# How to design an enclosure to maximise lower frequencies?



## bosher (Jun 16, 2012)

Ok basically i have a pair of 6X9 speakers and i bought an enclosure for them off ebay, about .31 cubic feet in size. Now, although their response is largely increased, the lower frequencies are not that good. How do you go about designing an enclosure to get better bass? I'm currently studying a bit of this at university, so i know about ported and sealed and a port is used to get a better bass response. I know there are other enclosure types like bandpass or passive radiator, but i assumed ported was the best? What i'm looking for is one that can really hit the low frequenices well. This is why i went with ported, because theres lots of stuff out there that helps you with tuning the ports to 'resonate?' at specific frequency.
However what really confuses me is that i have 2 shelf speakers, edirol MA-15D, and they are about half the size yet their response is soo much better. Why is this? is it to do with the size of the enclosure in relation to the cone/driver? And also with the material used to build the speaker? and baffle inside the speaker to absorb the rear soundwaves? 

The speakers i am trying to build an enclosure for are Infinity 9613i 

In short, what do you need to do to get an enclosure that significantly boost low frequencies?

Cheers


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## GranteedEV (Aug 8, 2010)

Welcome to HTS.



bosher said:


> Ok basically i have a pair of 6X9 speakers and i bought an enclosure for them off ebay, about .31 cubic feet in size. Now, although their response is largely increased, the lower frequencies are not that good. How do you go about designing an enclosure to get better bass? I'm currently studying a bit of this at university, so i know about ported and sealed and a port is used to get a better bass response. I know there are other enclosure types like bandpass or passive radiator, but i assumed ported was the best? What i'm looking for is one that can really hit the low frequenices well. This is why i went with ported, because theres lots of stuff out there that helps you with tuning the ports to 'resonate?' at specific frequency.


Ported is not the best, nor is passive radiator, nor is bandpass, nor is a transmission line, nor is sealed, and nor is a horn! Each has their own pros and cons. You can generally simulate these pros and cons in a CAD program like Jeff bagby's box modeller, or winISD. Once you've selected the tradeoffs which most appeal to you, the second step is the real world response. Sims may be accurate in an open field, but once placed inside a home or a car environment, a new variable is introduced.



> Why is this? is it to do with the size of the enclosure in relation to the cone/driver? And also with the material used to build the speaker? and baffle inside the speaker to absorb the rear soundwaves?


There are many factors at play - to the extent that I would tire myself out explaining them. It gets especially difficult when people talk about "low frequencies" as an all-encompassing term. Is 60hz a low frequency by your definition? Or are we talking about 20hz here? A strong understanding of the spectrum in your choice of music is necessary. Many of us here at HTS consider 15hz to be a low frequency, but you may not have any 15hz content you listen to. The most I can say is that the driver's Thiele Small Parameters will tell you how it will interact with different sorts of boxes, and a driver's thermal power handling and linear excursion capabilities will define its distortion limits at various frequencies. Balancing these things out the best you can is the most you can do when designing a box - you can't pull performance out of a driver which is not there.



> In short, what do you need to do to get an enclosure that significantly boost low frequencies?


Simply put, it's a combination of things, and there's no "magic enclosure". It starts with identifying the frequencies your driver is capable of, and then examining whether or not any enclosure can reproduce these frequencies at the desired output levels. Don't forget to factor in room gain or cabin gain. The ability to measure is never a bad thing either.


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## GranteedEV (Aug 8, 2010)

This is the driver being used:

http://a248.e.akamai.net/pix.crutchfield.com/Manuals/108/108421I.PDF

I don't have access to the specs where I am at the moment, but one thing seems clear to me just from eyeballing the driver page.

These speakers are designed to be in free air - mounted on the door or something along those lines. Therefore they probably have a high QTS at least _over_ .7 What this means is that around the point of driver resonance frequency (the fs), the driver has a lot of acoustic energy produced over a relatively narrow bandwidth.

A q of .7 is generally a decent frequency response for a box which has a good balance - it is "maximally flat".

What an enclosure - of any sort - will invariably do is _raise_ Q.

So if your driver has a qts of .7, your box may have have a qtc anywhere from 1 to 3 - determined by volume of the box (Vb) in relation to the free air compliance of the driver (VAS). 

What you normally want is a driver with a qts under .6 or so, so that the box can have a qtc under .8

Likely in this case, this is an impossible scenario as drivers designed for free air sound best with as much volume of air as possible - the free air quality factor (qts) will always be lower than the quality factor in-box (qtc). 

Additonally while a port can extend bass, it can always reduce bass extension depending on where it is tuned. 

When i get home I will take a better look at the T/S Parameters of the driver, but my suspicion is that this driver does not _want_ to be in a box.


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## bosher (Jun 16, 2012)

Thanks for your replies! 



GranteedEV said:


> The most I can say is that the driver's Thiele Small Parameters will tell you how it will interact with different sorts of boxes, and a driver's thermal power handling and linear excursion capabilities will define its distortion limits at various frequencies. .


So your saying the cone's T/S parameters will also influence the design of the enclosure? I never really understood a lot of the parameters, guess i need to do some research then!



GranteedEV said:


> It gets especially difficult when people talk about "low frequencies" as an all-encompassing term. Is 60hz a low frequency by your definition? Or are we talking about 20hz here? A strong understanding of the spectrum in your choice of music is necessary. .


Sorry about that, for me low frequency is between 20 - 120hz, perhaps more 40/50hz - 120hz since i doubt very few songs utilise the subsonic frequencies. 



GanteedEV said:


> These speakers are designed to be in free air - mounted on the door or something along those lines. Therefore they probably have a high QTS at least over .7 What this means is that around the point of driver resonance frequency (the fs), the driver has a lot of acoustic energy produced over a relatively narrow bandwidth. .


Yeah your right, they are car speakers. However i dont want to put them in my car at this point since its not really worth the hassle, so i was trying to find them a home in my home. 

I always get a bit confused when people talk about free air or infinite baffle. Does free air mean mounting the speaker in the middle of a piece of wood and thats it? Or am i missing something? 
I also though infinite baffle were sealed/vented enclosures but with foam lining all the inside of the speaker?

I've looked around trying to find T/S parameters for these speakers but i can't seem to find any?! The manufactures haven't really given much detail about them other than their wattage/sensitivity and frequency response. Is there a way to judge the T/S parameters based on this? 

They are:

4 Ohm Impedance 
Sensitivity 92dB
Watt RMS 100W
Watt Peak 300W
Frequency response 46 Hz - 21Khz

Thanks a lot!


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## GranteedEV (Aug 8, 2010)

bosher said:


> So your saying the cone's T/S parameters will also influence the design of the enclosure?


Also is not the the right word... rather "foremost" would be be correct



> I never really understood a lot of the parameters, guess i need to do some research then!


A lot of the parameters aren't really necessary for the layman to "understand". As long as you understand the relationship between Qts, VAS, and Fs, you should be good Playing around in box modelling software like winISD might help in this regard.



> Sorry about that, for me low frequency is between 20 - 120hz, perhaps more 40/50hz - 120hz since i doubt very few songs utilise the subsonic frequencies.


*just a pet peeve, but it's _infra_sonic frequencies. Subsonic would mean the sound pressure levels are below audibility, with no regard to frequency.



> Yeah your right, they are car speakers. However i dont want to put them in my car at this point since its not really worth the hassle, so i was trying to find them a home in my home.


That's simply not an option. THere's _main_ two reasons for this:

1) Due to their likely high qts and probably high vas, they will have too high a qtc in _any_ box.
2) Their crossover is not designed for a finite baffle, therefore they will lack baffle step compensation, and thus sound very thin in the lower mids.



> I always get a bit confused when people talk about free air or infinite baffle. Does free air mean mounting the speaker in the middle of a piece of wood and thats it? Or am i missing something?


Free air means that there is no enclosure acting as a spring upon the driver.
Infinite baffle implies that the driver is in free air, and that the rear wave of the driver is completely sealed off from the front wave of the driver.



> I also though infinite baffle were sealed/vented enclosures but with foam lining all the inside of the speaker?


They are not. 



> I've looked around trying to find T/S parameters for these speakers but i can't seem to find any?! The manufactures haven't really given much detail about them other than their wattage/sensitivity and frequency response.


That is a big sign that the driver is simply not meant for use in any enclosure.



> Is there a way to judge the T/S parameters based on this?
> 
> They are:
> 
> ...


None of this is useful information. If you reworked the crossover after baffle step losses, i'd expect the sensitivity to drop down to aroun 87db. Impedance is not a static number. Power handling means nothing without a context. And the frequency response shows no tolerance (+/- 2db being a good speaker)


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## fbov (Aug 28, 2008)

GranteedEV said:


> ...*just a pet peeve, but it's _infra_sonic frequencies. Subsonic would mean the sound pressure levels are below audibility, with no regard to frequency.
> ...


I think I have a new pet peeve... We've all used ultrasonic cleaners - too high to hear. Infra-red and Ultra-violet in E&M, too. This should make sense to everyone. It does to me... now. Glad you brought it up - thanks!

However, if I may return the favor, subsonic refers to speed, as in supersonic flight, not audibility. 

Have fun,
Frnak


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## bosher (Jun 16, 2012)

alright cheers for the help! learnt quite a lot about it then...guess i'm going to have to dig up some more speakers!


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