low-mid horn speaker compression ratio

[Cracky]

Hi,
me and my friends are trying to construct a horn speaker, with a bandwidth of 90-500 Hz. We chose to base our design on the B&C 12FW76 driver. We were surprised, that our calculations gave a compression ratio of about 1:4.88.


I used the following equations to calculate the throat area:


efficiency = Fs / (Fch * Qes)
St = efficiency * Re * density_of_air * c * Sd^2 / ((1 - efficiency) * Bl^2)


where:
St := throat area
Fch := upper cutoff frequency (500 Hz)
The other parameters are the Thiele-Small-Parameters of the driver and physical constants.



Are my equations wrong or do you think this is a reasonable compression ratio?
Can a high compression ratio damage the driver?



Thanks and have a nice day everyone.

 

[kipman725]

In my notes I have St = (P0)*C*R*Sd^2 / (Bl)^2

P0 = 1.21 kg/m^3
C = 343 m/s

I designed a very similar horn using a 15" driver and got a compression ratio of 1:2.523:
Mega midbass straight horn 139dB
I think your aiming for a specific upper cut off frequency though while I was aiming for the widest possible bandwidth. If you want to cut the horn off at 500 Hz you could add in the a throat chamber.

I recommend simulating in Hornresp, you will quickly see if your sums are correct!