What should you focus on and how does different calibration settings affect sound, including description of music samples for evaluation.
Even at low volume there is this sense of power and realism. And when you turn it up the physical impact is massive. No excessive bass-boom, just power and detail. The bass just sounds natural.
This is not possible to achieve with only a pair of “full-range” main speakers, even if you use dsp processing.
In Room2 I am testing and experimenting with bass-system calibration – different approaches and settings. To find out more about what is important to get right, so my customers can get better guidance for calibration. The current system consist of four Compact Horn subwoofers – two V110 up front, two V6030 at the back.
To be able to adjust anything, the system must have proper topology, with processing for crossover and delay on main speakers, and separate dsp on the bass system. This is quite easy to achieve today, several options for pre/processor, the bass-system amplifiers have dsp.
What was done
I will make a new system calibration, starting with positioning of subwoofer units, then continuing with dsp tuning to get the best possible sound from this system in this room.
First, I moved the V110 units so that the port opening is inside towards the center of the room, and there is a free view from the port to the listening position. They were previously placed with port against sidewalls in the corner, with the F2 main speakers obstructing the path for the sound.
The back units are not placed in the best position, but in this room this is the only location possible. Simply accept, and make the best of it.
Then I adjust eq on the front V110 units. I use a target with some lift towards the low end. To reduce the level of tactile feedback from the floor, level is reduced from 20hz and down. Especially for music, too much movement is unnatural and distracting, and low frequency noise becomes too noticeable on some recordings. Allpass-filter tunes the timing and phase at the upper end towards the crossover frequency.
The back V6030 units get eq adjustments, and corrected delay. This delay is important for the end result.
I never try to use eq to boost dips in the frequency response, and use decay and waterfall to look for resonances that can be cut down.
Integration with main speakers is the last step. Here I look at group delay, step response and spectrum plots to determine the correct time delay on main speakers. If it is not possible to get the system to sum up properly around the crossover frequency, I try to adjust the allpass-filter to change the phase on the bass-system. Crossover is set at 120Hz, note that changing crossover also requires changing delay on main speakers.
Final tuning of the delay for main speakers is determined by frequency response and group delay. There will be a small range that will work, and we can do fine tuning inside this range by listening.
Back V6030 units makes only a small difference on measurements, they do not add more capacity, but have huge impact on sound quality.
If the system is perfect, it is easy to see when everything is just right – frequency response will follow target perfectly, timing is perfect, the are no resonances. Real-world systems in ordinary rooms are never like this, and it is not always obvious to see what and how to adjust for best results.
To focus on timing is the key to get better bass. Use GD – group delay – to see if there are acoustic effects causing delays, and verify integration between main speakers and bass-system.
A more typical before-calibration example:
Then I look at early decay, to see how dead the bass range is, and find resonances. Here we see a resonance at 38Hz, which lifts the level around the resonance frequency and rings on for a long time, and surely this sounds very boomy – the bass guitar has only one string, all bass drums sound similar with no definition. This is the typical situation, before attempting to fix anything be it using dsp or acoustic remedies, regardless whether subwoofers or full-range mains are used. But this can be fixed.
The spectrum plot is important, here I can see how the sound starts and stops in time across the frequency range. I try to get a narrow line that starts simultaneously across the whole range.
If you thought that did not look so good, here is a typical real-world system, set up with wrong delay on main speakers:
The step-response is nice to look at, but the shape of the signal does not correlate well with sound quality, it is not necessary to have a perfect shape. A good system can be something like this, quite tidy and dies slowly resonating around the cut-off frequency of the system.
In a faulty system the signal is spread along a larger time interval. Here are some examples where you can see how the shape of the step is severly distorted. These samples are collected from typical real-world listening rooms, they are from customers – BEFORE calibration, or places I have visited. Something like this is what I usually find.
It may not be possible to remove all group delay, some resonances will remain, there may be dips in the frequency response that can not be filled. To choose the best compromise is where this objective technical exercise starts to be more like art.
If this exercise does not look tempting, it is still possible to get a good calibration of your system by getting assistance from an expert. To measure is easy, just send the data files and receive dsp files and settings in return.
It would be nice if I could just supply an app that fixes everything, but this is not possible now, there are simply too many variables to account for. Rooms are different – shape, absorption and resonance pattern of boundaries, placement of subwoofers and listening position, tactile effects are not in the measurement. So you need to listen, and adjust according to what is experienced and heard.
Tuning the bass character
The character of the bass is determined by frequency response and timing and sound field properties. By changing any of those parameters, it is possible to tune the sound according to what we hear in relation to how we want it.
Frequency response is easy to change, with the dsp in the system. A lift towards lower frequencies will give more weight, a heavier bass. Lifting the curve slightly in different frequency bands affect perception of punch. If there is too little weight on large drums, a lift around 30-50hz can be tried, too hard or thick attack can be reduced by lowering around 80-120hz.
Changing the delay on main speakers affects attack, listen to drums and change in steps around 0.5ms/20cm.
Listening is the most important “measurement”. The final saying is always listening. If it does not sound good, it does not help what the measurements show. But if the sound is far off, the cause can be found in measurements, you just need to know what to look for. And then go back, adjust, and listen again.
I listen at both low volume and louder, where low usually is -30dB, louder is 0db or more. A good system sounds good at any volume. But some properties of sound requires louder to be perceived correctly, such as tactile feel and the very deepest bass.
Daft Punk – Get Lucky
The combination of a tight, precise, powerful and slightly heavy kick, and the bassline, makes this a good number one just to get an idea if the system is worth your time to listen to. At -30dB master volume everything in the bass is audible, you can follow the bassline and hear all notes, and the tactile character of the bass drum makes it very exciting to listen to, makes you want to turn up the volume. At 0dB the bass is hard and powerful and physical. If I don’t get a physical sensation here, I may not bother to waste more time on that system.
Anne Bisson – September in Montreal
This is one of 2 samples I use for a quick overall evaluation of sound systems. In a few seconds the percussion, piano and vocal separates real performance from marketing mumbo. For bass, this one is rather simple – it is easy to get too much bass here, the bass drum should have a nice, mid-heavy, subtle punch. If the drum sounds like hitting a pillow, it is time for the microphone once more, or perhaps go shopping for a new bass-system.
Panda Dub – (Antilogy) Lost on the Moon
Here I pay attention to the lowest tones, they should give an increased sense of power, but not sound louder. Sound quality is very good on this recording, and the bass is exciting when done right.
This is the 2nd sample I use for quick evaluation. Lots of space, sounds are placed all over a large 3D landscape, and the bass is very powerful when done right.
Flashbulb – (Kirlian Selections) Autumn Insomnia Session
The explosive dynamics of the drums are totally tight and very physical. When the synth enters with its droning bass, the drums should remain tight and hard and powerful.
Flashbulb – (Reunion) Walking Irrevocable
Very tight and powerful drums makes this a good test. If it sounds exciting, that is a good indication of basic bass performance in place.
Also try Oak Lawn UFO – massive bass, tight.
Flashbulb – (Piety of Ashes) Leaves
This is the sub-bass test. The deep bass transients should be more noticeable than actually heard, a sense of push and heaviness, and quite powerful if the system is set up with some lift towards the low end. The deep bass is clean with little harmonic content, making it revealing for distortion and capacity problems.
This album has several tracks that will challenge your bass system, with its combination of hard midbass attacks and full-range all the way down.
Hot Sardines – Album Live at Joe’s Pub
This recording has a low-end boost, and can sound unnatural if the sub-bass is too strong. Character of the bass is exciting and dynamic, and it will sound quite heavy in the lower frequencies.
Vanessa Fernandez – (Use Me) Hard Times
Here I listen for the bass drum, with its powerful and hard attack – it is very easy to get too much. It should be tight and powerful, but not thick.
This album has very good sound quality, try Here But I’m Gone for some more good bass.
Infected Mushrooms – Avratz
This bass-test-classic lets you turn up the volume for insane bass without going deaf. Massive, heavy punch, feels like you are hit by a shock-wave and pushed backwards.
Sera Una Noche – Nublado
Yes, we need Nublado on the list. It is the mid-upper bass from the drums that makes this one interesting for bass. Play it loud, around 0 to +6dB, the drums will have hard, sharp feel if the system can reproduce it, but this also depends a lot on chosen target – if the level in upper bass is slightly moderate, it simply will not be as powerful. It is not only about getting the most powerful tactile feel, I find it more important to have proper balance and that the sound resembles that of real acoustic instruments. But it should always sound exciting.
A new one from 2019, now simply sit down and enjoy.
Good speakers and bass-system with decent capacity is only a starting point for achieving that addictive bass. The system must also be set up and calibrated properly to make it work.
Articles on bass calibration can be found on the net, but they are not very useful. They focus on frequency response, leaving timing and phase and tactile out in the blue. There are articles on multi-sub setup, where placement of subwoofers and eq settings are meticulously described to achieve a perfectly flat frequency response, with no concern at all for what is happening to the sound field or timing. Getting the timing right is more important than super-flat frequency response.
Everything can be measured, but it can be difficult to see how the graphs correlate to the sound we hear. To measure it easy, to know what to fix and how to do it is the difficult part.
A short-list for avoiding the most severe pitfalls:
- Fix the bass-system first.
- Focus on getting control of group delay and reduce decay.
- Before you attempt to boost dips in the frequency response, measure at different locations around the listening position, because cancellations can be very local relative to position.
- Look at timing on group delay and spectrum to determine delay for main speakers.
- Since you can not measure sound field properties (particle velocity, direction of sound, sound intensity), you need to listen to determine if it sounds good, especially for tactile feel.
- Target frequency response has huge impact on sound character, and when timing and decay is optimized, it will be much easier to adapt the system to your taste and preference.
- You need processing capability for crossover and delay on main speakers, if this is missing, your first step is to get an amplifier with sound processing, several options in all price ranges exist.
This was my attempt to give some good advice. I hope you find inspiration here for further exploration with bass and subwoofers and calibration.
If you have questions or comments, there is a facebook group called Kvålsvoll Design Community (Note the date on this article – if you read this several years later, facebook and the group may not be around).