# Bass Boost vs. Gain: What shoud I adjust?



## jack_e28 (Jan 20, 2016)

Hi I'm new to car audio but I've been researching lately and haven't found a definitive answer to my question.

A few months ago, I upgraded my 1984 BMW 533i with new speakers, a sub, 2 amps (one mono for the sub and one 4 channel for the mids), and a new head unit. I am usually good with electronics but with all the voltage, impedance, wattage, and other factors I was thoroughly confused and I decided tuning my speakers and sub was best left to a professional. So, I took my car down to a local audio shop and had them tune my two amps.

When I got the car back, I immediately was disappointed. My sub was not pushing out nearly as much sound as my friend's, who has the exact same sub. However, it was still louder than it was when I gave it to the shop. Also, the bass was rather muddy and didn't sound great overall. I decided to live with it for a few months but now I've decided to cut a hole behind my rear seat to bring more air to the sub (it's common to do this with cars like mine). 

When I removed the subwoofer amp today, I noticed the gain on the amp hadn't been touched since I gave the car to the shop, despite the owner specifically telling me that gain setting was the most important step they do in the tuning process. Also, the bass boost was turned all the way up (18dB for my amp). So, did the shop actually tune it correctly? I am almost positive that the gains should have been adjusted instead of cranking up the bass boost, but I've read different things on different sites.

My head unit is a Kenwood KDC-X399, my amp is a Kenwood KAC-9106D, and my Sub is an Alpine SWR-10d4.

The input sensitivity on my amp was set to 4 volts when I gave it to them and got it back, which is the preamp voltage of my head unit.
Any feedback would help. Thanks!


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## lurch (Jan 20, 2014)

That is exactly the opposite of what I would have done.

I would have zeroed the bass boost and increased the gain. Are you equipped with a remote bass level control? If so, increase the gain substantially and use the remote to control level.
Get rid of the bass boost, no wonder it sounds bad.


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## Weigel21 (Sep 8, 2014)

Absolutely not, they did a horrible job "tuning" the amp. And "tuning" isn't quite the word I'd use, but...

Alright, so what you need to do is dial that Bass Boost down to "0" or at least to no more than 2-3dB. 

Now, the gain may or may not be set correctly, it's an unknown at this point in time, but I have serious doubts based on how they set the bass boost so excessively. I'm surprised the sub didn't blow in the two month time period, as that amp was surly clipping every time you cranked the system (and surely well before that). 

So, you need to find the HU's maximum clean output level. You can do this a few ways, but there's really only one way you'll be able to do it without specialized equipment, and that's by playing test tones and listening. 

You'll want to dial the gains down on your amplifiers and then begin playing various test tones. I'd strongly recommend using hearing protection during this time, as it will get loud. Now the test tones you choose is completely up to you, but do play various tones, including some lower frequencies that will be played by the sub and higher frequencies to be played by the mids. 

After you find the HU's maximum clean volume level, you can then set the gains with the hU at said maximum clean volume level and the sub volume of the HU at +15. 

I'll leave a few links for you to look into with some good information. 

What Is Distortion?

Using test tones to set amplifier gain

https://www.youtube.com/watch?v=WhrDqke8BKo

Stereo System Test & Analysis Tones by Nino B.

Try to avoid excessive boosting, but if you must use the BB of the amp, then you may want to play a 40Hz test tone to see if the amp is clipping with said level of boost. 

I'd recommend using the 85Hz LPF of the HU with the amp's LPF set to it's 200Hz maximum level or set the HU's LPF to off and use the amp's LPF set around 80Hz or whatever seems to sound best to you. 

You'll want to have the HPF set slightly above the frequency used for the LPF, as it can create unwanted dips/spikes to have the crossovers overlap. No idea what your other amplifier happens to be. 

Now, no idea if the sub is in a ported or sealed enclosure, but if it's ported, I recommend setting the ISF to roughly 5Hz below the tuning of the enclosure. If the enclosure is sealed, then you can set the ISF to whatever you want, but I'd recommend around 30Hz as the sub won't play well below that in a sealed enclosure, so why waste the power. 

Think I covered most of what I wanted.


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## jack_e28 (Jan 20, 2016)

Thank you for your replies. 

I called the shop I took it to and asked them why they had tuned it this way, so they told me to bring my car down to see. The owner brought out the technician who had tuned my amp and he told me that he had set the bass boost so high because I asked for more bass, and my trunk was too sealed to get any more bass out of the sub without setting the bass boost up. However, when I took it to the shop the first time, I said I wanted more bass because I knew that the sub could push out more than it was now, and I knew I had set the gains too low. The guy told me the gains were already up as high as they could go. But, from my understanding, if the bass boost is up, shouldn't there be more room for the gains to be increased? I thought that bass boost is still amplifying the signal, just like gain, except to only a certain frequency range. I could be wrong, but it just doesn't seem to add up to me. 

Regardless, I'm tuning it myself later today or tomorrow with some test tones.Thanks for all your help


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## Weigel21 (Sep 8, 2014)

Bass Boost applies more gain to a particular bandwidth, centered around a specific frequency. For every 3dB of boost, an amp is asked to more/less double it's output around this frequency. With the BB set to 18dB, if a 40hz tone in a song is recorded at 0dB as a PEAK bass hit, then the amp would be asked to produce 1000's of watts more than it is capable of producing and thus it will clip like crazy. Unless they set your gain with a 40Hz test tone to ensure it doesn't clip worse than Edward scissor hands on crack, then I'd completely remove the boost from the mix. So to answer your question, the higher the BB, the lower the gain must be set to ensure clip free output. 

Again, it's easy enough to set gains properly, even if you doubt your abilities. 

Go through the links I listed, the first has a test in which a 400Hz tone is played at various levels of distortion. Surely you'll be able to hear 5%THD, but try to hear when 1%THD begins. That is the target THD one try's to avoid when setting gains. 

When you find your HU's lowest maximum clean volume level after playing various test tones, use said volume level to set the gain on the amp. Leave the crossovers set to full/off/through/or as high of a setting as possible so as to not affect the test tones being played. 

You can use a -5dB or -10dB test tone to set the gain on the sub amp. If you wish to use the bass boost, use a 40Hz test tone, as you'll see how much boost you can apply before distortion exceeds 1%THD. 

Once more, you'll want the gain's on the amps at minimum when playing test tones to locate the HU's max clean volume. And I urge wearing hearing protection. I have found that for me at least, wearing ear plugs does not affect my ability to hear the tone distort. I confirmed this by finding my ears detected the distortion at the same time my DSO201 pocket scope showed the wave begin to square off. 

If you really don't trust your ears, you can use your eyes, at least for the sub, as you can see the cone move differently when it begins to play 2nd and 3rd harmonics, as evident in the youtube link I left. 

Best of luck and let us know how things go.


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## Mikeb1302 (Jun 15, 2021)

jack_e28 said:


> Hi I'm new to car audio but I've been researching lately and haven't found a definitive answer to my question.
> A few months ago, I upgraded my 1984 BMW 533i with new speakers, a sub, 2 amps (one mono for the sub and one 4 channel for the mids), and a new head unit. I am usually good with electronics but with all the voltage, impedance, wattage, and other factors I was thoroughly confused and I decided tuning my speakers and sub was best left to a professional. So, I took my car down to a local audio shop and had them tune my two amps.
> When I got the car back, I immediately was dissapointed. My sub was not pushing out nearly as much sound as my friend's, who has the exact same sub. However, it was still louder than it was when I gave it to the shop. Also, the bass was rather muddy and didn't sound great overall. I decided to live with it for a few months but now I've decided to cut a hole behind my rear seat to bring more air to the sub (it's common to do this with cars like mine). When I removed the subwoofer amp today, I noticed the gain on the amp hadn't been touched since I gave the car to the shop, despite the owner specifically telling me that gain setting was the most important step they do in the tuning process. Also, the bass boost was turned all the way up (18dB for my amp). So, did the shop actually tune it correctly? I am almost positive that the gains should have been adjusted instead of cranking up the bass boost, but I've read different things on different sites.
> My head unit is a Kenwood KDC-X399, my amp is a Kenwood KAC-9106D, and my Sub is an Alpine SWR-10d4.
> ...


Sigh - as usual the internet is full of well meaning people positioning themselves as experts.

you cannot say that the shop did a bad job unless you look at the waveforms.
If your subwoofer input is derived from the main speaker line then it’s probable that this signal begins to roll off in the bass frequencies from around 80hz and below.
Let’s say that for some reason your cross over was set to 150hz. If you use gain to adjust amp output to not clip a 150hz tone then you’re going to see signals at 30-50hz down as much as 6-12db.
If you use gain to bring the level up at that frequency range, now your amp is going to be horribly clipping at 150hz. 

To solve this you would adjust your bass boost until your 40-50hz test tones are not clipping. Now your 150hz should also not be clipping. 
you’ve effectively flattened out the frequency response in the lower frequencies- just what you want to do!

this is no different to using a parametric equalizer dsp to correct frequency response.

In short - adding bass boost may be better at avoiding clipping then trying to turn up gain if you’re trying to amplify a signal that’s rolling off at the low end. 

Micro-Electronics Engineer (30+ years)


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## MikeyB (Apr 16, 2021)

jack_e28 said:


> Hi I'm new to car audio but I've been researching lately and haven't found a definitive answer to my question.
> A few months ago, I upgraded my 1984 BMW 533i with new speakers, a sub, 2 amps (one mono for the sub and one 4 channel for the mids), and a new head unit. I am usually good with electronics but with all the voltage, impedance, wattage, and other factors I was thoroughly confused and I decided tuning my speakers and sub was best left to a professional. So, I took my car down to a local audio shop and had them tune my two amps.
> When I got the car back, I immediately was dissapointed. My sub was not pushing out nearly as much sound as my friend's, who has the exact same sub. However, it was still louder than it was when I gave it to the shop. Also, the bass was rather muddy and didn't sound great overall. I decided to live with it for a few months but now I've decided to cut a hole behind my rear seat to bring more air to the sub (it's common to do this with cars like mine). When I removed the subwoofer amp today, I noticed the gain on the amp hadn't been touched since I gave the car to the shop, despite the owner specifically telling me that gain setting was the most important step they do in the tuning process. Also, the bass boost was turned all the way up (18dB for my amp). So, did the shop actually tune it correctly? I am almost positive that the gains should have been adjusted instead of cranking up the bass boost, but I've read different things on different sites.
> My head unit is a Kenwood KDC-X399, my amp is a Kenwood KAC-9106D, and my Sub is an Alpine SWR-10d4.
> ...


Realize this is an old post but let's put to rest the opinions on bass boost which are not in any way based on any engineering.
There's no magic to bass boost.
Typically if you look at the frequency response from the OEM head unit, it's not flat from 20Hz - 100Hz, it rolls off to protect cheap speakers or due to the natural bandwidth of the system.
If you shift the gain up then this roll off is just boosted equal amounts. So if 60Hz is at 10dB and 30Hz is at 3db then adding 3db of gain will give you 60Hz at 13db and 30Hz at 6db - still not a flat bass response.
Bass boost simply adds gain using an active filter that boosts lower frequencies more than upper frequencies - so bass boost might add 6db to 30Hz and 0db at 60Hz - now 30Hz is at 9db and 60Hz is at 10db - a much flatter response..
You might be able to do this using a parametric equalizer but depending upon the attenuation of the lower bass frequencies of the stock unit this may not get you were you want to be with the low frequencies out of the subwoofer unless you get the boost from 'bass boost' also.

Of course you still need to set this up carefully to avoid clipping. The way i do this:
1. Choose a frequency that's around 100 Hz or so with all low pass filtering on the amp set to the highest it'll go (typically 300 Hz or so).
Adjust your gain so that you're not clipping (Oscilloscope is best for this)
2. Use an RTA to adjust your base boost to try and get the bass response as flat as possible. If the adjustment causes the 100Hz level to rise a little then just adjust the gain down a little to get the 100Hz level back to where it was before you started playing with the bass boost. All this assumes any boosted frequencies did not get to levels higher than the 100Hz baseline.

If you follow this process then bass boost will not wreck your subs but will give you a bit more bass.


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## kory.jerrard.green (Oct 26, 2021)

Mikeb1302 said:


> Sigh - as usual the internet is full of well meaning people positioning themselves as experts.
> 
> you cannot say that the shop did a bad job unless you look at the waveforms.
> If your subwoofer input is derived from the main speaker line then it’s probable that this signal begins to roll off in the bass frequencies from around 80hz and below.
> ...


Hi, So I have LPF and HPF on my head unit. Should I set them at the same frequency or should the HPF be higher than the LPF? Because I've seen articles and people say both. Also, if I'm going to use the head unit to set my HPF/LPF levels, should set the LPF on the amp or turn it off? Also, what is the HPF and LPF slope? They have three different levels on my head unit, but I have no idea what they mean. Thanks.


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## Djaaz (12 mo ago)

terrible idea, gain is not a volume knob, gain is designed to match head unit voltage, if you don’t do this properly, you’ll get clipping, which sounds like **** to those of us who are serious about car audio, and can tell the difference between an honestly curious person, and someone acting like they know what they are talking about. Your advice would only cause someone to blow their subwoofer honestly.


lurch said:


> that is exactly the opposite of what i would have done.
> 
> i would have zeroed the bass boost and increased the gain.
> are you equipped with a remote bass level control ?
> ...


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