Bass enclosures: Increase stuffing rather than enclosure size?

[nat8808]

On DIYaudio.com in the course of a thread, Joachim Gerhard (quite a well known speaker designer e.g Audio Physic) said this:

In closed boxes for subwoofer use i stuff more because you can get an effective virtual doubling of the internal volume. Here glass wool or rock wool is most efficient.

I have some large kind of PA (could say high-end PA) speakers with dual 15" drivers. An expert in the field (studio engineer, equipment designer) came round to listen to them a while ago and commented on the lack of bass considering the 2x15" drivers. On the back of an envelope and with a tape measure, he said the enclosures were too small (made by an experienced professional cabinet builder but not necessarily a speaker designer) should be nearer 300 litres rather than the 170 litres or so that they are.

So! Can I get these to work properly simply by changing the speaker stuffing of the enclosures considering Joachim's comment above? Currently they are ported with four shallow ports and stuffing is that whispy white synthetic stuff, rough to the touch (know what I mean?) I think it is only attached to the sides..

Can it be calculated or more trial and error?

 

[Tenson]

You need to provide some data, please.

1) Please tell us the exact driver model in use (datasheet if possible).

2) The physical cabinet volume

3)The port cross-section area and it's length

Are the 2 drivers and 4 ports all in the same cabinet? No walls separating the drivers (walls or braces with holes are fine)?

 

[AmDismal]

Ideally you would look to model them from scratch, and experiment with the model, and then with the actual speaker and its positioning, to get the response you want. However, if the constraints are "I ain't changing the box, the drivers or the positioning" then you could add more (and more dense) stuffing and see what happened. Ideally you would be able to measure as well as listen.

The general point is correct - adding stuffing increases the effective volume of the cabinet.

 

[Tenson]

Porting could be adjusted easily too.

 

[nat8808]

Thanks folks. Definately it is a case of being that "the constraints are "I ain't changing the box, the drivers or the positioning" ". The box would be too much work, I might change the drivers one day but too expensive for now. As they are active, I wonder how the crossover on the amps has been set up, whether they are set as if it were the 300 litre enclosure or just adjusted to work as best a possible with the current design..

They are ATC PA375 drivers, I'll have to check the voice coil model. I believe they are in one cabinet with no divide.

I'm going to have to check as they are in storage very near by...

 

[Clubsport911]

Bass is air movement. The cabinet is critical, as is the compliance of the driver. Put simply... A massive driver, moving little, makes for little bass. You have to also consider the out of phase energy - currently coming out of the cabinet via the port. If incorrectly designed and tuned , it can significantly reduce bass as it might be resonating at the wrong frequency. Then of course, there is the crossover design and how that interacts with both the cabinet and the driver

Although the physics are not new, the subject is deceptively complex. Whatever happens, ports have some cancellation of bass at certain frequencies,

There are a ton of papers about bass and speakers, and whilst you can increase the effective volume with stuffing, you cannot double it. This is unheard of.

If you PM me, I will mail you a very well written whitepaper from KEF explaining all about drivers, bass and ports. To get more bass from your speakers, may not be so easy as just more stuffing.

 

[Tenson]

If incorrectly designed and tuned , it can significantly reduce bass as it might be resonating at the wrong frequency.

Only below the resonance frequency of the port will it act like an open hole and allow out of phase sound to pass. Above resonance it acts much like a closed box. At resonance the port increases the efficiency of coupling driver movement to the air.

I have a hunch the port tuning could be improved on these speakers. It is typically too high when people just add a port and do no calculations. 'Big Port = Big Bass' is generally the thinking, but frequently incorrect.