Featured Using your entire car as a HUGE ported enclosure?

1aespinoza said:

Yes. I have a 1999 Silverado and when I crack a window, the lower bass notes immediately sound louder. It definitely gets better with both windows fully open.

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Hmm. That's a car with a much smaller cabin with 1500 liters since it does not have the back seats, and it probably has a slightly bigger window (maybe 45x45). Throwing in those numbers tells me that your windows were being tuned to 37 Hz. That matches with the tuning of most sealed car boxes, and the range of all normal music tracks.

However, I'm targeting the lower range (close to 20 Hz) and I need it at the highest SPL. It's the sound that you only hear at the best nightclubs. Special music tracks are needed.

1aespinoza said:

As far as that wheel well experiment goes, I think a fully sealed mounting baffle would work great. It would also allow you to keep the subs a bit more protected from road debris.

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But sealed boxes are the least efficient in the lower end (20 Hz). It will not play 20 Hz loud. And you might not be able to throw 20 Hz at full power because of cone excursion.

For instance, my He15 sub fits well in a 33 L small box but there it makes just 70 dB/w at 20 Hz Whatever driver you test in a small sealed box will always give very similar output at 20 Hz.

In contrast, when simulating "whell well" setup, I can use three IB15 drivers in a manifold, and the predicted response is 95 dB/w at 20 Hz. This 25 dB difference makes it incomparable.

In regards to the debris, they would be safe with a manifold similar to this one:



Video with the car IB setup:


He has made a cut to the well, and uses a layer of foam material covering it:



But am pretty sure this foam at the bottom is restricting the bass output. I would try to find a different solution to hold the debris. Something that would not obstruct the airflow.

1aespinoza said:

What I meant is sealing the front of the baffle from the rear or exposed side.

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Oh, so you were still talking about IB. Sorry for my misunderstanding.

1aespinoza said:

Instead of a manifold I think it more pragmatic to make a raised floor.

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A raised floor takes too much space from the trunk, partially defeating the purpose of an IB. But a thin mesh seems enough. Like this "Galvanized Steel Mosquito Screen":

maylar said:

The long standing myth/legend is that this phenomenon is due to "reflections". Sound reflects off the windows and somehow ends up out of phase with the sub and therefore cancels some of its output.

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The measured car SPL reveals that there is a lot more going on in addition to the window port effect that I described.

In regards to the cancellation effect, can be observed in the mid-bass.

I believe that if the sub is placed at the center of the car (over the back seats) and the car is oval-shaped (like the one I had), that would minimize the overall resonance effects of the car:



However, usually it is placed at the end of the trunk and the car is long, so it causes a "pipe effect":



Those SPL measurements below are from JBL. They show the cancellation effect occurring slightly below 70 Hz in most vehicles:



This effect seems to fit the pipe harmonics calculation formula, which depends on the length that the sound takes to travel through the long-shaped car:

f_n = (n * c) / (2 * L)
or
L = (n * c) / (2 * f_n)

Where:

• f_n = frequency at which cancellation occurs for the nth harmonic (in Hz)
• n = harmonic number (1, 2, 3, ...)
• c = speed of sound in air (approximately 343 m/s at room temperature)
• L = effective internal length of the car (in meters)

So if the first and strongest dip in the response is at 68 Hz, we can assume it is the first harmonic, so the effective car length can be calculated as 343 / (2 * 68) = 2.5 meters, which seems to be correct if you consider that there is a curved trajectory from the back of the trunk to the floor of the driver/passenger, as I show in the image. In real pipes, the harmonics will also create great peaks, but since the car has a huge amount of absorption material, the resonations are more likely to cause dips than peaks.



Yet, the mid-bass resonance is not the most prominent car effect. What is more noticeable is the lower-end gain in the bass. And it's not just the "window port" effect. It goes much beyond that. The response that I posted before shows about +8 dB at 20 Hz, but the car measurements show much more, around +20 to +25 dB at 20 Hz (compared to the loudspeaker's reference SPL level). The huge gains are due to "directivity".

If you add a wall at the side of a subwoofer dividing the entire airspace around it in half, the subwoofer suddenly gains +3 dB in efficiency. Because the air around it will become "heavier". This is why adding a 2nd sub gives you +6 dB. This is +3 in the overall efficiency (because you gave half airspace for each sub) and another +3 dB because you doubled the overall input power.

But if you divide the air space again (by adding another wall or by doubling the subs again), you get another +3 dB in efficiency. You can do this multiple times. This is why horns are so efficient. Horns allow the loudspeaker to transfer a huge amount of power into the air with a small cone excursion, so it is a more efficient transfer. And since the car restricts the airspace so much, it produces a similar effect as a horn, in regards to the lower end frequencies. The efficiency gain happens when the wavelengths are long enough so that the wave generation becomes "heavier" inside the car than outside of it.

As noted by measurements, the car gains at the lower end begin around 60-85 Hz:



I believe the exact frequency where the gains start depends on the quarter wavelength effect. It's the length needed for the expansion phase of the wave. The quarter wavelength of 60 Hz is (343 / 60) / 4 = 1.4 meters, which is about the maximum width we find in cars. So waves above 60 Hz can spread to the sides but waves below 60 Hz start to feel forced in just one direction, similar to the effect of a horn. And for 85 Hz, we get (343 / 85) / 4 = 1 m for the quarter wavelength, which matches the width of many trunks, so there can be some gain below 85 Hz too.


Image source here.