dB/Oct. 3 PEQ filters are



needed: one cut for the peak between 30 and 55Hz, a slight boost (3 dB max) for the dip between 55 and 95

Hz, and sometimes a cut after 100 Hz.

z) CONCLUSIONIn this chapter, we've seen how to simulate a speaker response curve. We also have seen that filters setting,

for any type, cannot be evaluated one by one but as a whole, because they interact between themselves.

There is not a single filter but a combination of filters.

We've also seen that electrical filters are not placed in arbitrary order but they are setup depending on real

response curve of the speaker, in order to get the final acoustical response as close as possible to the target

curve we have chosen.

This chapter clearly shows the limit of tuning by ears and the superiority of the simulation systems. How to be

sure to choose the proper filter if we don't know what the response curve looks like and if we are not able to

visualize the action and interaction of multiple filters?

mooc waveflex part 7 - waveflex caraudio · mooc dsp a8 + rew v5 – part 7 ... and open the first...

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