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Questions from the net

In this thread I will comment on questions found other places on the net - mainly on audio/hifi forums. This may be one specific question, or a topic that often shows up.

?: Does a sealed box always have better transient response compared to ported?

The 2 problems with acoustic port loading for very low frequencies are non-linearities and pipe resonances.

At low f with insufficient port area the air velocity gets too high, which creates loss of efficiency, distortion and noise.

Trying to increase the area gives a very long port, which moves the 1. pipe resonance so low in frequency that it enters down into the pass-band of a subwoofer.

Both problems can be solved.

Small-signal transient response for a ported will be equal to a sealed with eq to bring the frequency response back in shape. This is a minimum-phase system, where the resulting step response simply is a result of the frequency response, the ringing at cut-off is a result of the roll-off characteristics.

What is more important, is the large signal transient response. Now the construction of the driver and for a ported/acoustically loaded its port design as well, is what will determine how it performs. Acoustic loading can increase output capacity significantly due to reduced cone displacement.

In all practical situations, what happens when the bass-system is placed inside a room, is what determines (small-signal) transient response. What we observe now, is a frequency response that is very different from the calculated, where now the room completely dominates. Time domain behavior follows the frequency response, with similar disastrous effects. Fortunately, this can often be significantly improved with eq. Then we understand that whatever frequency response the subwoofer was born with does not matter, it is the output capacity that limits what can be achieved.

The Compact Horn subwoofers solves both issues with port radiator area and pipe resonances. Still, laws of physics apply, so it is not possible to get both loud and low bass from a too small box. This article tries to explain how it works: https://www.kvalsvoll.com/Articles/CompactSubwooferTechnology.htm

?: What is a transmission line speaker?

The classic transmission-line speaker is a long acoustic channel with decreasing area, filled with damping material to absorb all sound. The claimed benefit is removal of all resonances and reflected sound from inside the cabinet, so the driver operates in a cabinet that is removed acoustically - like an acoustic black hole.

It was supposed to be driven by a driver with heavy moving mass, and low motor force. The length of the channel determines the low frequency cut-off, where this length equals 1/4 wavelength. This gives a speaker with quite low cut-off, and low efficiency.

Then someone came up with the idea of letting some of the sound radiate out from the end of the pipe, to increase efficiency. Since this sound is delayed from the sound from the driver diaphragm, the port output and driver output combine in-phase and the total sound output is then increased.

Of commercial designs utilizing this concept, the IMF were famous for its reproduction of the very lowest frequencies - in a time where subwoofers did not exist, this was among very few speakers that could reproduce sound down to and below 20Hz.

Transmission lines are complicated to design and build. And there are some technical challenges that can be difficult to solve. The channel itself can have resonances, especially upwards in frequency, above its intended frequency range. The port output is delayed and only sums correctly with the sound from the front of the driver across a limited frequency range - there will be cancellation above this range, and also problems with delayed sound output.

The ported variations will have the benefit of acoustic loading and increased efficiency due to the port output, similar to an ordinary ported bass-reflex box.

The closed-end variant does not have those issues with delayed sound and problematic summation, and it is possible to achieve resonance-free response, and this black-hole cabinet behavior. But the ordinary sealed cabinet can perform just as good, when filled appropriately with damping material.

Great care must be taken when designing a transmission line with acoustic port at the end of the channel, to ensure there are no resonances left, and attenuate the output from the port at higher frequencies. If done properly, it will perform quite similar to a ported box.

I do not have a transmission line to show, but I can use the F2 as an example of a ported horn design that works in a similar way, and presents the same challenges. The driver is mounted off-set into the channel of a short horn, and this horn is then ported at the bottom of the cabinet. Like this:

For this to work properly, the horn channel must be damped very accurately, in the right places with the right type and amount of damping material. Here is the frequency response of the port output - yellow is with no damping, red is damped and how the speakers is when finished:

We see that the undamped response is quite horrible and useless, with severe resonances. The damped is smooth and falls off towards higher frequencies.

Then the question is - why bother with such a complicated design, with all its potential issues? For the F2, the answer is simply that this bass driver needs a cabinet with acoustic reinforcement from around 200Hz and down, to provide any bass at all. The F2 now achieves reasonably flat response down to a roll-off at 60Hz, which gives typical in-room response down to around 40Hz.

The Compact Horn subwoofers can also be seen as a variation of transmission line - they have long, but narrow acoustic channels, ported at the end. Here, the resonance problem is solved differently, there is no damping material in the horn channel, and some of the problems with summation that is difficult with full-range speakers are not present since they are subwoofers with limited frequency range upwards.

Whether you call it transmission-line or something else, what we are dealing with here are cabinets with some sort of acoustic loading using a channel that is long compared to the wavelength. And they can have benefits like increased output capacity, extended low-frequency range and better acoustic coupling.

?: I am looking for a new DAC that can give me a 3-dimensional presentation of the instruments - which measurements, data to look for?

3-D presentation - holography - imaging, is not relevant for a DAC. No DAC can improve any of this. Actually, any DAC that is not defective is completely transparent sounding - it does not add or subtract or change anything to the sound.

What does matter for those spatial qualities, are loudspeakers, placement and room acoustics.

?: I want to add acoustic treatment to my room, to fix the frequency response - How?

Improving room acoustics is not about fixing the frequency response, often you will not see any improvement there at all. See this thread on tips for what to do, and how to analyze measurements:


?: What is Group Delay?

This is more of a technical question for us audio nerds, though.

Group Delay is the derivative of the phase. That is the technical definition, non-arguable.

What this mean, and how it relates to time delay, is that it shows how the signal is delayed compared to some other part of the frequency range.

Resonances, room reflections, any deviation from flat frequency response, all create group delay.

High group delay relates to sound quality as compromised transient response and reduced imaging accuracy.

Example of group delay, F205 speakers in Room2 (no smoothing):

Other group delay example, trad hifi speaker in typical room (1/12 octave smoothing):

?: How can a "good" DAC with a nice sound signature improve sound for active speaker systems?

In modern active speaker systems, like the F205+T6, all analog input signals are converted to digital, processed digitally, and then converted back to analog before the output power amplifiers. So the signal will always go through at least one additional AD-DA conversion anyway.

And no DAC of today of decent quality has a "sound" - it just is so good, that there are no audible contributions added, no audible loss of what is coming in.

The best would be to feed the system with digital source signal, but if that is not possible, it is not likely there will be any audible degradation to the sound by just using the analog input on the speaker system.

The new A502 is an example of a DSP amplifier with digital processing and both analog and digital input, and it is guaranteed sonically transparent - does not add or remove anything:


F205+T6 sound system:


Relevant articles:




?: Can speaker directivity affect room decay?

Radiation pattern of the speaker does not change the slope of the decay, but more controlled and narrow pattern will give a better initial early decay drop, and reduces the relative level of the reflected sound energy compared to the early direct sound.


Initial drop:

Wide vs. narrow speaker: