I've been given a small grant by my College to design some high/mid frequency horns.
It's my understanding that the throat (acoustic) impedance of that the horn presents to the compression driver should be equal to the (acoustic) source iimpedance of the driver; the reactive part of the source impedance when the driver is oscillating into open air is reduced to near 0 by the gradual flaring of the horn, meaning more power is transfered as sound. If the throat impedance of the horn isn't properly matched to the source imepedance, it will increase the reactive part of the impedance as well. This throat impedance is in turn determined by the size of the mouth, the horn's shape, and the length.
If this understanding is correct, my question is:
How do I calculate or measure this source (acoustic) impedance of my compression drivers? There doesn't seem to be any charts on the spec sheets for ACOUSTIC impedance, and none of the texts I've read so far go into any detail about measuring the source impedance short of solving the full circuit (which I'm not confident doing, but may end up being the avenue I have to take.)
I feel like I may be misunderstanding the relationship between the source impedance and the throat impedance; perhaps the source impedance is determined by the throat impedance (or the throat impedance presented to the source is really the only one that matters), and therefore the horn only needs to be designed with a mouth impedance, length, and profile that yield the desired throat imepdance response. This would mean that the throat impedance would just have to be determined in such a way that it was purely resistive through the desired frequency range.
Am I understanding this correctly, or am I completely off point?
It's my understanding that the throat (acoustic) impedance of that the horn presents to the compression driver should be equal to the (acoustic) source iimpedance of the driver; the reactive part of the source impedance when the driver is oscillating into open air is reduced to near 0 by the gradual flaring of the horn, meaning more power is transfered as sound. If the throat impedance of the horn isn't properly matched to the source imepedance, it will increase the reactive part of the impedance as well. This throat impedance is in turn determined by the size of the mouth, the horn's shape, and the length.
If this understanding is correct, my question is:
How do I calculate or measure this source (acoustic) impedance of my compression drivers? There doesn't seem to be any charts on the spec sheets for ACOUSTIC impedance, and none of the texts I've read so far go into any detail about measuring the source impedance short of solving the full circuit (which I'm not confident doing, but may end up being the avenue I have to take.)
I feel like I may be misunderstanding the relationship between the source impedance and the throat impedance; perhaps the source impedance is determined by the throat impedance (or the throat impedance presented to the source is really the only one that matters), and therefore the horn only needs to be designed with a mouth impedance, length, and profile that yield the desired throat imepdance response. This would mean that the throat impedance would just have to be determined in such a way that it was purely resistive through the desired frequency range.
Am I understanding this correctly, or am I completely off point?
It’s not something you can easily calculate for a compression driver due to the complexity of the internal assembly, such as the phase plug. It is possible to measure it for the purpose of generating a two-port model though, which can then be used to test potential horn designs.
This can be done using only electrical measurements, but you'll need some big lengths of tube and some maths.
Some relevant papers on this topic:
https://www.aes.org/e-lib/online/browse.cfm?elib=9817https://www.aes.org/e-lib/browse.cfm?elib=12202https://www.aes.org/e-lib/browse.cfm?elib=12764https://www.aes.org/e-lib/browse.cfm?elib=12845https://www.aes.org/e-lib/browse.cfm?elib=12743https://www.aes.org/e-lib/online/browse.cfm?elib=15636
https://www.comsol.jp/paper/download/290171/cinanni_paper.pdf (this one is an overview of the process using plane-wave tube measurements, but is open to anyone to read)
This can be done using only electrical measurements, but you'll need some big lengths of tube and some maths.
Some relevant papers on this topic:
https://www.aes.org/e-lib/online/browse.cfm?elib=9817https://www.aes.org/e-lib/browse.cfm?elib=12202https://www.aes.org/e-lib/browse.cfm?elib=12764https://www.aes.org/e-lib/browse.cfm?elib=12845https://www.aes.org/e-lib/browse.cfm?elib=12743https://www.aes.org/e-lib/online/browse.cfm?elib=15636
https://www.comsol.jp/paper/download/290171/cinanni_paper.pdf (this one is an overview of the process using plane-wave tube measurements, but is open to anyone to read)