Perfect bass with Equalizer APO and REW Tutorial

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Perfect bass with Equalizer APO and REW Tutorial

Welcome to this tutorial where we're going to use two free pieces of software and an affordable microphone to perform acoustical measurments, and then use those measurments to set up manual equalization filters to help you improve the bass response in your audio system. Hopefully, this will help you get the perfect bass!

In a nutshell the process you in this tutorial are as follows. We measure the bass response, then create filters to correct the flaws we find in those measurements. The goal is to achieve a smooth and fairly flat frequency response. A lot of people dislike the sound of a flat frequency response, so we go further by applying a bass boost filter to adjust the tonal balance to create warmth. If done correctly the net result should be detailed and punchy bass at a balance which you enjoy subjectively. To protect your speakers and subwoofers we will also look into adding a protection filter.

This tutorial may look long, but in actuality there's lots of screenshots here, so it's really not that lengthy. To see the actual optimization process without reading about the software needed and how great it is, you can jump directly to the beginning of the REW and Equalizer APO tutorial to see what you're up against.

Calibrated measurement microphone for REW

To do acoustical measurements you need a (preferably calibrated) measurement microphone intended for this kind of work. In this tutorial we will be using the very affordable miniDSP UMIK-1 which is an easy to use and effective USB microphone on the cheap. The UMIK-1 is available through my Amazon US and Amazon UK affiliate links, or you can get it directly from miniDSP.

Software used in this tutorial


To follow this tutorial you will need two pieces of software, both of which are completely free.

Firstly, you'll need REW which is available for download here. REW will enable you to make acoustic measurements (and many other things, such as creating house curves, setting up automatic parametric equalization filters and do impedance measurements) but in this tutorial we will only use REW to perform acoustical measurements. Furthermore, this tutorial assumes that you know how to install software and how to take acoustical measurements.

If you don't know how to take measurements, this helpful guide in getting started with REW (PDF) may come in handy as it details the basics of using Room EQ Wizard. Just for the record, the REW guide is in no way related to DIYGeezer, but it's an excellent resource deserving of being promoted. It's very detailed and the page count may scare you but it has a lot of screenshots, so it's really not that bad.

Secondly, you will need is Equalizer APO. This stellar software will turn your computer into an extremely powerful DSP engine and allow you to set up delay and create all sorts of filters such as peaking filters, low-pass filters, high-pass filters, band-pass filters, notch filters and many more. You can even use EQ APO to implement filters you create by using REW's automatic algorithms. However, in this tutorial we will use EQ APO to create manual filters as that affords us the most flexibility. Equalizer APO now has a fantastic and user friendly GUI, so there is no longer the need for a third party GUI like the PEACE GUI.

First caveat, Equalizer APO is Windows only and to leverage it you must be using your computer as source for all your audio and movie playback. One of the many great things about Equalizer APO is that it's system wide, so all audio going through your operating system, such as the audio from a Youtube video, listening to music streamed from Tidal, Spotify or even Soundcloud, will be processed.

Using DSP filters to improve the sound from other sources

If you want to implement equalization filters through other means than EQ APO, for example to manipulate the audio from your CD-player or standalone network streamer, you should check out miniDSP 4x10 HD or similar standalone devices, or you will need to somehow take the outputs of those sources and run them through your computer to have EQ APO apply the filters.

Important when setting up the Equalizer APO settings

Equalizer APO configurator window

Equalizer APO not working? A common mistake people make is forgetting to activate their playback device in EQ APO. In the screenshot above you can see that the AMD HDMI Output is activated, this is my interface of choice, and it's used to send audio to the surround processor. By activating it in this menu we can be sure that the filters we set will be applied if our interface is supported.

So please make sure that all your playback devices which you intend to do processing on is activated, and also don't forget to reboot your computer after installing Equalizer APO or else it will not work. If you need to change these settings sometime in the future, they are available through C:\Program Files\EqualizerAPO\Configurator.exe or wherever you installed the software.

To make sure that Equalizer APO is activated on your audio output after reboot, go to and start C:\Program Files\EqualizerAPO\Editor.exe (or wherever you installed the program). By the way, a tip is to create a shortcut for the EQ APO Editor on your desktop as this is where you will be changeing your filters.

Equalizer APO GUI and filter editor

In the top left corner you have your gain. Play some music and adjust the gain. If the volume goes up and down as you adjust your gain EQ APO is activated. Congratulations. :)

This may be different as you read this but the present version of Equalizer APO come with some filters activated through example.txt which is included in the second row from the top. Disable those filters and set the gain to 0. You should see a flat frequency response graph in the analysis panel at the bottom of the GUI. This means that no filters are effective. We don't want to pollute our measurements with some preset filters, now do we? On that note, make sure your surround sound processor or surround receiver does not have any active filters too.

How to use REW to set filters in Equalizer APO, a tutorial

Important info on fixing peaks and dips using EQ

First off, we need to talk about using equalization in general to avoid the potential of damaged equipment.

Fixing response peaks using EQ is straight forward. You just create a filter to lower the peak and the peak goes bye, bye. One downside is that you will need to play louder to achieve the same perceived SPL, because now that dip is gone and you need to crank your system louder. This will in turn give you a practical loss of headroom. Another potential downside is that you may lose some resolution if the peak is large and you need to subtract a lot of information. This is normally not a problem.

Fixing dips in the response on the other hand is very costly and may not even be possible by using PEQ filters alone. If the dips are a result of destructive interference between your room and the speakers no amount of bass boost will help you and will actually make the problem worse. The same goes for out of phase destructive interference between two sources, such as two subs. If they interact destructively there will be an acoustical short and the output from one subwoofer will cancel with the other sub's output. Boosting in any of these situations will be like trying to fill a black hole. It's impossible and must be taken care of by using placement, acoustical treatments and phase/delay to fix.


If on the other hand it is possible to fix those dips with parametric EQ filters you need to understand the implications of boosting. For every 3dB of boost you need double the amplifier power, so don't go nuts boosting that 15dB dip, that's 32 times the power required at those frequencies(!), so be cautious and do 3-6 dB of boost at max unless you have good reasons and knowledge to do more. As you can see, boosting quickly eats away at the amplifier headroom in your system. In addition to gobbling up amplifier power two more and very important limits must also be known. These limits are the speaker excursion capability and power handling capacity. If you boost too much you may damage your speaker mechanically by the voice coil being smashed against the motor or burn the voice coil by going beyond the power handling capacity.

Summing up: To get your target response subtract as much as you can by removing the peaks and then check if the remaining dips can be corrected with filters by applying small boost filters and see if the response improves. If the response doesn't improve with the boosting filters you must investigate if you have destructive interference through room interactions or phase issues between the components and use delay, placement, acoustical treatment or a combination thereof to improve the situation. If the dip does improve with the boost filter you can boost around 6dB, but if you want to boost more, be sure to know the limits of your amplifier, speaker excursion and speaker power handling to avoid damage.

Take acoustical measurements

Measurement with no filters except the crossover

Acoustical measurement of left, right and both speakers

Here we have three measurements from 20 to 100 Hz. The blue one is a dual 15" subwoofer in the right side of the room, the red one is an identical dual 15" sub in the left side of the room and the green line are both subwoofers combined, for a total of 4x15" woofers. The color scheme will be kept throughout unless stated otherwise.

Why measure the subs separately and combined? The answer is to see how they interact. If they had measured nicely alone but poorly together chances are we would need to change the phase, delay or place them differently. Since the green line looks like a sum of the blue and red line we know there are little destructive interference here and we can move on to fix the response by applying filters.

Our goal for the frequency response is to achieve a flat and smooth curve with the use of as few DSP filters as possible. After a flat frequency response is achieved we will use a low-shelf filter as bass boost in order to get a subjectively pleasing tonal balance, but more on that later.

We start by analyzing the frequency response measurements. Since there are no apparent destructive interactions going on, we limit our focus to the combined graph.

Analysing the bass response

Bass response measurement analysis

Notice the red line that shoots across the center of the measured response. That's what we're trying to achieve by correcting the 4 problem areas.

  1. As we can see the bass response is falling from around 35 Hz, correcting this far down can be risky, and to keep this tutorial simple we will not focus on that.
  2. This is a small peak we will try to pull down with a PEQ filter. As we can see it is above the red line between 35-50 Hz and the center of it is about 6dB in amplitude above the target.
  3. This is a dip and will be the last response issue we will tackle, so we dismiss that for now. Generally you should wait to correct dips until you've fixed the peaks as you may find that lowering peaks can also improve the dips due to room interactions.
  4. Here we have a lot of energy compared to the target, and particularly around 95 Hz where it's about 10dB louder.

So we're first going to tackle peak #2, then peak #4 and lastly see if we can do anything with the dip at #3.

Correcting for bass response peak #2

We open up the Equalizer APO filter editor

Equalizer APO add parametric EQ peaking filter

Do as the annotations tell you and you should get a peaking filter panel inside the editor.

Equalizer APO peaking filter panel

This begs for some explanations. The center frequency is what it says it is: The center frequency for the peak (or dip) you want to correct. The Gain controls how much you want to boost or subtract at that frequency and the Q factor is how wide or narrow the filter should be. A low Q creates a wide band filter and a high Q creates a narrow filter. Luckily, Equalizer APO has an excellent GUI showing the transfer function of our filters at the bottom of the program. Here are a couple of examples of a very high and a very narrow Q factor.

Small Q creates a wide filter

Equalizer APO's transfer function affected by a small Q factor

Large Q creates narrow filter

Equalizer APO's transfer function affected by a large Q factor

Try to keep your filters fairly wide, preferably above 0.5 or so, as these filters will also affect the phase and narrow filters will generally not sound good.

Play around with the controls and look at how the different settings affect the transfer function. Be careful not to play any audio while you're doing this though, as Equalizer APO takes effect immediately and if you do something silly it may cause damage.

Back to fixing the bass response peak #2 again

Looking at the 2nd peak the center frequency is around 44 Hz and it's about 5dB above the target. Using Equalizer APO's transfer function display I adjust the Q until I get something that looks like the inverse of the peak and end up with these settings.

EQ APO correcting 2nd bass response peak

To verify if this is a good filter we simply measure the frequency response again with the filter active,

REW frequency response measurement after correcting 2nd peak

and sure enough, the purple line is the new frequency response after the filter was applied. PROFIT!

A tip here is to do this iteratively, make small adjustments and measure all the time until you find the best possible filter, and again, try to avoid narrow filters. As this process will include a lot of tweaking and taking many measurements another great tip is to use remote control software such as TeamViewer or VNC to sit at another computer and adjust the audio PC while doing this. That way you can drink coffee and relax while performing this time consuming process.

Tackling peak #4 in the response

This is done in the exact same way as peak #2 and going through all the steps again would be redundant. The only thing worth mentioning is that you want to add another parametric EQ filter panel to Equalizer APO and work off of that.

Room EQ Wizard frequency response measurement after correcting for peak #4

After a lot of tinkering, the pink line is what the frequency response ended up looking like after correcting for the peak at around 90 Hz.

Trying to correct for dip #3

Now we have a decision to make. Should we leave it like this or try to do something with the dip between 50-60 Hz?

As already mentioned boosting comes at a cost in terms of headroom, speaker excursion and speaker power handling capacity. Furthermore, not all dips are equal and some can't be corrected, so a good idea is to first check if the dip is fixable through equalization, and if it is, then we create a filter to fix it and lastly we sit down and listen while turning that filter on and off to see what we like the most.

An important part of setting boost filters is to avoid clipping in the digital domain. When you've created a new peaking filter panel for your boost in Equalizer APO and start to play with the controls you may experience that you run the risk of clipping. This is indicated in the transfer function graph as the problem area is colored in red.

Equalizer APO warning of potential for clipping

If this happens to you, lower the gain until there is no red left. In this situation, we see that I'm 3.6dB above the limit and we need to lower the gain by that amount.

How to check if a frequency response dip can be corrected

From our measurements we can see that the center frequency of the dip is around 55 Hz, hence range from 50 to 60 Hz. Furthermore, we see that in comparison to the rest of the line it is about 6 dB lower in amplitude. To see if our filter will improve or worsen the case we create a new peaking filter looking like the opposite of the dip. If you have a large dip, of say 10dB, you can try to boost it with 3-4dB just to see if it has any effect. In this situation I'll use 6dB straight away since I have ample headroom and 6dB of bass boost is nothing in this system.

So setting a filter with a positive gain at 55 Hz and making sure to lower the gain enough that we are not clipping in the digital domain we measure the frequency response again.

Room EQ Wizard frequency response measurement after correcting for dip

The blue line is the new frequency response after applying the bass boost filter with Equalizer APO, and the result is excellent! Had the dip grown bigger as a result of the boost filter, you would know that you could not have done anything about the dip with equalization. If the dip had grown bigger as a result of a boost it is likely that you're having a room related issue or a phase issue, and those problems must be solved with solutions discussed earlier.

Summing up and concluding

Frequency response before and after equalization

So this is the frequency response before and after three pretty broad parametric equalization filters. Pretty impressive feat, and it sounds great, no doubt.

As you may have noticed we have been correcting for the frequency response universally, i.e.; we did not correct the left and right subs separately. The reason is that the wavelengths involved around these frequencies are so long that you will have a hard time perceiving where they are coming from. We could have corrected each of the subs individually too, but that would have made this tutorial far longer and it's already very long, but it's fully doable and now that you know how to use Equalizer APO together with REW you can give it a go on your own and see what you prefer.

Help! A flat frequency response sounds thin; let's bass boost!

After making your bass flat and smooth you may feel like the sound is thin and lacking. This is very normal for a number of reasons, one being human perception and the way our ears work in terms of sensitivity. The solution is to put on a low-shelf bass boost to restore a subjectively pleasing balance between lows, mids and highs.

How to create a low-shelf bass boost filter to improve the listening experience? To create a low-shelf filter in Equalizer APO click on the + sign, then Parametric filters and Low-shelf filter.

Creating a low-shelf filter in Equalizer APO

You will now get new filter controls for the low-shelf filter.

How does the low-shelf filter work?

Configure a low-shelf filter in Equalizer APO

This is intended to be an example, but I personally like low-shelf filters going from around 150-300 Hz with a boost of up to 6dB to create some meat on the bone. The center frequency you set will determine where the boost starts. The gain will determine the overall boost and if you change the filter type to include slope from the drop down you can also control how steep the slope of the filter will be.

How big the low-shelf boost filter should be depends on your subjective preferences and listening levels. The lower in volume you listen, the less sensitive to bass your ears become, so when you're listening to very low level music you need to boost more than when you are listening to loud music. This is part of the reason why Equalizer APO is genius; with a few clicks you can change your low-shelf gain setting. I recommend listening to a bit below normal listening levels while adjusting these settings for the first time, and of course make sure all your previous filters are activated and that there is no potential for clipping by lowering the preamplification gain in Equalizer APO, so pay close attention to the transfer function graph of all your filters.

Equalizer APO transfer function with all correction filters

Your final transfer function in EQ APO may look something like this after you've applied both correction filters and the low shelf boost filter to adjust the tonal balance.

Protecting your system against over excursion and damage

To add a layer of protection for your loudspeakers and subwoofers you can implement a high pass filter. A high pass filter will limit all frequencies below a set threshold. For example you may want to limit everything below 25 Hz or even higher than that depending on your system. Consumer subwoofers are often high-passed by the manufacturer, but passive speakers are not and if your speakers are ported they will begin unloading below the tuning frequency of the port and a high pass filter may be in place to protect those beautiful speaker cones against damage.

EQ APO adding high-pass filter to protect speakers and subwoofers

Play around with the controls of the high pass filter and notice how the response is affected below the set frequency.

High-pass filter initiated in Equalizer APO

We see a dramatic drop in level below the set frequency, so indeed everything below that threshold is limited, hence your speakers protected.

Alternative to positive low-shelf filter

An alternative to the positive low-shelf filter is a negative high-shelf filter. It basically does the same thing, but instead of boosting the low frequencies you limit or subtract the higher frequencies. The net result is that the tonal balance shifts towards the lows as you reduce the highs more and more, and in turn realize the same bass boost as before. The trouble is that a potential for loss of resolution may be the result if you lower the highs, and you potentially still need the low-pass filter to protect the highs anyways, so the best way of going about it is to use a positive low shelf filter.

Last words on this tutorial and the application of it

Please tell all your friends about this tutorial, and if you have any questions you are welcome to post them in the comment section below. I tried to keep this as short and sweet as possible, but as things tend to go I wrote a long tutorial. :) Hopefully, my emphasis on protecting your system against cooked voice coils and smashed speaker cones came through clearly. Watch that boost, especially when you're playing loud. At low level listening you have much more headroom, and hence you can boost more.

Don't forget to tell the World, have a nice day, and happy DSP'ing with Equalizer APO!


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Discussion on Perfect bass with Equalizer APO and REW Tutorial

I have downloaded many programs to increase sound volume only to be disappointed and then uninstalling them but the APO equlaizer is the best program to increase sound volume it works like a charm for me.

You did not mention, why did you decide that the target curve should be where it is? I have no idea where mine should be.

Hum, the HUGE help with eq-apo is the "delay" function, which is essential to get subs and front on phase ! You really should talk about this in this article.