@colmo you can download WinISD and play around with tuning but you may have to put in the parameters from the datasheet by hand. Start by putting in Troels parameters and see what it looks like. You can also look at the Purifi design and see if that works (15l 35hz tuning https://purifi-audio.com/wp-content/uploads/2019/11/SPK4-App-Note.pdf). Also look at @xrk971 bookshelf Transmission line speaker for the Purifi this may be a better solution if you have room for a bigger box.
https://www.diyaudio.com/community/...using-ptt6-5-and-rs28f-in-a-waveguide.354778/
https://www.diyaudio.com/community/...using-ptt6-5-and-rs28f-in-a-waveguide.354778/
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increase of box volume requires an increase of Qts to maintain a flat response. Luckily this is easily obtained by adding sim series resistance to the driver which increases Qes and thereby also Qts. This trades some sensitivity for deeper bass in an optimal way. a data point is that for Qts=0.4 we get a maximally flat Butterworth response (B4) for Vbox =Vas and port tuned to fs. Our PTT6.5 needs a bit of series resistance added to bring Qts up to 0.4. a lower Qts can also work but gives a more drooping response - which may work better in a room (room bass gain). Bass tuning is therefore very subjective and room dependent. We will add more apps info with tuning examples n the future data sheets.
cheers Lars@Purifi
Just wondering if it gets roomy in too big a box, or is it the room that causes that
I take it you meant boomy.
yes both. In addition the cabinet dimensions also play a part. What looks ideal or not ideal on a simulation (usually 2pi or anechoic) gets affected by the cabinet dimensions (baffle step) as well at the room dimensions and absorptive characteristics.
this is an area of research and psychoacoustics that continues.
Speaker designers tend to anticipate room size and placement into consideration
yes both. In addition the cabinet dimensions also play a part. What looks ideal or not ideal on a simulation (usually 2pi or anechoic) gets affected by the cabinet dimensions (baffle step) as well at the room dimensions and absorptive characteristics.
this is an area of research and psychoacoustics that continues.
Speaker designers tend to anticipate room size and placement into consideration
It's not a good idea to use a reflex box that is larger than it needs to be. What a lot of people notice is that tuning and box size can be controlled separately to the driver, and they get greedy in their expectation of bass extension, but not always to good effect.
Room modes also get in the way. Room gain is not a thing in domestic listening rooms.
Room modes also get in the way. Room gain is not a thing in domestic listening rooms.
How do you explain speakers bass response being different in a smaller rooms Vs bigger rooms.
the speaker in a larger room eg. 6x9x3.6m placed well away from walls eg. Front baffle >1m from front wall?
VS
the same speaker in a small room eg.
3x3mx2.4m bedroom and being placed right up again the wall
Do you call this room modes or room gain?
Room gain is about pressurisation, as in sealed rooms... so that excludes talking about individual walls.
Room mode is referring to the room's dimensions, so you typically have a lower room mode in a larger room. This means that the same speaker, in a larger room, will tend to have a "gain" at a lower frequency.
But - there is no gain. What really happens, is that the frequency is being reflected for a longer time - it takes longer to "die" out - which we perceive as louder bass. When we dampen these frequencies with big acoustic damping materials (lower frequencies require a lot more damping than higher ones to be effective), we absorb the sound, it reflects less and the response becomes more even/smooth.
This is why, when you use EQ to dampen the bass - the bass will still "ring" or bounce between all surfaces in the room, leaving the room mode unaffected, and this is also why room acoustics ultimately is the best way - but EQ is so much easier.
We are less likely to hear distortion(any difference from linear) at lower frequencies, so that means that there is more freedom to manipulate the signal, to make the speaker sound more neutral/correct.
This isn't quite right, the mode itself is always unaffected whether it is excited or not, but the response measured at any particular point in the room will be strongly affected by the position of the source and how it excites the modes. EQ'ing modal frequencies does fix both the magnitude and temporal ringing but EQ'ing does not reduce overall low frequency decay as treatment can.
Floyd Toole Section 8.3 3rd Edition Sound Reproduction
"Because low-frequency room resonances generally behave as minimum-phase systems, individual modes can be addressed with parametric-filter equalization to reduce the magnitude of the spectral bump associated with a room resonance. If there is a bump, there will be ringing. If the bump is attenuated by equalization, or in fact any of the alternative schemes, so is the ringing. Waterfall diagrams and impulse responses routinely confirm the improvements in time domain behavior."
A good write up on room/pressurization gain here https://www.audiosciencereview.com/forum/index.php?threads/analytical-analysis-room-gain.23211/
"We have a mode at 0 Hz that pressurizes the room the same in all positions, so no gradients, and it satisfies the wave equation as discussed already. Room gain is simply the effect of having an eigenmode at DC! With the increase in resulting pressure as the excitation frequency gets closer and closer to the modal frequency being second-order (see the denominator in the equation higher up), and the output from a closed sub goes down by an order of two, we can get a resulting pressure which is flat towards DC. So while we do not have sound at 0 Hz, since there are no oscillations, the effect of any mode extends away from it. Also, for a ported subwoofer we will not be able to have this flat response, because of its higher order roll-off, but the in room response will still be modified by the mode. If you actually do the calculations, be it analytically or numerically, the 'zeroth' mode will pop right out:"
What I meant, was that the room mode can be altered by damping - EQ can't do that. I think we mostly agree 👍 What can be tricky - is me writing so that it can be understood fully 🤣 But I will also read the book again.... can't hurt 🤓
René is luckily my mentor. Studying speaker box modes gave me a similar insight to using model analysis in rooms. Placing the sealed sub in a null to kill the lowest mode seems like a great idea. The fc of the sub should then be at the lowest mode frequency. It just takes an air tight listening room to keep the response to DC…
I think I'm going to agree to disagree on this comment.
Regardless of the cause ie. "cabin gain" or room modes, as well explained by Rene in the theoretical sealed room; the effect of gain or loss below the Schroeder frequency present a big challenge for the speaker designer.
In many, if not most cases, the actual in-room response of a speaker in an individual room, is different from the power response or predicted in-room response as suggested by the spinorama/CTA2034A.
The actual in-room response of any particular speaker in any particular room needs to be measured, and compensated for, because it is often FAR OFF from the power response or predicted in-room response.
There are commercial products eg. Dirac OR Acourate that automate the measurement of the actual in-room response, and provide corrections to allow a more neutral sound.
For the active/passive speaker designer, however, who has no intimate knowledge or control of the actual room it will be placed in, we must compensate for this with using various methods eg. various amounts of baffle step compensation (for passive speakers), use of boundary compensation switches for active analog, or parametric EQ/DSP.
Below is an example of a speaker's Measured In Room Response VS Predicted In Room response. I'm not singling out this speaker for any particular reason, other that not providing my own measurements and thus suggested of bias.
As one can see, below the Schroeder frequency, the predicted in room response is deviates significantly.
Whether it is caused by boundaries, carpets, open doors, hallways, or room modes in a closed sealed room, or "cabin gain", the practical effect is what matters.
This speaker will not sound neutral in this particular room. And will need in-situ compensation.
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Sometimes these discussions become semantic.
My current room is pretty far from a sealed cabin being open on two sides, but yet there is still a reasonable amount of measurable low frequency gain when compared to an outdoor measurement. Below 100Hz there is a gradual rise that peaks or dips along with the modes and boundaries.
Nice !
How do you compare Aluminium and composite woofers from Purifi ?
IMO "pressurization" below lowest room mode is beneficial! Especially with closed box woofers. In fluid's room lowest mode makes peak/dip at 45-50Hz and beleow that response is boosted 6dB. Easy give it a bit more boost with dsp, if cone excursion allwos. BR-tuned bass is much "weaker" this way, it doesn't even react to eq much below tuning.
how does a 'real room' differ and what is the 'it' being referred to in this context?