Typically the lowest frequencies I have come across for SAW devices operated at is 20MHz. I have come across suggested ranges for surface wave devices starting as low as 1MHz.

I know that at lower frequencies, the wavelength and the IDT widths increase, so the devices become larger.

And I think that due to the nature of surface waves penetration depth of one wavelength that that would impose practical limitations based on the substrate thickness.

I am just curious what are the lowest frequencies that have been demonstrated for a SAW device? As at 1 MHz assuming a wave velocity of 3000 m/s, that would be a 3 mm wavelength.

And what might be some of the limitations or factors to consider for a SAW device operating at the lower end of its frequency limits?