Acoustic Suspension - is bigger better?

[Ren Kitchener]

I loved my old Yamaha NS10's. I sold them a long time ago and I really miss them. So I'm going to make some, but not copy the slight peaking the NS10's had at between 1 kHz and 3 kHz (I may be able to do this with a little filtering where it's needed). I'm also aiming to make the enclosure larger - which I'm not sure if it deviates from what acoustic suspension is about. I have this idea that as the enclosure gets larger, the bass frequency and amplitude should extend downwards, proportionally to size, but it will still maintain that 'tight' bass, in the right phase. Is this 'sound' thinking?

 

[ryanosaur]

Others with more DIY experience may confirm my understanding...
The Vb (Volume of the cabinet) will be tailored to the Driver's T/S Parameters and the target System Q. Yes, generally larger Volume will equate to a lower tuning, however there are limits to how far you can safely push that.

My understanding is that for a true Acoustic Suspension, you need a woofer with a low Fs and a low Qts (around .3).

This article was published here earlier this year:

Sealed is Not Acoustic Suspension in Loudspeakers

Some vented speakers offer port plugs for sealing the cabinet and making them act like sealed speakers. This does not result in a true acoustic suspension. What's the difference? Read on to find out.

www.audioholics.com

Perhaps it will help.

 

[Ren Kitchener]

Thank you Ryanosaur - I have a 'cheap' 70 W, 8" woofer. fs = 37 Hz, Qts = 0.7 (which is on the high side, but I have no idea what it really means). I'll have a go with a 70 or 80 Litre box, full of pillow stuffing and keep the power down to 50 W max - I have a little more confidence now, after reading the article and taking your thoughts on board.

 

[Swerd]

I have this idea that as the enclosure gets larger, the bass frequency and amplitude should extend downwards, proportionally to size, but it will still maintain that 'tight' bass, in the right phase. Is this 'sound' thinking?

It's not quite as simple as that. For each woofer, there is a range of enclosure volumes that can work. Within that range, bigger is usually better, but not always. You must know the so-called Thiele/Small (T/S) Parameters for a woofer, decide whether a given woofer works better in a sealed or ported cabinet, decide on what overall Q value you want in your speaker, and then calculate the enclosure volume.

A good list of verbal definitions of the various Thiele/Small (T/S) Parameters can be found here:

Understanding Loudspeaker Data

Choose the most appropriate loudspeaker for a particular enclosure by understanding louspeaker performance data using Thiele-Small Parameters.

www.eminence.com

To design a woofer enclosure, you must know a woofer's Fs, Vas, and Qts

In general, Qts values of 0.4 or below indicates a woofer is well suited to a vented enclosure. Qts between 0.4 and 0.7 indicates suitability for a sealed enclosure. Qts of 0.7 or above indicates suitability for free-air or infinite baffle applications. However, there are exceptions.

The first 2 or 3 chapters of

Speaker Building 201 by Ray Alden

does a very good job of explaining how to calculate cabinet volume for bass drivers.

 

[Ren Kitchener]

Thank you Swerd!

The speaker's Qts is 0.7. So it should be okay for the large 'box' I'm planning to make for it (around 80 Litres). It will either work well or fail badly.

That is a really nice summary for the Qts Swerd - thank you!

~Ren.

 

[Swerd]

The speaker's Qts is 0.7. So it should be okay for the large 'box' I'm planning to make for it (around 80 Litres). It will either work well or fail badly.

That is a really nice summary for the Qts Swerd - thank you!

~Ren.

You might not need a box as big as 80 L. It's easier to run a number of calculations than doing trial & error with several test cabinets.

I'd give you the formula for sealed cabinet volumes, but right now I can't find my copy of Ray Alden. Basically, you can't just do the calculation once. You have to pick a range of overall Q values and run the numbers. I'd stick with overall Q values in the 0.5 to 0.7 range. They produce tighter or dry sounding bass, well damped bass. A bass note starts and stops rapidly, without ringing on and on.

Avoid larger Q values. The cabinet will be smaller and it will make louder bass, but it will be less damped, resulting in sloppy sounding bass that keeps ringing. That tends to muddy up the mid-range sound of the entire speaker. The decision for overall Q is a personal choice, but will directly impact the cabinet size and the speaker's sound. It's nice to know your choices before making sawdust.

There are plenty of online calculators that also do this. If you have Excel on your computer, try playing with this calculator:

Basic Enclosure Volume Calculator

Hey everyone. Here is a volume calculator I came up with. I figured it would be a big help for some of the newbies. This site does not have one and all the others seem to. It is in excel. Simply enter in the dimensions (internal) that you wish and it calculates exterior dimensions for...

forums.audioholics.com

 

[Verdinut]

With a sealed box, the situation is different from ported designs, and you won't most likely get any improvement in low frequency performance by making it bigger. There is an optimum box volume for each woofer which should be used.

 

[Ren Kitchener]

Thanks Verdinut - what you say is exactly what I'm seeing with the calculators, but in my mind, I'm not getting why a larger volume box wouldn't let the cone move a little further - I'm still sort of obsessing about the formula; P1/V1 = P2/V2 that I remembered from my Physics lessons at school (or something similar to that formula) - maybe I should let it go, and accept that I failed my Physics exam. ~Ren.

 

[Verdinut]

Ren,
The calculations with your software, I believe, would be the good figures. If you can simulate the response with a speaker building software, using a bigger box than that suggested by it, you will see that there might not be any improvement in LF response. The cut-off frequency depends on the overall box Qtc.

 

[Verdinut]

@Ren Kitchener,
There's a popular notion that cut-off frequency for sealed enclosures decreases with the increasing of box volume. This is true only for Qtc equal to 0.707 or greater. For values less than 0.707, increasing volume causes an increase in cut-off frequency.

You have some Physics background. I'm sure that you would find the following technical book interesting and detailed on loudspeaker design, and I strongly recommend it:

Loudspeaker Design Cookbook: Dickason, Vance: 9781882580330: Amazon.com: Books

Loudspeaker Design Cookbook [Dickason, Vance] on Amazon.com. *FREE* shipping on qualifying offers. Loudspeaker Design Cookbook

www.amazon.com

 

[Ren Kitchener]

Thanks @Verdinut,

This has been tremendously helpful.

My speaker has a Qtc of 0.700 - so it's it's just below the pivot point you mention (e.g. at <0.707, the cut off frequency should get higher with an increase in volume). I can test it - I still have my nice studio condenser mic, a good digital oscilloscope and an Agilent 33120A signal generator. No anechoic chamber, but I'll 'close-mic' the speaker. It'll be interesting to see what I can actually get from it in a month's time - or less if I can get the cut-list together soon. ~Ren. p.s. the book looks good - I may add it to my Christmas list, to look forwards to.

CORRECTION: The Qts is 0.55 (the stockist made the error on their website) - but I'll still have a go with a larger box to see what happens. Then add some pine blocks to reduce the volume.